Fast Ethernet Local Network. Ethernet equipment and Fast Ethernet. Quick Feature LAN FDDI

Note the main features of the development of Ethernet networks and the transition to Fast Ethernet networks (IEEE 802.3U standard):

- Tenth-time increase bandwidth;
- Saving the CSMA / CD Random Access Method;
- saving frame format;
- Support for traditional data media.

These properties, as well as support for two speeds and auto-detects 10/100 Mbps, embedded in network cards and Fast Ethernet switches, allow you to make a smooth transition from Ethernet networks to more high-speed FAST Ethernet networks, providing advantageous continuity compared to other technologies. Another additional factor of successful market conquest is the low cost of Fast Ethernet equipment.

Fast Ethernet Standard Architecture

Fast Ethernet level structure (including the MII interface and Fast Ethernet transceiver) is shown in Fig. 13. Even at the Stage of Stage 100Base-T, the IEEE 802.3u committee determined that there is no universal signal encoding scheme that would be ideal for all three physical interfaces (TX, FX, T4). If you compare with the Ethernet standard, then there the coding function (Manchester code) performs the level of physical alarm PLS (Fig. 5), which is located above the medium-dependent AUI interface. In the Fast Ethernet standard, the encoding functions performs the PCS encoding sublayer located below the medium-dependent MII interface. As a result, each transceiver must use its own coding schema set, which is best suited for the appropriate physical interface, for example, 4B / 5V and NRZI set for the 100Base-FX interface.

MII Interface and Fast Ethernet Transceivers. The MII interface (Medium Independent Interface) in the Fast Ethernet standard is an analogue of the AUI interface in the Ethernet standard. The MII interface provides the relationship between summons of matching and physical coding. Its main purpose is to simplify the use different types medium. The MII interface involves the further connection of the Fast Ethernet transceiver. A 40-pin connector is used for communication. The maximum distance in the MII interface cable should not exceed 0.5 m.

If the device has standard physical interfaces (for example, RJ-45), the structure of a physical layer reference can be hidden inside the chip with large logic integration. In addition, deviations are allowed in the protocols of intermediate suite in a single device, which put the main goal of the increase in speed.

Physical Interfaces Fast Ethernet

The Fast Ethernet IEEE 802.3u standard features three types of physical interface (Fig. 14, Table 6 The main characteristics of the physical interfaces of the FAST Ethernet IEEE 802.3u standard): 100Base-FX, 100Base-Tx and 100Base-T4.

100Base-FX. The standard of this fiber optic interface is fully identical to the FDDI PMD standard. The main optical connector of the 100Base-FX is Duplex SC. The interface allows duplex communication channel.

* - The distance is achieved only in duplex communication mode.
100Base-TX. The standard of this physical interface involves the use of unshielded twisted pair of category not lower than 5. It is completely identical to the FDDI UTP PMD standard. The physical port of RJ-45, as in the 10Base-T standard, may be two types: MDI (network cards, workstations) and MDI-X (Fast Ethernet repeatters, switches). The MDI port in single quantity can be available on the Fast Ethernet repeater.

For transmission over the copper cable, pairs of 1 and 3. Couples 2 and 4 are used - free. The RJ-45 port on the network card and on the switch can support, along with the 100Base-TX mode, and 10Base-T mode, or the function of auto definition of speed. Most modern network cards and switches support this feature by RJ-45 ports and, in addition, can work in duplex mode.

100Base-T4. This type of interface allows you to provide a half-duplex communication channel over a twisted pair UTP Cat. 3 and higher. It is the possibility of transition of an enterprise from the Ethernet standard to the Fast Ethernet standard without radical replacement of the existing Cable system based on UTP CAT.3 should be considered the main advantage of this standard.

Unlike the 100Base-Tx standard, only two twisted cable pairs are used, all four pairs are used in the 100Base-T4 standard. Moreover, when communicating the workstation and repeater by means of a direct cable, data from the workstation to the repeater goes along the twisted pairs 1, 3 and 4, and in the opposite direction - on pairs 2, 3 and 4, pairs 1 and 2 are used to detect conflicts like the Ethernet standard . The other two pairs 3 and 4 alternately, depending on the commands, can pass the signal or in one or in the other direction. Signal transmission In parallel with three twisted pairs is equivalent to inverse multiplexing, considered in Chapter 5. The bit rate per channel is 33.33 Mbps.

Symbolic coding 8B / 6T. If manchester coding was used, then bitmaps per one vitua couple Would be 33.33 Mbit / s, which would exceed the set limit of 30 MHz for such cables. Effective reduction in the modulation frequency is achieved if instead of a direct (two-level) binary code to use three-level (Ternary) code. This code is known as 8V / 6T; This means that before the transmission occurs, each set of 8 binary bits (symbol) is first converted in accordance with certain rules in 6 triple (three-level) characters.

The 100Base-T4 interface has one significant disadvantage - the fundamental impossibility of supporting the duplex transmission mode. And if during the construction of small Fast Ethernet networks using 10Base-TX repeaters, there is no advantage over 100Base-T4 (there is a collisional domain, the bandwidth is not more than 100 Mbps), then during the construction of networks using switches, the lack of an interface interface 100Vase-T4 becomes obvious and very serious. Therefore, this interface has not received such a large propagation as 100Base-TX and 100Base-FX.

Types of devices Fast Ethernet

The main categories of devices used in Fast Ethernet are the same as in Ethernet: transceivers; converters; network cards (for installation on workstations / file servers); repeaters; Switches.

Transceiver - a two-port device, covering the PCS, RMA, PMD and AUTONEG sublayer, and having, on the one hand, the MII interface, on the other - one of the medium-dependent physical interfaces (100Base-FX, 100Base-TX or 100Base-T4). Transceivers are used relatively rarely, as rarely used network cards, repeaters, switches with MII interface.

LAN card. The most widespread network cards with a 100Base-TX interface on the PCI bus were received. Optional, but extremely desirable, RJ-45 port functions are 100/10 Mbps autoconfiguration and duplex support. Most modern manufactured cards support these functions. Network cards are also available with 100Base-FX optical interface (IMC, Adaptec, Transition Networks, etc.) - the main standard optical is the SC connector (ST) on multimode OS.

Converter (Media Converter) is a two-port device, both ports of which represent medium-dependent interfaces. Converters, unlike repeaters, can work in duplex mode for excluding the case when there is 100Base-T4 port. 100Base-TX / 100Base-FX converters are distributed. By virtue of general trends in the growth of broadband extended networks using single-mode wok, the consumption of optical transceivers for single-mode ES sharply increased in recent decades. Converter chassis combining several separate modules 100Base-TX / 100Base-FX can connect the plurality of fiber-optic segments converging in the central node to the switch equipped with duplex ports RJ-45 (100Base-TX).

Repeater. By parameter of maximum time delays when repeatering frames, FAST Ethernet repeaters are divided into two classes:

- Class I. Double RTD delay should not exceed 130 W. For less than harsh requirements, this class repeaters may have T4 and TX / FX ports, as well as combine the stack.
- Class II. To repeat this class, more stringent dual-run delay requirements are imposed: RTD

Switch - Important device of corporate networks. Most of the modern Fast Ethernet switches support 100/10 Mbps autoconfiguration via RJ-45 ports and can provide a duplex communication channel over all ports (except 100Base-T4). Switches may have special additional slots to establish a UP-LINK module. Optical ports like FAST Ethernet 100Base-FX, FDDI, ATM (155 Mbps), Gigabit Ethernet, etc., can act as interfaces in such modules.

Large manufacturers of switches Fast Ethernet are companies: 3Com, Bay Networks, Cabletron, Dec, Intel, Nbase, Cisco, etc.

Introduction

The purpose of the creation of this report was a brief and affordable presentation of the basic principles of work and features of computer networks, on the example of Fast Ethernet.

The network is called a group of connected computers and other devices. The main purpose of computer networks is the sharing of resources and the implementation of interactive communication both within one company and abroad. Resources are data, applications and peripherals, such as an external drive, printer, mouse, modem or joystick. The concept of interactive communication of computers implies messaging in real time.

There are many sets of data standards in computer networks. One set is the Fast Ethernet standard.

From this material you will learn about:

· Fast Ethernet technologies
· Switches
· FTP cable
· Types of compounds
· Computer network topologies

In my work, I will show the principles of the network based on the Fast Ethernet standard.

Switching local computing networks (LAN) and FAST Ethernet technology have been developed in response to the need to improve the efficiency of Ethernet network operation. By increasing bandwidth, these technologies can eliminate "bottlenecks" on the network and support applications that require high data transfer rates. The attractiveness of these solutions is that you do not need to choose or another. They are complementary, so the efficiency of the network functioning is most often possible by using both technologies.

The collected information will be useful, both persons starting to learn computer networks and network administrators.

1. Network scheme

2. Fast Ethernet Technology

computer Network Fast Ethernet

Fast Ethernet - ETHERNET technology development result. Based and keeping in inviolability The same CSMA / CD method (collective access with channel poll and collision detection), FAST Ethernet devices operate at a speed, 10 times higher than the Ethernet speed. 100 Mbps. Fast Ethernet provides sufficient bandwidth for applications as automated design and production systems (CAD / CAM), graphics and image processing, multimedia. Fast Ethernet is compatible with 10 Mbps Ethernet, so that the Fast Ethernet integration into your LAN is more convenient to implement the switch, and not the router.

Switch (Switch)

Using switches Many working groups can be interconnected to form a large LAN (see scheme 1). Cheap switches work better than routers, ensuring higher efficiency of LAN functioning. Fast Ethernet Working Groups, including one or two hubs, can be connected via the Fast Ethernet switch to further increase the number of users, as well as coverage of a more extensive area.

As an example, consider the following switch:

Fig. 1 D-LINK-1228 / ME

The DES-1228 / ME switched series includes custom FAST Ethernet switches 2 "Premium" class. Possessing the advanced functionality, the DES-1228 / ME devices are an inexpensive solution to create a safe and high-performance network. Distinctive features of this switch are high port density, 4 UPLink Gigabit Port, a small step of changing the settings to control the bandwidth and improved network control. These switches allow you to optimize the network both in terms of functionality and in terms of cost characteristics. The Switches of the DES-1228 / ME series are the best solution for both the functionality and value characteristics.

FTP cable

LAN-5EFTP-BL cableconsists of 4 pairs of single-core copper conductors.

Conductor diameter 24AWG.

Each conductor is concluded in HDPE insulation (high density polyethylene).

Two conductors twisted with a specially selected step make up one twisted pair.

4 twisted pairs are wrapped with a polyethylene film and, together with a copper single-core grounding conductor, enclosed in a shared screen of foil and PVC shell (PVC).

Direct connection (Straight Through)

It serves:

1. To connect the computer to the switch (hub, switches) via the computer's network card
2. To connect to the switch (hub, switches) network peripheral equipment - printers, scanners
3. For uplink "And on the above-standing switch (hub, switch) - modern switches can automatically customize the inputs in the reception connector

Crossover (Crossover)

It serves:

1. For direct connection of 2 computers to the local network, without the use of switching equipment (hubs, switches, routers, etc.).
2. For uplink, connecting to the above-standing switch in a complex area of \u200b\u200bthe local network, for old types of switches (hubs, switches), they have a separate connector, and a marked "Uplink" or sign H.

Topology Star

Stars - The basic topology of the computer network, in which all network computers are attached to the central node (usually switch), forming a physical network segment. Such a network segment can function both separately and as part of a complex network topology (as a rule, "tree"). The whole exchange of information goes exclusively through a central computer, which is in such a way a very large load is assigned, so it cannot be engaged in anything else. As a rule, it is the central computer that is the most powerful, and it is precisely all the functions for the exchange management. No conflicts on the network with a topology star are not possible, because the management is fully centrally.

application

The classic 10 megabit ethernet arranged most users for about 15 years. However, in the early 1990s, its insufficient bandwidth began to be felt. For computers on the Intel 80286 or 80386 processors with ISA tires (8 MB / s) or EISA (32 MB / s), the bandwidth of the Ethernet segment was 1/8 or 1/32 channel "Memory-Disk", and this was well agreed with the ratio Data volumes processed locally and data transmitted over the network. For more powerful client stations with PCI bus (133 MB / s), this share fell to 1/133, which was clearly not enough. Therefore, many segments of 10 megabit ethernet have become overloaded, the response of servers in them has fallen significantly, and the frequency of the occurrence of collisions has increased significantly, even reducing the useful bandwidth.

There is a need to develop a "new" Ethernet, that is, technologies that would be as effective in terms of price / quality ratio with 100 Mbps performance. As a result of searches and research, specialists were divided into two camps, which eventually led to the emergence of two new technologies - Fast Ethernet and L00VG-Anylan. They are distinguished by the degree of continuity with the classic Ethernet.

In 1992, a group of manufacturers of network equipment, including such Ethernet technology, such as Synoptics, 3Com and a number of others, formed a non-commercial association Fast Ethernet Alliance to develop a standard new technologywhich was to preserve the features of Ethernet technology as much as possible.

The second camp was headed by Hewlett-Packard and AT & T, which were offered to take advantage of the convenient occasion to eliminate certain known disadvantages of Ethernet technology. After some time, IBM has joined these companies, which contributed proposal to ensure some compatibility with TKen Ring networks.

In the IEEE Institute Committee 802 at the same time a research team was formed to study the technical potential of new high-speed technologies. For the period from the end of 1992 and at the end of 1993, the IEEE group studied 100 megabit solutions proposed by various manufacturers. Along with the offers of Fast Ethernet Alliance, the Group also considered high-speed technology proposed by Hewlett-Packard and AT & T companies.

The discussion center had the problem of saving the random CSMA / CD access method. The Fast Ethernet Alliance proposal retained this method and thereby ensured the continuity and consistency of 10 Mbps and 100 Mbps networks. The HP and AT & T coalition that had supported a significantly smaller number of manufacturers in the network industry than the Fast Ethernet Alliance, offered a completely new access method called Demand Priority. - Priority access on demand. He significantly changed the picture of the behavior of nodes on the network, so I could not fit into Ethernet technology and Standard 802.3, and a new IEEE 802.12 Committee was organized for its standardization.

In the fall of 1995, both technologies have become IEEE standards. The IEEE 802.3 Committee adopted the Fast Ethernet specification as standard 802.3 and, which is not an independent standard, and is an addition to the existing standard 802.3 in the form of chapters from 21 to 30. Committee 802.12 adopted L00VG-AnyLAN technology, which uses a new Demand Priority access method And supports frames of two formats - Ethernet and Token Ring.

v. Physical level of technology Fast Ethernet

All differences between the Fast Ethernet technology from Ethernet are focused on the physical level (Fig. 3.20). Mac and LLC levels in Fast Ethernet remained absolutely the same, and they are described by the previous chapters of standards 802.3 and 802.2. Therefore, considering FAST Ethernet technology, we will study only a few options for its physical level.

The more complex structure of the physical level of FAST Ethernet technology is caused by the fact that it uses three options for cable systems:

· Fiber optic multimode cable, two fibers are used;
· Twisted pair of category 5, two pairs are used;
· Twisted pair of category 3, four pairs are used.

Coaxial cable, who gave the world to the first Ethernet network, in the number of allowed data transmission media, Fast Ethernet did not hit. This is a general trend of many new technologies, because at short distances, twisted pair of category 5 allows you to transmit data at the same speed as a coaxial cable, but the network is cheaper and more convenient to operate. At large distances, the optical fiber has a much broader bandwidth than the coaxial, and the network costs turns out not much higher, especially considering the high costs of troubleshooting and troubleshooting in a large cable coaxial system.

Differences Fast Ethernet technology from Ethernet technology

The rejection of the coaxial cable led to the fact that the Fast Ethernet networks always have a hierarchical tree structure constructed at hubs, as well as the L0Base-T / L0Base-F network. The main difference between the Fast Ethernet network configurations is to reduce the network diameter of about 200 m, which is explained by a decrease in the transfer time of the minimum length 10 times due to an increase in the transmission rate of 10 times compared with 10 megabit Ethernet.

Nevertheless, this circumstance does not greatly prevent the construction of large networks on FAST Ethernet technology. The fact is that the mid-90s is noted not only to the wide distribution of inexpensive high-speed technologies, but also the rapid development of local networks based on switches. When using switches, the Fast Ethernet protocol can operate in full-duplex mode, in which there are no restrictions on the total network length, but only restrictions on the length of the physical segments connecting adjacent devices (adapter - switch or switch - switch). Therefore, when creating high-length local networks, the Fast Ethernet technology is also actively used, but only in the full duplex version, together with switches.

This section discusses the half-duplex version of the Fast Ethernet technology, which fully complies with the definition of the access method described in Standard 802.3.

Compared with the options for the physical implementation of Ethernet (and there are six there are six), in the Fast Ethernet, the differences of each option from other deeper is variable as the number of conductors and coding methods. And since the physical options of Fast Ethernet were created simultaneously, and not evolutionally, as for Ethernet networks, it was possible to determine in detail those philodes of the physical layer that do not change from the variant to the option, and those suproes that are specific for each physical environment.

The official standard 802.3 and has set three different specifications for the physical level of Fast Ethernet and gave them the following names:

Fast Ethernet Physical Level Structure

· 100Base-TX for a two-party cable on unshielded twisted pair of UTP category 5 or shielded twisted pair STP Type 1;
· 100Base-T4 for a four-pane cable on unshielded twisted pair UTP category 3, 4 or 5;
· 100Base-FX for multimode fiber optic cable, two fiber are used.

For all three standards, the following statements and characteristics are valid.

· Formats of FAST Ethernetee technology differ from frame formats of 10 megabit ethernet technologies.
· Intercader interval (IPG) is 0.96 μs, and the bit interval is 10 ns. All temporal parameters of the access algorithm (deferred interval, minimum length transmission time, and the like), measured in bit intervals, remained the same, so the changes to the Standard sections relating to the MAC level were not made.
· The sign of the free state of the medium is the transmission of an IDLE symbol of the corresponding redundant code (and not the lack of signals as in Ethernet standards 10 Mbps). The physical layer includes three items:
o RECONCILITION SUBLAYER;
o independent of the media interface (MILIA INDEPENDENT INTERFACE, MIL);
o Physical Layer Device, PHY).

The level of matching is needed so that the MAC level designed for the AUI interface is able to work with the physical level through the MP interface.

The physical layer device (PHY) consists, in turn, from several sublevels (see Fig. 3.20):

· Logical coding of data that converts from MAC bytes coming from the 4B / 5B or 8B / 6T code symbols (both codes are used in Fast Ethernet technology);
· Pries of physical attachment and a dependence on the physical environment (PMD), which ensure the formation of signals in accordance with the method of physical coding, for example NRZI or MLT-3;
· Auto-traveler suite, which allows two interactive ports to automatically select the most efficient mode of operation, for example, half-duplex or full-duplex (this sublayer is optional).

The MP interface supports an independent physical environment of the data exchange between the MAC sublayer and PHY sublayer. This interface is similar to assigning the AUI interface of the classic Ethernet except that the AUI interface was located between the paragraph of the physical signal encoding (for any cable options, the same physical coding method is used - manchester code) and a paragraph of physical connection to the environment, and the MP interface is located between the MAC sublayer and a signal encoding sublevels that are three-FX, TX and T4 in the Fast Ethernet standard.

The MP connector unlike the AUI connector has 40 contacts, the maximum cable length of the MP is one meter. Signals transmitted over the MP interface have amplitude 5 V.

Physical level 100Base-FX - multimode fiber, two fibers

This specification determines the operation of the FAST Ethernet protocol on the multimode fiber in the half-duplex and complete duplex modes based on a well-proven FDDI encoding scheme. As in the FDDI standard, each node is connected to the network with two optical fibers that are from the receiver (R x) and from the transmitter (T x).

There are many common properties between the L00Base-FX and L00Base-TX specifications, therefore the common property for two specifications will be given under the generalized name L00Base-FX / TX.

While Ethernet with a transmission rate of 10 Mbit / s uses Manchester encoding to represent data when transmitted via cable, a different coding method is defined in the Fast Ethernet standard - 4V / 5B. This method has already shown its efficiency in the FDDI standard and changed to the L00Base-FX / TX specification. In this method, every 4 data bits of Mac sublayer (called symbols) are presented with 5 bits. Excess bit allows you to apply potential codes when representing each of the five bits in the form of electrical or optical pulses. The existence of prohibited combinations of symbols allows you to reject erroneous characters, which increases the stability of the operation of networks with L00Base-FX / TX.

To separate the Ethernet frame from IDLE characters, a combination of Start Delimiter characters is used (pair of symbols J (11000) and to (10001) 4B / 5B code, and after the frame is completed before the first idle character, the T. symbol is inserted.

Continuous data stream 100Base-Fx / TX specifications

After converting 4-bit portions of Mac codes into 5-bit portions of the physical layer, they must be represented as optical or electrical signals in a cable connecting network nodes. The L00Base-FX and L00Base-TX specification uses various physical coding methods for this - NRZI and MLT-3, respectively (as in FDDI technology when working through fiber optic and twisted pair).

Physical level 100Base-TX - twisted pair of DTP Cat 5 or STP Type 1, two pairs

As a data transfer environment, the L00Base-TX specification uses the UTP Category 5 cable or STP Type cable 1. The maximum cable length in both cases is 100 m.

The main differences from the L00Base-FX specification is the use of the MLT-3 method for transmitting 5-bit portions of the 4V / 5B code in twisted pair, as well as the availability of auto-track function (AUTO-NEGOTIATION) to select the port operation mode. The auto-track scheme allows two connected physically devices that support several standards of physical layer, characterized by bit speed and number of twisted pairs, select the most profitable mode of operation. Typically, the auto-bargoing procedure occurs when a network adapter is connected, which can operate at a speed of 10 and 100 Mbps, to a hub or switcher.

The AUTO-NEGOTIATION scheme described below is the L00BASE-T technology standard. Prior to this, manufacturers used various own schemes. automatic definition The speed of operation of interacting ports that were not compatible. AUTO-NEGOTIATION scheme asked as a standard, National Semiconductor is originally called NWAY.

In total, 5 different modes of operation are currently identified, which can support L00Base-TX or 100Base-T4 devices on twisted pairs;

· L0Base-T - 2 pairs of category 3;
· L0Base-T Full-duplex - 2 pairs of category 3;
· L00BASE-TX - 2 pairs of category 5 (or Type 1Astp);
· 100Base-T4 - 4 pairs of category 3;
· 100Base-TX Full-Duplex - 2 pairs of category 5 (or Type 1A STP).

The L0Base-T mode has the lowest priority in the negotiation process, and the total duplex mode 100Base-T4 is the highest. The negotiation process occurs when the device is turned on, and can also be initiated at any time the device control module.

The device that has begun the AUTO-NEGOTIATION process sends a package of special pulses to its partner. FAST LINK PULSE BURST (FLP)which contains an 8-bit word encoding the proposed interaction mode, starting from the priority supported by this node.

If the partner node supports the AUTO-NEGOTICATION function and can also support the proposed mode, it corresponds to the FLP pulse pack, in which this mode confirms, and the negotiations end. If the partner node can maintain less priority mode, then it indicates it in response, and this mode is selected as a worker. Thus, the most priority general mode of nodes is always selected.

A node that supports only L0Base-T technology, every 16 ms sends Manchester pulses to check the integrity of the line connecting it with the adjacent node. Such a node does not understand the FLP request, which makes it a node with the AUTO-NEGOTIATION function, and continues to send its impulses. The node that received in response to the FLP request only the impulses of the integrity of the line understands that its partner can only work according to the L0Base-T standard, and sets this mode of operation and for itself.

Physical level 100Base-T4 - twisted pair UTP Cat 3, four pairs

The 100Base-T4 specification was designed to be used for high-speed Ethernet available wiring on a twisted pair of category 3. This specification allows you to enhance the total bandwidth due to the simultaneous transmission of the bit streams on all 4 cable pairs.

The 100Base-T4 specification appeared later than other physical level specifications Fast Ethernet. The developers of this technology first wanted to create physical specifications that are closest to the L0Base-T and L0Base-F specifications that operated on two data transmission lines: two pairs or two fibers. For the implementation of work on two twisted pairs I had to go to a better Category 5 cable.

At the same time, the developers of the competing technology L00VG-Anylan initially made a bet on a twisted pair of category 3; The most important advantage was not so much in value, but in the fact that it was already laid in the overwhelming number of buildings. Therefore, after the release of the L00BASE-TX and L00BASE-FX specifications and L00BASE-FX developers, Fast Ethernet technology has implemented their own version of the physical layer for twisted category 3 pairs.

Instead of coding 4B / 5V, in this method, coding 8B / 6t is used, which has a narrower spectrum of the signal and at a speed of 33 Mbit / s fit into the 12 MHz band twisted a pair of category 3 (when encoding 4V / 5V, the signal spectrum does not fit into this strip) . Each 8 bits of the Mac level information are encoded with 6-Tropic figures (Ternary Symbols), that is, numbers having three states. Each tricious digit has a duration of 40 ns. A group of 6-terrible digits is then transmitted to one of the three transmitted twisted pairs, independently and sequentially.

The fourth pair is always used to listen to the carrier frequency in order to detect the collision. Data transfer rate for each of the three transmit steam is 33.3 Mbps, so the total speed of the 100Base-T4 protocol is 100 Mbps. At the same time, due to the accepted method of encoding, the signal change rate at each pair is only 25 mbeda, which allows the use of twisted pair of category 3.

In fig. 3.23 shows the connection of the MDI port of the MDI network adapter 100Base-T4 with the MDI-X of the hub (the console does not say that this connector connects the receiver and the transmitter changes in cable pairs compared to the network adapter connector, which makes it easier to connect pair of wires in the cable. without crossing). Couple 1 -2 always required for data transfer from MDI port to port MDI-X, steam 3 -6 - To receive data from the MDI port from the port of MDI-X, and pairs 4 -5 and 7 -8 They are bidirectional and used for both reception and transmission, depending on the need.

Connection of nodes by specification 100Base-T4

Fast Ethernet

Fast Ethernet - IEEE 802.3 U formally adopted on October 26, 1995 determines the standard of the channel-level protocol for networks of working when using both copper and fiber-optic cable at 100MB / s. The new specification is the heiress Ethernet standard IEE 802.3, using the same frame format, the mechanism of access to the CSMA / CD environment and the star topology. Evolution touched several elements of the configuration of physical layer tools, which made it possible to increase bandwidth, including the types of cable used, the length of the segments and the number of hubs.

Fast Ethernet structure

To better understand the work and understand the interaction of FAST Ethernet elements, we turn to Figure 1.

Figure 1. Fast Ethernet System

Logic Communication Management Subject (LLC)

In the IEEE 802.3 specification, the channel level functions are divided into two sublevels: logical link management (LLC) and the level of access to the environment (Mac), which will be discussed below. LLC, whose functions are defined by the IEEE 802.2 standard, actually provides interconnection with higher level protocols (for example, with IP or IPX), providing various communication services:

Service without establishing connections and admission confirmations. A simple service that does not provide data flow control or error control, and also does not guarantee the correct delivery of data.
Service with connecting. Absolutely reliable service that guarantees the correct delivery of data by establishing a connection to the receiver system prior to the start of data and the use of error control and data control mechanisms.
Service without establishing a connection confirmation. Middle-quality service that uses reception confirmation messages to ensure the guaranteed delivery, but does not establish connections before data transmission.

On the transmitting system, the data transmitted down from the network layer protocol is first encapsulated by the LLC sublayer. The standard calls them Protocol Data Unit (PDU, protocol data block). When the PDU is transmitted down the MAC sublayer, where the title and post-information is made again, from now on, it is technically possible to call it. For Ethernet package, this means that frame 802.3 in addition to the network layer data contains a three-byte LLC header. Thus, the maximum allowable data length in each packet decreases from 1500 to 1497 bytes.

The LLC header consists of three fields:

In some cases, LLC frames play a minor role in the process of networking. For example, in the network using TCP / IP along with other protocols, the only LLC function can be able to provide the possibility of frames 802.3 to contain the SNAP header, like EtherType Indicating the network layer protocol to which the frame must be transmitted. In this case, all PDU LLC will use a non-measured information format. However, other high-level protocols require a higher extended service from LLC. For example, NetBIOS sessions and several NetWare protocols use LLC services with a connection more widely.

Snap header

The receiving system must be determined which of the network layer protocols should receive incoming data. In packages 802.3, within the PDU LLC, another protocol is applied, called Sub- Network Access Protocol (SNAP, subnet access protocol).

The SNAP header has a length of 5 bytes and is located immediately after the LLC header in the frame data field 802.3, as shown in the figure. The title contains two fields.

Organization code.The identifier of the organization or manufacturer is a 3-byte field that takes the same value as the first 3 bytes of the sender MAC in the header 802.3.

Local code.Local code is a field of 2 bytes, which is functionally equivalent to the EtherType field in the Ethernet II header.

Site agreement

As mentioned earlier, Fast Ethernet is the evolving standard. Mac designed for AUI interface, you must convert for the MII interface used in Fast Ethernet, for which this type is designed.

Enable access control (Mac)

Each node in the Fast Ethernet network has an access controller MEDIA AccessController- Mac). Mac is key to Fast Ethernet and has three destinations:

The most important of the three Mac appointments is the first. For any network technology that uses the general environment, the rules for access to the environment defining when the node can transmit is its main characteristic. The development of the rules of access to the environment are engaged in several committees of Ieee. Committee 802.3, often referred to as the Ethernet Committee, determines the standards for the LANs in which the rules called CSMA / CD (Carrier Sense Multiple Access With Collision Detection - multiple access with the control of carrier and detection of conflicts).

CSMS / CDs are rules to access the environment for both Ethernet and Fast Ethernet. It is in this area that two technologies fully coincide.

Since all nodes in Fast Ethernet share the same environment, they can only pass when they occur. Define this queue CSMA / CD rules.

CSMA / CD.

The Mac Fast Ethernet controller before proceeding with the transfer, listens to the carrier. The carrier exists only when another node behaves. The PhY level determines the presence of carrier and generates a message for Mac. The presence of a carrier suggests that the environment is busy and listening to the node (or nodes) must yield to the transmitter.

Mac, having a frame for transmission, before passing it, should wait some minimum time interval after the end of the previous frame. This time is called interpocketry Shchel(IPG, Interpacket Gap) and continues 0.96 microseconds, that is, the tenth of the time transmission time of the ordinary Ethernet with a speed of 10 Mbps (IPG is the single time interval, always defined in microseconds, and not in time of the bit) Figure 2.

Figure 2. Interpacecate gap

After completing the package 1, all LAN nodes are required to wait during IPG time before they can transmit. The time interval between the packages 1 and 2, 2 and 3 in Fig. 2 is the IPG time. After completing the transmission of the package 3, no node had material for processing, so the time interval between the packages 3 and 4 is longer than the IPG.

All network nodes must comply with these rules. Even if there is a lot of frames for transmission and this node is the only transmitter, then after sending each package, it should wait for at least IPG time.

This is the CSMA part of the Fast Ethernet Environment Rules. In short, many nodes have access to the environment and use the carrier to control its employment.

In early experimental networks, these rules were used, and such networks worked very well. However, the use of only CSMA led to the emergence of a problem. Often two nodes, having a package for transfer and waiting for the IPG time, started to transmit simultaneously, which led to distortion of data on both sides. This situation is called collisia (Collision) or conflict.

To overcome this obstacle, early protocols used a fairly simple mechanism. Packages were divided into two categories: teams and reactions. Each command transmitted by the node required the reaction. If for some time (called a timeout period) after transferring the command the reaction to it was not received, the initial command was submitted again. This could occur several times (limit number of timeouts) before the transferring unit fixed the error.

This scheme could work perfectly, but only until a certain point. The emergence of conflicts led to a sharp decline in performance (usually measured in bytes per second), because the nodes were often simple in anticipation of responses to commands, never reach destination. The overload of the network, an increase in the number of nodes is directly related to the increasing number of conflicts and, therefore, with a decrease in network performance.

Early network designers quickly found a solution to this problem: each node must establish the loss of the transmitted packet by detecting the conflict (and not to expect a reaction that will never follow). This means that the packets lost due to conflict must be immediately transferred again until the time of the timeout time. If the node conveyed the last bit of the package without the occurrence of the conflict, it means that the package passed successfully.

The method of controlling the carrier is well combined with the function of detection of collisions. The collisions still continue to occur, but it does not reflect on network performance, as the nodes are rapidly getting rid of them. The DIX group by developing access rules for the CSMA / CD environment for Ethernet, designed them as a simple algorithm - Figure 3.

Figure 3. CSMA / CD work algorithm

Physical Level Device (PHY)

Since Fast Ethernet can use a different type of cable, then for each medium a unique signal pre-converting is required. The transformation is also required for efficient data transfer: make a transmitted code resistant to interference, possible losses, or distortions of individual elements (bodes), to ensure effective synchronization of clock generators on the transmitting or receiving side.

Coding Site (PCS)

Encodes / decodes the data coming from / to the MAC level using algorithms or.

Subjects of physical attachment and dependence on the physical environment (PMA and PMD)

The RMA and PMD subsensity communicate between the PSC sublayer and the MDI interface, providing the formation in accordance with the physical coding method: or.

AutoNEG (AUTONEG)

The auto-trailership fabric allows two interactive ports to automatically select the most efficient mode of operation: duplex or half-duplex 10 or 100 MB / s. Physical level

Fast Ethernet standard defines three types of Ethernet signal transmission medium at 100 Mbps.

100Base-TX - two twisted pairs of wires. Transmission is carried out in accordance with the data transfer standard in the twisted physical environment developed by ANSI (American National Standards Institute - American National Institute of Standards). Twisted data cable can be shielded or unshielded. Uses 4B / 5B data encoding algorithm and MLT-3 physical coding method.
100Base-FX - two veins, fiber optic cable. The transfer is also carried out in accordance with the data transfer standard in the fiber optic environment, which is developed by ANSI. Uses the 4B / 5B data encoding algorithm and the NRZI physical coding method.

100Base-TX and 100Base-FX specifications are also known as 100Base-X

100Base-T4 is a special specification developed by the IEEE 802.3u Committee. According to this specification, data transmission is carried out on the four twisted pairs of the telephone cable, which is called the UTP Cable Cable 3. Uses the 8V / 6T data encoding algorithm and the NRZI physical coding method.

Additionally, the Fast Ethernet standard includes recommendations for using a cable shielded twisted pair of category 1, which is a standard cable, traditionally used in TECK Ring networks. Organization of support and recommendations for using the STP cable in the Fast Ethernet network provide a method for switching to Fast Ethernet for buyers having a cable wiring STP.

The Fast Ethernet specification also includes a mechanism of autonotidation that allows the port of the node to automatically be configured to the data transfer rate - 10 or 100 Mbps. This mechanism is based on the exchange of a number of packets with a hub or switch port.

Wednesday 100Base-TX

As a transmission medium, 100Base-Tx uses two twisted pairs, and one pair is used to transmit data, and the second is for their reception. Since the ANSI TP - PMD specification contains descriptions of both shielded and unshielded twisted pairs, then the 100Base-TX specification includes support for both unshielded and shielded twisted pairs of type 1 and 7.

MDI connector (Medium Dependent Interface)

The 100Base-TX channel interface, depending on the medium, can be one of two types. For a cable on unshielded twisted pairs, an eight-contact connector RJ 45 of category 5 should be used as the MDI connector 5. The same connector is used in the 10Base-T network, which provides backward compatibility with existing Category 5. For shielded twisted pairs as the MDI connector is necessary Use a STP IBM type 1 connector, which is a shielded DB9 connector. Such a jack is usually applied in TKen Ring networks.

UTP Cable Category 5 (E)

In the UTP 100Base-Tx interface, two pairs of wires are used. To minimize crosspoints and possible signal distortion, the remaining four wires should not be used to transfer any signals. Transmission and reception signals for each pair are polarized, with one wire transmits positive (+), and the second is negative (-) signal. Color marking of cable wires and connector contact numbers for the 100Base-TX network are given in Table. 1. Although the Phy 100Base-TX level was developed after the adoption of the ANSI TP-PMD standard, but the contact numbers of the RJ 45 connector were changed to match the wiring diagram already used in the 10Base-T standard. In the ANSI TP-PMD standard, Contacts 7 and 9 are used to receive data, while in the 100Base-Tx and 10Base-T standards, contacts 3 and 6 are intended for this. This wiring provides the ability to use 100Base-TX adapters instead of 10 base adapters - T and connecting them to the same category 5 cables without wiring changes. In the RJ 45 connector, the wiring pairs used are connected to contacts 1, 2 and 3, 6. For proper connection Wiring should be guided by their color labeling.

Table 1. Purpose of connector contacts MDI Cable UTP. 100Base-TX.

Nodes interact with each other by sharing frames (Frames). The Fast Ethernet frame is a basic network exchange unit - any information transmitted between nodes is placed in the data field of one or more frames. Frame shipment from one node to another is possible only if there is a way to unique identification of all network nodes. Therefore, each node in the LAN has an address called its MAS-address. This address is unique: no two local network nodes can have the same MAC address. Moreover, none of the LAN technologies (with the exception of ArcNet) no two nodes in the world may have the same MAC address. Any frame contains at least three main portions of the information: the recipient address, the address of the sender and the data. Some frames have other fields, but only three listed are mandatory. Figure 4 reflects the FAST Ethernet frame structure.

Figure 4. Frame structure Fast. Ethernet

address of the recipient - indicates the address of the node receiving data;
address of the sender - indicates the address of the node sent data;
Length / type (L / T - LENGTH / TYPE) - contains information about the type of data transmitted;
Control summary (PCS - Frame Check Sequence) - designed to check the correctness of the frame received by the receiving node.

The minimum frame volume is 64 octets, or 512 bits (terms octetand byte -synonyms). The maximum frame volume is equal to 1518 octets, or 12144 bits.

Addressing personnel

Each node in the Fast Ethernet network has a unique number called the MAC address (Mac Address) or a node address. This number consists of 48 bits (6 bytes), assigned to the network interface during the manufacture of the device and is programmed during the initialization process. Therefore, network interfaces of all LANs, with the exception of ArcNet, which uses 8-bit addresses assigned by the network administrator, have a built-in unique MAC address, differing from all other MAC addresses on Earth and assigned by the manufacturer by coordination with IEEE.

To facilitate the network interface management process, IEEE has been proposed to divide the 48-bit address field into four parts, as shown in Figure 5. The first two bit characters (bits 0 and 1) are flags of the address type. The flag value determines the method of interpretation of the address part (bits 2 - 47).

Figure 5. Mas-Address format

BIT I / G called individual / group address flagand shows how (individual or group) is the address. The individual address is assigned only to one interface (or node) on the network. Addresses in which the I / G bit is set to 0 is Mas-addressesor addresses of the node.If the I / O bit is set to 1, the address refers to group and is usually called multipoint address(Multicast Address) or functional addressFUNCTIONAL ADDRESS). A group address can be assigned to one or multiple LAN network interfaces. Frames sent in a group address receive or copy all the LAN network interfaces. Multipoint addresses allow you to send a frame to a subset of the local network nodes. If the I / O bit is set to 1, the bits from 46 to 0 are interpreted as a multipoint address, and not as fields U / L, OUI and OUA of the usual address. Bit U / L called universal / local control flagand determines how the address of the network interface was assigned. If both bits, I / O and U / L are set to 0, the address is a unique 48-bit identifier described earlier.

OUI (Organizationally Unique Identifier - organizative unique identifier). IEEE assigns one or more OUI to each manufacturer of network adapters and interfaces. Each manufacturer is responsible for the correctness of the assignment of OUA (Organizationally Unique Address - organizationally unique address)which should have any device created by it.

When the U / L bit is set, the address is locally manageable. This means that it is not as a network interface manufacturer. Any organization can create its own MAC-address of the network interface by setting the U / L bit in 1, and the bits from the 2nd to the 47th to some selected value. Network interface, having received a frame, the first thing decodes the address of the recipient. When set to the I / O bit address, the MAC level will receive this frame only if the recipient's address is listed, which is stored on the node. This technique allows one node to send a frame to many nodes.

There is a special multipoint address called broadcast address.In a 48-bit broadcast IEEE address, all bits are set to 1. If the frame is transmitted to the recipient's broadcast address, then all network nodes will receive and process it.

Length field / type

The L / T (Length / Type - Length / Type) field is used for two different purposes:

to determine the length of the frame data field, excluding any addition to spaces;
to denote the data type in the data field.

The value of the L / T field located in the range between 0 and 1500 is the length of the frame data field; A higher value indicates the type of protocol.

In general, the L / T field is the historical sediment of the Ethernet standardization in IEEE, which has generated a number of problems with the compatibility of equipment released to 1983. Now Ethernet and Fast Ethernet never uses the L / T fields. The specified field serves only for coordination with software processing (that is, with protocols). But the only genuinely standard destination of the L / T field is to use it as a field of length - in Specifications 802.3 is not even mentioned about its possible application as a data type field. Standard reads: "Frames with a field of length exceeding defined in paragraph 4.4.2 can be ignored, discarded or used in a particular way. Using frame data is out of this standard."

Summing up this, we note that the L / T field is the primary mechanism for which it is determined frame type.FATERS FAST Ethernet and Ethernet, in which the value of the L / T field is set to the length (value L / T 802.3, frames in which the value of the field is set to the data type (value L / T\u003e 1500) is called frames Ethernet- II. or DIX..

Data field

In the data fieldthere is information that one node is sent to another. Unlike other fields that store highly specific information, the data field may contain almost any information, if only its volume was at least 46 and no more than 1,500 bytes. As the content field content is formatted and interpreted, protocols are determined.

If you need to send data with a length of less than 46 bytes, the LLC level adds bytes to their end with an unknown value called insignificant data(Pad Data). As a result, the field length becomes equal to 46 bytes.

If the frame is Type 802.3, then the L / T field indicates the value of the valid data. For example, if a 12-byte message is sent, the L / T field stores the value 12, and 34 additional incognizable bytes are also in the data field. Adding minor bytes initiates the LLC Fast Ethernet level, and is usually implemented hardware.

Mac levels do not specify the contents of the L / T field - it makes the software. Setting the value of this field is almost always done by the network interface driver.

Control summary

The Frame Check Sequence (PCS - Frame Check Sequence) allows you to make sure that the received frames are not damaged. When forming a transmitted frame at Mac, a special mathematical formula is used CRC.Cyclic Redundancy Check is a cyclic excess code) intended to calculate 32-bit values. The resulting value is placed in the FCS frame field. On the input of the MAC level element, calculating the CRC, the values \u200b\u200bof all frame bytes are fed. The FCS field is the primary and most important mechanism for detecting and correcting errors in Fast Ethernet. Starting from the first byte of the address of the recipient and ending with the last byte of the data field.

DSAP and SSAP fields

DSAP / SSAP values

Description

Indiv LLC SUBLAYER MGT

Group LLC SUBLAYER MGT

SNA PATH CONTROL

Reserved (DOD IP)

ISO CLNS IS 8473

The 8V6T encoding algorithm converts the eight-bitty data octet (8b) into a six-bit ternary symbol (6T). The code groups 6t are intended for transmission in parallel with three twisted cable pairs, so the effective data transfer rate for each twisted pair is one third of 100 Mbps, that is, 33.33 Mbps. The rate of transmission of ternary symbols for each twisted pair is 6/8 from 33.3 Mbps, which corresponds to a clock frequency of 25 MHz. It is with such a frequency that the MP interface timer works. Unlike binary signals that have two levels, ternary signals transmitted for each pair can have three levels.

Symbol encoding table

Linear code	Symbol















	MLT-3 MULTI LEVEL TRANSMISSION - 3 (multi-level transmission) is a bit similar to the NRZ code, but unlike the latter has three levels of the signal. The unit corresponds to the transition from one level signal to another, and the change in the signal level occurs consistently taking into account the previous transition. When the "zero" is not changed. This code, as well as NRZ needs pre-coding. Compiled by materials: Laem Queen, Richard Russell "Fast Ethernet"; K. Skler "Computer Networks"; V.G. and N.A. Olifer "Computer Networks";

Physical level

Fast Ethernet standard defines three types of Ethernet signal transmission medium at 100 Mbps.

· 100Base-TX - two twisted pairs of wires. Transmission is carried out in accordance with the data transfer standard in the twisted physical environment developed by ANSI (American National Standards Institute - American National Institute of Standards). Twisted data cable can be shielded or unshielded. Uses 4B / 5B data encoding algorithm and MLT-3 physical coding method.

· 100Base-FX - two veins, fiber optic cable. The transfer is also carried out in accordance with the data transfer standard in the fiber optic environment, which is developed by ANSI. Uses the 4B / 5B data encoding algorithm and the NRZI physical coding method.

· 100Base-T4 is a special specification developed by the IEEE 802.3u Committee. According to this specification, data transmission is carried out on the four twisted pairs of the telephone cable, which is called the UTP Cable Cable 3. Uses the 8V / 6T data encoding algorithm and the NRZI physical coding method.

Multimode cable

In a fiber-optic cable of this type, a fiber with a core diameter of 50, or 62.5 micrometers and an outer sheath of 125 micrometers thick, is used. Such a cable is called a multimode optical cable with fibers 50/125 (62.5 / 125) micrometers. To transfer the light signal over a multimode cable, a LED transceiver with a wavelength of 850 (820) of nanometers is used. If the multimode cable connects two ports of switches operating in full-duplex mode, it can have a length of up to 2000 meters.

Single mode cable

A single-mode fiber optic cable has a smaller than that of multimode, the core diameter is 10 micrometer, and a laser transceiver is used to transmit over a single-mode cable, which in the aggregate ensures efficient transmission to high distances. The wavelength of the transmitted light signal is close to the diameter of the core, which is 1300 nanometers. This number is known as the wavelength of zero dispersion. In a single-mode cable, the dispersion and loss of the signal is very insignificant, which allows you to transmit light signals over long distances than in the case of the use of multimode fiber.

38. Gigabit Ethernet technology, general characteristics, physical environment specification, basic concepts.
3.7.1. General characteristic standard

Quickly quickly after the Fast Ethernet products appeared, network integrators and administrators felt certain restrictions on the construction of corporate networks. In many cases, the servers connected along the 100 megabital channel overloaded the networks of networks, which also operate at a speed of 100 Mbps - FDDI and FAST Ethernet highway. The need for the next level of speed hierarchy was felt. In 1995 more high level Speeds could only be provided by ATM switches, and in the absence of convenient means of migrating this technology to local networks (although the LAN Emulation specification - Lane was adopted in early 1995, its practical implementation was ahead) almost no one decided to implement them into the local network . In addition, ATM technology differed in a very high level of value.

Therefore, the next step made by IEEE looked logical - 5 months after the final adoption of the Fast Ethernet Standard in June 1995, the IEEE high-speed technology research team was prescribed to consider the possibility of developing an Ethernet standard with even higher bit speed.

In the summer of 1996, it was announced the creation of a group of 802.3z to develop a protocol maximally similar to Ethernet, but with a bit rate of 1000 Mb / s. As in the case of Fast Ethernet, the message was perceived by Ethernet supporters with great enthusiasm.

The main reason for enthusiasm was the prospect of the same smooth translating network of networks on Gigabit Ethernet, just as the overloaded Ethernet segments located at the lower levels of the network hierarchy were translated into Fast Ethernet. In addition, the transmission of data on gigabit speeds has already been available, both in territorial networks (SDH technology) and in local - Fiber Channel technology, which is used mainly to connect high-speed peripherals to large computers and transmits data on fiber optic cable with The speed close to the gigabit, by overpowering 8V / 10V.

The first version of the standard was considered in January 1997, and finally the 802.3z standard was adopted on June 29, 1998 at a meeting of the IEEE 802.3 committee. Work on the implementation of Gigabit Ethernet on twisted pair of category 5 was transferred to the Special Committee 802.3AB, which has already considered several options for the draft of this standard, and since July 1998 the project has acquired a fairly stable nature. The final adoption of 802.3ab is expected in September 1999.

Without waiting for the standard, some companies have released the first Gigabit Ethernet equipment on the fiber optic cable for the summer of 1997.

The main idea of \u200b\u200bthe Gigabit Ethernet standard developers consists in maximizing the ideas of the classical Ethernet technology when the bit rate is 1000 Mbps reach.

Since, when developing a new technology, it is natural to expect some technical innovations that are in the general direction of the development of network technologies, it is important to note that Gigabit Ethernet, as well as its less high-speed fellow, at the protocol level will not besupport:

quality of service;
redundant communication;
testing the performance of nodes and equipment (in the latter case - with the exception of the communication testing port, as is done for Ethernet 10Base-T and 10Base-F and Fast Ethernet).

All three named properties are considered very promising and useful in modern networks, especially in the networks of the nearest future. Why do the authors of Gigabit Ethernet refuse them?

The main idea of \u200b\u200bGigabit Ethernet technology developers is that there is a very many networks in which high speed of the highway and the ability to assign priority packages in switches will be quite sufficient to ensure the quality of the transport service of all network customers. And only in those rare cases, when the highway is loaded enough, and the service quality requirements are very tough, it is necessary to apply ATM technology, which is really due to high technical complexity gives guarantees of service for all major types of traffic.

39. Structural cable system used in network technologies.
Structured Cabling System (Structured Cabling System, SCS) is a set of switching elements (cables, connectors, connectors, crossbar panels and cabinets), as well as a methodology for sharing, which allows you to create regular, easily expandable link structures in computer networks.

The structured cable system represents a kind of "constructor", with which the network designer builds the configuration you need from standard cables connected by standard connectors and switched on standard cross-panels. If you need to configure linkages, you can easily change - add a computer, segment, switch, withdraw unnecessary equipment, and also change connections between computers and concentrators.

When constructing a structured cable system, it is understood that each workplace in the enterprise must be equipped with sockets for connecting a phone and a computer, even if this moment is not required. That is, a good structured cable system is built redundant. In the future, this can save funds, since changes in the connection of new devices can be made by recoming already laid cables.

The typical hierarchical structure of the structured cable system includes:

horizontal subsystems (within the flood);
vertical subsystems (inside the building);
campus subsystem (within one territory with several buildings).

Horizontal subsystemconnects a crosslobe of the floor with user sockets. Subsystems of this type correspond to the floors of the building. Vertical subsystemconnects the cross cabinets of each floor from the central hardware building. The next step of the hierarchy is campus subsystem,which connects several buildings from the main hardware of the entire campus. This part of the cable system is usually called a highway (backbone).

The use of a structured cable system instead of chaotic laid cables gives enterprises a lot of advantages.

· Universality.The structured cable system at a well-thought-out organization can become a single environment for transmitting computer data on a local computer network, organization of a local telephone network, transmission of video information and even transmission of signals from fire safety sensors or security systems. This allows you to automate many control processes, monitoring and managing business services and life support systems.

· Increase service life.The term of moral aging of a well-structured cable system can be 10-15 years.

· Reducing the cost of adding new users and changes to their placement places.It is known that the cost of the cable system is significant and is mainly determined by the cost of the cable, but the cost of work on its laying. Therefore, it is more profitable to spend one-time work on the cable laying, possibly with a large margin in length than to perform a gasket, increasing the length of the cable. With this approach, all the work on adding or moving the user is reduced to connecting a computer to an existing outlet.

· The possibility of easy network expansion.The structured cable system is modular, so it is easy to expand. For example, you can add a new subnet to the highway without any influence on the existing subnets. It can be replaced in a separate subnet type of cable regardless of the rest of the network. The structured cable system is the basis for dividing the network on easily managed logic segments, as it is already divided into physical segments.

· Ensuring more efficient maintenance.Structured cable system facilitates maintenance and troubleshooting compared to the tire cable system. With the bus organization of the cable system, the failure of one of the devices or connecting elements leads to a difficult to localizable failure of the entire network. In structured cable systems, the failure of one segment does not affect others, since the combination of segments is carried out using hub. Hubs are diagnosed and localized a faulty area.

· Reliability.The structured cable system has enhanced reliability, since the manufacturer of such a system guarantees not only the quality of its individual components, but also their compatibility.

40. Concentrators and network adapters, principles, use, basic concepts.
Concentrators along with network adapters, as well as the cable system, represent the minimum of equipment with which you can create a local network. Such a network will be a common shared environment

Network Adapter (Network Interface Card, NIC)together with its driver implements the second, channel level of the open systems in the end node of the network. More precisely, in the pair operation system, the adapter and the driver performs only the functions of the physical and woofer, while the LLC level is usually implemented by the operating system module, one for all drivers and network adapters. Actually it should be in accordance with the IEEE 802 stack model model. For example, in Windows NT, the LLC level is implemented in the NDIS module, with all network adapter drivers, regardless of which technology is supported by the driver.

The network adapter, together with the driver, perform two operations: transmission and reception of the frame.

In adapters for client computers, a significant part of the work is shifted to the driver, thereby the adapter turns out to be easier and cheaper. The disadvantage of this approach is a high degree of loading of the computer's central processor with routine frameworks from the computer's RAM to the network. The central processor is forced to engage in this work instead of performing user application tasks.

Network adapter before installing the computer must be configured. When configuring the adapter, the IRQ interrupt number used is usually set by the adapter, direct access channel number of the DMA (if the adapter supports the DMA mode) and the basic I / O port.

In almost all modern local network technologies, a device has been defined that has several equal names - concentrator (Concentrator), Hub (Hub), Repeater (Repeater). Depending on the application of this device, the composition of its functions and constructive execution varies greatly. Only the main function remains unchanged - this is repetition of frameeither on all ports (as defined in Ethernet standard), or only on some ports, in accordance with the algorithm defined by the relevant standard.

The hub usually has several ports to which the end nodes of the network are connected using individual physical segments of the cable - computers. The hub combines separate network segments into a single shared environment, access to which is carried out in accordance with one of the considered local network protocols - Ethernet, token Ring, etc. Since the logic of access to the shared medium significantly depends on the technology, then for each type Technologies produced their hubs - Ethernet; Token Ring; FDDI and 100VG-AnyLAN. For a specific protocol, it is sometimes used, the highly specialized name of this device, more accurately reflecting its functions or the traditionally used by traditions, for example, for the TKEN Ring concentrators is characterized by MSAU.

Each hub performs some basic function defined in the corresponding protocol of the technology that it supports. Although this function is quite detailed in the standard standard, when it is implemented, the hubs of different manufacturers may differ in such details as the number of ports, support for several types of cables, etc.

In addition to the main function, the hub can perform a number of additional functions that are either not defined in the standard are or optional. For example, the TKEN Ring concentrator can perform the function of disconnecting incorrectly working ports and transition to a backup ring, although in the standard it is not described in the standard. The hub turned out to be a convenient device for performing additional functions that facilitate control and operation of the network.

41. Use of bridges and switches, principles, features, examples, restrictions
Structuring with bridges and switches

the network can be divided into logical segments using devices of two types - bridges (Bridge) and / or Switches (Switch, Switching Hub).

Bridge and switch are functional twins. Both of these devices promote frames on the basis of the same algorithms. Bridges and switches use two types of algorithms: algorithm transparent bridge (Transparent Bridge),described in the IEEE 802.1D standard or algorithm source Routing Bridge (Source Routing Bridge)iBM companies for TKEN Ring networks. These standards have been developed long before the first switch appears, so they use the term "bridge". When the first industrial model of the switch for Ethernet technology appeared on the light, it performed the same algorithm for promoting IEEE 802.ID, which was worked out with a dozen years worked out by bridges of local and global networks.

The main difference of the switch from the bridge is that the bridge processes the frames consistently, and the switch is parallel. This circumstance is due to the fact that the bridges appeared in those times when the network was divided into a small number of segments, and the intersegment traffic was small (he obeyed the rules 80 by 20%).

Today, bridges still work in networks, but only enough slow global connections between two remote local networks. Such bridges are called remote bridges (Remote Bridge), and the algorithm of their work is no different from the 802.1D standard or Source Routing.

Transparent bridges are able, in addition to transferring frames within a single technology, broadcast local networks protocols, such as Ethernet in Token Ring, FDDI in Ethernet, etc. This property of transparent bridges is described in the IEEE 802.1h standard.

In the future, we will call a device that promotes frames according to the bridge algorithm and works on a local network, a modern term "switch". When describing the 802.1D and source algorithms themselves, in the next section, we will call the device with a bridge, as it actually is called in these standards.

42. Switches for local networks, protocols, operation modes, examples.
Each of the 8 10Base-T ports are serviced by one Ethernet Packet Processor Packet Package Processor. In addition, the switch has a system module that coordinates all EPR processors. The system module leads a common switch address table and provides a switch on the SNMP protocol. To transfer frames between ports, a switching matrix is \u200b\u200bused, similar to those operating in telephone switches or multiprocessor computers, connecting multiple processors with multiple memory modules.

Switching matrix works on the principle of switching channels. For 8 ports, the matrix can provide 8 simultaneous interior channels With half-duplex mode of the ports and 16 - with a complete duplex, when the transmitter and receiver of each port are operating independently of each other.

When the frame is received in any port, the EPR processor buffers the several first bytes of the frame to read the destination address. After receiving the destination address, the processor immediately decides on the transfer of the package, without waiting for the arrival of the remaining bytes of the frame.

If the frame needs to be transferred to another port, the processor refers to the switching matrix and tries to install a path in it connecting its port with a port through which the route is route to the destination address. The switching matrix can only do this when the port address port at that moment is free, that is not connected to another port. If the port is busy, then, as in any channel switched, the matrix fails. In this case, the frame is completely buffered by the input port processor, after which the processor waits for the release of the output port and the formation of the switching matrix of the desired path. After the desired path is installed, the buffered bytes of the frame are sent to it, which are accepted by the output port processor. As soon as the output port processor accesss the Ethernet segment connected to the CSMA / CD algorithm, the frame bytes immediately begin to be transmitted to the network. The described method of transferring a frame without its complete buffering received the title of switching "on the fly" ("On-the-fly") or "Nutrole" ("Cut-Through"). The main reason for improving network performance when using the switch is parallelprocessing several frames. This effect illustrates Fig. 4.26. The figure shows the ideal situation in terms of improving performance when the four ports of eight transmit data from the maximum for the Ethernet protocol with a speed of 10 MB / s, and they transmit this data to the remaining four switter ports not conflicting - data streams between network nodes were distributed so that For each port receiving port, there is your output port. If the switch has time to process the input traffic, even with the maximum intensity of frame entering the input ports, then the overall performance of the switch in the example above will be 4x10 \u003d 40 Mbps, and when summoning the example for N ports - (N / 2) XLO Mbps. It is said that the switch provides each station or segment connected to its ports, the allocated bandwidth of the protocol. It is possible that the network does not always develop a situation that is depicted in Fig. 4.26. If two stations, such as stations connected to ports 3 and 4, at the same time, you need to record the data on the same server connected to the port. 8, the switch will not be able to select each station of the data stream of 10 Mbps, since port 5 cannot transmit data at a speed of 20 Mbps. Station frames will be expected in the internal queues of the input ports 3 and 4, when port is free 8 to transfer the next frame. Obviously, a good solution for such distribution of data streams would be to connect a server to a higher-speed port, for example, Fast Ethernet. So, as the main dignity of the switch, thanks to which he won very good positions in local networks, it is its high performance, the developers of switches are trying to produce Called non-blocking (non-blocking)switch models.

43. The algorithm of the transparent bridge.
Transparent bridges are invisible for network adapters of end-nodes, as they independently build a special address table, on the basis of which can be solved, you need to transmit a new segment to any other segment or not. Network adapters when using transparent bridges work in the same way as in the case of their absence, that is, they do not take any additional actions so that the frame passes through the bridge. The transparent bridge algorithm does not depend on the local network technology in which the bridge is installed, so the transparent Ethernet bridges work in the same way as transparent FDDi bridges.

The transparent bridge builds its address table based on passive monitoring of traffic circulating in the segments connected to its ports. At the same time, the bridge takes into account the addresses of data sources of data entering the ports of the bridge. At the address of the frame of the frame, the bridge concludes that this node belongs to this or another network segment.

Consider the process of automatic creation of the address table of the bridge and its use on the example of a simple network shown in Fig. 4.18.

Fig. 4.18. Principle of operation of a transparent bridge

The bridge connects two logical segments. Segment 1 make up computers connected with one segment of the coaxial cable to port 1 of the bridge, and segment 2 - computers connected using another segment of the coaxial cable to port 2 of the bridge.

Each port of the bridge works as a final node of its segment in one exception - the port of the bridge does not have its own MAC address. The port of the bridge works in the so-called insome (Promisquous)package capture mode when all packages entering ports are remembered in buffer memory. With this mode, the bridge is following all traffic transmitted in the segments attached to it, and uses packets passing through it to study the network composition. Since all packets are written to the buffer, the port address is not needed.

In the initial state, the bridge knows nothing about that computers with which MAC addresses are connected to each of its ports. Therefore, in this case, the bridge simply transmits any captured and buffered frame on all its ports except from which this frame is obtained. In our example, the bridge is only two ports, so it transmits frames from port 1 to port 2, and vice versa. When the bridge is going to transfer a frame from a segment to a segment, for example, from segment 1 to segment 2, it tries to access the segment 2 as the end node according to the access algorithm rules, in this example, according to the CSMA / CD algorithm rules.

Simultaneously with the transmission of frame to all ports, the bridge studies the address of the frame of the frame and does new record About his belonging in its address table, which is also called the filtering table or routing.

After the bridge passed the stage of learning, it can work more rationally. When receiving a frame, directed, for example, from a computer 1, 3, it browsing the address table for the coincidence of its addresses with the destination address 3. Since there is such an entry, the bridge performs the second stage of the table analysis - checks whether computers are checked with the source addresses ( In our case, this is the address 1) and the destination address (address 3) in one segment. Since in our example they are in different segments, the bridge performs the operation forwardingframe - transmits a frame to another port, having previously access to another segment.

If the destination address is unknown, the bridge transmits a frame to all its ports, except the port - the source of the frame, as in the initial stage of the learning process.

44. Bridges with routing from the source.
Source routing bridges are used to connect token Ring and FDDI rings, although transparent bridges can be used for the same purposes. Routing from the source (Source Routing, SR) is based on the fact that the sender station is placed in the frame sent to another ring all the address information about intermediate bridges and rings that the frame must pass before you get into the ring to which the station is connected recipient.

Consider the principles of work bridges Source Routing (hereinafter, SR-bridges) on the example of the network shown in Fig. 4.21. The network consists of three rings connected by three bridges. To set the row and bridges route have identifiers. SR-bridges do not build a target table, and when promoting frames, use the information available in the corresponding data frame fields.

RIC. 4.21.Source Routing Bridges

Upon receipt of each SR-bridge pack, you only need to view the route information field (field Routing Information Field, RIF, in the TOKEN Ring or FDDI frame) for its identifier in it. And if it is present there and is accompanied by the ID ID, which is connected to this bridge, then in this case the bridge copies the received frame into the specified ring. Otherwise, the frame in another ring is not copied. In any case, the source copy of the frame is returned on the source ring of the sender station, and if it was transferred to another ring, then bit A (address is recognized) and the bit C (frame is copied) The frame status fields are set to 1 to report the sender station, That the frame was received by the destination station (in this case, transferred to the bridge to another ring).

Since the route information in the frame is not always needed, but only for the transmission of the frame between the stations connected to different rings, the presence in the frame of the RIF field is indicated by setting 1 bits of the individual / group address (I / G) (while this bit is not used By destination, as the source address is always individual).

The RIF field has a managing subfield consisting of three parts.

Frame typespecifies the type of RIF field. There are various types of RIF fields used to find a route and send a frame to a well-known route.
Maximum frame length fieldused by the bridge for the connection of the rings, in which the different MTU value is set. With this field, the bridge notifies the station to the maximum possible length of the frame (that is, the minimum MTU value throughout the entire route).
Field length Rif.it is necessary because in advance the number of the route descriptors specifying the identifiers of intersected rings and bridges is unknown.

For the operation of the routing algorithm from the source, two additional types of frame are used - SRBF single-hour broadcast scorer (SINGLE-ROUTE Broadcast Frame) and multiple-hour broadcast scorer-explorer ARBF (All-Route Broadcast Frame).

All SR-bridges must be configured by the administrator manually to transmit ArBF frames to all ports, except for the frame source port, and for SRBF frames, some ports of bridges must be blocked so that there are no loops in the network.

Advantages and disadvantages of bridges with routing from the source

45. Switches: technical implementation, functions, characteristics affecting their work.
Features of the technical implementation of switches. Many first generation switches were similar to routers, that is, they were based on the central general purpose processor associated with interface ports on the internal high-speed tire. The main disadvantage of such switches was their low speed. The universal processor could not cope with a large amount of specialized framework for forwarding between interface modules. In addition to processor chips for successful non-blocking operation, the switch must also have a high-speed assembly for transferring frames between processor port chips. Currently, switches are used as a basic one of three schemes, on which such an exchange unit is built:

switching matrix;
shared multiple memory;
total bus.

The highest distribution among standard networks received an Ethernet network. For the first time it appeared in 1972 (the developer was the well-known Xerox firm). The network was quite successful, and as a result of this in 1980, such largest companies as DEC and Intel were supported in 1980 (the combination of these companies called DIX on the first letters of their names). Their efforts in 1985, the Ethernet network became an international standard, it was adopted by the largest international organizations on standards: IEEE and ELECTERONIC Engineers Committee (ECMA (European Computer Manufacturers Association).

The standard was called IEEE 802.3 (in English read as Eight Oh Two Dot Three). It defines multiple access to the tire type monocanal with conflict detection and transmission control, that is, with the already mentioned CSMA / CD access method. Some other networks satisfy this standard, since the level of its detail is low. As a result of the IEEE 802.3 standard, both constructive and electrical characteristics were often incompatible. However, recently, the IEEE 802.3 standard is considered to be the standard Ethernet network.

The main characteristics of the initial standard IEEE 802.3:

topology - tire;
transmission medium - coaxial cable;
transmission rate - 10 Mbps;
maximum network length - 5 km;
maximum number of subscribers - up to 1024;
network segment length - up to 500 m;
number of subscribers on one segment - up to 100;
access method - CSMA / CD;
the transmission is narrow-band, that is, without modulation (monocanal).

Strictly speaking, there are minor differences between IEEE 802.3 and Ethernet standards, but they usually prefer not to remember.

Ethernet network is now most popular in the world (more than 90% of the market), it is alleged that it will remain in the coming years. This consistently contributed to the fact that from the very beginning, the characteristics, parameters, network protocols were discovered from the very beginning, as a result of which the huge number of manufacturers around the world began to produce Ethernet equipment, fully compatible with each other.

In the classical Ethernet network, a 50-ohm coaxial cable of two types (thick and thin) was used. However, recently (from the beginning of the 90s), the highest distribution received the Ethernet version using twisted pairs as a medium. The standard is also defined for the application of the fiber optic cable. To account for these changes to the initial standard IEEE 802.3, appropriate additions were made. In 1995, an additional standard appeared on a faster version of Ethernet operating at 100 Mbit / s (the so-called Fast Ethernet, IEEE 802.3u standard), using a twin or fiber-optic cable as a medium. In 1997, the version for the speed of 1000 Mbps (Gigabit Ethernet, IEEE 802.3z standard) appeared.

In addition to the standard tire topology, topologies such as passive star and passive tree are increasingly used. This assumes the use of repeaters and repeater hubs connecting various parts (segments) of the network. As a result, a tree structure on segments of different types can be formed (Fig. 7.1).

Fig. 7.1. Classical Ethernet Topology

A classic tire or a single subscriber can be used as a segment (part of the network). For bus segments, a coaxial cable is used, and for the rays of the passive star (for attaching to a single computers) - twisted steam and fiber optic cable. The main requirement to the resulting topology is that there are no closed paths (loops). In fact, it turns out that all subscribers are connected to the physical bus, since the signal from each of them applies immediately to all parties and does not return back (as in the ring).

The maximum length of the network cable as a whole (the maximum signal path) theoretically can reach 6.5 kilometers, but practically does not exceed 3.5 kilometers.

The FAST Ethernet network does not provide a tire physical topology, only a passive star or passive tree is used. In addition, Fast Ethernet has much more stringent requirements for the maximum length of the network. After all, with an increase in 10 times the transmission rate and preservation of the package format, its minimum length becomes ten times shorter. Thus, 10 times the permissible value of the double time of the signal over the network is reduced (5.12 μs against 51.2 μs in Ethernet).

For information transfer to the Ethernet network uses a standard manchester code.

Access to the Ethernet network is carried out by random CSMA / CD method that ensures subscriber equality. The network uses variable length packets with the structure shown in Fig. 7.2. (numbers show the number of bytes)

Fig. 7.2. Ethernet Network Package Structure

Ethernet frame length (i.e., a package without a preamble) should be at least 512 bite intervals or 51.2 μs (this is exactly the limit value of the double time of passing on the network). Provided individual, group and broadcast addressing.

The Ethernet package includes the following fields:

The preamble consists of 8 bytes, the first seven are code 10101010, and the last byte - code 10101011. In the IEEE 802.3 standard, the eighth byte is called a sign of the start of the frame (SFD - Start of Frame Delimiter) and forms a separate packet field.
The recipient addresses (receiver) and the sender (transmitter) include 6 bytes and are built according to the standard described in the addressing of lecturing packages. These address fields are processed by the subscriber equipment.
Control field (L / T - LENGTH / TYPE) contains information about the length of the data field. It can also determine the type of protocol used. It is believed that if the value of this field is not more than 1500, then it indicates the length of the data field. If its value is more than 1500, then it defines the type of frame. The control field is processed programmatically.
The data field should include from 46 to 1500 bytes of data. If the package must contain less than 46 bytes of data, the data field is complemented by filling bytes. According to the IEEE 802.3 standard, a special filling field is allocated in the package structure (Pad Data - insignificant data), which may have zero length when data is sufficient (more than 46 bytes).
The checksum field (FCS - Frame Check Sequence) contains a 32-bit cyclic checksum package (CRC) and serves to verify the correctness of the packet transmission.

Thus, the minimum frame length (package without preamble) is 64 bytes (512 bits). It is this value that determines the maximum allowable double delay in the distribution of the signal over the network in 512 bite intervals (51.2 μs for Ethernet or 5.12 μs for Fast Ethernet). The standard assumes that the preamble can decrease when the package passes through various network devices, so it is not taken into account. The maximum frame length is equal to 1518 bytes (12144 bits, that is, 1214.4 μs for Ethernet, 121.44 μs for FAST Ethernet). This is important to select the size of the buffer memory of network equipment and to evaluate the total network load.

The choice of preamble format is not accidental. The fact is that the sequence of alternating units and zeros (101010 ... 10) in Manchester code is characterized by what has transitions only in the middle of the bit intervals (see Section 2.6.3), that is, only information transitions. Of course, the receiver simply tune in (synchronize) with such a sequence, even if it is shortening for several bits for some reason. The last two single bits of the preamble (11) differ significantly from the sequence 101010 ... 10 (transitions also appear on the border intervals). Therefore, the already configured receiver can easily highlight them and detect the beginning. useful information (Start of frame).

For an Ethernet network operating at a speed of 10 Mbps, the standard defines the four main types of network segments focused on different information transfer environments:

10Base5 (thick coaxial cable);
10Base2 (thin coaxial cable);
10Base-T (twisted pair);
10Base-FL (fiber optic cable).

The name of the segment includes three elements: a digit 10 means a transmission rate of 10 Mbps, the word BASE - transmission in the main frequency band (that is, without modulating a high-frequency signal), and the last element is the permissible length of the segment: 5 - 500 meters, 2 - 200 meters (more precisely, 185 meters) or communication type: T - twisted pair (from English twisted-pair), f - fiber optic cable (from English Fiber Optic).

In the same way for the Ethernet network operating at a speed of 100 Mbps (Fast Ethernet), the standard defines three types of segments that differ in the type of transmission medium:

100Base-T4 (quad twisted pair);
100Base-TX (dual twisted pair);
100Base-FX (fiber optic cable).

Here, the number 100 means the transfer rate of 100 Mbit / s, the letter T is a twisted pair, the letter F is the fiber optic cable. Types 100Base-TX and 100Base-FX are sometimes combined under the name 100Base-X, and 100Base-T4 and 100Base-TX - under the name 100Base-T.

Read more Features of Ethernet equipment, as well as the CSMA / CD exchange control algorithm and the cyclic checksum calculation algorithm (CRC) will be discussed later in the special sections of the course. Here it should be noted only that the Ethernet network is not different in record characteristics or optimal algorithms, it is inferior to other standard networks for a number of parameters. But thanks to powerful support, the highest standardization level, huge amounts of technical output, Ethernet is allocated beneficial among other standard networks, and therefore any other network technology is made to compare from Ethernet.

The development of Ethernet technology goes along the path of increasingly departing from the initial standard. The use of new transmission and switched media allows you to significantly increase the size of the network. The refusal to manchester code (on the Fast Ethernet and Gigabit Ethernet network) provides an increase in the data transfer rate and reduce the requirements for the cable. Refusal from the CSMA / CD control method (with full-duplex exchange mode) makes it possible to dramatically improve the efficiency of work and remove restrictions from the network length. However, all new network varieties are also called an Ethernet network.

Token-ring

The Taken-Ring network (marker ring) was proposed by IBM in 1985 (the first option appeared in 1980). It was intended to combine all types of computers manufactured by IBM. Already the fact that it supports IBM, the largest manufacturer of computer equipment, indicates that she needs to pay special attention. But no less important is that the token-ring is currently the international standard IEEE 802.5 (although there is minor differences between token-ring and IEEE 802.5). This puts this network for one level by status with Ethernet.

Taken-Ring was developed as a reliable Ethernet alternative. And although now Ethernet displaces all other networks, Taken-Ring can not be considered hopelessly outdated. More than 10 million computers around the world are combined with this network.

IBM has done everything for the widest possible dissemination of its network: detailed documentation has been released up to the adapter circuits. As a result, many companies, for example, 3Som, Novell, Western Digital., Proteon and others have begun to produce adapters. By the way, the NetBIOS concept was developed specifically for this network, as well as for another IBM PC NetBiOS network. If the NetBIOS PC Network network has been kept in the NetBiOS-built-in permanent memory adapter, the NetBIOS emulation program has already been used on the TOKEN-Ring network. This allowed to respond more flexibly to the features of the equipment and maintain compatibility with higher level programs.

The Taken-Ring network has a ring topology, although it looks more like a star. This is due to the fact that individual subscribers (computers) are attached to the network not directly, but through special hubs or multiple access devices (MSAU or MAU - MULTITIATION ACCESS UNIT). Physically, the network forms a stellar-ring topology (Fig. 7.3). In fact, the subscribers are combined after all the same in the ring, that is, each of them transmits information to one neighboring subscriber, and receives information from the other.

Fig. 7.3. Star-ring Topology TECKEN-RING

The hub (MAU) allows you to centralize the configuration task, disabling faulty subscribers, network control, etc. (Fig. 7.4). It does not produce any processing of information.

Fig. 7.4. Connection of network subscribers token-ring in a ring with a hub (MAU)

For each subscriber, a special connection to the hub is applied as part of the hub (TCU - Trunk Coupling Unit), which provides automatic switching on the subscriber to the ring if it is connected to the hub and is working. If the subscriber is disconnected from the hub or it is faulty, the TCU unit automatically restores the integrity of the ring without the participation of this subscriber. TCU triggers signal direct current (The so-called phantom current), which comes from the subscriber who wants to turn on the ring. The subscriber can also disconnect from the ring and conduct a self-test procedure (the extreme right subscriber in Fig. 7.4). The phantom current does not affect the information signal, since the signal in the ring does not have a constant component.

Constructively, the hub is an autonomous block with ten connectors on the front panel (Fig. 7.5).

Fig. 7.5. Taken-Ring hub (8228 MAU)

Eight central connectors (1 ... 8) are designed to connect subscribers (computers) using adapter (Adapter Cable) or radial cables. Two extreme connections: Input RI (Ring In) and Output RO (Ring Out) serve to connect to other concentrators using special trunk cables (Path Cable). Wall and desktop options are offered.

There are both passive and active MAU concentrators. The active hub restores the signal coming from the subscriber (that is, it works like an Ethernet hub). The passive hub does not restore the signal, only rebuitates the communication lines.

The hub in the network may be the only one (as in Fig. 7.4), in this case, only subscribers connected to it are closed into the ring. Externally, such a topology looks like a star. If you need to connect more than eight subscribers to the network, then several concentrators are connected by trunk cables and form a stellar-ring topology.

As already noted, the annular topology is very sensitive to the rings cable cliffs. To increase the vitality of the network, the TKEN-Ring provides the mode of the so-called rings folding, which allows us to bypass the breakdown.

In normal mode, the hubs are connected to the ring with two parallel cables, but the transmission of information is made at the same time only one of them (Fig. 7.6).

Fig. 7.6. Combining MAU concentrators in normal mode

In the case of single damage (cliff) of the cable, the network transmits on both cables, thereby bypassing the damaged area. At the same time, the procedure for bypassing subscribers connected to concentrators is preserved (Fig. 7.7). True, the total length of the ring increases.

In the case of multiple cable damage, the network decomposes several parts (segments), not interconnected, but retaining full performance (Fig. 7.8). The maximum part of the network remains associated as before. Of course, this does not save the network as a whole, but allows, with the correct distribution of subscribers on the concentrators, to maintain a significant part of the functions of the damaged network.

Several hubs can be constructively combined into a group, cluster (Cluster), inside which subscribers are also connected to the ring. Cluster use allows you to increase the number of subscribers connected to one center, for example, up to 16 (if two hub is included in the cluster).

Fig. 7.7. Ring folding when damaged cable

Fig. 7.8. Decay Rings with multiple cable damage

As an IBM Token-Ring transmission medium, a twisted pair was first used, both unshielded (UTP) and shielded (STP), but then the hardware options for the coaxial cable, as well as for the fiber optic cable in the FDDI standard appeared.

The main technical characteristics of the classic network of TECKEN-Ring:

maximum number of hubs type IBM 8228 MAU - 12;
the maximum number of subscribers in the network is 96;
the maximum cable length between the subscriber and the hub is 45 meters;
the maximum cable length between the hubs is 45 meters;
the maximum cable length connecting all hubs is 120 meters;
data transfer rate - 4 Mbps and 16 Mbps.

All specified characteristics relate to the use of unshielded twisted pair. If another transmission environment is applied, network characteristics may differ. For example, when using shielded twisted pair (STP), the number of subscribers can be increased to 260 (instead of 96), the cable length is up to 100 meters (instead of 45), the number of hubs - up to 33, and the full length of the ring connecting the hubs to 200 meters . The fiber optic cable allows you to increase the length of the cable to two kilometers.

To transfer information to TECKEN-Ring, biphasic code is used (more precisely, its option with a mandatory transition in the center of the bit interval). As in any star-like topology, no additional measures for electrical consignment and external grounding are required. Approval is performed by equipment of network adapters and hubs.

To attach cables in Token-Ring, RJ-45 connectors are used (for unshielded twisted pair), as well as MIC and DB9P. The wires in the cable connect the same connector contacts (that is, the so-called straight cables are used).

The TECKEN-RING network in the classic version is inferior to the Ethernet network both on the permissible size and the maximum number of subscribers. As for the transfer rate, currently there are versions of Token-Ring to the speed of 100 Mbps (High Speed \u200b\u200bTaken-Ring, Hstr) and 1000 Mbps (Gigabit Taken-Ring). Companies supporting Token-Ring (including IBM, Olicom, Madge) do not intend to refuse their network, considering it as a worthy competitor Ethernet.

Compared to Ethernet equipment, TECKE-Ring equipment is noticeably more expensive, as a more complex exchange management method is used, so the TKen-Ring network has not received so widespread.

However, unlike Ethernet, the token-ring network keeps a high level of load (more than 30-40%) and provides a guaranteed access time. This is necessary, for example, in industrial networks, in which the reaction delay to the external event can lead to serious accidents.

The TKEN-Ring network uses a classic marker access method, that is, the ring constantly circulates the marker to which subscribers can attach their data packets (see Fig. 7.8). This implies such an important dignity of this network as the lack of conflicts, but there are disadvantages, in particular the need to control the integrity of the marker and the dependence of the network functioning from each subscriber (in the event of a malfunction, the subscriber must be excluded from the ring).

Territory transfer time in TECKEN-Ring 10 ms. With the maximum number of subscribers 260, the full cycle of the ring will be 260 x 10 ms \u003d 2.6 s. During this time, all 260 subscribers will be able to transfer their packages (if, of course, they have something to transmit). During the same time, the free marker will necessarily reach each subscriber. The same interval is the upper Taken-Ring access time limit.

Each subscriber of the network (its network adapter) must perform the following functions:

detection of transmission errors;
network configuration control (network recovery upon failure of the subscriber who precedes it in the ring);
control of numerous temporal relations accepted on the network.

A large number of functions, of course, complicates and increases the apparatus of the network adapter.

To control the integrity of the marker in the network, one of the subscribers is used (the so-called active monitor). At the same time, its equipment is no different from the rest, but its software is monitored for temporary ratios on the network and form a new marker if necessary.

The active monitor performs the following functions:

launches the marker in the ring at the beginning of work and when it disappears;
regularly (once in 7 seconds) reports its presence with a special control package (AMP - Active Monitor Present);
removes a package from the ring that was not removed by his subscriber sent;
watch out for a permissible packet transmission time.

The active monitor is selected when the network is initialized, it can be any network of network, but, as a rule, the first subscriber included in the network becomes. The subscriber who has become an active monitor includes its own buffer (shear register), which ensures that the marker will fit in the ring even with the minimum ring length. The size of this buffer is 24 bits for a speed of 4 Mbps and 32 bits for 16 Mbps velocity.

Each subscriber constantly monitors how the active monitor performs its duties. If an active monitor for some reason fails, a special mechanism is included, through which all other subscribers (spare, reserve monitors) decide on the appointment of a new active monitor. To do this, the subscriber, detecting an accident of an active monitor, transmits the control packet to the ring (the marker request package) with its MAC address. Each next subscriber compares the MAC address from the package with its own. If his own address is less, it transmits the package further unchanged. If more, then it sets his MAC address in the package. An active monitor will be the subscriber who has the value of the MAC address more than that of the rest (it should get a back package back with its MAC address). A sign of the event of an active monitor is the failure to comply with it one of the listed functions.

The token-ring network marker is a control packet containing only three bytes (Fig. 7.9): an initial separator bytes (SD - Start Delimiter), an access control byte (AC-Access Control) and an end delimiter bytes (ED - END DELIMITER). All these three bytes also consist of the information package, however, the functions of them in the marker and in the package are somewhat different.

The initial and final separators are not just a sequence of zeros and units, but contain signals of a special type. This was done so that the separators could not be confused with any other packet bytes.

Fig. 7.9. Taken-Ring network marker format

The initial separator SD contains four non-standard bit intervals (Fig. 7.10). Two of them, indicating J, are a low signal level during the entire bit interval. Two other bits indicated by are a high level of signal during the entire bit interval. It is clear that such synchronization failures are easily detected by the receiver. Bits J and K can never meet among the bits of useful information.

Fig. 7.10. Initial (SD) and final (ED) separators

The final ED separator also contains four bits of a special type (two bits J and two bits k), as well as two single bits. But, in addition, it includes two information bits that make sense only in the composition of the information package:

Bit I (Intermediate) is a sign of an intermediate package (1 corresponds to the first in the chain or intermediate package, 0 is the last in the chain or single package).
BIT E (ERROR) is a sign of a detected error (0 corresponds to the absence of errors, 1 - their presence).

Access Control Byte (AC - Access Control) is divided into four fields (Fig. 7.11): The priority field (three bits), the marker bit, the monitor bit and the reservation field (three bits).

Fig. 7.11. Access Control Byte

The bits (field) of the priority allow the subscriber to assign priority to their packages or markers (a priority can be from 0 to 7, and 7 meets the highest priority, and 0 - lower). The subscriber can attach his package to the marker only when its own priority (the priority of its packages) is the same or higher than the priority of the Marker.

The marker bit determines whether the package is attached to the marker or not (the unit corresponds to the marker without a package, zero - marker with the package). The monitor bits installed in one says that this marker is transferred to the active monitor.

Bits (field) redundancy allow the subscriber to reserve their right to further capture the network, that is, take a service line. If the subscriber's priority (the priority of its packets) is higher than the current value of the reservation field, it can write its priority there instead of the previous one. After bypassing the ring in the backup field, the highest priority from all subscribers will be recorded. The contents of the backup field are similar to the content of the priority field, but speaks about the future priority.

As a result of the use of priority and reservation fields, it is possible to access the network only to subscribers with packets for transmission with the highest priority. Less priority packages will be served only on the exhaustion of more priority packages.

The format of the information package (frame) Token-Ring is presented in Fig. 7.12. In addition to the initial and final separators, as well as an access control byte, this package also includes a packet control byte, network address of the receiver and transmitter, data, checksum and packet status bytes.

Fig. 7.12. Package format (frame) TECKEN-RING network (field length is given in bytes)

Putting the packet fields (frame).

The initial separator (SD) is a sign of the start of the package, the format is the same as in the marker.
Access control byte (AC) has the same format as in the marker.
Package control panel (FC - Frame Control) defines the type of packet (frame).
Six-month MAC addresses of the sender and the package recipient have the standard format described in the Lecture 4.
Data field (DATA) includes the transmitted data (in the information package) or information for the exchange management (in the control packet).
The checksum field (FCS - Frame Check Sequence) is a 32-bit cyclic package checkline (CRC).
The final separator (ED), as in the marker, indicates the end of the package. In addition, it determines whether this package is intermediate or final in the sequence of transmitted packets, and also contains a feature of the package error (see Fig. 7.10).
Package status byte (FS - FRAME STATUS) indicates what happened with this package: whether it was seen by the receiver (that is, there is a receiver with a given address) and copied to the receiver's memory. According to him, the sender of the package will find out whether the package has come to the destination and without errors or it is necessary to transmit it again.

It should be noted that a greater permissible amount of transmitted data in one packet compared to the Ethernet network can be a decisive factor to increase network performance. Theoretically, 16 Mbps and 100 Mbps transmission rates of the data field can be achieved even 18 Kbytes, which is fundamentally transmitted by large amounts of data. But even at a speed of 4 Mbit / s thanks to a marker access method, the TECKEN-Ring network often provides a greater actual transmission rate than the Ethernet network (10 Mbps). Especially noticeable token-ring advantage at high loads (over 30-40%), since in this case the CSMA / CD method requires a lot of time to resolve repeated conflicts.

The subscriber who wants to transmit the package is waiting for the coming of a free marker and captures it. The captured marker turns into the frame of the information package. The subscriber then transfers the information packet into the ring and is waiting for it. After that, he frees the marker and again sends it to the network.

In addition to the marker and the usual package on the TOKEn-Ring network, a special control packet can be transmitted to interrupt transmission (ABORT). It can be sent at any time and anywhere in the data stream. This package consists of two single-byte fields - initial (SD) and final (ED) separators of the described format.

Interestingly, in a faster version of Token-Ring (16 Mbit / s and above), the so-called event of the early formation of the marker (ETR - Early Taken Release) is used. It allows you to avoid unproductive network usage at the time until the data packet returns along the ring to its sender.

The ETR method is reduced to the fact that immediately after the transfer of its package attached to the marker, any subscriber issues a new free marker to the network. Other subscribers can start the transfer of their packages immediately after the completion of the package of the previous subscriber, without waiting until it completes bypassing the entire rings of the network. As a result, several packages may be in the network at the same time, but there will always be no more than one free marker. This conveyor is particularly effective in high-length networks that have a significant propagation delay.

When connecting the subscriber to the concentrator, it performs the procedure for autonomous self-testing and testing of the cable (in the ring it does not turn on, since there is no phantom current signal). The subscriber sends itself a number of packets and checks the correctness of their passage (its input is directly connected to its own output of the TCU unit, as shown in Fig. 7.4). After that, the subscriber includes itself in the ring, sending a phantom current. At the time of inclusion, the packet transmitted over the ring can be spoiled. Next, the subscriber sets up synchronization and checks the availability of an active monitor in the network. If there is no active monitor, the subscriber begins to match the right to become them. The subscriber then checks the uniqueness of its own address in the ring and collects information about other subscribers. After that, he becomes a full participant in the network exchange.

In the exchange process, each subscriber follows the health of the previous subscriber (by ring). If he suspects the failure of the previous subscriber, it launches the procedure automatic recovery rings. A special control package (Bucken) speaks to the previous subscriber about the need to conduct self-testing and, possibly, disconnect from the ring.

The Taken-Ring network also provides the use of bridges and switches. They are used to separate a large ring into several ring segments that have the ability to exchange packages among themselves. This reduces the load on each segment and increase the share of time provided to each subscriber.

As a result, you can form a distributed ring, that is, the combination of several ring segments with one large main ring (Fig. 7.13) or a stellar-ring structure with a central switch to which the ring segments are connected (Fig. 7.14).

Fig. 7.13. Combining segments by a trunk ring with bridges

Fig. 7.14. Communion of segments by the central switch

ARCNET network (or ArcNet from the English Attached Resource Computer Net, computer network of the United resources) is one of the oldest networks. It was developed by Datapoint Corporation back in 1977. There are no international standards for this network, although it is precisely it is considered the generic team of the marker access method. Despite the lack of standards, the ArcNet network until recently (in 1980 - 1990) was popular, even seriously competing with Ethernet. A large number of companies (for example, DataPoint, Standard Microsystems, Xircom and others) produced equipment for the network of this type. But now the production of ARCnet equipment is almost discontinued.

Among the main advantages of the ArcNet network compared to Ethernet, you can call a limited amount of access time, high reliability of communication, ease of diagnostics, as well as a relatively low cost of adapters. The most significant disadvantages of the network include low information transfer rate (2.5 Mbps), addressing system and package format.

A rather rare code is used to transmit information on the ARCNet network, in which the logical unit corresponds to two pulses during the bit interval, and a logical zero is one impulse. Obviously, it is a self-crying code that requires even greater cable bandwidth than even Manchester.

As a transmission medium, a coaxial cable with a wave resistance of 93 ohms is used, for example, the RG-62A / U brand. Options with twisted pair (shielded and unshielded) were not widely used. Options for fiber optic cable were also proposed, but they also did not save ArcNet.

As a topology, the ARCNET network uses a classic bus (ArcNet-Bus), as well as a passive star (ArcNet-Star). Hubs (hubs) are used in the star. It is possible to combine with the help of tire and stellar segments in the tree topology (as in Ethernet). The main limitation - in the topology should not be closed paths (loops). Another limitation: the number of segments connected by a sequential chain with hubs should not exceed three.

Hubs are two types:

Active hubs (restore the shape of the incoming signals and enhance them). Number of ports - from 4 to 64. Active hubs can be connected to each other (cascaded).
Passive concentrators (simply mix the incoming signals without amplification). Number of ports - 4. Passive hubs cannot be connected with each other. They can only associate active hubs and / or network adapters.

Tire segments can only be connected to active concentrators.

Network adapters are also two types:

High-impedance (BUS), intended for use in tire segments:
Low-impedance (STAR) intended for use in the passive star.

Low-imaginary adapters differ from highly pressed the fact that they contain in their composition matching 93-ohm terminators. When applying, external approval is not required. In tire segments, low-impedance adapters can be used as terminal to match the tire. High-impedance adapters require the use of external 93-ohm terminators. Some network adapters have the ability to switch from high-impedance state to low-imaginary, they can also work in the bus, and in the star.

Thus, the topology of the ArcNet network has the following form (Fig. 7.15).

Fig. 7.15. Topology ARCNET Type Type Type (B - Tire Adapters, S - Adapters for Work in the Star)

The main technical characteristics of the ArcNet network are as follows.

Transmission medium - coaxial cable, twisted steam.
Maximum network length - 6 kilometers.
The maximum cable length from the subscriber to the passive concentrator is 30 meters.
The maximum cable length from the subscriber to the active concentrator is 600 meters.
The maximum cable length between active and passive concentrators is 30 meters.
The maximum cable length between the active concentrators is 600 meters.
Maximum amount Subscribers online - 255.
The maximum number of subscribers on the bus segment is 8.
The minimum distance between the subscribers in the bus is 1 meter.
The maximum length of the bus segment is 300 meters.
Data transfer rate - 2.5 Mbps.

When creating complex topologies, it is necessary to ensure that the delay in the propagation of signals in the network between subscribers has not exceeded 30 μs. The maximum attenuation of the signal in the cable at a frequency of 5 MHz should not exceed 11 dB.

The ArcNet network uses a marker access method (transfer method), but it is somewhat different from the token-Ring network. The closest of this method is to the one that is provided in the IEEE 802.4 standard. Subscriber's sequence for this method:

1. The subscriber who wants to transmit is waiting for the parish of the Marker.

2. Having received a marker, it sends a request to send the information receiving subscriber (asks whether the receiver is ready to accept his package).

3. The receiver, receiving a request, sends the answer (confirms its readiness).

4. Having received a readiness confirmation, the transmitter subscriber sends its package.

5. After receiving the package, the receiver sends a package reception confirmation.

6. The transmitter, receiving a package reception confirmation, finishes its communication session. After that, the marker is transmitted to the following subscriber in order of decreasing network addresses.

Thus, in this case, the package is transmitted only when there is confidence in the receiver's readiness to take it. This significantly increases the reliability of the transfer.

Just as in the case of Token-Ring, conflicts in ArcNet are completely excluded. Like any marker network, Arcnet keeps the load well and guarantees the amount of network access time (unlike Ethernet). The total time for bypassing the marker of all subscribers is 840 ms. Accordingly, the same interval determines the upper limit of the network access time.

The marker is formed by the Special Subscriber - the network controller. They are a subscriber with a minimum (zero) address.

If the subscriber does not receive a free marker for 840 ms, it sends a long bit sequence to the network (for the guaranteed destruction of the spoiled old marker). After that, the network control and destination (if necessary) of the new controller is performed.

The size of the ARCNET network package is 0.5 kB. In addition to the data field, it also includes 8-bit address receiver and transmitter and a 16-bit cyclic checksum (CRC). Such a small package size is not too convenient at high intensity exchange over the network.

ArcNet network adapters differ from the adapters of other networks in that they need to install their own switches or jumpers network address (All of them can be 255, since the last, 256th address is applied on the network for a wide broadcast mode). Control of the uniqueness of each network address is fully imposed on network users. The connection of new subscribers becomes quite difficult at the same time, as it is necessary to set the address that has not yet been used. Selecting an 8-bit address format limits the permissible number of subscribers in the network - 255, which may not be enough for large companies.

As a result, all this led to the almost complete abandonment of the ARCNET network. There were variants of the ARCNET network, calculated on the transfer rate of 20 Mbps, but they were not widespread.