Fast ethernet data transfer speed. Fast Ethernet technology, its features, physical layer, construction rules. DSAP and SSAP field values

The ComputerPress testing laboratory tested Fast Ethernet network cards for the PCI bus intended for use in 10/100 Mbit/s workstations. The currently most common cards with a throughput of 10/100 Mbit/s were selected, since, firstly, they can be used in Ethernet, Fast Ethernet and mixed networks, and, secondly, the promising Gigabit Ethernet technology ( throughput up to 1000 Mbit/s) is still most often used to connect powerful servers to the network equipment of the network core. It is extremely important what quality of passive network equipment (cables, sockets, etc.) is used in the network. It is well known that if for Ethernet networks a twisted pair cable of category 3 is sufficient, then category 5 is already required for Fast Ethernet. Signal scattering and poor noise immunity can significantly reduce network throughput.

The purpose of testing was to determine, first of all, the effective performance index (Performance/Efficiency Index Ratio - hereinafter P/E index), and only then - the absolute value of throughput. The P/E index is calculated as the ratio of the network card throughput in Mbit/s to the CPU load as a percentage. This index is the industry standard for measuring network adapter performance. It was introduced in order to take into account the use of CPU resources by network cards. The fact is that some network adapter manufacturers try to achieve maximum performance by using more computer processor cycles to perform network operations. Minimum processor load and relatively high throughput are essential for running mission-critical business, multimedia, and real-time applications.

We tested the cards that are currently most often used for workstations in corporate and local networks:

1. D-Link DFE-538TX
2. SMC EtherPower II 10/100 9432TX/MP
3. 3Com Fast EtherLink XL 3C905B-TX-NM
4. Compex RL 100ATX
5. Intel EtherExpress PRO/100+ Management
6. CNet PRO-120
7. NetGear FA 310TX
8. Allied Telesyn AT 2500TX
9. Surecom EP-320X-R

The main characteristics of the tested network adapters are given in Table. 1 . Let us explain some of the terms used in the table. Automatic connection speed detection means that the adapter itself determines the maximum possible operating speed. In addition, if auto-speed detection is supported, no additional configuration is required when moving from Ethernet to Fast Ethernet and back. That is, from system administrator There is no need to reconfigure the adapter or reload drivers.

Support for Bus Master mode allows you to transfer data directly between the network card and computer memory. This frees up the central processor to perform other operations. This property has become a de facto standard. It’s no wonder that all well-known network cards support Bus Master mode.

Remote turn-on (Wake on LAN) allows you to turn on your PC over a network. That is, it becomes possible to service the PC during non-working hours. For this purpose, three-pin connectors are used on the motherboard and the network adapter, which are connected with a special cable (included in the package). In addition, special control software is required. Wake on LAN technology was developed by the Intel-IBM alliance.

Full duplex mode allows you to transmit data simultaneously in both directions, half duplex - only in one direction. Thus, the maximum possible throughput in full duplex mode is 200 Mbit/s.

The DMI (Desktop Management Interface) makes it possible to obtain information about the configuration and resources of a PC using network management software.

Support for the WfM (Wired for Management) specification ensures the interaction of the network adapter with network management and administration software.

To remotely boot a computer OS over a network, network adapters are equipped with special BootROM memory. This allows diskless workstations to be used effectively on a network. Most of the tested cards only had a BootROM slot; The BootROM chip itself is usually a separately ordered option.

ACPI (Advanced Configuration Power Interface) support helps reduce power consumption. ACPI is a new technology that powers the power management system. It is based on the use of both hardware and software. In principle, Wake on LAN is part of ACPI.

Proprietary performance tools allow you to increase the efficiency of your network card. The most famous of them are Parallel Tasking II from 3Com and Adaptive Technology from Intel. These products are usually patented.

Support for major operating systems is provided by almost all adapters. The main operating systems include: Windows, Windows NT, NetWare, Linux, SCO UNIX, LAN Manager and others.

The level of service support is assessed by the availability of documentation, a floppy disk with drivers and the ability to download latest versions drivers from the company website. Packaging also plays an important role. From this point of view, the best, in our opinion, are the network adapters from D-Link, Allied Telesyn and Surecom. But overall the level of support turned out to be satisfactory for all cards.

Typically, the warranty covers the entire life of the AC adapter (lifetime warranty). Sometimes it is limited to 1-3 years.

Testing methodology

All tests used the latest versions of network card drivers, which were downloaded from the Internet servers of the respective manufacturers. In the case where the network card driver allowed any settings and optimization, the default settings were used (except for the Intel network adapter). Note that the cards and corresponding drivers from 3Com and Intel have the richest additional capabilities and functions.

Performance measurements were performed using Novell's Perform3 utility. The principle of operation of the utility is that a small file is copied from the workstation to a shared one network drive server, after which it remains in the server’s file cache and is read from there many times over a given period of time. This allows for memory-network-memory interoperability and eliminates the impact of latency associated with disk operations. The utility parameters include initial file size, final file size, resizing step, and testing time. The Novell Perform3 utility displays performance values ​​for different file sizes, average and maximum performance(in KB/s). The following parameters were used to configure the utility:

• Initial file size - 4095 bytes
• Final file size - 65,535 bytes
• File increment step - 8192 bytes

The testing time with each file was set to twenty seconds.

Each experiment used a pair of identical network cards, one running on the server and the other running on the workstation. This seems to be inconsistent with common practice, as servers typically use specialized network adapters that come with a number of additional features. But this is exactly the way - the same network cards are installed on both the server and workstations - testing is carried out by all well-known test laboratories in the world (KeyLabs, Tolly Group, etc.). The results are somewhat lower, but the experiment turns out to be clean, since only the analyzed network cards work on all computers.

Compaq DeskPro EN client configuration:

• Pentium II 450 MHz processor
• cache 512 KB
• RAM 128 MB
• hard drive 10 GB
• operating system Microsoft Windows NT Server 4.0 c 6 a SP
• TCP/IP protocol.

Compaq DeskPro EP server configuration:

• Celeron processor 400 MHz
• RAM 64 MB
• hard drive 4.3 GB
• operating room Microsoft system Windows NT Workstation 4.0 c c 6 a SP
• TCP/IP protocol.

Testing was carried out in conditions where the computers were connected directly with a UTP Category 5 crossover cable. During these tests, the cards operated in 100Base-TX Full Duplex mode. In this mode, the throughput is slightly higher due to the fact that part of the service information (for example, confirmation of reception) is transmitted simultaneously with useful information, the volume of which is estimated. Under these conditions, it was possible to record fairly high throughput values; for example, for the 3Com Fast EtherLink XL 3C905B-TX-NM adapter, the average is 79.23 Mbps.

CPU load was measured on the server using Windows utilities NT Performance Monitor; the data was recorded in a log file. The Perform3 utility was run on the client so as not to affect the server's processor load. The server computer processor was an Intel Celeron, whose performance is significantly lower than the performance of Pentium II and III processors. Intel Celeron was used deliberately: the fact is that since the processor load is determined with a fairly large absolute error, in the case of large absolute values ​​the relative error is smaller.

After each test, the Perform3 utility places the results of its work in a text file in the form of a data set of the following form:

65535 bytes. 10491.49 KBps. 10491.49 Aggregate KBps. 57343 bytes. 10844.03 KBps. 10844.03 Aggregate KBps. 49151 bytes. 10737.95 KBps. 10737.95 Aggregate KBps. 40959 bytes. 10603.04 KBps. 10603.04 Aggregate KBps. 32767 bytes. 10497.73 KBps. 10497.73 Aggregate KBps. 24575 bytes. 10220.29 KBps. 10220.29 Aggregate KBps. 16383 bytes. 9573.00 KBps. 9573.00 Aggregate KBps. 8191 bytes. 8195.50 KBps. 8195.50 Aggregate KBps. 10844.03 Maximum KBps. 10145.38 Average KBp.

It displays the file size, the corresponding throughput for the selected client and for all clients (in this case there is only one client), as well as the maximum and average throughput for the entire test. The obtained average values ​​for each test were converted from KB/s to Mbit/s using the formula:
(KB x 8)/1024,
and the P/E index value was calculated as the ratio of throughput to processor load as a percentage. Subsequently, the average value of the P/E index was calculated based on the results of three measurements.

The following problem arose when using the Perform3 utility on Windows NT Workstation: in addition to writing to a network drive, the file was also written to the local file cache, from where it was subsequently read very quickly. The results were impressive, but unrealistic, since there was no data transfer as such over the network. In order for applications to treat shared network drives as regular local drives, operating system a special network component is used - a redirector that redirects I/O requests over the network. Under normal operating conditions, when performing the procedure of writing a file to a shared network drive, the redirector uses the Windows NT caching algorithm. That is why when writing to the server, writing also occurs to the local file cache of the client machine. And to carry out testing, it is necessary that caching be carried out only on the server. To ensure that there is no caching on the client computer, Windows registry NT, the parameter values ​​were changed, which made it possible to disable caching performed by the redirector. Here's how it was done:

1. Path to Registry:

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Rdr\Parameters

   Parameter name:

   UseWriteBehind enables write-behind optimization for files being written

   Type: REG_DWORD

   Value: 0 (default: 1)

2. Path to Registry:

   HKEY_LOCAL_MACHINE\SYSTEM\CurrentControlSet\Services\Lanmanworkstation\parameters

   Parameter name:

   UtilizeNTCaching specifies whether the redirector will use the Windows NT cache manager to cache file contents.

   Type: REG_DWORD Value: 0 (Default: 1)

Intel EtherExpress PRO/100+Management Network Adapter

This card's throughput and CPU utilization were found to be almost the same as the 3Com's. The settings windows for this card are shown below.

The new Intel 82559 controller installed on this card provides very high performance, especially in Fast Ethernet networks.

The technology that Intel uses in its Intel EtherExpress PRO/100+ card is called Adaptive Technology. The essence of the method is to automatically change the time intervals between Ethernet packets depending on the network load. As network congestion increases, the distance between individual Ethernet packets dynamically increases, which reduces the number of collisions and increases throughput. When the network load is light, when the probability of collisions is low, the time intervals between packets are reduced, which also leads to increased performance. The greatest benefits of this method should be seen in large collision Ethernet segments, that is, in cases where the network topology is dominated by hubs rather than switches.

Intel's new technology, called Priority Packet, allows traffic to be regulated through network card, according to the priorities of individual packages. This makes it possible to increase data transfer rates for mission-critical applications.

Provides support for virtual local networks VLAN (IEEE 802.1Q standard).

There are only two indicators on the board - work/connection, speed 100.

www.intel.com

Network adapter SMC EtherPower II 10/100 SMC9432TX/MP

The architecture of this card uses two promising technologies: SMC SimulTasking and Programmable InterPacket Gap. The first technology is similar to 3Com Parallel Tasking technology. By comparing the test results for cards from these two manufacturers, we can draw a conclusion about the degree of effectiveness of the implementation of these technologies. We also note that this network card showed the third result both in terms of performance and P/E index, ahead of all cards except 3Com and Intel.

There are four LED indicators on the card: speed 100, transmission, connection, duplex.

The company's main website address is: www.smc.com

Introduction

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

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 communications both within one company and outside it. Resources are data, applications and peripherals, such as external drive, printer, mouse, modem or joystick. The concept of interactive communication between computers implies the exchange of messages in real time.

There are many sets of standards for data transmission in computer networks. One of the sets 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 operation 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 throughput, these technologies can eliminate " narrow places» on the network and support applications that require high data transfer rates. The appeal of these solutions is that you don't have to choose one or the other. They are complementary, so network efficiency can often be improved by using both technologies.

The collected information will be useful both to people starting to study computer networks and to network administrators.

1. Network diagram

2. Fast Ethernet technology

computer network fast ethernet

Fast Ethernet is the result of the development of Ethernet technology. Based on and retaining the same CSMA/CD (channel polling multiple access and collision detection) technique, 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 image processing, and multimedia. Fast Ethernet is compatible with 10 Mbps Ethernet, so it's easier to integrate Fast Ethernet into your LAN using a switch rather than a router.

Switch

Using switches many workgroups can be connected to form a large LAN (see Diagram 1). Inexpensive switches perform better than routers, providing better LAN performance. Fast Ethernet workgroups consisting of one or two hubs can be connected through a Fast Ethernet switch to further increase the number of users as well as cover a larger area.

As an example, consider the following switch:

Rice. 1 D-Link-1228/ME

The DES-1228/ME series of switches includes premium, configurable Layer 2 Fast Ethernet switches. With advanced functionality, DES-1228/ME devices are inexpensive solution to create a secure and high-performance network. Distinctive Features The features of this switch are high port density, 4 Gigabit Uplink ports, small step change settings for bandwidth management, and improved network management. These switches allow you to optimize your network both in terms of functionality and cost characteristics. Switches of the DES-1228/ME series are the optimal solution both in terms of functionality and cost characteristics.

FTP cable

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

Conductor diameter 24AWG.

Each conductor is encased in HDPE (High Density Polyethylene) insulation.

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

The 4 twisted pairs are wrapped in polyethylene film and, together with a single-core copper ground conductor, are enclosed in a common foil shield and PVC sheath.

Straight through

It serves:

• 1. To connect a computer to a switch (hub, switch) via the computer’s network card
• 2. To connect network peripheral equipment - printers, scanners - to the switch (hub, switch)
• 3. for UPLINK on a higher 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-level switch in a local network with a complex structure, for older types of switches (hubs, switches), they have a separate connector, also marked “UPLINK” or an 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 segment of the network. Such a network segment can function either separately or as part of a complex network topology (usually a “tree”). All information exchange takes place exclusively through the central computer, which is subject to a very large load in this way, so it cannot do anything else except the network. As a rule, it is the central computer that is the most powerful, and it is on it that all functions for managing the exchange are assigned. In principle, no conflicts in a network with a star topology are possible, because management is completely centralized.

Application

Classic 10 Mbit Ethernet suited most users for about 15 years. However, in the early 90s, its insufficient capacity began to be felt. For computers on Intel processors 80286 or 80386 with ISA (8 MB/s) or EISA (32 MB/s) buses, the Ethernet segment bandwidth was 1/8 or 1/32 of the memory-to-disk channel, and this was well consistent with the ratio of data volumes processed locally , and data transmitted 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 10Mbps Ethernet segments became overloaded, server responsiveness dropped significantly, and collision rates increased significantly, further reducing usable throughput.

There is a need to develop a “new” Ethernet, that is, a technology that would be equally cost-effective with a performance of 100 Mbit/s. As a result of searches and research, experts 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 network equipment manufacturers, including Ethernet technology leaders such as SynOptics, 3Com and several others, formed the Fast Ethernet Alliance, a non-profit association, to develop a standard for a new technology that would preserve the features of Ethernet technology to the maximum extent 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 shortcomings of Ethernet technology. After some time, these companies were joined by IBM, which contributed by proposing to provide some compatibility with Token Ring networks in the new technology.

At the same time, IEEE Committee 802 formed a research group to study the technical potential of new high-speed technologies. Between late 1992 and late 1993, the IEEE team studied 100-Mbit solutions offered by various vendors. In addition to the Fast Ethernet Alliance proposals, the group also reviewed high-speed technology proposed by Hewlett-Packard and AT&T.

The discussion centered on the issue of maintaining the random CSMA/CD access method. The Fast Ethernet Alliance proposal preserved this method and thereby ensured continuity and consistency between 10 Mbps and 100 Mbps networks. The HP-AT&T coalition, which had the support of significantly fewer vendors in the networking industry than the Fast Ethernet Alliance, proposed an entirely new access method called Demand Priority- priority access on demand. It significantly changed the behavior of nodes on the network, so it could not fit into 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 the 802.3 standard, which is not a standalone standard, but is an addition to the existing 802.3 standard in the form of chapters 21 to 30. The 802.12 committee adopted the l00VG-AnyLAN technology, which uses a new Demand Priority access method and supports two frame formats - Ethernet and Token Ring.

v Physical layer of Fast Ethernet technology

All 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, when 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 due to the fact that it uses three types of cabling systems:

• · fiber optic multimode cable, two fibers are used;
• · Category 5 twisted pair, two pairs are used;
• · Category 3 twisted pair, four pairs are used.

Coaxial cable, which gave the world the first Ethernet network, was not included in the list of permitted data transmission media of the new Fast Ethernet technology. This is a common trend in many new technologies because short distances Category 5 twisted pair allows you to transmit data at the same speed as coaxial cable, but the network is cheaper and easier to operate. Over long distances, optical fiber has much higher bandwidth than coax, and the cost of the network is not much higher, especially when you consider the high troubleshooting costs of a large coaxial cable system.

Differences between Fast Ethernet technology and Ethernet technology

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

Nevertheless, this circumstance does not really hinder the construction of large networks using 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 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, but only restrictions on the length of the physical segments connecting neighboring devices (adapter - switch or switch - switch). Therefore, when creating long-distance local network backbones, Fast Ethernet technology is also actively used, but only in the full-duplex version, in conjunction with switches.

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

Compared to the physical implementation options for Ethernet (and there are six of them), in Fast Ethernet the differences between each option and the others are deeper - both the number of conductors and coding methods change. And since the physical variants 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 variant to variant, and those sublayers that are specific to each variant 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 four-pair UTP Category 3, 4 or 5 UTP cable;
• · 100Base-FX for multimode fiber optic cable, two fibers are used.

The following statements and characteristics are true for all three standards.

• · Fast Ethernetee technology frame formats are different from 10 Mbit Ethernet technology frame formats.
• · The interframe interval (IPG) is 0.96 µs and the bit interval is 10 ns. All timing parameters of the access algorithm (backoff interval, minimum frame length transmission time, etc.), measured in bit intervals, remained the same, so no changes were made to the sections of the standard relating to the MAC level.
• · A sign of a free state of the medium is the transmission of the Idle symbol of the corresponding redundant code (and not the absence of signals, as in the 10 Mbit/s Ethernet standards). The physical layer includes three elements:
• o reconciliation sublayer;
• o media independent interface (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 MP interface.

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

• · logical data encoding sublevel, 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 connection sublayers and physical media dependence (PMD) sublayers, which provide signal generation in accordance with a physical coding method, for example NRZI or MLT-3;
• · autonegotiation sublayer, which allows two communicating ports to automatically select the most efficient operating mode, for example, half-duplex or full-duplex (this sublayer is optional).

The MP interface supports a 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 physical signal coding sublayer (for all cable options the same physical coding method was used - Manchester code) and the physical connection sublayer to the medium, and the MP 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 length of the MP cable is one meter. Signals transmitted via the MP interface have an amplitude of 5 V.

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

This specification defines the operation of the Fast Ethernet protocol over multimode fiber in half-duplex and full-duplex modes based on the well-proven FDDI encoding 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 properties common to the two specifications will be given under the generic name l00Base-FX/TX.

While 10 Mbps Ethernet uses Manchester encoding to represent data over a cable, the Fast Ethernet standard defines a different encoding method - 4V/5V. This method has already proven 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 potential codes to be applied by representing each of the five bits as electrical or optical pulses. The existence of prohibited symbol combinations allows erroneous symbols to be rejected, which increases the stability of networks with l00Base-FX/TX.

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

Continuous data flow of 100Base-FX/TX specifications

Once the 4-bit chunks of MAC codes are converted into 5-bit chunks of the physical layer, they need to be represented as optical or electrical signals in the cable connecting the network nodes. The l00Base-FX and l00Base-TX specifications use different physical encoding methods for this - NRZI and MLT-3, respectively (as in FDDI technology when operating over optical fiber and twisted pair).

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

The l00Base-TX specification uses UTP Category 5 cable or STP Type 1 cable as the data transmission medium. Maximum length cable in both cases - 100 m.

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

The Auto-negotiation scheme described below is the l00Base-T technology standard today. Previously, manufacturers used various proprietary schemes to automatically determine the speed of communicating ports that were not compatible. The Auto-negotiation scheme adopted as a standard was originally proposed by National Semiconductor under the name NWay.

A total of 5 different operating modes are currently defined that can support l00Base-TX or 100Base-T4 devices on twisted pairs;

• · l0Base-T - 2 pairs of category 3;
• l0Base-T full-duplex - 2 pairs of category 3;
• · l00Base-TX - 2 pairs of category 5 (or Type 1ASTP);
• · 100Base-T4 - 4 pairs of category 3;
• · 100Base-TX full-duplex - 2 pairs of category 5 (or Type 1A STP).

The l0Base-T mode has the lowest priority in the negotiation process, and the full-duplex 100Base-T4 mode has the highest. The negotiation process occurs when the device is turned on, and can also be initiated at any time by the device control module.

The device that has started the auto-negotiation process sends a packet of special impulses to its partner Fast Link Pulse burst (FLP), which contains an 8-bit word encoding the proposed interaction mode, starting with the highest priority supported by the node.

If the peer 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 the given mode, and this ends the negotiation. 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 mode. Thus, the highest priority common node mode is always selected.

A node that only supports l0Base-T technology sends Manchester pulses every 16 ms to check the integrity of the line connecting it to a neighboring node. Such a node does not understand the FLP request that a node with the Auto-negotiation function makes to it, and continues to send its pulses. A node that receives only line integrity pulses in response to an FLP request understands that its partner can only operate using the l0Base-T standard, and sets this operating mode for itself.

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

The 100Base-T4 specification was designed to allow high-speed Ethernet to use existing Category 3 twisted pair wiring. This specification increases overall throughput by simultaneously carrying bit streams over all 4 pairs of cable.

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

At the same time, the developers of the competing technology l00VG-AnyLAN initially relied on working over twisted pair cable of category 3; the most important advantage was not so much the cost, but the fact that it was already installed in the vast 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 encoding, this method uses 8V/6T encoding, which has a narrower signal spectrum and, at a speed of 33 Mbit/s, fits into the 16 MHz band of category 3 twisted pair cable (when encoding 4V/5V, the signal spectrum does not fit into this band) . Every 8 bits of MAC level information are encoded by 6 ternary symbols, that is, numbers that have three states. Each ternary digit has a duration of 40 ns. The group of 6 ternary digits is then transmitted onto one of the three transmit twisted pairs, independently and sequentially.

The fourth pair is always used for listening carrier frequency for collision detection purposes. The data transfer rate on 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 category 3 twisted pair.

In Fig. Figure 3.23 shows the connection between the MDI port of a 100Base-T4 network adapter and the MDI-X port of a hub (the prefix X indicates that for this connector, the receiver and transmitter connections are exchanged in cable pairs compared to the network adapter connector, which makes it easier to connect 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 the pair 4 -5 And 7 -8 are bidirectional and are used for both reception and transmission, depending on the need.

Connection of nodes according to the 100Base-T4 specification

Fast Ethernet

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

Fast Ethernet structure

To better understand the operation and understand the interaction of Fast Ethernet elements, let's turn to Figure 1.

Figure 1. Fast Ethernet system

Logical Link Control (LLC) Sublayer

The IEEE 802.3u specification divides the link layer functions into two sublayers: logical link control (LLC) and media access layer (MAC), which will be discussed below. LLC, whose functions are defined by the IEEE 802.2 standard, actually interconnects with higher-level protocols (for example, IP or IPX), providing various communication services:

• Service without connection establishment and reception confirmations. A simple service that does not provide data flow control or error control, and does not guarantee correct delivery of data.
• Connection-based service. An absolutely reliable service that guarantees correct data delivery by establishing a connection to the receiving system before data transmission begins and using error control and data flow control mechanisms.
• Connectionless service with reception confirmations. A medium-complex service that uses acknowledgment messages to provide guaranteed delivery, but does not establish a connection before transmitting the data.

On the sending system, data passed down from the protocol Network layer, are first encapsulated by the LLC sublayer. The standard calls them Protocol Data Unit (PDU). When the PDU is passed down to the MAC sublayer, where it is again surrounded by header and post information, from that point on it can technically be called a frame. 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 allowed 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 using TCP/IP along with other protocols, the sole function of the LLC may be to allow 802.3 frames to contain a SNAP header, like 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 high-level protocols require more advanced services from the LLC. For example, NetBIOS sessions and several NetWare protocols use connection-oriented LLC services more widely.

SNAP header

The receiving system needs to determine which Network Layer protocol should receive the incoming data. 802.3 packets within LLC PDUs use another protocol called Sub-NetworkAccessProtocol (SNAP (Subnetwork 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 Manufacturer 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.

Negotiation sublayer

As stated earlier, Fast Ethernet is an evolved standard. The MAC designed for the AUI interface must be converted for the MII interface used in Fast Ethernet, which is what this sublayer is designed for.

Media Access Control (MAC)

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

The most important of the three MAC assignments is the first. For anyone network technology, which uses a shared medium, media access rules that determine when a node can transmit are its main characteristic. Several IEEE committees are involved in developing rules for access to the medium. 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 - multiple access with carrier sensing and 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.

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

CSMA/ CD

The Fast Ethernet MAC controller listens to 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 to the MAC. The presence of a carrier indicates that the medium is busy and the listening node (or nodes) must yield to the transmitting one.

A MAC that has a frame to transmit must wait some 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 regular Ethernet packet at a speed of 10 Mbit/s (IPG is a single time interval, always defined in microseconds, not in bit time) Figure 2.

Figure 2. Interpacket gap

After packet 1 ends, all LAN nodes are required to wait for IPG time before they can transmit. The time interval between packets 1 and 2, 2 and 3 in Fig. 2 is IPG time. After packet 3 has completed transmitting, no node has any material to process, so the time interval between packets 3 and 4 is longer than the IPG.

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

This is the CSMA portion of the Fast Ethernet media access rules. In short, many nodes have access to the medium and use the carrier to monitor its occupancy.

Early experimental networks used exactly these rules, and such networks worked very well. However, using only CSMA created a problem. Often two nodes, having a packet to transmit and waiting for the IPG time, began to transmit simultaneously, which led to data corruption on both sides. This situation is called collision(collision) or conflict.

To overcome this obstacle, early protocols used a fairly simple mechanism. Packets were divided into two categories: commands and reactions. Every command sent by a node required a response. If no response was received for some time (called the time-out period) after the command was sent, then the original command was issued again. This could happen several times (the maximum number of timeouts) before the sending node recorded the error.

This scheme could work perfectly, but only up to a certain point. The occurrence of conflicts resulted in a sharp decrease in performance (usually measured in bytes per second) because nodes were often idle waiting for responses to commands that never reached their destination. Network congestion and an increase in the number of nodes are directly related to an increase in the number of conflicts and, consequently, a decrease in network performance.

Early network designers quickly found a solution to this problem: each node must determine whether a transmitted packet has been lost by detecting a collision (rather than waiting for a response that never comes). This means that packets lost due to collision must be immediately retransmitted before the timeout expires. If the node transmitted the last bit of the packet without causing a collision, then the packet was transmitted successfully.

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

Figure 3. CSMA/CD operating algorithm

Physical layer device (PHY)

Since Fast Ethernet can use different type cable, each medium requires unique signal pre-conditioning. Conversion is also required for effective data transmission: to make the transmitted code resistant to interference, possible losses, or distortion of its individual elements (baud), to ensure effective synchronization of clock generators on the transmitting or receiving side.

Coding Sublayer (PCS)

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

Sublevels of physical connection and dependence on the physical environment (PMA and PMD)

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

Autonegotiation sublayer (AUTONEG)

The auto-negotiation sublayer allows two communicating ports to automatically select the most efficient operating mode: 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. The 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). Twisted data cable can be shielded or unshielded. Uses 4V/5V data encoding algorithm and MLT-3 physical encoding method.
• 100Base-FX - two cores of fiber optic cable. Transmission is also carried out in accordance with the Fiber Optic Communications Standard developed by ANSI. Uses 4V/5V data encoding algorithm and NRZI physical encoding method.

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

• 100Base-T4 is a specific 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 the 8V/6T data encoding algorithm and the NRZI physical encoding method.

Additionally, the Fast Ethernet standard includes recommendations for the use of Category 1 shielded twisted pair cable, which is the standard cable traditionally used in Token Ring networks. Support and guidance for using STP cabling on a Fast Ethernet network provides 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 configure itself to a data rate of 10 or 100 Mbit/s. This mechanism is based on the exchange of a series of packets with a hub or switch port.

100Base-TX environment

The 100Base-TX transmission medium uses two twisted pairs, with one pair used to transmit data and the other to receive it. Since the ANSI TP - PMD specification contains both shielded and unshielded twisted pair cables, the 100Base-TX specification includes support for both unshielded and shielded twisted pair cables, Types 1 and 7.

MDI (Medium Dependent Interface) connector

The 100Base-TX link interface, depending on the environment, can be one of two types. For unshielded twisted pair cabling, the MDI connector must be an eight-pin RJ 45 Category 5 connector. This connector is also used in 10Base-T networks, providing backward compatibility with existing Category 5 cabling. For shielded twisted pair cables, the MDI connector must be Use the IBM Type 1 STP connector, which is a shielded DB9 connector. This connector is usually 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 transmitting the positive (+) signal and the other wire transmitting the negative (-) signal. The color coding of cable wires and connector pin numbers for the 100Base-TX network are given in table. 1. Although the 100Base-TX PHY layer was developed after the adoption of the ANSI TP-PMD standard, the RJ 45 connector pin numbers were changed to match the wiring already used in the 10Base-T standard. 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 purpose. This layout allows the use of 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. To correctly connect the wires, you should be guided by their color markings.

Table 1. Connector pin assignmentsMDIcableUTP100Base-TX

Nodes communicate with each other by exchanging frames. In Fast Ethernet, a frame is the basic unit of communication over a network - any information transferred 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, each 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) can no two nodes in the world have the same MAC address. Any frame contains at least three main pieces of information: the recipient's address, the sender's 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;
• check sum frame(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 a unique number called a MAC address or host address. This number consists of 48 bits (6 bytes), is assigned to the network interface during device manufacture and is programmed during the initialization process. 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, different from all other MAC addresses on Earth and assigned by the manufacturer in agreement with IEEE.

To make the process of managing network interfaces easier, IEEE has proposed dividing 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 value of the flags determines how the address portion (bits 2 - 47) is interpreted.

Figure 5. MAC address format

The I/G bit is called individual/group address checkbox and shows what type of address (individual or group) it is. A unicast address is assigned to only one interface (or node) on a 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 group address can be assigned to one or more LAN network interfaces. Frames sent to a multicast address are received or copied by all LAN network interfaces that have it. Multicast addresses allow a frame to be sent to a subset of nodes on the local network. If the I/O bit is set to 1, then bits 46 through 0 are treated as a multicast address rather than as the U/L, OUI, and OUA fields of a regular address. The U/L bit is called universal/local control flag and determines how the address was assigned to the network interface. If both I/O and U/L bits are set to 0, then the address is the unique 48-bit identifier described earlier.

OUI (organizationally unique identifier - organizationally unique identifier). IEEE assigns one or more OUIs to each network adapter and interface manufacturer. Each manufacturer is responsible for the correct assignment of OUA (organizationally unique address - organizationally unique address), which any device created by him must have.

When the U/L bit is set, the address is locally controlled. This means that it is not set by the network interface manufacturer. 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 selected value. Network interface, having received the frame, first of all decodes the recipient address. When the I/O bit in an address is set, the MAC layer will only receive the frame if the destination address is in a list maintained by the host. This technique allows one node to send a frame to many nodes.

There is a special multipoint address called broadcast address. In a 48-bit 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 is used for two different purposes:

• to determine the length of the frame data field, excluding any padding by spaces;
• to indicate the data type in a data field.

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

In general, the L/T field is a historical remnant of Ethernet standardization in IEEE, which gave rise to a number of problems with the compatibility of equipment released before 1983. Now Ethernet and Fast Ethernet never use L/T fields. The specified field serves only to coordinate with the software that processes the frames (that is, with the protocols). But the only truly standard use for the L/T field is as a length field—the 802.3 specification doesn't 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 used privately. Use of these frames is outside the scope of this standard."

To summarize 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 length is specified by the value of the L/T field (L/T value 802.3, 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, the data field can contain almost any information, as long as its size is at least 46 and no more than 1500 bytes. Protocols determine how the contents of a data field are formatted and interpreted.

If it is necessary to send data less than 46 bytes in length, the LLC layer adds bytes with an unknown value, 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 sent, the L/T field stores the value 12, and the data field contains 34 additional non-significant bytes. The addition of non-significant bytes initiates the Fast Ethernet LLC layer, and is usually implemented in hardware.

MAC level facilities do not set the contents of the L/T field - this does software. Setting the value of this field is almost always done by the network interface driver.

Frame checksum

The frame checksum (PCS - Frame Check Sequence) allows you to ensure that the received frames are not damaged. When forming a 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 input of the MAC layer element that calculates the CRC is the values ​​of all bytes of the frame. The FCS field is the primary and most important error detection and correction mechanism in Fast Ethernet. Starting from the first byte of the recipient 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 encoding algorithm converts an eight-bit data octet (8B) into a six-bit ternary character (6T). 6T code groups are designed to be transmitted in parallel over three twisted pairs of cable, so the effective data transfer rate on each twisted pair is one third of 100 Mbps, that is, 33.33 Mbps. The ternary symbol rate on each twisted pair is 6/8 of 33.3 Mbps, which corresponds to a clock frequency of 25 MHz. This is the frequency at which the MP interface timer operates. 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) - is slightly similar to the NRZ code, but unlike the latter it has three signal levels. One corresponds to a transition from one signal level to another, and the change in signal level occurs sequentially, taking into account the previous transition. When transmitting “zero” the signal does not change. This code, like NRZ, requires pre-coding. Compiled from materials: Laem Queen, Richard Russell "Fast Ethernet"; K. Zakler "Computer networks"; V.G. and N.A. Olifer "Computer networks";

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 interface 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 that require efficient work high data rates 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 has high throughput (theoretically up to 264 MB/s), which fully satisfies the requirements of not only Fast Ethernet, but also the 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. The PC Card interface allows for easy connection of miniature expansion cards to a computer, and the exchange speed with these cards is quite high. However, more and more laptop computers are equipped with built-in network adapters, as network connectivity becomes an integral part of the standard feature set. These built-in adapters are again connected to the internal PCI bus computer.

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.

Other 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;
• possibility of installing microcircuits on the board permanent memory for remote boot (BootROM).
• 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 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.

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).

The segment (part of the network) can be a classic bus or a single subscriber. For bus segments, a coaxial cable is used, and for passive star beams (for connection to a hub single computers) – twisted pair and fiber optic cable. 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.

Rice. 7.1. Classic Ethernet network topology.

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.

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.

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.

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 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:

· 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.

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 transmit 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, viewing it as worthy competitor 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 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).

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.

Arcnet network

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 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.

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.

· Maximum cable length between active concentrators– 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.

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.

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 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 the English Fiber Distributed Data Interface, fiber-optic distributed data interface) is one of the latest developments in local network standards. 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 networks, the FDDI standard was initially focused on high transmission speeds (100 Mbit/s) and the use of the most promising fiber optic cable. Therefore, in this case, the developers were not constrained by the framework of old standards, focused on low speeds and electrical cable.

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 the attenuation of signals in the cable, but to the need to limit the time it takes for a signal to completely travel 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 in electric 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 function of converting electrical signals into optical signals 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). Maximum size network 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 port top level(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, useful information due to the use of the 5B/6B code 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.