Fast Ethernet technology. Description of Fast Ethernet technology Data transfer speed fast ethernet

Ethernet, but also to equipment of other, less popular networks.

Ethernet and Fast Ethernet Adapters

Adapter Specifications

Network adapters (NIC, Network Interface Card) Ethernet and Fast Ethernet can be interfaced with a computer through one of standard interfaces:

ISA (Industry Standard Architecture) bus;
PCI bus (Peripheral Component Interconnect);
PC Card bus (aka PCMCIA);

Adapters designed for the ISA system bus (backbone) were not so long ago the main type of adapters. The number of companies producing such adapters was large, which is why the devices of this type were the cheapest. Adapters for ISA are available in 8- and 16-bit. 8-bit adapters are cheaper, while 16-bit adapters are faster. True, information exchange on the ISA bus cannot be too fast (in the limit - 16 MB/s, in reality - no more than 8 MB/s, and for 8-bit adapters - up to 2 MB/s). Therefore, Fast Ethernet adapters, which require high transfer rates for effective operation, are practically not produced for this system bus. The ISA bus is becoming a thing of the past.

The PCI bus has now practically replaced the ISA bus and is becoming the main expansion bus for computers. It provides 32- and 64-bit data exchange and is highly throughput(theoretically up to 264 MB/s), which fully satisfies the requirements of not only Fast Ethernet, but also faster Gigabit Ethernet. It is also important that the PCI bus is used not only in IBM PC computers, but also in PowerMac computers. In addition, it supports Plug-and-Play automatic hardware configuration. Apparently, in the near future, the majority of computers will be oriented towards the PCI bus. network adapters. The disadvantage of PCI compared to the ISA bus is that the number of expansion slots in a computer is usually small (usually 3 slots). But exactly network adapters connect to PCI first.

The PC Card bus (old name PCMCIA) is currently used only in Notebook class portable computers. In these computers, the internal PCI bus is usually not routed to the outside. PC Card interface allows for easy connection to a computer miniature circuit boards expansion, and the exchange speed with these boards is quite high. However, more and more laptop computers equipped with built-in network adapters, as network connectivity becomes an integral part of the standard feature set. These onboard adapters are again connected to the computer's internal PCI bus.

When choosing network adapter oriented to a particular bus, you must first of all make sure that there are free expansion slots for this bus in the computer connected to the network. You should also evaluate the complexity of installing the purchased adapter and the prospects for producing boards of this type. The latter may be needed if the adapter fails.

Finally, they meet again network adapters, connecting to a computer via a parallel (printer) LPT port. The main advantage of this approach is that you do not need to open the computer case to connect adapters. In addition, in this case, adapters do not occupy computer system resources, such as interrupt channels and DMAs, as well as memory addresses and I/O devices. However, the speed of information exchange between them and the computer in this case is much lower than when using the system bus. In addition, they require more processor time to communicate with the network, thereby slowing down the computer.

Recently, there are more and more computers in which network adapters built into system board. The advantages of this approach are obvious: the user does not have to buy a network adapter and install it in the computer. You just need to connect the network cable to the external connector of your computer. However, the disadvantage is that the user cannot select the adapter with the best characteristics.

To others the most important characteristics network adapters can be attributed:

adapter configuration method;
the size of the buffer memory installed on the board and the exchange modes with it;
the ability to install a permanent memory chip for remote booting (BootROM) on the board.
the ability to connect the adapter to different types of transmission media (twisted pair, thin and thick coaxial cable, fiber optic cable);
the network transmission speed used by the adapter and the availability of its switching function;
the adapter can use full-duplex exchange mode;
compatibility of the adapter (more precisely, the adapter driver) with the network software used.

User configuration of the adapter was used primarily for adapters designed for the ISA bus. Configuration involves setting up the use of computer system resources (input/output addresses, interrupt channels and direct memory access, buffer memory addresses and remote boot memory). Configuration can be carried out by setting switches (jumpers) to the desired position or using the DOS configuration program supplied with the adapter (Jumperless, Software configuration). When starting such a program, the user is prompted to set the hardware configuration using a simple menu: select adapter parameters. The same program allows you to make self-test adapter The selected parameters are stored in the adapter's non-volatile memory. In any case, when choosing parameters, you must avoid conflicts with system devices computer and with other expansion cards.

The adapter can also be configured automatically in Plug-and-Play mode when the computer is turned on. Modern adapters usually support this particular mode, so they can be easily installed by the user.

In the simplest adapters, exchange with the internal buffer memory of the adapter (Adapter RAM) is carried out through the address space of input/output devices. In this case, no additional configuration of memory addresses is required. The base address of buffer memory operating in shared memory mode must be specified. It is assigned to the computer's upper memory area (

The most widespread among standard networks is the Ethernet network. It first appeared in 1972 (developed by the famous company Xerox). The network turned out to be quite successful, and as a result, in 1980 it was supported by such major companies as DEC and Intel (the association of these companies was called 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: IEEE Committee 802 (Institute of Electrical and Electronic Engineers) and ECMA (European Computer Manufacturers Association).

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

Main characteristics of the original IEEE 802.3 standard:

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

Strictly speaking, there are minor differences between the IEEE 802.3 and Ethernet standards, but they are usually ignored.

The Ethernet network is now the most popular in the world (more than 90% of the market), and presumably it will remain so in the coming years. This was greatly facilitated by the fact that from the very beginning the characteristics, parameters, and 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.

The classic Ethernet network used 50-ohm coaxial cable of two types (thick and thin). However, recently (since the early 90s), the most widely used version of Ethernet is that using twisted pairs as a transmission medium. A standard has also been defined for use in fiber optic cable networks. 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 (Gigabit Ethernet, IEEE 802.3z standard) also appeared.

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

Rice. 7.1. Classic Ethernet network topology

The segment (part of the network) can be a classic bus or a single subscriber. Coaxial cable is used for bus segments, and twisted pair and fiber optic cable is used for passive star spokes (for connecting single computers to a hub). The main requirement for the resulting topology is that it should not contain closed paths (loops). 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.

A Fast Ethernet network 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. After all, with a 10-fold increase in transmission speed and preservation of the packet format, its minimum length becomes ten times shorter. Thus, the permissible value of double signal transmission 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 the random CSMA/CD method, ensuring equality of subscribers. The network uses packets of variable length with the structure shown in Fig. 7.2. (numbers show number of bytes)

Rice. 7.2. Ethernet Packet Structure

Length Ethernet frame(that is, a packet without a preamble) must be at least 512 bit intervals or 51.2 μs (this is exactly the limit of double travel time in the network). Individual, group and broadcast addressing is provided.

The Ethernet packet contains the following fields:

The preamble consists of 8 bytes, the first seven are code 10101010, and the last byte is 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 each contain 6 bytes and are built according to the standard described in the Addressing Packets section of Lecture 4. These address fields are processed by subscriber equipment.
The control field (L/T – Length/Type) contains information about the length of the data field. It may 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 greater than 1500, then it determines the frame type. The control field is processed by software.
The data field must contain from 46 to 1500 bytes of data. If the packet must 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 zero length when there is enough data (more than 46 bytes).
The Frame Check Sequence (FCS) field contains the packet's 32-bit cyclic checksum (CRC) and is used to verify that the packet was transmitted correctly.

Thus, the minimum frame length (packet without preamble) is 64 bytes (512 bits). It is this value that determines the maximum permissible 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 may decrease as the packet passes through various network devices, so it is not taken into account. 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 network equipment and to estimate the overall network load.

The choice of preamble format is not accidental. The fact 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 bit intervals (see section 2.6.3), that is, only information transitions. Of course, it is easy for the receiver to tune in (synchronize) with such a sequence, even if for some reason it is shortened by several bits. The last two single bits of the preamble (11) differ significantly from the sequence 101010...10 (transitions also appear at the boundary of bit intervals). Therefore, an 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 information transmission media:

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

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

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

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

Here, the number 100 means a transmission speed of 100 Mbit/s, the letter T means twisted pair, and the letter F means fiber optic cable. The types 100BASE-TX and 100BASE-FX are sometimes combined under the name 100BASE-X, and 100BASE-T4 and 100BASE-TX are called 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. Here it should only be noted that the Ethernet network is not distinguished by either record-breaking characteristics or optimal algorithms; it is inferior in a number of parameters to other standard networks. But thanks to powerful support, the highest level of standardization, and huge volumes of technical equipment, Ethernet stands out among other standard networks, and therefore any other network technology is usually compared with Ethernet.

The development of Ethernet technology is moving further and further away from the original standard. The use of new transmission media and switches makes it possible to significantly increase the size of the network. Elimination of the Manchester code (in Fast Ethernet and Gigabit Ethernet networks) provides increased data transfer speeds and reduced cable requirements. Refusal of the CSMA/CD control method (with full-duplex exchange mode) makes it possible to dramatically increase operating efficiency and remove restrictions on network length. However, all new varieties of network are also called Ethernet network.

Token-Ring Network

The Token-Ring network was proposed by IBM in 1985 (the first version appeared in 1980). It was intended to network all types of computers produced by IBM. The mere fact that it is supported by IBM, the largest manufacturer computer equipment, indicates that she needs to give Special attention. But equally important is 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 of status as Ethernet.

Token-Ring was developed as a reliable alternative to Ethernet. And although Ethernet is now replacing all other networks, Token-Ring cannot be considered hopelessly outdated. More than 10 million computers around the world are connected by this network.

IBM has done everything to ensure the widest possible distribution of its network: detailed documentation has been released up to circuit diagrams adapters. As a result, many companies, for example, 3COM, Novell, Western Digital, Proteon and others have begun producing adapters. By the way, the NetBIOS concept was developed specifically for this network, as well as for another network, the IBM PC Network. If in the previously created PC Network NetBIOS programs were stored in the built-in read-only memory of the adapter, then in the Token-Ring network a program emulating NetBIOS was already used. This made it possible to respond more flexibly to hardware features and maintain compatibility with higher-level programs.

The Token-Ring network has a ring topology, although outwardly it looks more like a star. This is due to the fact that individual subscribers (computers) connect to the network not directly, but through special hubs or multiple access devices (MSAU or MAU - Multistation Access Unit). Physically, the network forms a star-ring topology (Fig. 7.3). In reality, the subscribers are still 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 topology of the Token-Ring network

The hub (MAU) allows you to centralize configuration settings, disconnecting faulty subscribers, monitoring network operation, etc. (Fig. 7.4). It does not perform any information processing.

Rice. 7.4. Connecting Token-Ring network subscribers into a ring using a hub (MAU)

For each subscriber as part of the concentrator, special block connection to the trunk (TCU - Trunk Coupling Unit), which ensures automatic inclusion of the subscriber in the ring if it is connected to the hub and is working properly. If a subscriber disconnects from the concentrator or it is faulty, the TCU automatically restores the integrity of the ring without intervention. of this subscriber. The TCU is triggered by a signal direct current(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 perform 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 designed for connecting subscribers (computers) using adapter cables or radial cables. The two outermost connectors: input RI (Ring In) and output RO (Ring Out) are used for connection to other hubs using special trunk cables (Path cable). The concentrator is available in wall-mounted and desktop versions.

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

The hub in the network can be the only one (as in Fig. 7.4), in this case only the subscribers connected to it are closed in the ring. Externally, this topology looks like a star. If you need to connect more than eight subscribers 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 mode of so-called ring folding, 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. Merging MAU hubs in normal mode

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

In case of multiple cable damages, the network breaks up into several parts (segments) that are not interconnected, but remain fully operational (Fig. 7.8). Maximum part The network remains connected as before. Of course, this no longer saves the network as a whole, but it allows, with the correct distribution of subscribers among hubs, 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. Rolling up the ring if the cable is damaged

Rice. 7.8. Ring disintegration due to multiple cable damages

The transmission medium in the IBM Token-Ring network was initially twisted pair, both unshielded (UTP) and shielded (STP), but then equipment options appeared for coaxial cable, as well as for fiber optic cable in the FDDI standard.

Basic specifications classic version of the Token-Ring network:

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

All characteristics given refer to the case of using unshielded twisted pair cable. If a different transmission medium is used, network performance may vary. For example, when using shielded twisted pair (STP), the number of subscribers can be increased to 260 (instead of 96), the cable length can be increased to 100 meters (instead of 45), the number of hubs can be increased to 33, and the total length of the ring connecting the hubs can be up to 200 meters . Fiber optic cable allows you to increase the cable length up to two kilometers.

To transfer information to 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 measures are required. Negotiation is performed by the hardware of network adapters and hubs.

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

The Token-Ring network in its classic version is inferior to the Ethernet network both in terms of permissible size and the maximum number of subscribers. In terms of transfer speed, Token-Ring is currently available in 100 Mbps (High Speed Token-Ring, HSTR) and 1000 Mbps (Gigabit Token-Ring) versions. Companies supporting Token-Ring (including IBM, Olicom, Madge) do not intend to abandon their network, considering it as a worthy competitor to Ethernet.

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

However, unlike Ethernet, the Token-Ring network holds up much better high level load (more than 30-40%) and provides guaranteed access time. This is necessary, for example, in industrial networks, where a delay in the response 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 time for transmitting a packet to Token-Ring is 10 ms. With a maximum number of subscribers of 260, the full ring cycle will be 260 x 10 ms = 2.6 s. During this time, all 260 subscribers will be able to transmit their packets (if, of course, they have something to transmit). During this same time, the free token will definitely reach each subscriber. This same interval is the upper limit of the Token-Ring access time.

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

identification of transmission errors;
network configuration control (network restoration if the subscriber who precedes him in the ring fails);
control of numerous time relationships adopted in the network.

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

To monitor the integrity of the token in the network, one of the subscribers (the so-called active monitor) is used. At the same time, his equipment is no different from the others, but his software monitor time relationships in the network and create a new marker if necessary.

The active monitor performs the following functions:

launches a marker into the ring at the beginning of work and when it disappears;
regularly (once every 7 s) reports its presence with a special control package (AMP - Active Monitor Present);
removes a packet from the ring that was not removed by the subscriber who sent it;
monitors the permissible 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 his own buffer (shift register) in the network, which ensures that the token will fit in the ring even with a minimum ring length. The size of this buffer is 24 bits for a speed of 4 Mbit/s and 32 bits for a speed of 16 Mbit/s.

Each subscriber constantly monitors how the active monitor performs its duties. If the active monitor for some reason fails, then a special mechanism is activated, through which all other subscribers (spare, backup monitors) decide to assign a new active monitor. To do this, the subscriber that detects a failure of the active monitor transmits a control packet (token request packet) with its MAC address along the ring. Each subsequent subscriber compares the MAC address from the packet with its own. If its own address is smaller, it forwards the packet unchanged. If it is more, then it sets its MAC address in the packet. The active monitor will be the subscriber whose MAC address is greater than 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.

A Token-Ring network token is a control packet containing only three bytes (Fig. 7.9): a start delimiter byte (SD - Start Delimiter), an access control byte (AC - Access Control) and an end delimiter byte (ED - End Delimiter). All these three bytes are also part of the information package, although their functions in the marker and in the package are somewhat different.

The start and end separators are not just a sequence of zeros and ones, but contain special types of signals. This was done to ensure that the delimiters could not be confused with any other bytes in the packets.

Rice. 7.9. Token-Ring Network Token Format

The initial SD delimiter contains four non-standard bit intervals (Figure 7.10). Two of them, designated J, represent a low signal level throughout the entire bit interval. The other two bits, designated K, represent the high signal level for the entire bit interval. It is clear that such synchronization failures are easily detected by the receiver. The J and K bits can never appear among the payload bits.

Rice. 7.10. Leading (SD) and trailing (ED) delimiters 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 that make sense only as part of the information package:

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

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

Rice. 7.11. Access control byte format

The priority bits (field) allow a subscriber to assign priority to its packets or token (priority can be from 0 to 7, with 7 being the highest priority and 0 being the lowest). A subscriber can attach its packet to a token only if its own priority (the priority of its packets) is the same or higher than the priority of the token.

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

Reservation bits (field) allow the subscriber to reserve their right to further take over the network, that is, take a turn 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 going around the ring, the highest priority of all subscribers will be recorded in the reservation field. The contents of the reservation field are similar to the contents of the priority field, but indicate future priority.

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

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

Rice. 7.12. Packet (frame) format of the Token-Ring network (field lengths are given in bytes)

Purpose of packet (frame) fields.

The leading delimiter (SD) is an indication of the beginning of the packet, the format is the same as in the token.
The access control (AC) byte has the same format as in the token.
The packet control byte (FC – Frame Control) determines the type of packet (frame).
The six-byte MAC addresses of the sender and recipient of the packet have standard format, described in lecture 4.
The Data field includes the data to be transmitted (in the information packet) or information to control the exchange (in the control packet).
The Frame Check Sequence (FCS) field is a 32-bit packet cyclic checksum (CRC).
The terminating delimiter (ED), as in a token, indicates the end of the packet. In addition, it determines whether a given packet is intermediate or final in the sequence of transmitted packets, and also contains an indication that the packet was erroneous (see Figure 7.10).
The packet status byte (FS - Frame Status) tells what happened with this packet: whether it was seen by the receiver (that is, whether there is a receiver with the given address) and copied into the receiver's memory. Using it, the sender of the packet finds out whether the packet reached its destination and without errors or whether it needs to be transmitted again.

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

A subscriber who wants to transmit a packet waits for the arrival of a free token and captures it. The captured marker turns into a frame for the information package. The subscriber then transmits the information packet to the ring and waits for its return. After that, it releases the token and sends it back to the network.

In addition to the token and the regular packet, a special control packet can be transmitted in the Token-Ring network, which serves to interrupt the transmission (Abort). It can be sent at any time and anywhere in the data stream. This packet 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 Mbit/s and higher) uses the so-called Early Token Release (ETR) method. It avoids wasted network usage while the data packet is looping 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 begin transmitting their packets immediately after the previous subscriber's packet ends, without waiting for him to complete traversing the entire network ring. As a result, several packets can be on the network at the same time, but there will always be at most one free token. This pipeline is especially effective in long-distance networks that have significant propagation delay.

When a subscriber connects to the hub, he performs an autonomous self-test and cable testing procedure (it is not yet included in the ring, since there is no phantom current signal). The subscriber sends himself a series of packets and checks the correctness of their passage (his input is directly connected to his output by the TCU block, as shown in Fig. 7.4). After this, the subscriber includes himself in the ring, sending a phantom current. At the moment of switching on, the packet transmitted along the ring may be damaged. Next, the subscriber configures synchronization and checks for the presence of an active monitor on the network. If there is no active monitor, the subscriber begins a 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 network exchange.

During the exchange process, each subscriber monitors the health of the previous subscriber (on the ring). If it suspects a failure of the previous subscriber, it initiates an automatic ring recovery procedure. A special control packet (buoy) tells the previous subscriber to perform 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 share of time provided to each subscriber.

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

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

Rice. 7.14. Consolidation of segments by a central switch

Arcnet network (or ARCnet from English Attached Resource Computer Net, computer network connected resources) is one of the oldest networks. It was developed by Datapoint Corporation back in 1977. There are no international standards for this network, although it is 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.) produced equipment for this type of network. But now production of Arcnet equipment has practically ceased.

Among the main advantages of the Arcnet network compared to Ethernet are the limited access time, high reliability of communication, ease of diagnosis, and the relatively low cost of adapters. The most significant disadvantages of the network include low information transmission speed (2.5 Mbit/s), addressing system and packet format.

To transmit information on 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 a self-timed code that requires even more cable bandwidth than even Manchester.

The transmission medium in the network is a coaxial cable with a characteristic impedance of 93 Ohms, for example, brand RG-62A/U. Options with twisted pair (shielded and unshielded) are not widely used. Fiber optic cable options were also proposed, but they also did not save Arcnet.

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

There are two types of concentrators:

Active concentrators (restore the shape of incoming signals and amplify them). The number of ports is from 4 to 64. Active hubs can be connected to each other (cascaded).
Passive hubs (simply mix incoming signals without amplification). Number of ports – 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.

Network adapters also come in two types:

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

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

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

Rice. 7.15. Arcnet network topology is bus type (B – adapters for working in a 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 network length is 6 kilometers.
The maximum cable length from the subscriber to the passive hub is 30 meters.
The maximum cable length from the subscriber to the 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.
Maximum amount subscribers in the network – 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 speed – 2.5 Mbit/s.

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

The Arcnet network uses a token access method (transfer of rights method), but it is somewhat different from that of the Token-Ring network. This method is closest to the one provided in the IEEE 802.4 standard. The sequence of actions of subscribers when this method:

1. The subscriber who wants to transmit waits for the token to arrive.

2. Having received the token, it sends a request to transfer information to the receiving subscriber (asks if the receiver is ready to accept its packet).

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

4. Having received confirmation of readiness, the transmitting subscriber sends its packet.

5. Having received the packet, the receiver sends an acknowledgment of the packet.

6. The transmitter, having received confirmation of receipt of the packet, ends its communication session. After this, the token is transferred to the next subscriber in descending order of network addresses.

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

Just like with Token-Ring, conflicts are completely eliminated in Arcnet. Like any token network, Arcnet carries the load well and guarantees long access times to the network (unlike Ethernet). The total time for the marker to bypass all subscribers is 840 ms. Accordingly, the same interval determines the upper limit of network access time.

The marker is formed special subscriber– network controller. This is the subscriber with the minimum (zero) address.

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

The Arcnet network packet 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 when the intensity of network exchange is high.

Arcnet network adapters differ from other network adapters in that they require switches or jumpers to set their own network address(there can be 255 of them in total, since the last, 256th address is used in the network for broadcast mode). Control of the uniqueness of each network address rests entirely with network users. Connecting new subscribers becomes quite complicated, since it is necessary to set an address that has not yet been used. Choosing an 8-bit address format limits the allowed number of subscribers on the network 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 variants of the Arcnet network designed for transmission speeds of 20 Mbit/s, but they were not widely used.

Articles to read:

Lecture 6: Standard Ethernet/Fast Ethernet Network Segments

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

The standard is called IEEE 802.3 (read in English as “eight oh two dot three”). It defines multiple access to a mono bus type channel with collision detection and transmission control, that is, with the already mentioned CSMA/CD access method.

Main characteristics of the original IEEE 802.3 standard:

· topology – bus;

· transmission medium – coaxial cable;

· transmission speed – 10 Mbit/s;

· maximum network length – 5 km;

· maximum number of subscribers – up to 1024;

· network segment length – up to 500 m;

· number of subscribers on one segment – up to 100;

· access method – CSMA/CD;

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

Strictly speaking, there are minor differences between the IEEE 802.3 and Ethernet standards, but they are usually ignored.

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

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

Access to the Ethernet network is carried out using the random CSMA/CD method, ensuring 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 information transmission media:

· 10BASE5 (thick coaxial cable);

· 10BASE2 (thin coaxial cable);

· 10BASE-T (twisted pair);

· 10BASE-FL (fiber optic cable).

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

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

· 100BASE-T4 (quad twisted pair);

· 100BASE-TX (dual twisted pair);

· 100BASE-FX (fiber optic cable).

Here, the number “100” means a transmission speed of 100 Mbit/s, the letter “T” means twisted pair, and the letter “F” means fiber optic cable. The types 100BASE-TX and 100BASE-FX are sometimes combined under the name 100BASE-X, and 100BASE-T4 and 100BASE-TX are called 100BASE-T.

Token-Ring Network

The Token-Ring network was proposed by IBM in 1985 (the first version appeared in 1980). It was intended to network all types of computers produced by IBM. The very fact that it is supported by IBM, the largest manufacturer of computer equipment, suggests that it needs to be given special attention. But equally important is 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 of status as Ethernet.

Rice. 7.3. Star-ring topology of the Token-Ring network.

Main technical characteristics of the classic version of the Token-Ring network:

· maximum number of IBM 8228 MAU type hubs – 12;

· maximum number of subscribers in the network – 96;

· maximum cable length between the subscriber and the hub is 45 meters;

· maximum cable length between hubs is 45 meters;

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

· data transfer speed – 4 Mbit/s and 16 Mbit/s.

However, unlike Ethernet, the Token-Ring network can handle high load levels (more than 30-40%) much better and provides guaranteed access time. This is necessary, for example, in industrial networks, where a delay in the response 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. 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

The Arcnet network (or ARCnet from the English Attached Resource Computer Net, a computer network of connected resources) is one of the oldest networks. It was developed by Datapoint Corporation back in 1977. There are no international standards for this network, although it is 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 produced equipment for this type of network. But now production of Arcnet equipment has practically ceased.

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

Rice. 7.15. Arcnet network topology is bus type (B – adapters for working in a 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 network length is 6 kilometers.

· The maximum cable length from the subscriber to the passive hub is 30 meters.

· The maximum cable length from the subscriber to the 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 the bus segment is 8.

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

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

· Data transfer speed – 2.5 Mbit/s.

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

FDDI network

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

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

The choice of optical fiber as a transmission medium determined the following advantages new network, such as high noise immunity, maximum confidentiality of information transmission and excellent galvanic isolation of subscribers. High transmission speeds, which are much easier to achieve in the case of fiber optic cables, make it possible to solve many tasks that are not possible with lower-speed networks, for example, transmitting images in real time. In addition, fiber optic cable easily solves the problem of transmitting data over a distance of several kilometers without relaying, which makes it possible to build large networks that even cover entire cities and have 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 for by the international standard IEEE 802.5 (Token-Ring). Minor differences from this standard are determined by the need to ensure high speed information transfer over long distances. The FDDI network topology is ring, the most suitable topology for fiber optic cable. The network uses two multi-directional fiber optic cables, one of which is usually in reserve, but this solution allows the use of full-duplex information transmission (simultaneously in two directions) with double the effective speed of 200 Mbit/s (with each of the two channels operating at the speed 100 Mbit/s). A star-ring topology with hubs included in the ring (as in Token-Ring) is also used.

Main 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 (possibly using electrical twisted pair).

· Access method – token.

· Information transfer speed – 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 100 Mbps bandwidth cannot match FDDI in terms of network size allowance. In addition, the FDDI token access method, unlike CSMA/CD, provides guaranteed access time and the absence of conflicts at any load level.

The limitation on the total network length of 20 km is not due to signal attenuation in the cable, but to the need to limit time complete passage signal along the ring to ensure maximum permissible access time. But the maximum distance between subscribers (2 km with a multimode cable) is determined precisely by the attenuation of the signals in the cable (it should not exceed 11 dB). It is also possible to use single-mode cable, in which case the distance between subscribers can reach 45 kilometers, and the total ring length can be 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. Electrical versions of FDDI are much less standardized than fiber optic ones, so compatibility between equipment from different manufacturers is not guaranteed.

To transmit data in FDDI, a 4B/5B code specially developed for this standard is used.

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

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

· Class B subscribers (stations) (single connection subscribers, SAS – Single-Attachment Stations) are connected to only one (external) network ring. They are simpler and cheaper than Class A adapters, but do not have their capabilities. They can only be connected to the network through a hub or bypass switch, which turns them off in the event of an emergency.

In addition to the subscribers themselves (computers, terminals, etc.), the network uses Wiring Concentrators, the inclusion of which allows all connection points to be collected in one place for the purpose of monitoring network operation, diagnosing faults and simplifying reconfiguration. When using different types of cables (for example, fiber optic cable and twisted pair), the hub also performs the conversion function electrical signals to optical and vice versa. Concentrators also come in dual connection (DAC - Dual-Attachment Concentrator) and single connection (SAC - Single-Attachment Concentrator).

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

Rice. 8.1. Example of FDDI network configuration.

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

In conclusion, it should be noted that despite the obvious advantages of FDDI, this network has 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 now is basic, core (Backbone) networks that combine several networks. FDDI is also used to connect powerful workstations or servers that require high-speed communication. It is expected that Fast Ethernet can supplant FDDI, but the advantages of fiber optic cable, token management and the record-breaking permissible network size currently put FDDI ahead of the competition. And in cases where the cost of the equipment is critical, a twisted-pair version of FDDI (TPDDI) can be used in non-critical areas. In addition, the cost of FDDI equipment can greatly decrease as its production volume increases.

100VG-AnyLAN network

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

Its main advantages are high exchange speed, 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 Mbps, the letters VG indicate low-cost unshielded twisted pair cable of category 3 (Voice Grade), and AnyLAN (any network) indicates that the network is compatible with the two most common networks.

Main technical characteristics of the 100VG-AnyLAN network:

· Transfer speed – 100 Mbit/s.

· Topology – star with expandability (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 are quad unshielded twisted pair (UTP Category 3, 4 or 5 cable), dual twisted pair (UTP Category 5 cable), dual shielded twisted pair (STP), and fiber optic cable. Nowadays, quad twisted pair cables are mostly common.

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

· The maximum number of subscribers is 1024, recommended – up to 250.

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

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

The 100VG-AnyLAN network consists of a central (main, root) Level 1 hub, to which both individual subscribers and Level 2 hubs can be connected, to which subscribers and Level 3 hubs, in turn, can be connected, etc. In this case, 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 unshielded twisted pair cable.

Rice. 8.8. Network structure 100VG-AnyLAN.

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

Each of the hubs can be configured to work with Ethernet or Token-Ring packet formats. In this case, the hubs of the entire network must work with packets of only one format. Bridges are required to communicate with Ethernet and Token-Ring networks, but the bridges are quite simple.

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

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

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

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

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

When using quad twisted pair cable, each of the four twisted pair cables transmits at a speed of 30 Mbps. The total transmission speed is 120 Mbit/s. However helpful information Due to the use of the 5B/6B code, it is transmitted at only 100 Mbit/s. Thus, the cable bandwidth must be at least 15 MHz. Category 3 twisted pair cable (16 MHz bandwidth) satisfies this requirement.

Thus, the 100VG-AnyLAN network provides an affordable solution for increasing transmission speeds up to 100 Mbps. However, it is not fully compatible with any of the standard networks, so its future 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.

When it comes to the most common 100Mbps Fast Ethernet network, 100VG-AnyLAN provides twice the Category 5 UTP cable length (up to 200 meters), as well as a contention-free method of traffic management.

Today it is almost impossible to find a laptop or motherboard without an integrated network card, or even two. All of them have the same connector - RJ45 (more precisely, 8P8C), but the speed of the controller can differ by an order of magnitude. In cheap models it is 100 megabits per second (Fast Ethernet), in more expensive ones it is 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 a processor such as an Intel Pentium 4 or AMD Athlon XP, 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 connector, since the buses PCI Express did not exist at the time of their birth. But also for the PCI bus (33 MHz), “network cards” are produced that support the most current Gigabit Ethernet standard, although its throughput may not be enough to fully unleash the speed potential of a gigabit controller.

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

What advantages will the transition from Fast Ethernet to Gigabit Ethernet bring in practice? How different is the actual data transfer speed of PCI versions of network cards and PCI Express? Is normal speed enough? hard drive to fully load a gigabit channel? You will find answers to these questions in this material.

Test participants

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

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

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

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

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 “boost” gigabit network cards to full speed, but in practice it may still not be enough. The fact is that this “channel” is shared by all PCI devices; in addition, it transmits service information on servicing the bus itself. Let's see if this assumption is confirmed by real speed measurements.

Another nuance: the vast 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 fully load 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), and the speed can almost double. The second is to use SSD drives, speed parameters which are significantly 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 chipset 770 + AMD SB700);
RAM: Hynix DDR2 4 x 2048 GB PC2 8500 1066 MHz (dual-channel mode);
video card: AMD Radeon HD 4890 1024 MB DDR5 PCI Express 2.0;
network card: Realtek RTL8111DL 1000 Mbit/s (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 (dual-channel mode);
video card: AMD Radeon HD 3100 256 MB (integrated into the chipset);
HDD: Seagate 7200.10 160 GB SATA2;
operating system: Microsoft Windows XP Home SP3 (32-bit version).

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

Data transfer rate (hard drive - hard drive, Mbit/s)

The actual data transfer speed through the 100-megabit Acorp L-100S network card fell just short of the theoretical maximum. But both gigabit cards, although they outperformed the first by about six times, were unable to show the maximum possible speed. It is clearly seen that the speed is limited by the performance of Seagate 7200.10 hard drives, which, when directly tested on a computer, averages 79 megabytes per second (632 Mbit/s).

The fundamental difference in speed between network cards for PCI buses(Acorp L-1000S) and PCI Express (TP-LINK) are not observed in this case; the slight advantage of the latter can be explained by measurement error. Both controllers were operating at about sixty percent of their capacity.

Data transfer rate (RAM disk - RAM disk, Mbit/s)

Acorp L-100S expectedly showed the same low speed and when copying data from high-speed RAM disks. This is understandable - the Fast Ethernet standard has not corresponded to modern realities for a long time. Compared to the “hard drive-to-hard drive” testing mode, the Acorp L-1000S gigabit PCI card significantly increased performance - the advantage was approximately 36 percent. The TP-LINK TG-3468 network card showed an even more impressive lead - the increase was about 55 percent.

This is where the higher bandwidth of the PCI Express bus showed 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 almost an order of magnitude less for the copying to complete (compared to Fast Ethernet). For example, the time it takes to copy a 25 GB file (typical HD rip with good quality) from computer to computer will be less than four minutes, and with a previous generation adapter - 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 only have 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 high-performance SSD drives, preference should be given to cards with a PCI Express interface, which will provide the highest possible data transfer speed.

Naturally, it should be taken into account that other devices in the network “path” (switch, router...) must support the Gigabit Ethernet standard, and the category of twisted pair (patch cord) must be at least 5e. Otherwise, the real speed will remain at 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 to a gigabit network. network card, this will not affect the speed of other computers on the network.

Ethernet, despite
for all his success, it was never elegant.
Network cards have only rudimentary
concept of intelligence. They really
first send the packet, and only then
look to see if anyone else has transmitted data
at the same time as them. Someone compared Ethernet with
a society in which people can communicate
with each other only when everyone is screaming
simultaneously.

Like him
predecessor, Fast Ethernet uses a method
CSMACD (Carrier Sense Multiple Access with
Collision Detection - Multiple access to the environment with
carrier sensing and collision detection).
Behind this long and obscure acronym
hiding a very simple technology. When
the Ethernet card must send a message, then
first she waits for silence, then
sends a packet and listens at the same time, not
did anyone send a message
at the same time as him. If this happened, then
both packets do not reach the destination. If
there was no collision, but the board should continue
transmit data, she is still waiting
a few microseconds before
will try to send a new portion. This
made so that other boards also
could work and no one could capture
channel is exclusive. In case of a collision, both
devices go silent for a short time
time period generated
randomly and then take
new attempt to transfer data.

Due to collisions
Ethernet nor Fast Ethernet will ever be able to achieve
his maximum performance 10
or 100 Mbit/s. As soon as it starts
network traffic increases, temporary
delays between sending individual packets
are reduced, and the number of collisions
increases. Real
Ethernet performance cannot exceed
70% of its potential throughput
abilities, and maybe even lower if the line
seriously overloaded.

Ethernet uses
packet size is 1516 bytes which is fine
approached when it was first created.
Today this is considered a disadvantage when
Ethernet is used for communication
servers, since servers and communication lines
tend to exchange more
the number of small packages that
overloads the network. In addition, Fast Ethernet
imposes a limit on the distance between
connected devices – no more than 100
meters and it makes you show
extra caution when
designing such networks.

First Ethernet was
designed based on bus topology,
when all devices connected to a common
cable, thin or thick. Application
twisted pair only partially changed the protocol.
When using coaxial cable
the collision was determined by everyone at once
stations. In the case of twisted pair
the "jam" signal is used as soon as
station detects a collision, then it
sends a signal to the hub, the latter in
in turn sends out "jam" to everyone
devices connected to it.

In order to
reduce congestion, Ethernet networks
are divided into segments that
united through bridges and
routers. This allows you to transfer
between segments only the necessary traffic.
A message sent between two
stations in the same segment, there will be no
transferred to another and will not be able to call in it
overload.

Today at
construction of a central highway,
unifying servers use
switched Ethernet. Ethernet switches can be
consider as high speed
multi-port bridges that are able
independently determine which of it
ports the packet is addressed to. Switch
looks at packet headers and so on
thus compiles a table defining
where is this or that subscriber with this
physical address. This allows
limit the distribution scope of a package
and reduce the likelihood of overflow,
sending it only to the required port. Only
broadcast packets are sent over
all ports.

100BaseT
- big brother 10BaseT

Technology idea
Fast Ethernet was born in 1992. In August
next year group of producers
merged into the Fast Ethernet Alliance (FEA).
The FEA's goal was to obtain as quickly as possible
formal approval of Fast Ethernet from the committee
Institute of Electrical and Electrical Engineers 802.3
radio electronics (Institute of Electrical and Electronic
Engineers, IEEE), since it is this committee
deals with standards for Ethernet. Luck
accompanied by new technology and
to its supporting alliance: in June 1995
all formal procedures have been completed, and
Fast Ethernet technology was given the name
802.3u.

With a light hand IEEE
Fast Ethernet is called 100BaseT. This is explained
simple: 100BaseT is an extension
10BaseT standard with throughput from
10 Mbps to 100 Mbps. 100BaseT standard includes
includes a protocol for processing multiple
carrier sense access and
CSMA/CD (Carrier Sense Multiple) collision detection
Access with Collision Detection), which is also used in
10BaseT. In addition, Fast Ethernet can operate on
cables of several types, including
twisted pair Both of these properties are new
standards are very important for potential
buyers, and it is thanks to them that 100BaseT
turns out to be a successful way to migrate networks
based on 10BaseT.

Main
selling point for 100BaseT
is that Fast Ethernet is based on
inherited technology. Since in Fast Ethernet
the same transmission protocol is used
messages as in older versions of Ethernet, and
cable systems of these standards
compatible, to transition to 100BaseT from 10BaseT
required

smaller
capital investment than for installation
other types of high-speed networks. Except
Moreover, since 100BaseT is
continuation of the old Ethernet standard, everything
tools and procedures
analysis of network operation, as well as all
software running on
old Ethernet networks should use this standard
maintain functionality.
Therefore, the 100BaseT environment will be familiar
network administrators with experience
with Ethernet. This means that staff training will take
less time and will cost significantly
cheaper.

PRESERVATION
PROTOCOL

Perhaps,
the greatest practical benefit of the new
technology brought the decision to leave
message transfer protocol unchanged.
Message transfer protocol, in our case
CSMA/CD, defines the way in which data
transmitted over a network from one node to another
through the cable system. In ISO/OSI model
CSMA/CD protocol is part of the layer
media access control (MAC).
At this level, the format is determined, in
in which information is transmitted over the network, and
the way a network device receives
network access (or network management) for
data transmission.

Name CSMA/CD
can be broken down into two parts: Carrier Sense Multiple Access
and Collision Detection. From the first part of the name you can
conclude how a node with a network
adapter determines the moment when it
message should be sent. In accordance with
With the CSMA protocol, the network node first “listens”
network to determine whether it is being transmitted to
this moment some other message.
If a carrier tone is heard,
this means that the network is currently occupied by someone else
message - the network node goes into mode
waiting and remains in it until the network
will be released. When the network comes
silence, the node begins transmission.
In fact, the data is sent to all nodes
network or segment, but are accepted only by those
the node to which they are addressed.

Collision Detection -
the second part of the name is used to resolve
situations where two or more nodes are trying to
transmit messages simultaneously.
According to the CSMA protocol, everyone is ready for
transmission, the node must first listen to the network,
to determine if she is available. However,
if two nodes are listening at the same time,
they will both decide that the network is free and start
transmit your packets simultaneously. In this
situations transmitted data
overlap each other (network
engineers call it a conflict), and not a single
messages do not reach the point
appointments. Collision Detection requires that the node
I also listened to the network after the transmission
package. If a conflict is detected, then
the node repeats the transmission through a random
the chosen period of time and
checks again to see if a conflict has occurred.

THREE TYPES OF FAST ETHERNET

Along with
maintaining the CSMA/CD protocol, other important
The solution was to design 100BaseT this way
in such a way that it can be used
cables of different types - like those that
used in older versions of Ethernet, and
newer models. The standard defines three
modifications to ensure work with
different types of Fast Ethernet cables: 100BaseTX, 100BaseT4
and 100BaseFX. Modifications 100BaseTX and 100BaseT4 are calculated
on twisted pair, and 100BaseFX was designed for
optical cable.

100BaseTX standard
requires the use of two UTP or STP pairs. One
a pair serves for transmission, the other for
reception. These requirements are met by two
Main cable standard: EIA/TIA-568 UTP
IBM Category 5 and STP Type 1. In 100BaseTX
attractive security
full duplex mode when working with
network servers, as well as the use
only two out of four pairs of eight-core
cable - the other two pairs remain
free and can be used in
further to expand opportunities
networks.

However, if you
are going to work with 100BaseTX, using for
this is Category 5 wiring, then you should
know about its shortcomings. This cable
more expensive than other eight-core cables (for example
Category 3). In addition, to work with it
requires the use of punchdown blocks
blocks), connectors and patch panels,
meeting the requirements of Category 5.
It should be added that for support
full duplex mode should be
install full duplex switches.

100BaseT4 standard
has more lenient requirements for
the cable being used. The reason for this is
the fact that 100BaseT4 uses
all four pairs of eight-core cable: one
for transmitting, another for receiving, and
the remaining two work as transmission,
and at the reception. Thus, in 100BaseT4 and reception,
and data transfer can be carried out via
three couples By splitting 100 Mbit/s into three pairs,
100BaseT4 reduces the signal frequency, so
for its transmission is enough and less
high quality cable. For implementation
100BaseT4 networks are suitable for UTP Category 3 and
5, as well as UTP Category 5 and STP Type 1.

Advantage
100BaseT4 is less rigid
wiring requirements. Category 3 and
4 are more common, and in addition they
significantly cheaper than cables
Category 5, what should not be forgotten before
start of installation work. The disadvantages
are that 100BaseT4 requires all four
pairs and that full duplex mode is this
not supported by the protocol.

Fast Ethernet includes
also a standard for multimode operation
optical fiber with a 62.5-micron core and 125-micron
shell. The 100BaseFX standard is focused on
mainly on the highway - for connection
Fast Ethernet repeaters within one
building. Traditional benefits
optical cable are also inherent in the standard
100BaseFX: Electromagnetic immunity
noise, improved data protection and large
distances between network devices.

RUNNER
FOR SHORT DISTANCES

Although Fast Ethernet
is a continuation of the Ethernet standard,
transition from a 10BaseT to 100BaseT network is not possible
considered as a mechanical replacement
equipment - for this they can
changes to the network topology will be required.

Theoretical
Fast Ethernet segment diameter limit
is 250 meters; it's only 10
percent of theoretical size limit
Ethernet networks (2500 meters). This limitation
stems from the nature of the CSMA/CD protocol and
transfer speed 100Mbit/s.

What already
noted earlier, transmitting data
the workstation must listen to the network in
passage of time to ensure
that the data has reached the destination station.
On an Ethernet network with a bandwidth of 10
Mbit/s (for example 10Base5) period of time,
required workstation for
listening to the network for conflicts,
determined by the distance that 512-bit
frame (frame size specified in the Ethernet standard)
will pass during the processing of this frame by
workstation. For an Ethernet network with bandwidth
with a capacity of 10 Mbit/s this distance is equal to
2500 meters.

On the other side,
the same 512-bit frame (802.3u standard
specifies a frame of the same size as 802.3, then
is in 512 bits), transmitted to the working
station in the Fast Ethernet network, will travel only 250 m,
before the workstation completes it
processing. If the receiving station were
far from the transmitting station
distance over 250 m, then the frame could
come into conflict with another frame on
the lines are somewhere further, and the transmitting
the station, having completed the transmission, no longer
would perceive this conflict. That's why
The maximum diameter of a 100BaseT network is
250 meters.

To
use the permissible distance,
you will need two repeaters to connect
all nodes. According to the standard,
maximum distance between node and
repeater range is 100 meters; in Fast Ethernet,
as in 10BaseT, the distance between
hub and workstation are not
must exceed 100 meters. Because the
connecting devices (repeaters)
introduce additional delays, real
the working distance between nodes can
turn out to be even smaller. That's why
it seems reasonable to take everything
distances with some reserve.

To work on
over long distances you will have to purchase
optical cable. For example, equipment
100BaseFX in half duplex mode allows
connect a switch to another switch
or terminal station located on
up to 450 meters apart.
By installing full duplex 100BaseFX, you can
connect two network devices to
distance up to two kilometers.

HOW
INSTALL 100BASET

In addition to cables,
which we have already discussed, to install Fast
Ethernet will require network adapters for
workstations and servers, hubs
100BaseT and maybe some
100BaseT switches.

Adapters,
necessary for organizing a 100BaseT network,
are called 10/100 Mbit/s Ethernet adapters.
These adapters are capable (this is a requirement
100BaseT standard) independently distinguish 10
Mbit/s from 100 Mbit/s. To serve the group
servers and workstations transferred to
100BaseT, you will also need a 100BaseT hub.

When turned on
server or personal computer With
adapter 10/100 the latter produces a signal,
announcing what he can provide
bandwidth 100Mbit/s. If
receiving station (most likely this is
there will be a hub) is also designed for
work with 100BaseT, it will respond with a signal
to which both the hub and the PC or server
automatically switch to 100BaseT mode. If
The hub only works with 10BaseT, it does not
gives a response signal, and the PC or server
will automatically switch to 10BaseT mode.

When
small-scale 100BaseT configurations are possible
use a 10/100 bridge or switch that
will provide connection to the part of the network working with
100BaseT, with an existing network
10BaseT.

DECEIVING
RAPIDITY

To sum it all up
above, we note that, as it seems to us,
Fast Ethernet is best for solving problems
high peak loads. For example, if
some of the users work with CAD or
image processing programs and
needs to increase bandwidth
abilities, then Fast Ethernet may be
a good way out of the situation. However, if
problems are caused by excess numbers
users on the network, then 100BaseT begins
slow down the exchange of information at about 50 percent
network load - in other words, on the same
level as 10BaseT. But in the end it's
after all, it’s nothing more than an expansion.

Model	Chipset	Data transfer rate	Tire	Network interface	Connectors	Jumbo Frame support	Wake-on-LAN support	Duplex
Acorp L-100S	Realtek RTL8139D	10 / 100 Mbit/s	PCI 2.3 (32 bits)	10Base-T, 100Base-TX	RJ45 (8P8C)	Eat	Eat	Full
Acorp L-1000S	Realtek RTL8169SC	10 / 100 / 1000 Mbit/s	PCI 2.3 (32 bits)		RJ45 (8P8C)	Eat	Eat	Full
	Realtek RTL8168B	10 / 100 / 1000 Mbit/s	PCI Express 1.0a	10Base-T, 100Base-TX, 1000Base-T	RJ45 (8P8C)	Eat	Eat	Full