Access Methods

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Access Methods

Access method is the term given to the set of rules by which networks arbitrate the use of a common medium. It is the way the LAN keeps different streams of data from crashing into each other as they share the network.

Networks need access methods for the same reason streets need traffic lights-to keep people from hitting each other. Think of the access method as traffic law. The network cable is the street. Traffic law (or the access method) regulates the use of the street (or cable), determining who can drive (or send data) where and at what time. On a network, if two or more people try to send data at exactly the same time, their signals will interfere with each other, ruining the data being transmitted. The access method prevents this.

The access method works at the data-link layer (layer 2) because it is concerned with the use of the medium that connects users. The access method doesn't care what is being sent over the network, just like the traffic law doesn't stipulate what you can carry. It just says you have to drive on the right side of the road and obey the traffic lights and signs.

Three traditional access methods are used today, although others exist and may become increasingly important. They are Ethernet, Token Ring, and ARCnet. Actually, these technologies encompass wider- ranging standards than their access methods. They also define other features of network transmission, such as the electrical characteristics of signals, and the size of data packets sent. Nevertheless, these standards are best known by the access methods they employ .

Ethernet

Ethernet is the most common network access method. It was developed by Xerox in the mid-1970s. It describes data transmission at 10Mbits/sec, 100Mbits/sec, 1Gbits/sec, and other throughput rates using the CSMA/CD protocol. Ethernet gained its popularity in engineering, scientific, and university environments. The shipments of Ethernet interface adapters grew substantially faster than those of Token Ring through the 1990s. ARCnet unit shipments have declined, and little new development of ARCnet-based solutions should be expected. Flurries of interest in FDDI and ATM for high speed backbones occurred in the late 1990s, but the multiple flavors of Ethernet dominate the great majority of LAN environments at the turn of the millennium .

The Ethernet access method is Carrier Sense Multiple Access/Collision Detection (CSMA/CD). This is a broadcast access method, which means every computer "hears" every transmission, but not every computer "listens" to every transmission.

Here's how CSMA/CD works. When a computer wants to send a message it does, as long as the cable isn't in use by another transmitting node. (This is the carrier sense part.) The signal it sends moves up and down the cable in every direction, passing every computer on the network segment. (This is multiple access.) Every computer can hear the message, but unless the message is addressed to it, the computer ignores it. Only the computer to which the message is addressed receives the message. The message is recognized because it contains the address of the destination computer.

A "collision" occurs if two computers send at the same time (because there is a narrow window of time in which the second computer may have begun transmitting but the "busy signal" has not yet reached the first computer, which blithely begins to transmit). A collision doesn't make any noise, but the signals become garbled and the messages can't be understood . In fact, nodes that detect a collision automatically transmit a special "jam" signal, which unambiguously destroys the colliding transmissions. When this happens, each of the colliding computers "backs off" or waits for a random amount of time, then tries to retransmit. This wait/retransmission sequence can repeat until both messages are transmitted successfully. The whole process takes a small fraction of a second.

Ethernet's detractors characterize it as an inefficient access method because frames are prone to collisions. But while collisions occur, they don't consume very much throughput capacity in most cases. Since the whole process of transmitting, colliding, and retransmitting takes place so quickly, the delay a collision causes is normally minuscule. Of course, as the traffic on a network approaches the total throughput capacity, the number of collisions will mount and the network will slow considerably. This happens with some large-scale imaging or engineering applications, or network segments with too many nodes. As long as an Ethernet network has a low traffic load, traditionally the most common environment, delay caused by collisions is seldom noticeable. In a switched environment, especially on Ethernet backbones, most links will have only two nodes, and the incidence of collisions on these segments will be minuscule.

Token Ring

When Token Ring was introduced in 1984, it was not the first token-passing, ring network, but because it was endorsed by IBM, it has had a tremendous impact on the network industry. Token Ring became part of IBM's connectivity solution for all its computers-personal, midrange , and mainframe. IBM's specifications match those of the IEEE 802.5 standard.

Token Ring unit shipments are still increasing in 1995, though this growth is unlikely to continue for long. IBM has had a stranglehold on the Token Ring market, though it no longer supplies the 90 percent share of Token Ring network interface cards that characterized this market in the 1980s.

The original Token Ring system transmitted data at 4Mbits/sec; the newer specification calls for 16Mbit/sec transmission. In Token Ring, the computers are arranged in a logical ring, but all data passing between work stations is routed through a hub. A multi-station access unit (MSAU or MAU) acts as the hub, and each work station is connected to it. Token Ring uses a token-passing access method to prevent data collisions-a token being a series of data bits created by one of the computers. The token moves around the ring, giving successive computers the right to transmit. When a computer receives the token, it may transmit a message of any length as long as the time to send does not exceed the token-holding timer (this combination of token and data is called a frame). As this message (frame) moves around the network, each computer regenerates the signal. Only the receiving computer copies the message into its memory, then marks the message as received. The sending computer removes the message from the token and recirculates it.

Token Ring's advantages include reliability and ease of maintenance. It uses a star-wired ring topology in which all computers are directly wired to a MAU. The MAU allows malfunctioning computers to be disconnected from the network. This overcomes one disadvantage of token-passing, which is that one malfunctioning computer can bring down the network, since all computers are actively passing signals around the ring.

Arcnet

ARCnet was developed by Datapoint in the early 1970s. It was especially popular in very small networks, since it was inexpensive and easy to maintain. ARCnet uses a token-passing access method that works on a star-bus topology. Data is transmitted at 2.5Mbits/sec. The network cable is laid out as a series of stars. Each computer is attached to a hub at the center of a star, and the hubs are connected in a bus or line. ARCnetPlus was designed as a backbone technology and can transmit data at 20Mbits/sec.

When a computer wants to send data on an ARCnet network, it must have the token. The token moves around the network in a given pattern, which in ARCnet's case is a logical ring. All computers on the network are numbered with an address from 0 to 255. (The maximum number of computers on each ARCnet segment is thus 256.) The token moves from computer to computer in numerical order, even if adjacent numbers are at physically opposite ends of the network. When the token reaches the highest number on the network it moves to the lowest , thus creating a logical ring.

Once a computer has the token, it can send one 512-byte packet. A packet is composed of the destination address, its own address, up to 508 bytes of data, and other information. The packet moves from node to node in sequential order until it reaches the destination node. At the destination, the data is removed and the token released to the next node.

The advantage of token passing is predictability. Because the token moves through the network in a determined path , it is possible to calculate the best and worst cases for data transmission. This makes network performance predictable. It also means introduction of new network nodes will have a predictable effect. This differs from Ethernet, where the addition of new nodes may or may not seriously affect performance. However, claims for a "predictable" network can be misleading-for example, lost tokens will affect worst-case delivery times.

A disadvantage of the token-passing access method is the fact that each node acts as a repeater, accepting and regenerating the token as it passes around the network in a specific pattern. If there is a malfunctioning node, the token may be destroyed or simply lost, bringing down the whole network. The token must then be regenerated.

This tutorial, number 4, by Aaron Brenner, was originally published in the November 1988 issue of LAN Magazine/Network Magazine.

 
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Network Tutorial
Lan Tutorial With Glossary of Terms: A Complete Introduction to Local Area Networks (Lan Networking Library)
ISBN: 0879303794
EAN: 2147483647
Year: 2003
Pages: 193

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