Let's note the main features of the development of Ethernet networks and the transition to Fast Ethernet networks (IEEE 802.3u standard):
- - tenfold increase in throughput;
- - preservation of the random access method CSMA / CD;
- - preservation of the frame format;
- - support for traditional data transmission media.
The specified properties, as well as support for two speeds and auto-sensing 10/100 Mbit / s, built into network cards and Fast Ethernet switches, allow smooth transition from Ethernet networks to faster Fast Ethernet networks, providing an advantageous continuity over other technologies. Another additional factor for successful market penetration is the low cost of Fast Ethernet equipment.
Fast Ethernet architecture
Structure Fast levels Ethernet (including MII interface and Fast Ethernet transceiver) is shown in fig. 13. At the stage of development of the 100Base-T standard, the IEEE 802.3u committee determined that there is no universal signal coding scheme that would be ideal for all three physical interfaces (TX, FX, T4). Compared to the Ethernet standard, the encoding function (Manchester code) is performed by the physical signaling layer PLS (Fig. 5), which is located above the medium-independent interface AUI. In the Fast Ethernet standard, the encoding functions are performed by the PCS encoding sublayer located below the medium-independent MII interface. As a result, each transceiver must use its own set of coding schemes, the best way suitable for the corresponding physical interface, such as a 4V / 5V kit and NRZI for a 100Base-FX interface.
MII interface and Fast Ethernet transceivers. MII (medium independent interface) in Fast Ethernet is analogous to AUI in Ethernet. The MII interface provides communication between the negotiation and physical coding sublayers. Its main purpose is to simplify the use of different types of environments. MII interface assumes further connection of Fast Ethernet transceiver. A 40-pin connector is used for communication. The maximum distance over the MII interface cable should not exceed 0.5 m.
If the device has standard physical interfaces (for example, RJ-45), then the structure of the physical layer sublayers can be hidden inside the microcircuit with a large logic integration. In addition, deviations in the protocols of intermediate sublevels in a single device are permissible, with the main goal of increasing performance.
Physical interfaces Fast Ethernet
The Fast Ethernet IEEE 802.3u standard establishes three types of physical interface (Fig. 14, Table 6 Main characteristics of physical interfaces of the IEEE 802.3u Fast Ethernet standard): 100Base-FX, 100Base-TX and 100Base-T4.
100Base-FX. The standard for this fiber optic interface is completely identical to the FDDI PMD standard. The main optical connector of the 100Base-FX standard is Duplex SC. The interface allows a full duplex communication channel.
- * - the distance is reached only in duplex communication mode.
- 100Base-TX. The standard for this physical interface assumes the use of an unshielded twisted pair of category 5 or higher. It is completely identical to the FDDI UTP PMD standard. The physical RJ-45 port, as in the 10Base-T standard, can be of two types: MDI (network cards, workstations) and MDI-X (Fast Ethernet repeaters, switches). A single MDI port may be present on a Fast Ethernet repeater.
Pairs 1 and 3 are used for copper transmission. Pairs 2 and 4 are free. The RJ-45 port on the network card and on the switch can support, in addition to the 100Base-TX mode, the 10Base-T mode, or the auto-sensing function. Most modern network cards and switches support this function over RJ-45 ports and, in addition, can operate in full duplex mode.
100Base-T4. This type of interface allows providing a half-duplex communication channel over a twisted pair UTP сat. 3 and higher. It is the ability of an enterprise to migrate from Ethernet to Fast Ethernet without radical replacement of the existing cabling system based on UTP cat.3 that should be considered the main advantage of this standard.
Unlike the 100Base-TX standard, where only two twisted pairs of cable are used for transmission, the 100Base-T4 standard uses all four pairs. Moreover, when connecting a workstation and a repeater via a direct cable, data from the workstation to the repeater goes through twisted pairs 1, 3 and 4, and in the opposite direction - along pairs 2, 3 and 4, Pairs 1 and 2 are used to detect collisions like the Ethernet standard ... The other two pairs 3 and 4, alternately, depending on the commands, can pass the signal either in one direction or in the other direction. Transmitting a signal in parallel over three twisted pairs is equivalent to inverse multiplexing discussed in Chapter 5. The bit rate per channel is 33.33 Mbps.
Character coding 8B / 6T... If Manchester coding were used, the bit rate per twisted pair would be 33.33 Mbit / s, which would exceed the established 30 MHz limit for such cables. An effective reduction in the modulation frequency is achieved by using a ternary code instead of a direct (two-level) binary code. This code is known as 8B / 6T; this means that before transmission takes place, each set of 8 binary bits (character) is first converted according to certain rules into 6 triple (three-level) characters.
The 100Base-T4 interface has one significant drawback - the fundamental impossibility of supporting the duplex transmission mode. And if during the construction of small Fast Ethernet networks using 100Base-TX repeaters it has no advantages over 100Base-T4 (there is a collision domain, the bandwidth of which is not more than 100 Mbit / s), then during the construction of networks using switches, the lack of the 100Base-T4 interface becomes obvious and very serious. Therefore, this interface is not as widespread as 100Base-TX and 100Base-FX.
Fast Ethernet device types
The main categories of devices used in Fast Ethernet are the same as in Ethernet: transceivers; converters; network cards (for installation on workstations / file servers); repeaters; switches.
Transceiver- a two-port device covering the PCS, PMA, PMD and AUTONEG sublevels, and having, on the one hand, an MII interface, on the other, one of the environment-dependent physical interfaces (100Base-FX, 100Base-TX or 100Base-T4). Transceivers are used relatively rarely, as well as network cards, repeaters, switches with an MII interface are rarely used.
LAN card. The most widely used network cards today are 100Base-TX interface cards to the PCI bus. Optional, but highly desirable, RJ-45 port features include 100/10 Mbps autoconfiguration and full duplex support. Most of the current cards being released support these features. There are also network cards with 100Base-FX optical interface (manufacturers IMC, Adaptec, Transition Networks, etc.) - the main standard optical connector is SC (ST allowed) for multimode fiber.
Converter(media converter) is a two-port device, both ports of which represent media-dependent interfaces. Converters, unlike repeaters, can operate in full duplex mode, except for the case when there is a 100Base-T4 port. 100Base-TX / 100Base-FX converters are widespread. Due to the general growth trends of long-range broadband networks using single-mode FOC, consumption optical transceivers on a single-mode optical fiber has increased sharply in the last decade. Converter chassis that combine multiple separate 100Base-TX / 100Base-FX modules allow multiple converging fiber segments to be connected to a switch equipped with RJ-45 (100Base-TX) duplex ports.
Repeater. According to the parameter of maximum time delays during frame retransmission, Fast Ethernet repeaters are divided into two classes:
- - Class I. Delay on double run RTD should not exceed 130W. Due to less stringent requirements, repeaters of this class can have T4 and TX / FX ports, and can also be stacked.
- - Class II. Repeaters of this class have more stringent double run delay requirements: RTD
Switch- an important device of corporate networks. Most modern Fast Ethernet switches support 100/10 Mbit / s autoconfiguration over RJ-45 ports and can provide full duplex communication on all ports (except for 100Base-T4). Switches can have special additional slots for installing an up-link module. Optical ports such as Fast Ethernet 100Base-FX, FDDI, ATM (155 Mbit / s), Gigabit Ethernet, etc. can act as interfaces for such modules.
Large switch manufacturers Fast Ethernet companies are: 3Com, Bay Networks, Cabletron, DEC, Intel, NBase, Cisco, etc.
Today it is almost impossible to find a laptop or motherboard on sale without an integrated network card, or even two. All of them have one connector - RJ45 (more precisely, 8P8C), but the speed of the controller may differ by an order of magnitude. In cheap models it is 100 megabits per second (Fast Ethernet), in more expensive models - 1000 (Gigabit Ethernet).
If your computer does not have a built-in LAN controller, then it is most likely already an "old man" based on an Intel Pentium 4 processor 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 slot, since PCI Express buses did not exist at the time of their birth. But even for the PCI bus (33 MHz), network cards are produced that support the most current Gigabit Ethernet standard, although its bandwidth may not be enough to fully unleash the high-speed potential of a gigabit controller.
But even in the case of a 100-megabit integrated network card, a discrete adapter will have to be purchased by those who are going to "upgrade" to 1000 megabits. The best option purchase of a PCI Express controller, which will provide the maximum speed of the network, if, of course, the corresponding connector is present in the computer. True, many will give preference to a PCI card, since they are much cheaper (the cost starts literally from 200 rubles).
What are the practical benefits of switching from Fast Ethernet to Gigabit Ethernet? How different is the actual data transfer rate of PCI versions of network cards and PCI Express? Will the usual speed be enough hard disk for a full download of a gigabit channel? You will find the answers to these questions in this material.
Test participants
Three of the cheapest discrete network cards (PCI - Fast Ethernet, PCI - Gigabit Ethernet, PCI Express - Gigabit Ethernet) were selected for testing, since they are in the greatest demand.
The 100 Mbps PCI network card is represented by the Acorp L-100S model (the price starts at 110 rubles), which uses the Realtek RTL8139D chipset, the most popular for cheap cards.
The 1000 Mbps PCI network card is represented by the Acorp L-1000S model (the price starts from 210 rubles), which is based on the Realtek RTL8169SC chip. This is the only card with a heatsink on the chipset - the rest of the test participants do not need additional cooling.
The 1000 Mbps PCI Express network card is represented by the TP-LINK TG-3468 model (the price starts from 340 rubles). And it is no exception - it is based on the RTL8168B chipset, which is also produced by Realtek.
The appearance of the network card
Chipsets from these families (RTL8139, RTL816X) can be seen not only on discrete network cards, but also integrated on many motherboards.
The characteristics of all three controllers are shown in the following table:
Show table
The PCI bus bandwidth (1066 Mbit / s) should theoretically be enough to "swing" gigabit network cards to full speed, but in practice it may still not be enough. The point is that this "channel" is shared by all PCI devices; in addition, it transmits service information on the maintenance of the bus itself. Let's see if this assumption is confirmed by real speed measurements.
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 provide a full load of the gigabit channel of the network card, the speed of which is 125 megabytes per second (1000: 8 = 125). There are two ways to get around this limitation. The first is to combine a pair of such hard drives into a RAID array (RAID 0, striping), while the speed can almost double. The second is to use SSD-drives, the speed parameters of which are noticeably higher than those of hard drives.
Testing
A computer with the following configuration was used as a server:
- processor: AMD Phenom II X4 955 3200 MHz (quad-core);
- motherboard: ASRock A770DE AM2 + (AMD 770 + AMD SB700 chipset);
- RAM: Hynix DDR2 4 x 2048 GB PC2 8500 1066 MHz (in dual-channel mode);
- video card: AMD Radeon HD 4890 1024 MB DDR5 PCI Express 2.0;
- LAN card: Realtek RTL8111DL 1000 Mbps (integrated on the motherboard);
- operating system: Microsoft Windows 7 Home Premium SP1 (64-bit).
A computer with the following configuration was used as a client into which the tested network cards were installed:
- processor: AMD Athlon 7850 2800 MHz (dual core);
- motherboard: MSI K9A2GM V2 (MS-7302, AMD RS780 + AMD SB700 chipset);
- RAM: Hynix DDR2 2 x 2048 GB PC2 8500 1066 MHz (in dual channel mode);
- video card: AMD Radeon HD 3100 256 MB (integrated into the chipset);
- HDD: Seagate 7200.10 160GB SATA2;
- operating system: Microsoft Windows XP Home SP3 (32-bit version).
The tests were carried out in two modes: reading and writing via a network connection from hard disks (this should show that they can be a bottleneck), as well as from RAM disks in the RAM of computers imitating fast SSD-drives. The network cards were connected directly using a three-meter patch cord (eight-core twisted pair, category 5e).
Data transfer rate (hard disk - hard disk, Mbps)
The real data transfer rate through the 100-megabit Acorp L-100S network card did not quite reach the theoretical maximum. Although both gigabit cards outperformed the first one by about six times, they failed to show the maximum possible speed. It is clearly seen that the speed "rested" on the performance of Seagate 7200.10 hard drives, which, when directly tested on a computer, averages 79 megabytes per second (632 Mbps).
There is no fundamental difference in speed between network cards for the PCI bus (Acorp L-1000S) and PCI Express (TP-LINK) in this case, the insignificant advantage of the latter can be explained by the measurement error. Both controllers worked at about sixty percent of their capacity.
Data transfer rate (RAM disk - RAM disk, Mbps)
Acorp L-100S, as expected, showed the same low speed when copying data from high-speed RAM disks. It is understandable - the Fast Ethernet standard does not correspond to modern realities for a long time. Compared to the "hard drive-hard drive" test mode, the Acorp L-1000S Gigabit PCI card noticeably improved performance - the advantage was about 36 percent. An even more impressive lead was demonstrated by the TP-LINK TG-3468 network card - an increase of about 55 percent.
This is where the higher throughput of the PCI Express bus manifested itself - it outperformed the Acorp L-1000S by 14 percent, which can no longer be attributed to an error. The winner fell slightly short of the theoretical maximum, but the speed of 916 megabits per second (114.5 Mb / s) still looks impressive - this means that you will have to wait for the copying to finish almost an order of magnitude less (compared to Fast Ethernet). For example, the time to copy a 25 GB file (typical HD rip with good quality) from computer to computer will take less than four minutes, and with the 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 have only hard drives that are not combined into a striping array (RAID 0), then there will be no fundamental difference in speed between PCI and PCI Express cards. Otherwise, as well as when using high-performance SSD-drives, preference should be given to cards with the PCI Express interface, which will provide the highest possible data transfer rate.
Naturally, it should be borne in mind that other devices in the network "path" (switch, router ...) must support the Gigabit Ethernet standard, and the twisted pair (patch cord) category must be at least 5e. Otherwise, the real speed will remain at the level of 100 megabits per second. By the way, backward compatibility with the Fast Ethernet standard remains: for example, a laptop with a 100-megabit network card can be connected to a gigabit network; this will not affect the speed of other computers on the network.
The ComputerPress test laboratory tested 10/100 Mbit / s network cards for the PCI bus, designed for use in 10/100 Mbit / s workstations. The most common currently used cards were selected with throughput 10/100 Mbit / s, since, firstly, they can be used in Ethernet, Fast Ethernet and mixed networks, and, secondly, the promising Gigabit Ethernet technology (bandwidth up to 1000 Mbit / s) is still used more often just to connect powerful servers to the network equipment of the network core. It is extremely important what quality passive network equipment (cables, sockets, etc.) is used on the network. It is well known that while Category 3 twisted pair cable is sufficient for Ethernet networks, Category 5 is required for Fast Ethernet. Signal scattering, poor noise immunity can significantly reduce network bandwidth.
The purpose of testing was to determine, first of all, the index of effective performance (Performance / Efficiency Index Ratio - hereinafter P / E-index), and only then - the absolute value of the throughput. The P / E index is calculated as the ratio of the bandwidth of the network card in Mbps to the percentage of the CPU utilization. This index is the industry standard for determining performance network adapters... It was introduced in order to take into account the use by network cards of CPU resources. This is because some network adapter manufacturers try to maximize performance by using more CPU cycles on the computer to perform network operations. Low CPU usage and relatively high bandwidth are essential for running mission-critical business and multimedia applications, as well as real-time tasks.
We have tested the cards that are currently most often used for workstations in corporate and local networks:
- D-Link DFE-538TX
- SMC EtherPower II 10/100 9432TX / MP
- 3Com Fast EtherLink XL 3C905B-TX-NM
- Compex RL 100ATX
- Intel EtherExpress PRO / 100 + Management
- CNet PRO-120
- NetGear FA 310TX
- Allied Telesyn AT 2500TX
- Surecom EP-320X-R
The main characteristics of the tested network adapters are shown in Table. one . Let us explain some of the terms used in the table. Automatic detection of the connection speed means that the adapter itself determines the maximum possible speed of operation. In addition, if autosensing is supported, no additional configuration is required when switching from Ethernet to Fast Ethernet and vice versa. That is from system administrator no need to reconfigure the adapter and reload drivers.
Bus Master support allows data transfer directly between the network card and the computer memory. This frees up the central processor to perform other operations. This property has become the de facto standard. No wonder all known network cards support the Bus Master mode.
Remote wake-on (Wake on LAN) allows you to turn on the PC over the network. That is, it becomes possible to service the PC outside of working hours. For this purpose, three-pin connectors on the motherboard and network adapter are used, which are connected with a special cable (included in the delivery set). In addition, special control software is required. Wake on LAN technology is developed by the Intel-IBM alliance.
Full duplex mode allows data to be transmitted simultaneously in both directions, half duplex - in only one. Thus, the maximum possible throughput in full duplex mode is 200 Mbps.
DMI (Desktop Management Interface) provides the ability to obtain information about the configuration and resources of the PC using network management software.
Support for the WfM (Wired for Management) specification enables a network adapter to interact with network management and administration software.
To remotely boot a computer's OS over a network, network adapters are supplied with a special BootROM memory. This allows for efficient use of diskless workstations on the network. Most tested cards only had a BootROM slot; the BootROM itself is usually a separately ordered option.
ACPI (Advanced Configuration Power Interface) support helps reduce power consumption. ACPI is new technology that ensures the operation of the power management system. It is based on the use of both hardware and software tools... Basically, Wake on LAN is an integral part of ACPI.
Proprietary means of increasing productivity can increase the efficiency of the network card. The most famous of them are Parallel Tasking II by 3Com and Adaptive. Technology company Intel. These funds 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 diskette with drivers and the ability to download the latest drivers from the company's website. Packaging also plays an important role. From this point of view, the best, in our opinion, are D-Link, Allied Telesyn and Surecom network adapters. But in general, the level of support was satisfactory for all cards.
Typically, the warranty covers the entire life of the power adapter (lifetime warranty). Sometimes it is limited to 1-3 years.
Testing methodology
All tests used the latest NIC drivers downloaded from the respective vendors' Internet servers. In the case when the driver of the network card allowed any adjustments and optimizations, the default settings were used (except for the Intel network adapter). Note that the richest additional features and features are available in cards and related drivers from 3Com and Intel.
Performance was measured using Novell's Perform3 utility. The principle of operation of the utility is that a small file is copied from a workstation to a shared one. network drive server, after which it remains in the file cache of the server and is read from there repeatedly within a specified period of time. This allows for memory-to-memory-to-memory interactions and eliminates the impact of disk latency. The utility parameters include initial file size, final file size, resizing step, and test time. Novell Perform3 utility displays performance values with 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 - 8192 bytes
The test time with each file was set to twenty seconds.
Each experiment used a pair of identical network cards, one running on a server and the other running on a workstation. This does not seem to be in line with common practice, since servers usually use specialized network adapters with a number of additional features. But this is exactly how - the same network cards are installed both on the server and on 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 room Microsoft system Windows NT Server 4.0 c 6 a SP
- TCP / IP protocol.
Compaq DeskPro EP server configuration:
- Celeron 400 MHz processor
- RAM 64 MB
- hard drive 4,3 GB
- operating system Microsoft Windows NT Workstation 4.0 c c 6 a SP
- TCP / IP protocol.
Testing was conducted with computers connected directly with a UTP Category 5 crossover cable. During these tests, the cards were operating in 100Base-TX Full Duplex mode. In this mode, the throughput turns out to be somewhat higher due to the fact that part of the service information (for example, acknowledgment of receipt) is transmitted simultaneously with the useful information, the amount of which is estimated. In these conditions, it was possible to fix rather high values of the throughput; for example, 3Com Fast EtherLink XL 3C905B-TX-NM adapter averages 79.23 Mbps.
The processor load was measured on the server using the Windows NT Performance Monitor utility; the data was written to a log file. The Perform3 utility was run on the client so as not to affect the server processor load. Intel Celeron was used as the processor of the server computer, the performance of which is significantly lower than the performance of Pentium II and III processors. Intel Celeron was used intentionally: the fact is that, since the processor load is determined with a sufficiently large absolute error, in the case of large absolute values, the relative error turns out to be smaller.
After each test, Perform3 utility places the results of its work in a text file as a dataset 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.
The file size is displayed, 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 throughout the test. The resulting average values for each test were converted from KB / s to Mbit / s using the formula:
(KB x 8) / 1024,
and the value of the P / E index was calculated as the ratio of the throughput to the processor load as a percentage. Subsequently, the average value of the P / E index was calculated based on the results of three measurements.
Using the Perform3 utility on Windows NT Workstation, the following problem arose: in addition to writing to a network drive, the file was also written to the local file cache, from which it was subsequently read very quickly. The results were impressive, but unrealistic, as there was no data transfer per se over the network. To enable applications to treat shared network drives as normal local drives, the operating system uses a special network component - a redirector that redirects I / O requests over the network. Under normal operating conditions, when executing the procedure for writing a file to a shared network drive, the redirector uses the Windows NT caching algorithm. That is why, when writing to the server, it also writes to the local file cache of the client machine. And for testing, it is necessary that caching is carried out only on the server. To prevent caching on the client computer, the values of the parameters in the Windows NT registry were changed, which made it possible to disable the caching performed by the redirector. Here's how it was done:
- Registry path:
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)
- Registry path:
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
The card's throughput and processor utilization are nearly the same as that of 3Com. The windows for setting the parameters of this map are shown below.
The new Intel 82559 controller in 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 collisions and increases throughput. With a low network load, when the probability of collisions is low, the time intervals between packets are reduced, which also leads to increased performance. The advantages of this method should be most pronounced in large collisional Ethernet segments, that is, in those cases when hubs rather than switches prevail in the network topology.
New Intel technology, called Priority Packet, allows you to regulate the traffic passing through the network card according to the priorities of individual packets. This provides the ability to increase data transfer rates for mission-critical applications.
VLAN support is provided (IEEE 802.1Q standard).
There are only two indicators on the board - work / connection, speed 100.
www.intel.com
SMC EtherPower II 10/100 SMC9432TX / MP Network Adapter
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. Comparing the test results for cards from these two manufacturers, we can conclude about the degree of efficiency of these technologies implementation. Note also that this network card showed the third result in terms of performance and P / E index, outperforming all cards except 3Com and Intel.
There are four LED indicators on the card: speed 100, transmission, connection, duplex.
The company's main Web site is www.smc.com
The most widespread among standard networks is the Ethernet network. It appeared in 1972 and became the international standard in 1985. It was adopted by the largest international standards organizations: IEEE 802 Committee (Institute of Electrical and Electronic Engineers) and ECMA (European Computer Manufacturers Association).
The standard was named IEEE 802.3 (in English it reads as "eight oh two dot three"). It defines multiple access to a mono channel of the bus type with collision detection and transmission control, that is, with the already mentioned CSMA / CD access method.
Key features of the original IEEE 802.3 standard:
· Topology - bus;
· Transmission medium - coaxial cable;
· Transmission speed - 10 Mbit / s;
· Maximum network length - 5 km;
· The maximum number of subscribers - up to 1024;
· The length of the network segment - up to 500 m;
· The number of subscribers on one segment - up to 100;
· Access method - CSMA / CD;
· Transmission is narrowband, that is, without modulation (mono channel).
Strictly speaking, there are minor differences between the IEEE 802.3 and Ethernet standards, but they usually prefer not to be remembered.
Ethernet is now the most popular in the world (over 90% of the market) and it is expected to remain so in the coming years. This was largely facilitated by the fact that from the very beginning the characteristics, parameters, protocols of the network were open, as a result of which a huge number of manufacturers around the world began to produce Ethernet equipment that is fully compatible with each other.
In a classic Ethernet network, a 50-ohm coaxial cable of two types (thick and thin) was used. However, in recent years (since the beginning of the 90s), the most widespread version of the Ethernet is using twisted pairs as a transmission medium. A standard has also been defined for the use of fiber optic cable in a network. Additions have been made to the original IEEE 802.3 standard to accommodate these changes. In 1995, an additional standard appeared for a faster version of Ethernet, operating at a speed of 100 Mbit / s (the so-called Fast Ethernet, IEEE 802.3u standard), using twisted pair or fiber-optic cable as the transmission medium. In 1997, a version with a speed of 1000 Mbit / s appeared (Gigabit Ethernet, IEEE 802.3z standard).
In addition to the standard bus topology, passive star and passive tree topologies are increasingly being used. This assumes the use of repeaters and repeater hubs connecting different parts (segments) of the network. As a result, a tree structure can be formed on segments of different types (Figure 7.1).
A segment (part of a network) can be a classic bus or a single subscriber. For bus segments, coaxial cable is used, and for passive star beams (for connecting single computers to a hub), twisted pair and fiber optic cables are used. The main requirement for the resulting topology is that there are no closed paths (loops) in it. In fact, it turns out that all subscribers are connected to a physical bus, since the signal from each of them propagates in all directions at once and does not return back (as in a ring).
Maximum length the network cable as a whole (maximum signal path) can theoretically reach 6.5 kilometers, but practically does not exceed 3.5 kilometers.
Rice. 7.1. Classic Ethernet network topology.
Fast Ethernet does not have a physical bus topology, only a passive star or passive tree is used. In addition, Fast Ethernet has much more stringent requirements for the maximum network length. Indeed, when the transmission speed is increased by 10 times and the format of the packet is preserved, its minimum length becomes ten times shorter. Thus, the permissible value of the double signal transit time through the network is reduced by 10 times (5.12 μs versus 51.2 μs in Ethernet).
The standard Manchester code is used to transmit information on the Ethernet network.
Access to the Ethernet network is carried out using a random CSMA / CD method, which ensures the equality of subscribers. The network uses packets of variable length.
For an Ethernet network operating at a speed of 10 Mbit / s, the standard defines four main types of network segments, focused on different media:
10BASE5 (thick coaxial cable);
10BASE2 (thin coaxial cable);
10BASE-T (twisted pair);
10BASE-FL (fiber optic cable).
The segment name includes three elements: the number "10" means the transmission rate of 10 Mbit / s, the BASE word - transmission in the main frequency band (that is, without modulation of the high-frequency signal), and the last element - the allowable segment length: "5" - 500 meters, "2" - 200 meters (more precisely, 185 meters) or the type of communication line: "T" - twisted pair (from English "twisted-pair"), "F" - fiber optic cable (from English "fiber optic").
Likewise, for an Ethernet network operating at a speed of 100 Mbps (Fast Ethernet), the standard defines three types of segments, differing in the types of transmission media:
· 100BASE-T4 (twisted pair);
100BASE-TX (double twisted pair);
· 100BASE-FX (fiber optic cable).
Here, the number "100" means a transmission rate of 100 Mbit / s, the letter "T" is a twisted pair, the letter "F" is a fiber-optic cable. The types 100BASE-TX and 100BASE-FX are sometimes combined under the name 100BASE-X, and 100BASE-T4 and 100BASE-TX under the name 100BASE-T.
Token-Ring network
The Token-Ring (token ring) network was proposed by IBM in 1985 (the first option appeared in 1980). It was designed to network all types of computers made by IBM. The very fact that it is supported by IBM, largest manufacturer computer technology, suggests that it needs to be given special attention. But no less important is the fact that Token-Ring is currently the international standard IEEE 802.5 (although there are minor differences between Token-Ring and IEEE 802.5). This puts this network on the same level as Ethernet in status.
Developed by Token-Ring as a reliable alternative to Ethernet. Although Ethernet is now superseding all other networks, Token-Ring is not hopelessly obsolete. More than 10 million computers worldwide are connected by this network.
The Token-Ring network has a ring topology, although it looks more like a star in appearance. This is due to the fact that individual subscribers (computers) are not connected to the network directly, but through special hubs or multi-station access devices (MSAU or MAU - Multistation Access Unit). Physically, the network forms a star-ring topology (Figure 7.3). In reality, the subscribers are nevertheless united in a ring, that is, each of them transmits information to one neighboring subscriber, and receives information from another.
Rice. 7.3. Star-ring token-ring network topology.
At first, twisted pair, both unshielded (UTP) and shielded (STP), were used as a transmission medium in the IBM Token-Ring network, but then there were options for equipment for coaxial cable, as well as for fiber-optic cable in the FDDI standard.
The main specifications classic version of the Token-Ring network:
· The maximum number of concentrators such as IBM 8228 MAU - 12;
· The maximum number of subscribers in the network - 96;
· Maximum cable length between the subscriber and the hub - 45 meters;
· Maximum cable length between hubs - 45 meters;
· The maximum length of the cable connecting all the hubs is 120 meters;
· Data transfer rate - 4 Mbit / s and 16 Mbit / s.
All specifications are based on the use of an unshielded twisted pair cable. If a different transmission medium is used, the characteristics of the network may differ. For example, when using shielded twisted pair (STP), the number of subscribers can be increased to 260 (instead of 96), the cable length - up to 100 meters (instead of 45), the number of hubs - up to 33, and the total length of the ring connecting the hubs - up to 200 meters ... Fiber optic cable allows to extend the cable length up to two kilometers.
To transfer information in Token-Ring, a biphase code is used (more precisely, its version with a mandatory transition in the center of the bit interval). As with any star topology, no additional electrical termination or external grounding is required. Negotiation is performed by the hardware of the network adapters and hubs.
Token-Ring cables use RJ-45 (unshielded twisted pair) connectors, MIC and DB9P connectors. The wires in the cable connect the same pins of the connectors (that is, the so-called "straight" cables are used).
The Token-Ring network in the classic version is inferior to the Ethernet network both in the allowable size and in the maximum number of subscribers. In terms of transmission speed, there are currently 100 Mbps (High Speed Token-Ring, HSTR) and 1000 Mbps (Gigabit Token-Ring) versions of Token-Ring. Companies that support Token-Ring (including IBM, Olicom, Madge) do not intend to abandon their network, seeing it as a worthy competitor to Ethernet.
Compared to Ethernet hardware, Token-Ring hardware is noticeably more expensive, since it uses a more complex method of exchange control, so the Token-Ring network is not so widespread.
However, unlike Ethernet, Token-Ring network maintains a high load level much better (more than 30-40%) and provides guaranteed access time. This is necessary, for example, in industrial networks, where a delay in reaction to an external event can lead to serious accidents.
The Token-Ring network uses the classic token access method, that is, a token constantly circulates around the ring, to which subscribers can attach their data packets (see Fig. 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 network operation on each subscriber (in the event of a malfunction, the subscriber must be excluded from the ring).
The maximum packet transfer time in Token-Ring is 10 ms. With a maximum number of 260 subscribers, the full cycle of the ring will be 260 x 10 ms = 2.6 s. During this time, all 260 subscribers will be able to transfer their packages (if, of course, they have something to transfer). During this time, a free marker will surely reach every subscriber. This interval is also the upper limit of the Token-Ring access time.
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 the Datapoint Corporation back in 1977. There are no international standards for this network, although it is she who is considered the ancestor of the token access method. Despite the lack of standards, the Arcnet network until recently (in 1980 - 1990) was popular, even seriously competing with Ethernet. A large number of companies produced equipment for this type of network. But now the production of Arcnet equipment has practically ceased.
Among the main advantages of the Arcnet network in comparison with Ethernet are the limited amount of access time, high reliability of communication, ease of diagnostics, as well as the relatively low cost of adapters. The most significant disadvantages of the network include low data transfer rate (2.5 Mbit / s), addressing system and packet format.
To transmit information in the Arcnet network, a rather rare code is used, in which a logical one corresponds to two pulses during a bit interval, and a logical zero corresponds to one pulse. Obviously this is self-timing code that requires even more cable bandwidth than even Manchester's.
The network uses a coaxial cable as a transmission medium. wave impedance 93 Ohm, for example, brand RG-62A / U. Twisted pair options (shielded and unshielded) are not widely used. Fiber optic options have been proposed, but they haven't saved Arcnet either.
As a topology, the Arcnet network uses the classic bus (Arcnet-BUS) as well as a passive star (Arcnet-STAR). Hubs are used in the star. It is possible to combine bus and star segments into a tree topology using hubs (as in Ethernet). The main limitation is that there should be no closed paths (loops) in the topology. Another limitation is that the number of daisy chained segments using hubs must not exceed three.
Thus, the topology of the Arcnet network is as follows (Figure 7.15).
Rice. 7.15. Arcnet network topology of bus type (B - adapters for working in the bus, S - adapters for working in a star).
The main technical characteristics of the Arcnet network are as follows.
· Transmission medium - coaxial cable, twisted pair.
· The maximum length of the network is 6 kilometers.
· The maximum cable length from the subscriber to the passive concentrator is 30 meters.
· The maximum cable length from the subscriber to the active concentrator is 600 meters.
· The maximum cable length between active and passive hubs is 30 meters.
· The maximum cable length between active hubs is 600 meters.
· The maximum number of subscribers in the network is 255.
· The maximum number of subscribers on the bus segment is 8.
· The minimum distance between subscribers in the bus is 1 meter.
· The maximum length of a bus segment is 300 meters.
· Data transfer rate - 2.5 Mbit / s.
When creating complex topologies, it is necessary to ensure that the delay in the propagation of signals in the network between subscribers does not exceed 30 μs. The maximum attenuation of the signal in the cable at a frequency of 5 MHz should not exceed 11 dB.
The Arcnet network uses a token access method (transfer of rights), but it differs slightly from that of Token-Ring. This method is closest to the one provided in the IEEE 802.4 standard.
As with Token-Ring, conflicts are completely eliminated in Arcnet. Like any token network, Arcnet holds the load well and guarantees the amount of network access time (as opposed to Ethernet). The total round trip time of all subscribers by the marker is 840 ms. Accordingly, the same interval determines the upper limit of the network access time.
The token is formed by a special subscriber - the network controller. It is the subscriber with the minimum (zero) address.
FDDI network
The FDDI network (from the English Fiber Distributed Data Interface, fiber-optic distributed data interface) is one of the latest developments local area network standards. The FDDI standard was proposed by the American National Standards Institute ANSI (ANSI X3T9.5 specification). Then the ISO 9314 standard was adopted, corresponding to the ANSI specifications. The level of network standardization is quite high.
Unlike other standard local area networks, the FDDI standard was initially focused on high transmission rates (100 Mbit / s) and on the use of the most promising fiber-optic cable. Therefore, in this case, the developers were not constrained by the framework of the old standards, focused on low speeds and an electric cable.
The choice of fiber as a transmission medium determined such advantages new network, as high noise immunity, maximum secrecy of information transmission and excellent galvanic isolation of subscribers. The high transmission speed, which is much easier to achieve in the case of fiber-optic cable, allows you to solve many problems that are not available on lower-speed networks, for example, transferring images in real time. In addition, fiber optic cable easily solves the problem of transmitting data over a distance of several kilometers without retransmission, which makes it possible to build large networks, covering even entire cities and having all the advantages of local networks (in particular, a low error rate). All this determined the popularity of the FDDI network, although it is not yet as widespread as Ethernet and Token-Ring.
The FDDI standard was based on the token access method provided by the international standard IEEE 802.5 (Token-Ring). Insignificant differences from this standard are determined by the need to provide a high speed of information transmission over long distances. FDDI network topology is a ring, the most suitable topology for fiber optic cable. The network uses two multidirectional fiber-optic cables, one of which is usually in reserve, but this solution also allows the use of full-duplex information transmission (simultaneously in two directions) with twice the effective speed of 200 Mbit / s (with each of the two channels operating at a speed 100 Mbps). A star-ring topology with hubs included in the ring (as in Token-Ring) is also used.
Basic technical characteristics of the FDDI network.
· The maximum number of network subscribers is 1000.
· The maximum length of the network ring is 20 kilometers.
· The maximum distance between network subscribers is 2 kilometers.
· Transmission medium - multimode fiber-optic cable (electrical twisted pair can be used).
· Access method - marker.
· Information transfer rate - 100 Mbit / s (200 Mbit / s for duplex transmission mode).
The FDDI standard has significant advantages over all the previously discussed networks. For example, a Fast Ethernet network with the same bandwidth of 100 Mbps cannot match FDDI in terms of network size. In addition, the FDDI token access method provides, unlike CSMA / CD, guaranteed access time and the absence of conflicts at any load level.
The limitation on the total network length of 20 km is associated not with the attenuation of signals in the cable, but with the need to limit the time for the complete passage of the signal along the ring to ensure the maximum allowable access time. But the maximum distance between subscribers (2 km with a multimode cable) is determined precisely by the attenuation of the signals in the cable (it should not exceed 11 dB). The possibility of using a single-mode cable is also provided, in which case the distance between subscribers can reach 45 kilometers, and the total length of the ring is 200 kilometers.
There is also an implementation of FDDI on an electrical cable (CDDI - Copper Distributed Data Interface or TPDDI - Twisted Pair Distributed Data Interface). This uses a Category 5 cable with RJ-45 connectors. The maximum distance between subscribers in this case should be no more than 100 meters. The cost of network equipment on an electric cable is several times less. But this version of the network no longer has such obvious advantages over competitors as the original fiber-optic FDDI. The electrical versions of FDDI are much less standardized than the fiber optic ones, so the compatibility of equipment from different manufacturers is not guaranteed.
For data transmission in FDDI, a 4V / 5V code is used, specially developed for this standard.
To achieve high network flexibility, the FDDI standard provides for the inclusion of two types of subscribers in the ring:
· Subscribers (stations) of class A (subscribers of dual connection, DAS - Dual-Attachment Stations) are connected to both (internal and external) rings of the network. At the same time, the possibility of exchange at a speed of up to 200 Mbit / s or redundancy of the network cable is realized (if the main cable is damaged, the reserve cable is used). Equipment of this class is used in the most critical parts of the network from the point of view of performance.
· Class B subscribers (stations) (subscribers of a single connection, SAS - Single-Attachment Stations) are connected to only one (external) ring of the network. They are simpler and cheaper than class A adapters, but lack their capabilities. They can be connected to the network only through a hub or bypass switch, which turns them off in case of an emergency.
In addition to the actual subscribers (computers, terminals, etc.), the network uses Wiring Concentrators, the inclusion of which allows you to collect all connection points in one place in order to monitor the operation of the network, diagnose faults and simplify reconfiguration. When using different types of cables (for example, fiber-optic cable and twisted pair), the hub also performs the function of converting electrical signals into optical ones and vice versa. Hubs are also available as Dual-Attachment Concentrator (DAC) and Single-Attachment Concentrator (SAC).
An example of an FDDI network configuration is shown in Fig. 8.1. The principle of combining network devices is illustrated in Figure 8.2.
Rice. 8.1. An example of an FDDI network configuration.
Unlike the access method offered by the IEEE 802.5 standard, FDDI uses what is known as multiple token passing. If, in the case of Token-Ring network, a new (free) token is transmitted by the subscriber only after returning his packet to him, then in FDDI a new token is transmitted by the subscriber immediately after the end of his transmission of the packet (similar to how it is done with the ETR method in the Token- Ring).
In conclusion, it should be noted that despite the obvious advantages of FDDI this network did not become widespread, which is mainly due to the high cost of its equipment (on the order of several hundred and even thousands of dollars). The main area of application of FDDI today is backbone networks that connect multiple networks. FDDI is also used to connect powerful workstations or servers that require high-speed communication. It is assumed that the Fast Ethernet network may overtake FDDI, but the advantages of fiber optic cable, token control method and record allowable network size currently put FDDI out of competition. And in cases where hardware cost is critical, twisted-pair version of FDDI (TPDDI) can be used in non-critical areas. In addition, the cost of FDDI equipment can greatly decrease with the growth of its production volume.
100VG-AnyLAN network
100VG-AnyLAN is one of the latest high-speed local area networks that has recently entered the market. It complies with the international standard IEEE 802.12, so the level of its standardization is quite high.
Its main advantages are a high exchange rate, a relatively low cost of equipment (about twice as expensive as the equipment of the most popular Ethernet 10BASE-T network), a centralized method of managing exchange without conflicts, as well as compatibility at the level of packet formats with Ethernet networks and Token-Ring.
In the name of the 100VG-AnyLAN network, the number 100 corresponds to a speed of 100 Mbps, the letters VG denote a cheap unshielded twisted pair cable of category 3 (Voice Grade), and AnyLAN (any network) denotes that the network is compatible with the two most common networks.
The main technical characteristics of the 100VG-AnyLAN network:
· Transfer rate - 100 Mbps.
· Topology - a star with the possibility of extension (tree). The number of cascading levels of concentrators (hubs) is up to 5.
· Access method - centralized, conflict-free (Demand Priority - with a priority request).
· The transmission medium is quad unshielded twisted pair (UTP category 3, 4, or 5 cables), double twisted pair (UTP category 5 cable), double shielded twisted pair (STP), and fiber optic cable. Nowadays, quad twisted pair is mainly widespread.
· The maximum cable length between the hub and the subscriber and between the hubs is 100 meters (for UTP category 3 cable), 200 meters (for UTP category 5 cable and shielded cable), 2 kilometers (for fiber optic cable). Maximum possible size networks - 2 kilometers (determined by permissible delays).
· Maximum number of subscribers - 1024, recommended - up to 250.
Thus, the parameters of the 100VG-AnyLAN network are quite close to those of the Fast Ethernet network. However, the main advantage of Fast Ethernet is full compatibility with the most common Ethernet network (in the case of 100VG-AnyLAN, this requires a bridge). In the same time, centralized management 100VG-AnyLAN, which eliminates conflicts and guarantees a limit value for access time (which is not provided in an Ethernet network), also cannot be discounted.
An example of the structure of a 100VG-AnyLAN network is shown in Fig. 8.8.
The 100VG-AnyLAN network consists of a central (main, root) layer 1 hub, to which both individual subscribers and layer 2 hubs can be connected, to which subscribers and layer 3 hubs, etc. are connected, etc. Moreover, the network can have no more than five such levels (in the original version there were no more than three). The maximum network size can be 1000 meters for an unshielded twisted pair cable.
Rice. 8.8. Network structure 100VG-AnyLAN.
Unlike non-intelligent hubs on other networks (e.g. Ethernet, Token-Ring, FDDI), 100VG-AnyLAN hubs are intelligent controllers that control network access. To do this, they continuously monitor requests coming to all ports. Hubs accept incoming packets and send them only to the subscribers to whom they are addressed. However, they do not perform any information processing, that is, in this case, it is still not an active, but not a passive star. Hubs cannot be called full-fledged subscribers.
Each of the hubs can be configured to accept Ethernet or Token-Ring packet formats. At the same time, the hubs of the entire network must work with packets of only one format. Bridging is required to communicate with Ethernet and Token-Ring networks, but bridging is fairly simple.
Hubs have one port top level(for connecting it to a higher-level hub) and several lower-level ports (for connecting subscribers). A computer ( work station), server, bridge, router, switch. Another hub can also be connected to the lower port.
Each port of the hub can be set to one of two possible modes of operation:
· Normal mode assumes forwarding to the subscriber connected to the port only packets addressed to him personally.
· Monitor mode assumes forwarding to the subscriber connected to the port of all packets arriving at the hub. This mode allows one of the subscribers to control the operation of the entire network as a whole (to perform the monitoring function).
The 100VG-AnyLAN access method is typical for star networks.
When using a quad twisted pair, each of the four twisted pairs is transmitted at a speed of 30 Mbps. The total transfer rate is 120 Mbps. but helpful information due to the use of the 5B / 6B code, it is transmitted at a speed of only 100 Mbit / s. Thus, the bandwidth of the cable must be at least 15 MHz. Category 3 twisted pair cable (bandwidth - 16 MHz) meets this requirement.
Thus, the 100VG-AnyLAN network is an affordable solution for increasing the transmission speed up to 100 Mbps. However, it does not have full compatibility with any of the standard networks, so its further fate is problematic. In addition, unlike the FDDI network, it does not have any record parameters. Most likely, 100VG-AnyLAN, despite the support of reputable companies and a high level of standardization, will remain just an example of interesting technical solutions.
If we talk about the most common 100 Mbps Fast Ethernet network, then 100VG-AnyLAN provides twice the length of UTP Category 5 cable (up to 200 meters), as well as a conflict-free method of exchange control.
Ethernet, but also to the equipment of other, less popular networks.Ethernet and Fast Ethernet Adapters
Adapter characteristics
Network adapters (NIC, Network Interface Card) Ethernet and Fast Ethernet can interface with a computer through one of the standard interfaces:
- ISA bus (Industry Standard Architecture);
- 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 that produced such adapters was large, which is why the devices of this type were the cheapest. ISA adapters are available in 8- and 16-bit. 8-bit adapters are cheaper, while 16-bit adapters are faster. True, the exchange of information via 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 baud rates for efficient operation, are practically not produced for this system bus. The ISA bus is a thing of the past.
The PCI bus has now practically supplanted the ISA bus and is becoming the main expansion bus for computers. It provides 32- and 64-bit data exchange and has a high 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 PCs, but also in PowerMac computers. In addition, it supports Plug-and-Play automatic hardware configuration. Apparently, in the near future, the majority of network adapters... The disadvantage of PCI in comparison with the ISA bus is that the number of its expansion slots in a computer, as a rule, is small (usually 3 slots). But it is precisely network adapters connect to PCI first.
The PC Card bus (the old name PCMCIA) is used so far only in notebook computers of the Notebook class. In these computers, the internal PCI bus is usually not routed out. The PC Card interface provides a simple connection to a computer of miniature expansion cards, and the exchange rate with these cards is quite high. However, more and more laptop computers equipped with built-in network adapters as the ability to access the network becomes an integral part of the standard set of functions. 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. It is also necessary to evaluate the laboriousness of installing the purchased adapter and the prospects for the release of boards of this type. The latter may be needed in the event of an adapter failure.
Finally, there are more network adapters connecting to the computer via the parallel (printer) LPT port. The main advantage of this approach is that you do not need to open the computer case to connect the adapters. In addition, in this case, the adapters do not occupy the system resources of the computer, such as interrupt channels and DMAs, as well as addresses of memory 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, more and more computers are found in which network adapters embedded in system board... The advantages of this approach are obvious: the user does not have to buy a network adapter and install it in a computer. You just need to connect network cable to an external connector on your computer. However, the disadvantage is that the user cannot select the adapter with the best performance.
To others essential characteristics network adapters can be attributed:
- way to configure the adapter;
- size of the board buffer memory and modes of exchange with it;
- the ability to install a permanent memory chip on the board 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);
- used by the adapter transmission speed over the network and the presence of the function of its switching;
- the possibility of using the adapter of the full-duplex exchange mode;
- compatibility of the adapter (more precisely, the adapter driver) with the network software used.
User configuration of the adapter was mainly used for adapters designed for the ISA bus. Configuration implies tuning to the use of computer system resources (I / O addresses, interrupt channels and direct memory access, buffer memory and remote boot memory). Configuration can be carried out by setting the switches (jumpers) to the desired position or using the DOS configuration program supplied with the adapter (Jumperless, Software configuration). When launching 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 powered on. Modern adapters usually support this mode, so they can be easily installed by the user.
In the simplest adapters, the exchange with the adapter's internal buffer memory (Adapter RAM) is carried out through the address space of the I / O devices. In this case, no additional configuration of memory addresses is required. The base address of the shared memory buffer must be specified. It is assigned to the area of the computer's upper memory (
