Fast Ethernet technology, its features, physical layer, construction rules. Description of Fast Ethernet Technology Purpose of Token Ring Network Technology

The most widespread among standard networks is the Ethernet network. It first appeared in 1972 (developed by the well-known company Xerox). The network turned out to be quite successful, and as a result of this, in 1980 it was supported by such major companies as DEC and Intel (the merger of these companies was named DIX after the first letters of their names). Through their efforts in 1985, the Ethernet network became an international standard, it was adopted by the largest international standards organizations: the 802 IEEE committee (Institute of Electrical and Electronic Engineers) and ECMA (European Computer Manufacturers Association).

The standard was named IEEE 802.3 (in English it reads as eight oh two dot three). It defines multiple access to a mono-channel of the bus type with collision detection and transmission control, that is, with the already mentioned CSMA / CD access method. Some other networks also met this standard, since the level of detail is low. As a result, networks of the IEEE 802.3 standard were often incompatible with each other in terms of both design and electrical characteristics. Recently, however, the IEEE 802.3 standard has been considered the standard for the Ethernet network.

Key features of the original IEEE 802.3 standard:

topology - bus;
transmission medium - coaxial cable;
transmission speed - 10 Mbit / s;
maximum network length - 5 km;
the maximum number of subscribers is up to 1024;
network segment length - up to 500 m;
number of subscribers on one segment - up to 100;
access method - CSMA / CD;
transmission is narrowband, that is, without modulation (mono channel).

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

Ethernet is now the most popular in the world (over 90% of the market), and it is expected to remain so in the coming years. This was largely due to the fact that from the very beginning the characteristics, parameters, protocols of the network were open, as a result of which a huge number of manufacturers around the world began to produce Ethernet equipment that was fully compatible with each other.

A classic Ethernet network used a 50-ohm coaxial cable of two types (thick and thin). However, in recent years (since the beginning of the 90s), the most widespread version of the Ethernet is using twisted pairs as a transmission medium. A standard has also been defined for the use of fiber optic cable in a network. Additions have been made to the original IEEE 802.3 standard to accommodate these changes. In 1995, an additional standard appeared for a faster version of Ethernet operating at a speed of 100 Mbit / s (the so-called Fast Ethernet, IEEE 802.3u standard), using twisted pair or fiber-optic cable as the transmission medium. In 1997, a version with a speed of 1000 Mbit / s appeared (Gigabit Ethernet, IEEE 802.3z standard).

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

Rice. 7.1. Classic Ethernet topology

A segment (part of a network) can be a classic bus or a single subscriber. The bus segments use coaxial cable, and the passive star beams (for connecting single computers to the hub) use twisted pair and fiber optic cables. The main requirement for the resulting topology is that there are no closed paths (loops) in it. In fact, it turns out that all subscribers are connected to a physical bus, since the signal from each of them propagates in all directions at once and does not return back (as in a ring).

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

Fast Ethernet does not include physical topology bus, only passive star or passive tree is used. In addition, Fast Ethernet has much more stringent requirements for the maximum network length. Indeed, if the transmission speed is increased by 10 times and the format of the packet is preserved, its minimum length becomes ten times shorter. Thus, the permissible value of the double signal transit time through the network is reduced by 10 times (5.12 μs versus 51.2 μs in Ethernet).

The standard Manchester code is used to transmit information on an Ethernet network.

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

Rice. 7.2. Ethernet packet structure

The Ethernet frame length (that is, the packet without the preamble) must be at least 512 bit intervals or 51.2 µs (this is the limit for the double transit time in the network). Provides individual, multicast and broadcast addressing.

The Ethernet packet includes the following fields:

The preamble consists of 8 bytes, the first seven are the code 10101010, and the last byte is the code 10101011. In the IEEE 802.3 standard, the eighth byte is called the Start of Frame Delimiter (SFD) and forms a separate field of the packet.
The recipient (receiver) and sender (transmitter) addresses are 6 bytes each and are built according to the standard described in the Packet Addressing section of Lecture 4. These address fields are processed by the subscribers' equipment.
The 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 generally accepted 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 determines the frame type. The control field is processed programmatically.
The data field must contain between 46 and 1500 bytes of data. If the packet is to contain less than 46 bytes of data, then the data field is padded with padding bytes. According to the IEEE 802.3 standard, a special padding field (pad data) is allocated in the packet structure, which can have a length of zero when there is enough data (more than 46 bytes).
The Frame Check Sequence (FCS) field contains a 32-bit cyclic packet checksum (CRC) and is used to check the correctness of the packet transmission.

Thus, the minimum frame length (packet without preamble) is 64 bytes (512 bits). It is this value that determines the maximum allowable double delay of signal propagation over the network in 512 bit intervals (51.2 μs for Ethernet or 5.12 μs for Fast Ethernet). The standard assumes that the preamble can be reduced as the packet passes through various network devices so it is not counted. The maximum frame length is 1518 bytes (12144 bits, i.e. 1214.4 μs for Ethernet, 121.44 μs for Fast Ethernet). This is important for choosing the size of the buffer memory of network equipment and for assessing the overall network load.

The choice of the preamble format is not accidental. The point is that the sequence of alternating ones and zeros (101010 ... 10) in the Manchester code is characterized by the fact that it has transitions only in the middle of the bit intervals (see Section 2.6.3), that is, only information transitions. Of course, it is easy for the receiver to tune (synchronize) with such a sequence, even if for some reason it is shortened by a few bits. The last two unit bits of the preamble (11) differ significantly from the sequence 101010 ... 10 (transitions also appear at the border of the bit intervals). Therefore, the already tuned receiver can easily select them and thereby detect the beginning of useful information (the beginning of the frame).

For an Ethernet network operating at a speed of 10 Mbit / s, the standard defines four main types of network segments, focused on different media:

10BASE5 (thick coaxial cable);
10BASE2 (thin coaxial cable);
10BASE-T (twisted pair);
10BASE-FL (fiber optic cable).

The segment name includes three elements: the number 10 means the transmission rate of 10 Mbit / s, the word BASE means transmission in the main frequency band (that is, without modulation of the high-frequency signal), and the last element means the permissible segment length: 5 - 500 meters, 2 - 200 meters (more precisely, 185 meters) or the type of communication line: T - twisted pair (from English twisted-pair), F - fiber optic cable (from English fiber optic).

Likewise, for an Ethernet network operating at a speed of 100 Mbps (Fast Ethernet), the standard defines three types of segments, differing in the types of transmission media:

100BASE-T4 (twisted pair);
100BASE-TX (twisted pair);
100BASE-FX (fiber optic cable).

Here, the number 100 stands for a transmission rate of 100 Mbps, the letter T for a twisted pair, and the letter F for a fiber optic cable. The 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.

The features of Ethernet equipment, as well as the CSMA / CD exchange control algorithm and the cyclic checksum (CRC) calculation algorithm will be discussed in more detail later in special sections of the course. It should be noted here only that the Ethernet network does not differ in either record characteristics or optimal algorithms; it is inferior in a number of parameters to other standard networks. But thanks to its powerful support, the highest level of standardization, huge volumes of production of technical means, Ethernet favorably stands out among other standard networks, and therefore it is customary to compare any other network technology with Ethernet.

The evolution of Ethernet technology is moving away from the original standard. The use of new transmission media and switches can significantly increase the size of the network. Abandoning the Manchester code (on Fast Ethernet and Gigabit Ethernet) results in higher data rates and reduced cable requirements. Rejection of the CSMA / CD control method (with full-duplex exchange mode) makes it possible to dramatically increase the efficiency of work and remove restrictions on the length of the network. However, all of the newer types of networking are also referred to as Ethernet.

Token-Ring network

The Token-Ring (token ring) network was proposed by IBM in 1985 (the first option appeared in 1980). It was designed to network all types of computers made by IBM. The very fact that it is supported by IBM, largest manufacturer computer technology, suggests that it needs to be given special attention. But no less important is the fact that Token-Ring is currently the international standard IEEE 802.5 (although there are minor differences between Token-Ring and IEEE 802.5). This puts this network on the same level as Ethernet in status.

Developed by Token-Ring as a reliable alternative to Ethernet. Although Ethernet is now superseding all other networks, Token-Ring is not hopelessly obsolete. More than 10 million computers worldwide are connected by this network.

IBM has done everything to make its network as widespread as possible: detailed documentation has been released up to schematic diagrams adapters. As a result, many companies, for example, 3COM, Novell, Western Digital, Proteon and others, started to manufacture adapters. By the way, the NetBIOS concept was developed specifically for this network, as well as for another IBM PC Network. Whereas in the previously created PC Network, NetBIOS programs were stored in the read-only memory built into the adapter, in the Token-Ring network, a NetBIOS emulation program was already used. This made it possible to more flexibly respond to the peculiarities of the hardware and maintain compatibility with higher-level programs.

The Token-Ring network has a ring topology, although it looks more like a star in appearance. This is due to the fact that individual subscribers (computers) are not connected to the network directly, but through special hubs or multi-station access devices (MSAU or MAU - Multistation Access Unit). Physically, the network forms a star-ring topology (Figure 7.3). In reality, the subscribers are nevertheless united in a ring, that is, each of them transmits information to one neighboring subscriber, and receives information from another.

Rice. 7.3. Star-ring token-ring network topology

At the same time, the hub (MAU) allows you to centralize the configuration task, disconnect faulty subscribers, monitor network operation, etc. (fig. 7.4). It does not perform any information processing.

Rice. 7.4. Ringing Token-Ring Subscribers Using a Hub (MAU)

A special Trunk Coupling Unit (TCU) is used for each subscriber as part of the hub, which provides automatic inclusion of the subscriber in the ring if it is connected to the hub and is working properly. If the subscriber disconnects from the hub or is faulty, the TCU automatically restores the integrity of the ring without participation this subscriber... The TCU is triggered by a DC signal (the so-called phantom current), which comes from a subscriber who wants to join the ring. The subscriber can also disconnect from the ring and carry out a self-test procedure (the far right subscriber in Fig. 7.4). The phantom current does not affect the information signal in any way, since the signal in the ring does not have a constant component.

Structurally, the hub is a self-contained unit with ten connectors on the front panel (Fig. 7.5).

Rice. 7.5. Token-Ring Hub (8228 MAU)

Eight central connectors (1 ... 8) are intended for connecting subscribers (computers) using adapter cables or radial cables. The two extreme connectors: input RI (Ring In) and output RO (Ring Out) are used to connect to other hubs using special trunk cables (Path cables). Wall-mount and desktop-mount options are available.

There are both passive and active MAUs. An active hub recovers the signal coming from the subscriber (that is, it acts as an Ethernet hub). The passive hub does not perform signal recovery, it only re-switches the communication lines.

The hub in the network can be the only one (as in Figure 7.4), in this case only the subscribers connected to it are closed in the ring. Outwardly, this topology looks like a star. If more than eight subscribers need to be connected to the network, then several hubs are connected by trunk cables and form a star-ring topology.

As noted, ring topology is very sensitive to ring cable breaks. To increase the survivability of the network, Token-Ring provides a so-called ring folding mode, which allows you to bypass the break point.

In normal mode, the hubs are connected in a ring by two parallel cables, but information is transmitted only through one of them (Fig. 7.6).

Rice. 7.6. Combining MAUs in Normal Mode

In the event of a single damage (breakage) of the cable, the network transmits through both cables, thereby bypassing the damaged section. At the same time, the order of bypassing subscribers connected to concentrators is even preserved (Fig. 7.7). True, the total length of the ring increases.

In the event of multiple damage to the cable, the network splits into several parts (segments) that are not connected to each other, but remain fully operational (Fig. 7.8). The maximum part of the network remains connected, as before. Of course, this no longer rescues the network as a whole, but it allows, with the correct distribution of subscribers to concentrators, to preserve a significant part of the functions of the damaged network.

Several hubs can be structurally combined into a group, a cluster, within which subscribers are also connected in a ring. The use of clusters allows you to increase the number of subscribers connected to one center, for example, up to 16 (if the cluster includes two hubs).

Rice. 7.7. Collapsing the ring when the cable is damaged

Rice. 7.8. Ring disintegration with multiple cable damage

At first, twisted pair, both unshielded (UTP) and shielded (STP), were used as a transmission medium in the IBM Token-Ring network, but then there were options for equipment for coaxial cable, as well as for fiber optic cable in the FDDI standard.

The main technical characteristics of the classic version of the Token-Ring network:

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

All specifications are based on the use of an unshielded twisted pair cable. If a different transmission medium is used, the characteristics of the network 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 - up to 100 meters (instead of 45), the number of hubs - up to 33, and the total length of the ring connecting the hubs - up to 200 meters ... Fiber optic cable allows to extend the cable length up to two kilometers.

To transfer information in Token-Ring, a biphase code is used (more precisely, its version with a mandatory transition in the center of the bit interval). As with any star topology, no additional electrical termination or external grounding is required. Negotiation is performed by the hardware of the network adapters and hubs.

Token-Ring cables use RJ-45 (unshielded twisted pair) connectors, MIC and DB9P connectors. The wires in the cable connect the same pins of the connectors (that is, the so-called straight cables are used).

The Token-Ring network in the classic version is inferior to the Ethernet network both in the allowable size and in the maximum number of subscribers. In terms of transmission speed, there are currently 100 Mbps (High Speed Token-Ring, HSTR) and 1000 Mbps (Gigabit Token-Ring) versions of Token-Ring. Companies that support Token-Ring (including IBM, Olicom, Madge) do not intend to abandon their network, seeing it as a worthy competitor to Ethernet.

Compared to Ethernet hardware, Token-Ring hardware is noticeably more expensive, since it uses a more complex method of exchange control, so the Token-Ring network is not so widespread.

However, unlike Ethernet, Token-Ring network maintains a high load level much better (more than 30-40%) and provides guaranteed access time. This is necessary, for example, in industrial networks, where a delay in reaction to an external event can lead to serious accidents.

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

The maximum packet transfer time in Token-Ring is 10 ms. With a maximum number of 260 subscribers, the full cycle of the ring will be 260 x 10 ms = 2.6 s. During this time, all 260 subscribers will be able to transfer their packages (if, of course, they have something to transfer). During this time, a free marker will surely reach every subscriber. This interval is also the upper limit of the Token-Ring access time.

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

identification of transmission errors;
network configuration control (network restoration in case of failure of the subscriber that precedes him in the ring);
control of numerous time relationships adopted in the network.

The large number of functions, of course, complicates and increases the cost of the network adapter hardware.

To control the integrity of the token in the network, one of the subscribers is used (the so-called active monitor). Moreover, his equipment is no different from the rest, but his software monitor the temporal relationships in the network and form, if necessary, a new marker.

The active monitor performs the following functions:

launches a marker into the ring at the beginning of work and when it disappears;
regularly (every 7 seconds) informs about his presence with a special control package (AMP - Active Monitor Present);
removes from the ring a packet that was not removed by the subscriber who sent it;
monitors the allowed packet transmission time.

The active monitor is selected when the network is initialized; it can be any computer on the network, but, as a rule, it becomes the first subscriber connected to the network. The subscriber, who has become an active monitor, includes its buffer (shift register) into the network, which guarantees that the marker will fit into the ring even with the minimum ring length. The size of this buffer is 24 bits for 4 Mbps and 32 bits for 16 Mbps.

Each subscriber constantly monitors how the active monitor performs its duties. If the active monitor fails for some reason, a special mechanism is activated through which all other subscribers (spare, backup monitors) decide on the appointment of a new active monitor. To do this, the subscriber who detects the failure of the active monitor transmits a control packet (token request packet) over the ring with its MAC address. Each subsequent subscriber compares the MAC address from the packet with its own. If its own address is less, it passes the packet on unchanged. If more, then it sets its own MAC address in the packet. The active monitor will be the subscriber whose MAC-address value is higher than that of the others (he must receive back a packet with his MAC-address three times). A sign of failure of the active monitor is its failure to perform one of the listed functions.

The Token-Ring network token is a control packet containing only three bytes (Figure 7.9): the Start Delimiter byte (SD), Access Control byte (AC), and End Delimiter byte (ED). All these three bytes are also included in the information package, although their functions in the marker and in the package are somewhat different.

The leading and trailing separators are not just a sequence of zeros and ones, but contain signals of a special kind. This was done so that the delimiters could not be confused with any other packet bytes.

Rice. 7.9. Token-Ring Token Format

The initial SD delimiter contains four non-standard bit intervals (Figure 7.10). Two of them, denoted by J, represent a low signal level during the entire bit interval. The other two bits, labeled K, represent a high signal level for the entire bit interval. It is clear that such timing failures are easily detected by the receiver. Bits J and K can never occur among the bits of useful information.

Rice. 7.10. Leading (SD) and Ending (ED) Delimiter Formats

The final delimiter ED also contains four special bits (two J bits and two K bits), as well as two one bits. But, in addition, it also includes two information bits, which are meaningful only as part of an information package:

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

The Access Control (AC) byte is divided into four fields (Figure 7.11): a priority field (three bits), a marker bit, a monitor bit, and a reservation field (three bits).

Rice. 7.11. Access Control Byte Format

The priority bits (field) allow the subscriber to assign priority to his packets or token (priority can be from 0 to 7, with 7 being the highest priority and 0 being the lowest). The subscriber can attach his package to the marker only when his own priority (the priority of his packages) is the same or higher than the priority of the token.

The marker bit determines whether a packet is attached to the marker or not (one corresponds to a marker without a packet, zero - to a marker with a packet). The monitor bit, set to one, indicates that this marker was transmitted by the active monitor.

Reservation bits (field) allow the subscriber to reserve his right to further seize the network, that is, to take a queue for service. If the subscriber's priority (the priority of his packets) is higher than the current value of the reservation field, then he can write his priority there instead of the previous one. After looping around the ring, the highest priority of all subscribers will be recorded in the reservation field. The content of the reservation field is similar to the content of the priority field, but indicates the future priority.

As a result of the use of priority and reservation fields, only subscribers with the highest priority packets for transmission are able to access the network. Lower priority packets will be served only when higher priority packets are exhausted.

The format of the information packet (frame) Token-Ring is shown in Fig. 7.12. In addition to the start and end delimiters, and the access control byte, this packet also includes the packet control byte, receiver and transmitter network addresses, data, checksum, and packet status byte.

Rice. 7.12. Packet (frame) format of the Token-Ring network (the length of the fields is given in bytes)

Purpose of the fields of the packet (frame).

The leading delimiter (SD) is the start of the packet, the format is the same as in the marker.
The Access Control (AC) byte has the same format as the token.
The Packet Control Byte (FC - Frame Control) defines the type of packet (frame).
The six-byte source and destination MAC addresses of a packet follow the standard format described in Chapter 4.
The data field (Data) includes the transmitted data (in an information packet) or information for control of the exchange (in a control packet).
The Frame Check Sequence (FCS) field is a 32-bit cyclic packet checksum (CRC).
The trailing separator (ED), as in the marker, indicates the end of the packet. In addition, it determines whether Current Package intermediate or final in the sequence of transmitted packets, and also contains a sign of packet error (see Fig. 7.10).
The packet status byte (FS - Frame Status) tells what happened to the given packet: whether it was seen by the receiver (that is, whether there is a receiver with the specified address) and copied into the receiver's memory. From it, the sender of the packet knows whether the packet arrived at its destination and without errors, or if it needs to be transmitted again.

It should be noted that the larger allowable size of the transmitted data in one packet compared to an Ethernet network can be a decisive factor in increasing network performance. Theoretically, for transfer rates of 16 Mbit / s and 100 Mbit / s, the length of the data field can even reach 18 Kbytes, which is essential when transferring large amounts of data. But even at 4 Mbps, Token-Ring often delivers faster actual transfer rates than 10 Mbps Ethernet, thanks to token-based access. The advantage of Token-Ring is especially noticeable at high loads (over 30-40%), since in this case the CSMA / CD method requires a lot of time to resolve repeated conflicts.

A subscriber wishing to transmit a packet waits for a free token to arrive and captures it. The captured marker is transformed into the frame of the information packet. Then the subscriber transmits information package into the ring and awaits his return. It then releases the token and sends it back to the network.

In addition to the token and the usual packet, a special control packet can be transmitted in the Token-Ring network, which serves to interrupt the transmission (Abort). It can be sent anytime and anywhere in the data stream. This package consists of two one-byte fields - the initial (SD) and final (ED) delimiters of the described format.

Interestingly, the faster version of Token-Ring (16 Mbps and higher) uses the so-called Early Token Release (ETR) method. It avoids network overhead while the data packet is looped back to its sender.

The ETR method boils down to the fact that immediately after transmitting its packet attached to the token, any subscriber issues a new free token to the network. Other subscribers can start transmitting their packets immediately after the end of the packet of the previous subscriber, without waiting for him to complete the traversal of the entire network ring. As a result, there can be several packets on the network at the same time, but there will always be no more than one free token. This pipeline is especially effective on long-haul networks that have significant propagation delay.

When a subscriber is connected to the hub, it performs the procedure of autonomous self-test and cable testing (it does not turn on in the ring yet, since there is no phantom current signal). The subscriber sends himself a number of packets and checks the correctness of their passage (his input is directly connected to his output by the TCU, as shown in Fig. 7.4). After that, the subscriber includes himself in the ring, sending a phantom current. At the moment of switching on, the packet transmitted over the ring can be corrupted. Next, the subscriber sets up synchronization and checks for an active monitor on the network. If there is no active monitor, the subscriber starts the competition for the right to become one. Then the subscriber checks the uniqueness of his own address in the ring and collects information about other subscribers. After which he becomes a full participant in the exchange over the network.

In the course of the exchange, each subscriber monitors the health of the previous subscriber (around the ring). If he suspects a failure of the previous subscriber, he starts the automatic ring recovery procedure. A special control package (buoy) tells the previous subscriber to conduct a self-test and, possibly, disconnect from the ring.

The Token-Ring network also provides for the use of bridges and switches. They are used to divide a large ring into several ring segments that can exchange packets with each other. This allows you to reduce the load on each segment and increase the proportion of time provided to each subscriber.

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

Rice. 7.13. Connecting segments with a trunk ring using bridges

Rice. 7.14. Aggregation of segments with a central switch

Arcnet (or ARCnet from Attached Resource Computer Net) is one of the oldest networks. It was developed by the Datapoint Corporation back in 1977. There are no international standards for this network, although it is she who is considered the ancestor of the token 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, etc.) have produced equipment for this type of network. But now the production of Arcnet equipment is practically discontinued.

Among the main advantages of the Arcnet network in comparison with Ethernet are the limited amount of access time, high reliability of communication, ease of diagnostics, as well as the relatively low cost of adapters. The most significant disadvantages of the network include low data transfer rate (2.5 Mbit / s), addressing system and packet format.

To transfer information in the Arcnet network, a rather rare code is used, in which a logical one corresponds to two pulses during a bit interval, and a logical zero corresponds to one pulse. Obviously this is self-timing code that requires even more cable bandwidth than even Manchester's.

As a transmission medium in the network, a coaxial cable with a characteristic impedance of 93 Ohm is used, for example, of the RG-62A / U brand. Twisted pair options (shielded and unshielded) are not widely used. Fiber optic options have been proposed, but they haven't saved Arcnet either.

As a topology, the Arcnet network uses the classic bus (Arcnet-BUS) as well as a passive star (Arcnet-STAR). Hubs are used in the star. It is possible to combine bus and star segments into a tree topology using hubs (as with Ethernet). The main limitation is that there should be no closed paths (loops) in the topology. Another limitation is that the number of daisy chained segments using hubs must not exceed three.

Hubs are of two types:

Active concentrators (restore the shape of incoming signals and amplify them). The number of ports is from 4 to 64. Active hubs can be interconnected (cascaded).
Passive hubs (just mix the incoming signals without amplification). The number of ports is 4. Passive hubs cannot be connected to each other. They can only link active hubs and / or network adapters.

Bus segments can only be connected to active hubs.

There are also two types of network adapters:

High impedance (Bus) for use in bus segments:
Low impedance (Star) designed for use in a passive star.

Low impedance adapters differ from high impedance adapters in that they contain 93-ohm matching terminators. When using them, external approval is not required. In bus segments, low impedance adapters can be used as terminating adapters for bus termination. High impedance adapters require external 93 ohm termination. Some network adapters have the ability to switch from a high impedance state to a low impedance state, they can work in the bus and in the star.

Thus, the topology of the Arcnet network looks like this (Figure 7.15).

Rice. 7.15. Arcnet network topology of bus type (B - adapters for working in the bus, S - adapters for working in a star)

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

Transmission medium - coaxial cable, twisted pair.
The maximum length of the network is 6 kilometers.
The maximum cable length from the subscriber to the passive hub is 30 meters.
The maximum cable length from a subscriber to an active hub is 600 meters.
The maximum cable length between active and passive hubs is 30 meters.
The maximum cable length between active hubs is 600 meters.
The maximum number of subscribers in the network is 255.
The maximum number of subscribers on a bus segment is 8.
The minimum distance between subscribers in the bus is 1 meter.
The maximum length of a bus segment is 300 meters.
The data transfer rate is 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 does not exceed 30 μs. The maximum attenuation of the signal in the cable at a frequency of 5 MHz should not exceed 11 dB.

Arcnet uses token access (pass-through), but is slightly different from Token-Ring. This method is closest to the one provided in the IEEE 802.4 standard. The sequence of actions of subscribers with this method:

1. The subscriber who wants to transmit is waiting for the arrival of the token.

2. Having received the token, he sends a request to transmit information to the receiving subscriber (asks if the receiver is ready to receive his packet).

3. The receiver, having received the request, sends a response (confirms its readiness).

4. Having received confirmation of readiness, the sender subscriber sends his packet.

5. On receiving the packet, the receiver sends an acknowledgment of the packet.

6. The transmitter, having received an acknowledgment of packet reception, ends its communication session. After that, the token is passed to the next subscriber in descending order of network addresses.

Thus, in this case, the packet is transmitted only when there is confidence in the readiness of the receiver to receive it. This significantly increases the reliability of the transmission.

As with Token-Ring, conflicts are completely eliminated in Arcnet. Like any token network, Arcnet holds the load well and guarantees the amount of network access time (as opposed to Ethernet). The total round trip time of all subscribers by the marker is 840 ms. Accordingly, the same interval determines the upper limit of the network access time.

The token is generated by a special subscriber - the network controller. It is the subscriber with the minimum (zero) address.

If the subscriber does not receive a free token within 840 ms, then he sends a long bit sequence to the network (to ensure the destruction of the damaged old token). After that, the procedure for monitoring the network and assigning (if necessary) a new controller is carried out.

The Arcnet package size is 0.5 KB. In addition to the data field, it also includes 8-bit receiver and transmitter addresses and a 16-bit cyclic checksum (CRC). Such a small packet size turns out to be not very convenient with a high traffic intensity over the network.

Arcnet network adapters differ from other network adapters in that they must be set to their own using switches or jumpers. network address(there can be 255 of them, since the last, 256th address is used in the network for broadcasting). The control over the uniqueness of each network address is entirely the responsibility of the network users. Connecting new subscribers becomes quite difficult at the same time, since it is necessary to set the address that has not yet been used. The choice of the 8-bit address format limits the number of network subscribers to 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 20 Mbit / s versions of the Arcnet network, but these were not widely adopted.

Articles to read:

Lecture 6: Standard Ethernet / Fast Ethernet Segments

Today it is almost impossible to find a laptop or motherboard on sale without an integrated network card, or even two. All of them have one connector - RJ45 (more precisely, 8P8C), but the speed of the controller may differ by an order of magnitude. In cheap models it is 100 megabits per second (Fast Ethernet), in more expensive ones - 1000 (Gigabit Ethernet).

If your computer does not have a built-in LAN controller, then it is most likely already an "old man" based on an Intel Pentium 4 or AMD Athlon XP processor, as well as their "ancestors". Such "dinosaurs" can be "made friends" with a wired network only by installing a discrete network card with a PCI slot, since PCI Express buses did not exist at the time of their birth. But even for the PCI bus (33 MHz), network cards are produced that support the most current Gigabit Ethernet standard, although its bandwidth may not be enough to fully unleash the high-speed potential of a gigabit controller.

But even in the case of a 100-megabit integrated network card, a discrete adapter will have to be purchased by those who are going to "upgrade" to 1000 megabits. The best option would be to buy a PCI Express controller, which will provide the maximum network speed, if, of course, the corresponding connector is present in the computer. True, many will give preference to a PCI card, since they are much cheaper (the cost starts literally from 200 rubles).

What are the practical benefits of switching from Fast Ethernet to Gigabit Ethernet? How different is the actual data transfer rate of PCI versions of network cards and PCI Express? Will the usual speed be enough hard disk for a full download of a gigabit channel? You will find the answers to these questions in this material.

Test participants

Three of the cheapest discrete network cards (PCI - Fast Ethernet, PCI - Gigabit Ethernet, PCI Express - Gigabit Ethernet) were selected for testing, since they are in the greatest demand.

The 100 Mbps PCI network card is represented by the Acorp L-100S model (the price starts at 110 rubles), which uses the Realtek RTL8139D chipset, the most popular for cheap cards.

The 1000 Mbps PCI network card is represented by the Acorp L-1000S model (the price starts from 210 rubles), which is based on the Realtek RTL8169SC chip. This is the only card with a heatsink on the chipset - the rest of the test participants do not need additional cooling.

The 1000 Mbps PCI Express network card is represented by the TP-LINK TG-3468 model (the price starts at 340 rubles). And it was no exception - it is based on the RTL8168B chipset, which is also produced by Realtek.

The appearance of the network card

Chipsets from these families (RTL8139, RTL816X) can be seen not only on discrete network cards, but also integrated on many motherboards.

The characteristics of all three controllers are shown in the following table:

Show table

The bandwidth of the PCI bus (1066 Mbit / s) should theoretically be enough to "swing" gigabit network cards to full speed, but in practice it may still not be enough. The point is that this "channel" is shared by all PCI devices; in addition, it transmits service information on the maintenance of the bus itself. Let's see if this assumption is confirmed by real speed measurements.

One more nuance: the overwhelming majority of modern hard drives have an average read speed of no more than 100 megabytes per second, and often even less. Accordingly, they will not be able to provide a full load of the gigabit channel of the network card, the speed of which is 125 megabytes per second (1000: 8 = 125). There are two ways to get around this limitation. The first is to combine a pair of such hard drives into a RAID array (RAID 0, striping), while the speed can almost double. The second is to use SSD-drives, the speed parameters of which are noticeably higher than those of hard drives.

Testing

A computer with the following configuration was used as a server:

processor: AMD Phenom II X4 955 3200 MHz (quad-core);
motherboard: ASRock A770DE AM2 + (AMD 770 + AMD SB700 chipset);
RAM: Hynix DDR2 4 x 2048 GB PC2 8500 1066 MHz (in dual-channel mode);
video card: AMD Radeon HD 4890 1024 MB DDR5 PCI Express 2.0;
network card: Realtek RTL8111DL 1000 Mbps (integrated on the motherboard);
operating system: Microsoft Windows 7 Home Premium SP1 (64-bit version).

A computer with the following configuration was used as a client into which the tested network cards were installed:

processor: AMD Athlon 7850 2800 MHz (dual core);
motherboard: MSI K9A2GM V2 (MS-7302, AMD RS780 + AMD SB700 chipset);
RAM: Hynix DDR2 2 x 2048 GB PC2 8500 1066 MHz (in dual-channel mode);
video card: AMD Radeon HD 3100 256 MB (integrated into the chipset);
hard drive: Seagate 7200.10 160 GB SATA2;
operating system: Microsoft Windows XP Home SP3 (32-bit version).

The tests were carried out in two modes: reading and writing via a network connection from hard disks (this should show that they can be a bottleneck), as well as from RAM disks in the RAM of computers imitating fast SSD-drives. The network cards were connected directly using a three-meter patch cord (eight-core twisted pair, category 5e).

Data transfer rate (hard disk - hard disk, Mbps)

The real data transfer speed through the 100-megabit Acorp L-100S network card did not quite reach the theoretical maximum. Although both gigabit cards outperformed the first by about six times, they failed to show the maximum possible speed. It is clearly seen that the speed "rested" on the performance of Seagate 7200.10 hard drives, which in direct testing on a computer averages 79 megabytes per second (632 Mbps).

There is no fundamental difference in speed between network cards for the PCI bus (Acorp L-1000S) and PCI Express (TP-LINK) in this case, the insignificant advantage of the latter can be explained by the measurement error. Both controllers worked at about sixty percent of their capacity.

Data transfer rate (RAM disk - RAM disk, Mbps)

Acorp L-100S, as expected, showed the same low speed when copying data from high-speed RAM disks. This is understandable - the Fast Ethernet standard does not correspond to modern realities for a long time. Compared to the "hard drive-hard drive" test mode, the Acorp L-1000S Gigabit PCI card noticeably improved performance - the advantage was about 36 percent. An even more impressive lead was demonstrated by the TP-LINK TG-3468 network card - an increase of about 55 percent.

This is where the higher throughput of the PCI Express bus manifested itself - it outperformed the Acorp L-1000S by 14 percent, which can no longer be attributed to an error. The winner fell slightly short of the theoretical maximum, but the speed of 916 megabits per second (114.5 Mb / s) still looks impressive - this means that you will have to wait for the copying to finish almost an order of magnitude less (compared to Fast Ethernet). For example, the time to copy a 25 GB file (typical HD rip with good quality) from computer to computer will be less than four minutes, and with the previous generation adapter it will take more than half an hour.

Testing has shown that Gigabit Ethernet network cards have a huge advantage (up to tenfold) over Fast Ethernet controllers. If your computers have only hard drives that are not combined into a striping array (RAID 0), then there will be no fundamental difference in speed between PCI and PCI Express cards. Otherwise, as well as when using productive SSD-drives, preference should be given to cards with the PCI Express interface, which will provide the highest possible data transfer speed.

Naturally, it should be borne in mind that other devices in the network "path" (switch, router ...) must support the Gigabit Ethernet standard, and the twisted pair (patch cord) category must be at least 5e. Otherwise, the real speed will remain at the level of 100 megabits per second. By the way, backward compatibility with the Fast Ethernet standard remains: you can connect, for example, a laptop with a 100-megabit network card to a gigabit network, this will not affect the speed of other computers on the network.

Introduction

The purpose of this report was a short and accessible presentation of the basic principles of operation and features of computer networks, using the example of Fast Ethernet.

A network is 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 firm and outside it. Resources are data, applications and peripherals such as an external drive, printer, mouse, modem, or joystick. Interactive communication between computers implies real-time messaging.

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

From this material you will learn about:

Fast Ethernet technologies
Switches
FTP cable
Connection types
Computer network topologies

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

Local switching computer networks(LAN) and Fast Ethernet technologies were developed in response to the need to improve the efficiency of Ethernet networks. By increasing bandwidth, these technologies can eliminate network bottlenecks and support applications that require high data rates. The appeal of these solutions is that you don't have to choose one or the other. They are complementary, so the efficiency of the network can most often be improved by using both technologies.

The collected information will be useful both to persons beginning to study computer networks and to network administrators.

1. Network diagram

2. Fast Ethernet technology

fast ethernet computer network

Fast Ethernet is the result of the evolution of Ethernet technology. Based on and keeping intact the same CSMA / CD (Channel Polling and Collision Detection Shared Access) method, Fast Ethernet devices operate at 10 times the speed of Ethernet. 100 Mbps. Fast Ethernet provides sufficient bandwidth for applications such as computer-aided design and manufacturing (CAD / CAM), graphics and imaging, and multimedia. Fast Ethernet is compatible with 10 Mbps Ethernet, so integrating Fast Ethernet into your LAN is more conveniently done using a switch rather than a router.

Switch

Using switches many workgroups can be linked together to form a large LAN (see Figure 1). Inexpensive switches perform better than routers for better LAN performance. Fast Ethernet workgroups of one or two hubs can be connected via a Fast Ethernet switch to further increase the number of users as well as cover a wider area.

As an example, consider the following switch:

Rice. 1 D-Link-1228 / ME

The DES-1228 / ME series of switches include configurable Fast Ethernet switches of layer 2 "premium" class. With advanced functionality, the DES-1228 / ME devices provide a low-cost solution for creating a secure and high-performance network. The switch features high port density, 4 Gigabit Uplink ports, small increments for bandwidth management, and improved network management. These switches allow you to optimize the network both in terms of functionality and cost characteristics. The DES-1228 / ME series switches are optimal solution both in terms of functionality and cost characteristics.

FTP cable

LAN-5EFTP-BL cable consists of 4 pairs of solid copper conductors.

Conductor diameter 24AWG.

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

Two conductors twisted at a specially selected pitch form one twisted pair.

4 twisted pairs are wrapped in plastic wrap and enclosed in general screen foil and PVC sheath.

Straight through

It serves:

1. To connect a computer to a switch (hub, switch) through the computer's network card
2. To connect to a switch (hub, switch) of network peripheral equipment - printers, scanners
3. for UPLINK "and on the upstream switch (hub, switch) - modern switches can automatically configure the inputs in the connector for reception and transmission

Crossover

It serves:

1. For direct connection of 2 computers to a local network, without the use of switching equipment (hubs, switches, routers, etc.).
2. for uplink, connection to a higher-standing switch in a complex local network structure, for old types of switches (hubs, switches), they have a separate connector, or marked "UPLINK" or X.

Star topology

To the stars- the basic topology of a computer network, in which all computers on the network are connected to a central node (usually a switch), forming a physical network segment. Such a network segment can function both separately and as part of a complex network topology (usually a “tree”). All information exchange is carried out exclusively through the central computer, on which a very large load is imposed in this way, therefore it cannot be engaged in anything other than the network. As a rule, it is the central computer that is the most powerful, and it is on it that all the functions of managing the exchange are entrusted. In principle, no conflicts in a network with a star topology are possible, because the management is completely centralized.

Application

Classic 10 megabit Ethernet has been satisfying for most users for about 15 years. However, in the early 90s, its insufficient bandwidth began to be felt. For computers based on Intel 80286 or 80386 processors with ISA (8 MB / s) or EISA (32 MB / s) buses, the throughput of the Ethernet segment was 1/8 or 1/32 of the memory-to-disk channel, and this was in good agreement with the ratio the amount of data processed locally and the data transferred over the network. For more powerful client stations with a PCI bus (133 MB / s), this share dropped to 1/133, which was clearly not enough. As a result, many 10Mbit Ethernet segments became congested, server responsiveness dropped dramatically, and collision rates increased dramatically, further reducing usable bandwidth.

There is a need to develop a "new" Ethernet, that is, a technology that would be as efficient in terms of price / quality ratio at a performance of 100 Mbps. As a result of searches and research, specialists were divided into two camps, which ultimately led to the emergence of two new technologies - Fast Ethernet and l00VG-AnyLAN. They differ in the degree of continuity with classic Ethernet.

In 1992, a group of networking equipment manufacturers, including leaders in Ethernet technology such as SynOptics, 3Com and others, formed the Fast Ethernet Alliance, a nonprofit alliance, to standardize on a new technology that would preserve the features of Ethernet as much as possible.

The second camp was led by Hewlett-Packard and AT&T, which offered to take advantage of the opportunity to address some of the known flaws in Ethernet technology. Some time later, IBM joined these companies, which contributed to the proposal to provide some compatibility with Token Ring networks in the new technology.

At the same time, a research group was formed in committee 802 of the IEEE to study the technical potential of new high-speed technologies. Between the end of 1992 and the end of 1993, the IEEE group examined 100-megabit solutions from various manufacturers. In addition to the Fast Ethernet Alliance offering, the group also reviewed high-speed technology offered by Hewlett-Packard and AT&T.

Discussion focused on the issue of preserving the random CSMA / CD access method. The Fast Ethernet Alliance proposal maintained this method and thereby ensured the continuity and consistency of 10 Mbps and 100 Mbps networks. A coalition of HP and AT&T, which had the backing of significantly fewer vendors in the networking industry than the Fast Ethernet Alliance, proposed a completely new access method called Demand Priority- priority access on demand. It significantly changed the picture of the behavior of nodes in the network, so it could not fit into the Ethernet technology and the 802.3 standard, and a new IEEE 802.12 committee was organized to standardize it.

In the fall of 1995, both technologies became IEEE standards. The IEEE 802.3 committee adopted the Fast Ethernet specification as an 802.3 standard and is not a stand-alone standard, but an addition to the existing 802.3 standard in the form of chapters 21 to 30. The 802.12 committee adopted l00VG-AnyLAN technology, which uses the new Demand Priority access method and supports frames in two formats - Ethernet and Token Ring.

v Physical layer of Fast Ethernet technology

All the differences between Fast Ethernet technology and Ethernet are concentrated on the physical layer (Fig. 3.20). The MAC and LLC layers in Fast Ethernet remain exactly the same, and are described in the previous chapters of the 802.3 and 802.2 standards. Therefore, considering Fast Ethernet technology, we will study only a few options for its physical layer.

The more complex structure of the physical layer of Fast Ethernet technology is caused by the fact that it uses three variants of 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, which gave the world the first Ethernet network, was not included in the number of allowed data transmission media of the new Fast Ethernet technology. This is a common trend in many new technologies, since over short distances, Category 5 twisted pair can transmit data at the same speed as coaxial cable, but the network is cheaper and easier to use. Over long distances, optical fiber has much higher bandwidth than coax, and the network cost is not much higher, especially when you consider the high troubleshooting costs of a large coaxial cabling system.

Differences between Fast Ethernet technology and Ethernet technology

The rejection of coaxial cable has led to the fact that Fast Ethernet networks always have a hierarchical tree structure built on hubs, like l0Base-T / l0Base-F networks. The main difference between Fast Ethernet network configurations is the reduction of the network diameter to about 200 m, which is explained by a 10-fold reduction in the transmission time of a minimum frame length due to a 10-fold increase in the transmission speed compared to 10-megabit Ethernet.

Nevertheless, this circumstance does not really impede the construction of large networks based on Fast Ethernet technology. The fact is that the mid-90s were marked not only by the widespread use of inexpensive high-speed technologies, but also by the rapid development of local area 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 length of the network, and only restrictions on the length of the physical segments connecting neighboring devices (adapter - switch or switch - switch) remain. Therefore, when creating long-distance LAN backbones, Fast Ethernet technology is also actively used, but only in a full-duplex version, together with switches.

This section discusses the half-duplex variant of Fast Ethernet operation, which fully complies with the definition of an access method described in the 802.3 standard.

Compared to the options for the physical implementation of Ethernet (and there are six of them), in Fast Ethernet, the differences between each option from the others are deeper - both the number of conductors and the coding methods change. And since the physical versions of Fast Ethernet were created simultaneously, and not evolutionarily, as for Ethernet networks, it was possible to define in detail those sublayers of the physical layer that do not change from version to version, and those sublevels that are specific to each version of the physical environment.

The official 802.3 standard established three different specifications for the Fast Ethernet physical layer and gave them the following names:

Fast Ethernet physical layer structure

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

The following statements and characteristics apply to all three standards.

· Fast Ethernetee frame formats are different from 10Mbit Ethernet frames.
· The Inter-Frame Gap (IPG) is 0.96 µs and the Bit Gap is 10 ns. All the time parameters of the access algorithm (backoff interval, transmission time of the minimum frame length, etc.), measured in bit intervals, remained the same, therefore, no changes were made to the sections of the standard concerning the MAC level.
· A sign of the free state of the medium is the transmission of the Idle symbol of the corresponding redundancy code over it (and not the absence of signals, as in the Ethernet 10 Mbit / s standards). The physical layer includes three elements:
o reconciliation sublayer;
o media independent interface (Mil);
o physical layer device (PHY).

The negotiation layer is needed so that the MAC layer, designed for the AUI interface, can work with the physical layer through the IP interface.

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

· A sublayer of logical data coding, which converts bytes coming from the MAC level into 4B / 5B or 8B / 6T code symbols (both codes are used in Fast Ethernet technology);
· Physical interconnection sublayers and physical media dependency (PMD) sublayers that provide signaling in accordance with a physical coding technique such as NRZI or MLT-3;
· An auto-negotiation sublayer that allows two communicating ports to automatically select the most efficient mode of operation, such as half or full duplex (this sublayer is optional).

The IP interface supports a physical medium independent way of exchanging data between the MAC sublayer and the PHY sublayer. This interface is similar in purpose to the AUI interface of classic Ethernet, except that the AUI interface was located between the sublayer of physical signal coding (for all cable variants, the same physical coding method was used - Manchester code) and the sublayer of physical connection to the medium, and the IP interface is located between the MAC sublayer and signal coding sublevels, of which there are three in the Fast Ethernet standard - FX, TX and T4.

The MP connector, unlike the AUI connector, has 40 pins, the maximum cable length for the MP is one meter. The signals transmitted via the MP interface have an amplitude of 5 V.

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

This specification defines Fast Ethernet operation over multimode fiber in half and full duplex modes based on the well-proven FDDI coding scheme. As in the FDDI standard, each node is connected to the network by two optical fibers coming from the receiver (R x) and from the transmitter (T x).

There are many similarities between the l00Base-FX and l00Base-TX specifications, so the properties common to the two specifications will be given under the generic name l00Base-FX / TX.

While 10 Mbps Ethernet uses Manchester coding to represent data when transmitted over a cable, Fast Ethernet defines a different coding method, 4V / 5V. This method has already shown its effectiveness in the FDDI standard and has been transferred without changes to the l00Base-FX / TX specification. In this method, every 4 bits of MAC sublayer data (called symbols) are represented by 5 bits. The redundant bit allows candidate codes to be applied by representing each of the five bits as electrical or optical pulses. The existence of prohibited combinations of characters allows you to reject erroneous characters, which increases the stability of networks with l00Base-FX / TX.

To separate the Ethernet frame from the Idle symbols, a combination of Start Delimiter symbols is used (a pair of symbols J (11000) and K (10001) of the 4B / 5B code, and after the end of the frame, a T symbol is inserted before the first Idle symbol.

Continuous data stream of 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 the cable connecting the network nodes. The l00Base-FX and l00Base-TX specifications use different physical coding methods for this - NRZI and MLT-3, respectively (as in FDDI technology when working through fiber and twisted pair).

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

The l00Base-TX specification uses a Category 5 UTP cable or an STP Type 1 cable as a transmission medium. The maximum cable length in both cases is 100 m.

The main differences from the l00Base-FX specification are the use of the MLT-3 method to transmit signals of 5-bit 4V / 5V code portions over a twisted pair, as well as the presence of the Auto-negotiation function to select the port operation mode. The auto-negotiation scheme allows two physically connected devices that support several physical layer standards, differing in bit rate and number of twisted pairs, to choose the most advantageous mode of operation. Usually, the auto-negotiation procedure occurs when you connect a network adapter, which can operate at speeds of 10 and 100 Mbps, to a hub or switch.

The Auto-negotiation scheme described below is now the standard for l00Base-T technology. Prior to this, manufacturers used various proprietary schemes for automatically detecting the speed of the interacting ports, which were not compatible. The standard Auto-negotiation scheme was originally proposed by National Semiconductor under the name NWay.

A total of 5 different operating modes are currently defined that can be supported by l00Base-TX or 100Base-T4 twisted pair devices;

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).

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

The device that started the auto-negotiation process sends a burst of special pulses to its partner. Fast Link Pulse burst (FLP), which contains an 8-bit word that encodes the proposed communication mode, starting with the highest supported by this node.

If the partner node supports the auto-negotuiation function and can also support the proposed mode, it responds with a burst of FLP pulses, in which it confirms this mode, and the negotiation ends there. If the partner node can support a lower priority mode, then it indicates it in the response, and this mode is selected as the working one. Thus, the highest priority common mode of the nodes is always selected.

A node that only supports l0Base-T technology sends Manchester pulses every 16 ms to check the continuity of the line connecting it to the neighboring node. Such a node does not understand the FLP request that the Auto-negotiation node makes to it and continues to send its pulses. A node that has received only continuity check pulses in response to the FLP request, realizes that its partner can only work according to the l0Base-T standard, and sets this mode of operation for itself.

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

The 100Base-T4 specification was designed to leverage existing Category 3 twisted-pair wiring for high-speed Ethernet. This specification improves overall throughput by simultaneously transmitting bit streams across all 4 cable pairs.

The 100Base-T4 specification came later than other Fast Ethernet physical layer specifications. The developers of this technology primarily wanted to create physical specifications that were closest to the l0Base-T and l0Base-F specifications, which worked on two data lines: two pairs or two fibers. To implement work on two twisted pair I had to switch to a higher quality Category 5 cable.

At the same time, the developers of the competing l00VG-AnyLAN technology initially relied on Category 3 twisted pair cables; the main advantage was not so much in cost, but in the fact that it had already been laid in the overwhelming majority of buildings. Therefore, after the release of the l00Base-TX and l00Base-FX specifications, the developers of Fast Ethernet technology implemented their own version of the physical layer for twisted pair Category 3.

Instead of 4V / 5V coding, this method uses 8V / 6T coding, which has a narrower signal spectrum and at a rate of 33 Mbps fits into the 16 MHz band of a twisted pair cable of category 3 (when coding 4V / 5V, the signal spectrum does not fit into this band) ... Every 8 bits of MAC layer information are encoded with 6 ternary symbols, that is, digits with three states. Each ternary digit is 40 ns long. A group of 6 ternary digits is then transmitted to one of the three transmit twisted pairs, independently and sequentially.

The fourth pair is always used to listen to the carrier frequency for collision detection. The data rate for each of the three transmit pairs is 33.3 Mbps, so the total speed of the 100Base-T4 protocol is 100 Mbps. At the same time, due to the adopted coding method, the signal change rate on each pair is only 25 Mbaud, which allows the use of a Category 3 twisted pair cable.

In fig. 3.23 shows the connection of the MDI port of the 100Base-T4 network adapter with the MDI-X port of the hub (the prefix X says that at this connector the connections of the receiver and transmitter are swapped in pairs of the cable compared to the connector of the network adapter, which makes it easier to connect the pairs of wires in the cable - without crossing). Pair 1 -2 always required to transfer data from MDI port to MDI-X port, pair 3 -6 - to receive data by the MDI port from the MDI-X port, and pairs 4 -5 and 7 -8 are bi-directional and are used for both receiving and transmitting, depending on the need.

Connection of nodes according to the 100Base-T4 specification

The most widespread among standard networks is the Ethernet network. It appeared in 1972 and became the international standard in 1985. It was adopted by the largest international standards organizations: IEEE 802 Committee (Institute of Electrical and Electronic Engineers) and ECMA (European Computer Manufacturers Association).

The standard was named IEEE 802.3 (in English it reads as "eight oh two dot three"). It defines multiple access to a mono-channel of the bus type with collision detection and transmission control, that is, with the already mentioned CSMA / CD access method.

Key features of the original IEEE 802.3 standard:

· Topology - bus;

· Transmission medium - coaxial cable;

· Transmission speed - 10 Mbit / s;

· Maximum network length - 5 km;

· The maximum number of subscribers - up to 1024;

· The length of the network segment - up to 500 m;

· The number of subscribers on one segment - up to 100;

· Access method - CSMA / CD;

· Transmission is narrowband, that is, without modulation (mono channel).

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

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

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

Rice. 7.1. Classic Ethernet network topology.

Fast Ethernet does not have a physical bus topology, only a passive star or passive tree is used. In addition, Fast Ethernet has much more stringent requirements for the maximum network length. Indeed, if the transmission speed is increased by 10 times and the format of the packet is preserved, its minimum length becomes ten times shorter. Thus, the permissible value of the double signal transit time through the network is reduced by 10 times (5.12 μs versus 51.2 μs in Ethernet).

The standard Manchester code is used to transmit information on an Ethernet network.

Access to the Ethernet network is carried out using a random CSMA / CD method, which ensures the equality of subscribers. The network uses packets of variable length.

For an Ethernet network operating at a speed of 10 Mbit / s, the standard defines four main types of network segments, focused on different media:

10BASE5 (thick coaxial cable);

10BASE2 (thin coaxial cable);

10BASE-T (twisted pair);

10BASE-FL (fiber optic cable).

The segment name includes three elements: the number "10" means the transmission rate of 10 Mbit / s, the BASE word - transmission in the main frequency band (that is, without modulation of the high-frequency signal), and the last element - the allowable segment length: "5" - 500 meters, "2" - 200 meters (more precisely, 185 meters) or the type of communication line: "T" - twisted pair (from English "twisted-pair"), "F" - fiber optic cable (from English "fiber optic").

Likewise, for an Ethernet network operating at a speed of 100 Mbps (Fast Ethernet), the standard defines three types of segments, differing in the types of transmission media:

· 100BASE-T4 (twisted pair);

· 100BASE-TX (double twisted pair);

· 100BASE-FX (fiber optic cable).

Here, the number "100" stands for a transmission rate of 100 Mbps, the letter "T" stands for a twisted pair, and the letter "F" stands for a fiber-optic cable. The 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.

Token-Ring network

The Token-Ring (token ring) network was proposed by IBM in 1985 (the first option appeared in 1980). It was designed to network all types of computers made by IBM. The very fact that it is supported by IBM, the largest manufacturer of computer technology, suggests that it needs special attention. But no less important is the fact that Token-Ring is currently the international standard IEEE 802.5 (although there are minor differences between Token-Ring and IEEE 802.5). This puts this network on the same level as Ethernet in status.

Rice. 7.3. Star-ring token-ring network topology.

The main technical characteristics of the classic version of the Token-Ring network:

· The maximum number of concentrators such as IBM 8228 MAU - 12;

· The maximum number of subscribers in the network - 96;

· Maximum cable length between the subscriber and the concentrator - 45 meters;

· Maximum cable length between hubs - 45 meters;

· The maximum length of the cable connecting all the hubs is 120 meters;

· Data transfer rate - 4 Mbit / s and 16 Mbit / s.

Compared to Ethernet hardware, Token-Ring hardware is noticeably more expensive, since it uses a more complex method of exchange control, so the Token-Ring network is not so widespread.

The Token-Ring network uses the classic token access method, that is, a token constantly circulates around the ring, to which subscribers can attach their data packets (see Fig. 4.15). This implies such an important advantage of this network as the absence of conflicts, but there are also disadvantages, in particular, the need to control the integrity of the token and the dependence of the functioning of the network on each subscriber (in the event of a malfunction, the subscriber must be excluded from the ring).

Arcnet network

Arcnet (or ARCnet from Attached Resource Computer Net) is one of the oldest networks. It was developed by the Datapoint Corporation back in 1977. There are no international standards for this network, although it is she who is considered the ancestor of the token 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 have produced equipment for this type of network. But now the production of Arcnet equipment is practically discontinued.

Thus, the topology of the Arcnet network is as follows (Figure 7.15).

Rice. 7.15. Arcnet network topology of bus type (B - adapters for working in the bus, S - adapters for working in a star).

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

· Transmission medium - coaxial cable, twisted pair.

· The maximum length of the network is 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 hubs is 30 meters.

· The maximum cable length between active hubs is 600 meters.

· The maximum number of subscribers in the network is 255.

· The maximum number of subscribers on the bus segment is 8.

· The minimum distance between subscribers in the bus is 1 meter.

· The maximum length of a bus segment is 300 meters.

· Data transfer rate - 2.5 Mbit / s.

Arcnet uses token access (pass-through), but is slightly different from Token-Ring. This method is closest to the one provided in the IEEE 802.4 standard.

The token is generated by a special subscriber - the network controller. It is the subscriber with the minimum (zero) address.

FDDI network

The FDDI network (from the English Fiber Distributed Data Interface, fiber-optic distributed data interface) is one of the latest developments in local area network standards. The FDDI standard was proposed by the American National Standards Institute ANSI (ANSI X3T9.5 specification). Then the ISO 9314 standard was adopted, corresponding to the ANSI specifications. The level of network standardization is quite high.

Unlike other standard local area networks, the FDDI standard was initially focused on high transmission rates (100 Mbit / s) and on the use of the most promising fiber-optic cable. Therefore, in this case, the developers were not constrained by the framework of the old standards, focused on low speeds and electrical cable.

The choice of fiber as a transmission medium determined such advantages of the new network as high noise immunity, maximum secrecy of information transmission and excellent galvanic isolation of subscribers. The high transmission speed, which is much easier to achieve in the case of fiber-optic cable, allows you to solve many problems that are not available in lower-speed networks, for example, the transmission of images in real time. In addition, fiber optic cable easily solves the problem of transmitting data over a distance of several kilometers without retransmission, which makes it possible to build large networks, covering even entire cities and having all the advantages of local networks (in particular, a low error rate). All this determined the popularity of the FDDI network, although it is not yet as widespread as Ethernet and Token-Ring.

The FDDI standard was based on the token access method provided by the international standard IEEE 802.5 (Token-Ring). Insignificant differences from this standard are determined by the need to provide a high speed of information transmission over long distances. FDDI network topology is a ring, the most suitable topology for fiber optic cable. The network uses two multidirectional fiber-optic cables, one of which is usually in reserve, but this solution also allows the use of full-duplex information transmission (simultaneously in two directions) with twice the effective speed of 200 Mbit / s (with each of the two channels operating at a speed 100 Mbps). A star-ring topology with hubs included in the ring (as in Token-Ring) is also used.

Basic technical characteristics of the FDDI network.

· The maximum number of network subscribers is 1000.

· The maximum length of the network ring is 20 kilometers.

· The maximum distance between network subscribers is 2 kilometers.

· Transmission medium - multimode fiber-optic cable (electrical twisted pair can be used).

· Access method - marker.

· Information transfer rate - 100 Mbit / s (200 Mbit / s for duplex transmission mode).

The FDDI standard has significant advantages over all previously discussed networks. For example, a Fast Ethernet network with the same bandwidth of 100 Mbps cannot match FDDI in terms of network size. In addition, the FDDI token access method provides, unlike CSMA / CD, guaranteed access time and the absence of conflicts at any load level.

The limitation on the total network length of 20 km is associated not with the attenuation of signals in the cable, but with the need to limit the time for the complete passage of the signal along the ring to ensure the maximum allowable access time. But the maximum distance between subscribers (2 km with a multimode cable) is determined precisely by the attenuation of signals in the cable (it should not exceed 11 dB). The possibility of using a single-mode cable is also provided, in which case the distance between subscribers can reach 45 kilometers, and the total length of the ring is 200 kilometers.

There is also an implementation of FDDI on an electrical cable (CDDI - Copper Distributed Data Interface or TPDDI - Twisted Pair Distributed Data Interface). This uses a Category 5 cable with RJ-45 connectors. The maximum distance between subscribers in this case should be no more than 100 meters. The cost of network equipment on an electric cable is several times less. But this version of the network no longer has such obvious advantages over competitors as the original fiber-optic FDDI. The electrical versions of FDDI are much less standardized than fiber optic versions, so the compatibility of equipment from different manufacturers is not guaranteed.

For data transmission in FDDI, a 4V / 5V code is used, specially developed for this standard.

To achieve high network flexibility, the FDDI standard provides for the inclusion of two types of subscribers in the ring:

· Class A subscribers (stations) (subscribers of dual connection, DAS - Dual-Attachment Stations) are connected to both (internal and external) rings of the network. At the same time, the possibility of exchange at a speed of up to 200 Mbit / s or redundancy of the network cable is realized (if the main cable is damaged, the reserve cable is used). Equipment of this class is used in the most critical parts of the network from the point of view of performance.

· Class B subscribers (stations) (subscribers of a single connection, SAS - Single-Attachment Stations) are connected to only one (external) ring of the network. They are simpler and cheaper than class A adapters, but lack their capabilities. They can be connected to the network only through a hub or bypass switch, which turns them off in case of an emergency.

In addition to the actual subscribers (computers, terminals, etc.), the network uses Wiring Concentrators, the inclusion of which allows you to collect all connection points in one place in order to monitor the operation of the network, diagnose faults and simplify reconfiguration. When using different types of cables (for example, fiber-optic cable and twisted pair), the hub also performs the function of converting electrical signals into optical ones and vice versa. Hubs are also available in Dual-Attachment Concentrator (DAC) and Single-Attachment Concentrator (SAC).

An example of an FDDI network configuration is shown in Fig. 8.1. The principle of combining network devices is illustrated in Figure 8.2.

Rice. 8.1. An example of an FDDI network configuration.

Unlike the access method offered by the IEEE 802.5 standard, FDDI uses what is known as multiple token passing. If, in the case of Token-Ring network, a new (free) token is transmitted by the subscriber only after returning his packet to him, then in FDDI a new token is transmitted by the subscriber immediately after the end of the packet transmission to him (similar to how it is done with the ETR method in the Token- Ring).

In conclusion, it should be noted that despite the obvious advantages of FDDI this network did not become widespread, which is mainly due to the high cost of its equipment (on the order of several hundred and even thousands of dollars). The main area of application of FDDI today is backbone networks that connect multiple networks. FDDI is also used to connect powerful workstations or servers that require high-speed communication. It is assumed that the Fast Ethernet network may overtake FDDI, but the advantages of fiber optic cable, token control method and record allowable network size currently put FDDI out of competition. And in cases where hardware cost is critical, twisted pair version of FDDI (TPDDI) can be used in non-critical areas. In addition, the cost of FDDI equipment can greatly decrease with an increase in its production volume.

100VG-AnyLAN network

100VG-AnyLAN is one of the latest high-speed local area networks that has recently entered the market. It complies with the international standard IEEE 802.12, so the level of its standardization is quite high.

Its main advantages are a high exchange rate, a relatively low cost of equipment (about twice as expensive as the equipment of the most popular Ethernet 10BASE-T network), a centralized method of managing exchange without conflicts, as well as compatibility at the level of packet formats with Ethernet and Token-Ring networks.

In the name of the 100VG-AnyLAN network, the number 100 corresponds to a speed of 100 Mbit / s, the letters VG denote a cheap unshielded twisted pair cable of category 3 (Voice Grade), and AnyLAN (any network) denotes that the network is compatible with the two most common networks.

The main technical characteristics of the 100VG-AnyLAN network:

· Transfer rate - 100 Mbps.

· Topology - a star with the possibility of extension (tree). The number of cascading levels of concentrators (hubs) is up to 5.

· Access method - centralized, conflict-free (Demand Priority - with a priority request).

· Transmission media - quad unshielded twisted pair (UTP category 3, 4, or 5 cables), double twisted pair (UTP category 5 cable), double shielded twisted pair (STP), and fiber optic cable. Nowadays, quad twisted pair is mainly widespread.

· The maximum cable length between the hub and the subscriber and between the hubs is 100 meters (for UTP category 3 cable), 200 meters (for UTP category 5 cable and shielded cable), 2 kilometers (for fiber optic cable). The maximum possible network size is 2 kilometers (determined by the allowable delays).

· Maximum number of subscribers - 1024, recommended - up to 250.

Thus, the parameters of the 100VG-AnyLAN network are quite close to those of the Fast Ethernet network. However, the main advantage of Fast Ethernet is full compatibility with the most common Ethernet network (in the case of 100VG-AnyLAN, this requires a bridge). At the same time, the centralized management of 100VG-AnyLAN, which eliminates conflicts and guarantees the maximum amount of access time (which is not provided for in the Ethernet network), also cannot be discounted.

An example of the structure of a 100VG-AnyLAN network is shown in Fig. 8.8.

The 100VG-AnyLAN network consists of a central (main, root) level 1 concentrator, to which both individual subscribers and level 2 concentrators can be connected, to which subscribers and level 3 concentrators, etc., can be connected, etc. Moreover, the network can have no more than five such levels (in the original version there were no more than three). The maximum network size can be 1000 meters for an unshielded twisted pair cable.

Rice. 8.8. Network structure 100VG-AnyLAN.

Unlike non-intelligent hubs on other networks (eg Ethernet, Token-Ring, FDDI), 100VG-AnyLAN hubs are intelligent controllers that control network access. To do this, they continuously monitor requests coming to all ports. Hubs accept incoming packets and send them only to the subscribers to whom they are addressed. However, they do not perform any information processing, that is, in this case, it is still not an active, but not a passive star. Hubs cannot be called full-fledged subscribers.

Each of the hubs can be configured to accept Ethernet or Token-Ring packet formats. In this case, the hubs of the entire network must work with packets of only one format. Bridging is required to communicate with Ethernet and Token-Ring networks, but bridging is fairly simple.

Hubs have one upper-level port (for connecting it to a higher-level hub) and several lower-level ports (for connecting subscribers). A computer (workstation), server, bridge, router, switch can act as a subscriber. Another hub can also be connected to the lower port.

Each port of the hub can be set to one of two possible modes of operation:

· Normal mode assumes forwarding to the subscriber connected to the port only packets addressed to him personally.

· Monitor mode assumes forwarding to the subscriber connected to the port of all packets arriving at the hub. This mode allows one of the subscribers to control the operation of the entire network as a whole (to perform the monitoring function).

The 100VG-AnyLAN access method is typical for star networks.

When using a quad twisted pair, each of the four twisted pairs is transmitted at a speed of 30 Mbps. The total transfer rate is 120 Mbps. However, useful information due to the use of the 5B / 6B code is transmitted only at a speed of 100 Mbps. Thus, the bandwidth of the cable must be at least 15 MHz. Category 3 twisted pair cable (bandwidth - 16 MHz) meets this requirement.

Thus, the 100VG-AnyLAN network is an affordable solution for increasing the transmission speed up to 100 Mbps. However, it does not have full compatibility with any of the standard networks, so its further fate is problematic. In addition, unlike the FDDI network, it does not have any record parameters. Most likely, 100VG-AnyLAN, despite the support of reputable companies and a high level of standardization, will remain just an example of interesting technical solutions.

If we talk about the most common 100-megabit Fast Ethernet network, then 100VG-AnyLAN provides twice the length of UTP category 5 cable (up to 200 meters), as well as a conflict-free method of exchange control.

Fast Ethernet

Fast Ethernet - the IEEE 802.3 u specification, officially adopted on October 26, 1995, defines a data link protocol standard for networks operating using both copper and fiber-optic cables at a speed of 100 Mb / s. The new specification is the successor to the Ethernet IEEE 802.3 standard, using the same frame format, CSMA / CD media access mechanism and star topology. Several physical layer configuration elements have evolved to increase throughput, including cable types, segment lengths, and number of hubs.

Fast Ethernet structure

To better understand the operation and understand the interaction of Fast Ethernet elements, refer to Figure 1.

Figure 1. Fast Ethernet System

Logic Link Control (LLC) Sublayer

The IEEE 802.3 u specification breaks down link layer functions into two sublayers: logical link control (LLC) and medium access layer (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, IP or IPX), providing various communication services:

Service without connection and acknowledgment of receipt. A simple service that does not provide flow control or error control, and does not guarantee correct delivery of data.
Connection-oriented service. An absolutely reliable service that guarantees correct data delivery by establishing a connection to the receiving system before the data transfer begins and using error control and data flow control mechanisms.
Connectionless service with acknowledgments. A moderately complex service that uses acknowledgment messages to ensure delivery, but does not establish connections until data is sent.

On the transmitting system, the downstream data from the Network Layer protocol is first encapsulated by the LLC sublayer. The standard calls them Protocol Data Unit (PDU). When the PDU is handed down to the MAC sublayer, where it is again framed with a header and post information, it can technically be called a frame at this point. For an Ethernet packet, this means that the 802.3 frame contains a three-byte LLC header in addition to the Network Layer data. Thus, the maximum allowable data length in each packet is reduced from 1500 to 1497 bytes.

The LLC header consists of three fields:

In some cases, LLC frames play a minor role in the network communication process. For example, on a network that uses TCP / IP along with other protocols, the only function of LLC might be to allow 802.3 frames to contain a SNAP header, like an Ethertype, indicating the Network Layer protocol to which the frame should be sent. In this case, all LLC PDUs use the unnumbered information format... However, other higher-level protocols require more advanced services from the LLC. For example, NetBIOS sessions and several NetWare protocols use LLC connection-oriented services more broadly.

SNAP header

The receiving system needs to determine which of the Network Layer protocols should receive the incoming data. 802.3 packets within the LLC PDU use another protocol called Sub -NetworkAccessProtocol (SNAP, Subnetting Access Protocol).

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

Organization code. The Organization or Vendor ID is a 3-byte field that takes the same value as the first 3 bytes of the sender's MAC address in the 802.3 header.

Local code. The local code is a 2-byte field that is functionally equivalent to the Ethertype field in the Ethernet II header.

Matching sublevel

As stated earlier, Fast Ethernet is an evolutionary standard. A MAC designed for the AUI interface needs to be mapped for the MII interface used in Fast Ethernet, which is what this sublayer is for.

Media Access Control (MAC)

Each node in a Fast Ethernet network has a media access controller (MediaAccessController- MAC). MAC is key to Fast Ethernet and has three purposes:

The most important of the three MAC assignments is the first. For any network technology that uses a common medium, the medium access rules that determine when a node can transmit are its primary characteristic. Several IEEE committees are involved in the development of rules for access to the environment. The 802.3 committee, often referred to as the Ethernet committee, defines LAN standards that use rules called CSMA /CD(Carrier Sense Multiple Access with Collision Detection).

CSMS / CD are media access rules for both Ethernet and Fast Ethernet. It is in this area that the two technologies completely coincide.

Since all nodes in Fast Ethernet share the same medium, they can only transmit when it is their turn. This queue is defined by CSMA / CD rules.

CSMA / CD

The MAC Fast Ethernet controller listens on the carrier before transmitting. The carrier exists only when another node is transmitting. The PHY layer detects the presence of a carrier and generates a message for the MAC. The presence of a carrier indicates that the environment is busy and the listening node (or nodes) must yield to the transmitting one.

A MAC that has a frame to transmit must wait a minimum amount of time after the end of the previous frame before transmitting it. This time is called interpacket gap(IPG, interpacket gap) and lasts 0.96 microseconds, that is, a tenth of the transmission time of a normal Ethernet packet at 10 Mbps (IPG is the only time interval, always specified in microseconds, not bit time) Figure 2.

Figure 2. Interpacket gap

After the end of packet 1, all LAN nodes must wait for the IPG time before being able to transmit. The time interval between packets 1 and 2, 2 and 3 in Fig. 2 is IPG time. After the transmission of packet 3 was complete, no nodes had material to process, so the time interval between packets 3 and 4 is longer than the IPG.

All nodes on the network must comply with these rules. Even if a node has many frames to transmit and this node is the only transmitting one, then after sending each packet it must wait for at least IPG time.

This is part of the CSMA Fast Ethernet Media Access Rules. In short, many nodes have access to the medium and use the carrier to keep track of whether it is busy.

The early experimental networks applied exactly these rules, and such networks worked very well. However, the use of CSMA alone led to a problem. Often, two nodes, having a packet to transmit and waiting for IPG time, would start transmitting at the same time, resulting in data corruption on both sides. This situation is called collision(collision) or conflict.

To overcome this obstacle, early protocols used a fairly simple mechanism. Packages were divided into two categories: commands and reactions. Each command sent by the node demanded a reaction. If no response was received for some time (called a timeout period) after the command was sent, the original command was re-issued. This could happen several times ( limit amount timeouts) before the sending node has recorded the error.

This scheme could work fine, but only up to a certain point. Conflicts caused dramatic performance degradation (usually measured in bytes per second) because nodes often stood idle, waiting for commands to never reach their destination. Network congestion, an increase in the number of nodes are directly related to an increase in the number of conflicts and, consequently, to a decrease in network performance.

Early network designers quickly found a solution to this problem: each node must detect the loss of a transmitted packet by detecting a conflict (and not wait for a reaction that will never follow). This means that packets lost due to the conflict must be retransmitted immediately before the timeout expires. If the host transmitted the last bit of the packet without a conflict, then the packet was transmitted successfully.

Carrier sense can be combined well with collision detection. Collisions still continue to occur, but this does not affect the performance of the network, as the nodes quickly get rid of them. The DIX group, having developed the rules for accessing the CSMA / CD environment for Ethernet, formalized them in the form of a simple algorithm - Figure 3.

Figure 3. Algorithm of CSMA / CD operation

Physical layer device (PHY)

Since Fast Ethernet can use different type cable, a unique signal preconversion is required for each medium. Conversion is also required for efficient data transmission: to make the transmitted code resistant to interference, possible loss, or distortion of its individual elements (baud), to ensure effective synchronization of clocks on the transmitting or receiving side.

Coding Sub-Layer (PCS)

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

Physical interconnection and physical media dependency sublayers (PMA and PMD)

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

Auto-negotiation sublevel (AUTONEG)

The auto-negotiation sublayer allows two communicating ports to automatically select the most efficient mode of operation: full-duplex or half-duplex 10 or 100 Mb / s. Physical layer

The Fast Ethernet standard defines three types of 100 Mbps Ethernet signaling media.

100Base-TX - two twisted pairs of wires. Transmission is carried out in accordance with the standard for data transmission in a twisted physical medium, developed by ANSI (American National Standards Institute - American National Standards Institute). Coiled data cable can be shielded or unshielded. Uses 4B / 5B data coding algorithm and MLT-3 physical coding method.
100Base-FX is a two-core fiber optic cable. Transmission is also carried out in accordance with the ANSI standard for data transmission in fiber optic media. Uses 4B / 5B data coding algorithm and 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 over four twisted pairs of telephone cable, which is called UTP Category 3 cable. It uses 8B / 6T data coding algorithm and NRZI physical coding method.

Additionally, the Fast Ethernet standard includes guidelines for Category 1 shielded twisted pair cable, which is the standard cable traditionally used in Token Ring networks. The support organization and guidelines for using STP cable on Fast Ethernet provide a path to Fast Ethernet for customers with STP cabling.

The Fast Ethernet specification also includes an auto-negotiation mechanism that allows a host port to automatically adjust to a data transfer rate of 10 Mbps or 100 Mbps. This mechanism is based on the exchange of a number of packets with a port of a hub or switch.

100Base-TX environment

Two twisted pairs are used as the transmission medium for 100Base-TX, with one pair being used to transmit data and the other to receive them. Since the ANSI TP-PMD specification contains descriptions of both shielded and unshielded twisted pairs, the 100Base-TX specification includes support for both unshielded and shielded type 1 and 7 twisted pairs.

MDI (Medium Dependent Interface) connector

The media-dependent 100Base-TX link interface can be one of two types. For unshielded twisted-pair cable, use an 8-pin RJ 45 Category 5 connector as the MDI connector. The same connector is used on a 10Base-T network to provide backward compatibility with existing Category 5 cabling. use IBM STP type 1 connector, which is a shielded DB9 connector. This connector is commonly used in Token Ring networks.

Category 5 (e) UTP cable

The UTP 100Base-TX media interface uses two pairs of wires. To minimize crosstalk and possible signal distortion, the remaining four wires should not be used to carry any signals. The transmit and receive signals for each pair are polarized, with one wire carrying the positive (+) and the other negative (-) signal. The color coding of the cable wires and the pin numbers of the connector for the 100Base-TX network are shown in table. 1. Although the 100Base-TX PHY layer was developed after the ANSI TP-PMD standard, the RJ 45 connector pin numbers have been changed to align with the 10Base-T pinouts already used. The ANSI TP-PMD standard uses pins 7 and 9 to receive data, while the 100Base-TX and 10Base-T standards use pins 3 and 6 for this. This wiring allows you to use 100Base-TX adapters instead of 10 Base adapters - T and connect them to the same Category 5 cables without changing the wiring. In the RJ 45 connector, the pairs of wires used are connected to pins 1, 2 and 3, 6. For the correct connection of the wires, follow their color coded.

Table 1. Purpose of connector contactsMDIcableUTP100Base-TX

Nodes interact with each other by exchanging frames (frames). In Fast Ethernet, a frame is the basic unit of exchange over a network - any information transmitted between nodes is placed in the data field of one or more frames. Forwarding frames from one node to another is possible only if there is a way to uniquely identify all network nodes. Therefore, every node on a LAN has an address called its MAC address. This address is unique: no two nodes on the local network can have the same MAC address. Moreover, in no LAN technology (with the exception of ARCNet) no two nodes in the world can have the same MAC address. Any frame contains at least three main pieces of information: recipient address, sender address, and data. Some frames have other fields, but only the three listed are required. Figure 4 shows the Fast Ethernet frame structure.

Figure 4. Frame structureFastEthernet

address of the recipient- the address of the node receiving the data is indicated;
sender's address- the address of the node that sent the data is indicated;
length / type(L / T - Length / Type) - contains information about the type of transmitted data;
frame checksum(PCS - Frame Check Sequence) - designed to check the correctness of the frame received by the receiving node.

The minimum frame size is 64 octets, or 512 bits (terms octet and byte - synonyms). The maximum frame size is 1518 octets, or 12144 bits.

Frame addressing

Each node on a Fast Ethernet network has unique number, which is called the MAC address or host address. This number consists of 48 bits (6 bytes), assigned to the network interface during device manufacture and programmed during initialization. Therefore, the 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 that differs from all other MAC addresses on Earth and is assigned by the manufacturer in agreement with the IEEE.

To facilitate the management of network interfaces, it has been proposed by the IEEE to divide the 48-bit address field into four parts, as shown in Figure 5. The first two bits of the address (bits 0 and 1) are address type flags. The meaning of the flags determines how the address part is interpreted (bits 2 - 47).

Figure 5. Format of the MAC address

The I / G bit is called individual / group address flag and shows what (individual or group) the address is. An individual address is assigned to only one interface (or node) on the network. Addresses with the I / G bit set to 0 are MAC addresses or node addresses. If the I / O bit is set to 1, then the address belongs to the group and is usually called multipoint address(multicast address) or functional address(functional address). A multicast address can be assigned to one or more LAN network interfaces. Frames sent to a multicast address receive or copy all LAN network interfaces that have it. Multicast addresses allow a frame to be sent to a subset of hosts on a local network. If the I / O bit is set to 1, then bits 46 to 0 are treated as a multicast address and not as the U / L, OUI, and OUA fields of the normal address. The U / L bit is called universal / local control flag and determines how the address was assigned to the network interface. If both bits, I / O and U / L, are set to 0, then the address is the unique 48-bit identifier described earlier.

OUI (organizationally unique identifier - organizationally unique identifier). The IEEE assigns one or more OUIs to each manufacturer of network adapters and interfaces. Each manufacturer is responsible for the correct assignment of the OUA (organizationally unique address - organizationally unique address), which should have any device it creates.

When the U / L bit is set, the address is locally managed. This means that it is not specified by the manufacturer of the network interface. Any organization can create its own MAC address for a network interface by setting the U / L bit to 1, and bits 2 through 47 to some chosen value. The network interface, having received the frame, first of all decodes the destination address. When the I / O bit is set in the address, the MAC layer will receive this frame only if the destination address is in the list that is stored on the node. This technique allows one node to send a frame to many nodes.

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

Field Length / Type

The L / T (Length / Type) field serves two different purposes:

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

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

In general, the L / T field is a historical residue of the Ethernet standardization in the IEEE, which gave rise to a number of compatibility problems for equipment released before 1983. Nowadays Ethernet and Fast Ethernet never use L / T fields. The specified field serves only for coordination with the software that processes frames (that is, with protocols). But the only truly standard purpose of the L / T field is to use it as a length field - the 802.3 specification does not even mention its possible use as a data type field. The standard states: "Frames with a length field value greater than that specified in clause 4.4.2 may be ignored, discarded, or privately used. The use of these frames is outside the scope of this standard."

Summarizing what has been said, we note that the L / T field is the primary mechanism by which frame type. Fast Ethernet and Ethernet frames in which the L / T field value specifies the length (L / T 802.3 value, frames in which the data type is set by the value of the same field (L / T value> 1500) are called frames Ethernet- II or DIX.

Data field

In the data field contains information that one node sends to another. Unlike other fields that store very specific information, a data field can contain almost any information, as long as its size is at least 46 and no more than 1500 bytes. How the content of a data field is formatted and interpreted is determined by the protocols.

If you need to send data less than 46 bytes in length, the LLC layer adds bytes with an unknown value to the end of the data, called insignificant data(pad data). As a result, the field length becomes 46 bytes.

If the frame is of type 802.3, then the L / T field indicates the amount of valid data. For example, if a 12-byte message is being sent, then the L / T field contains the value 12, and the data field contains 34 additional insignificant bytes. Adding insignificant bytes initiates the Fast Ethernet LLC layer, and is usually implemented in hardware.

The MAC layer facility does not specify the contents of the L / T field — the software does. Setting the value of this field is almost always done by the network interface driver.

Frame checksum

The Frame Check Sequence (PCS) ensures that the received frames are not corrupted. When forming the transmitted frame at the MAC level, a special mathematical formula is used CRC(Cyclic Redundancy Check), designed to calculate a 32-bit value. The resulting value is placed in the FCS field of the frame. The values of all bytes of the frame are supplied to the input of the MAC layer element that calculates the CRC. The FCS field is the primary and most important Fast Ethernet error detection and correction mechanism. Starting from the first byte of the destination address and ending with the last byte of the data field.

DSAP and SSAP Field Values

DSAP / SSAP Values

Description

Indiv LLC Sublayer Mgt

Group LLC Sublayer Mgt

SNA Path Control

Reserved (DOD IP)

ISO CLNS IS 8473

The 8B6T coding algorithm converts an eight-bit data octet (8B) to a six-bit ternary symbol (6T). Code groups 6T are designed to be transmitted in parallel over three twisted cable pairs, so the effective data rate for each twisted pair is one third of 100 Mbit / s, that is, 33.33 Mbit / s. The ternary symbol rate for each twisted pair is 6/8 of 33.3 Mbps, which corresponds to a clock rate of 25 MHz. It is with this frequency that the timer of the MP interface works. Unlike binary signals, which have two levels, ternary signals transmitted on each pair can have three levels.

Character encoding table

Linear code	Symbol















	MLT-3 Multi Level Transmission - 3 (multilevel transmission) - a bit similar to the NRZ code, but unlike the latter, it has three signal levels. The unit corresponds to the transition from one signal level to another, and the change in the signal level occurs sequentially taking into account the previous transition. When transmitting "zero", the signal does not change. This code, like NRZ, needs to be pre-encoded. Compiled on the basis of materials: Laem Queen, Richard Russell "Fast Ethernet"; K. Zakler "Computer Networks"; V.G. and N.A. Olifer "Computer Networks";

Model	Chipset	Baud rate	Tire	Network interface	Connectors	Jumbo Frame Support	Wake-on-LAN support	Duplex
Acorp L-100S	Realtek RTL8139D	10/100 Mbps	PCI 2.3 (32 bit)	10Base-T, 100Base-TX	RJ45 (8P8C)	There is	There is	Full
Acorp L-1000S	Realtek RTL8169SC	10/100/1000 Mbps	PCI 2.3 (32 bit)		RJ45 (8P8C)	There is	There is	Full
	Realtek RTL8168B	10/100/1000 Mbps	PCI Express 1.0a	10Base-T, 100Base-TX, 1000Base-T	RJ45 (8P8C)	There is	There is	Full