ARCNET

ARCNET (also camel cased as ARCnet) is a local area network (LAN) protocol, similar in purpose to Ethernet or Token Ring. ARCNET was the first widely available networking system for microcomputers and became popular in the 1980s for office automation tasks. It has since gained a following in the embedded systems market, where certain features of the protocol are especially useful.

History

ARCNET was developed by Datapoint Corporation in 1977, originally intended to allow groups of their Datapoint 2200 terminalss to talk to a shared 8" floppy disk system. As microcomputers took over from the Datapoint, ARCNET was re-purposed as LAN for these machines.

ARCNET remained proprietary until the late 1980s. This did not cause concern at the time, as Token Ring and Ethernet were essentially proprietary as well (controlled by IBM and 3Com respectively). ARCNET was less expensive than either, often much less, and by the late 1980s it had a market share about equal to that of Ethernet.

As more companies started producing Ethernet, the prices started to fall rapidly, and ARCNET disappeared over the course of a few short years. The same was largely true of Token Ring, although IBM's immense power managed to keep it in the market for some time longer.

ARCNET was eventually standardized as ANSI ARCNET 878.1. It appears this was when the name changed from ARCnet to ARCNET. Other companies entered the market, notably Standard Microsystems who produced systems based on a single VLSI chip which were cheaper than the originals. Datapoint soon found itself in financial trouble and eventually moved into custom programming in the embedded market.

Description

ARCNET uses a bus technology, in which messages are handed from peer to peer along the network. This is as opposed to the Ethernet model where the messages are broadcast to everyone on the network at the same time. Each approach has its advantages: ARCNET adds a small delay as the message is inspected by each peer, but Ethernet's performance can degrade drastically if too many peers attempt to broadcast at the same time. ARCNET has slightly lower best-case performance, but is much more predictable.

To mediate access to the bus, ARCNET uses a token-passing scheme, similar to that used by Token Ring. When peers are inactive, a single "token" message is passed around the network from machine to machine, and no peer is allowed to use the bus unless it has the token. If a particular peer wishes to send a message, it waits for the token to appear, sends its message, and then places a modified token packet back onto the end of the message. Since the token is modified, no one else will "see" it. When the receiver is finished reading the message, it strips off the token and returns it to its original state and passes it along.

The advantage to this system is that it guarantees access to the bus by everyone on the network. Although it might take some time to get the token depending on the size of the messages currently being sent about, you will always receive it within a certain time; thus it is deterministic. This makes it an ideal real-time networking system, which explains its use in the embedded systems and process control markets. Token Ring has similar qualities, but is much more expensive to implement than ARCNET.

In spite of ARCNET's deterministic operation and suitability for real-time environments, such as process control, Ethernet is gaining popularity in the process control industry. Proponents of ARCNET argue that this is silly, because Ethernet does not have deterministic delivery as ARCNET does.

At first the system was deployed using coax cable, but has since added support for twisted-pair and fibre. At lower speeds (2.5 Mbit/s and down), Cat-3 is enough to run ARCNET on twisted-pair.

External link

ARCNET Trade Association

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