From Academic Kids

ARCNET (also camel cased as ARCnet, an acronym from Attached Resource Computer NETwork) 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.


ARCNET was developed by Datapoint Corporation in 1977. It was the first LAN-based clustering solution, originally developed as an alternative to larger, more expensive computer systems. An application could be developed in DATABUS, Datapoint's proprietary COBOL-like language and deployed on a single computer with dumb terminals. When the number of users outgrew the capacity of the original computer, additional 'compute' resource computers could be attached via ARCNET, running the same applications and accessing the same data. If more storage was needed, additional disk resource computers could also be attached. Unfortunately, this incremental approach was ahead of its time and wasn't a commercial success. As microcomputers took over the industry, ARCNET was re-purposed as an inexpensive 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.


Original ARCNET used RG-62/U coax cable and active hubs in a star-wired bus topology, similar in layout to modern twisted pair Ethernet LANs. At the time of its greatest popularity ARCNET enjoyed two major advantages over Ethernet. One was the star-wired bus, this was much easier to build and expand than the coax-based linear bus Ethernet of the time. Another was cable distance – ARCNET coax cable runs could extend 2000 feet between active hubs or between an active hub and an end node, while the RG-58 ‘thin’ Ethernet most widely used at that time was limited to a maximum run of 600 feet from end to end. Of course, ARCNET required either an active or passive hub between nodes if there were more than two nodes in the network, while thin Ethernet allowed nodes to be spaced anywhere along the linear coax cable, but the ARCNET passive hubs were very inexpensive.

To mediate access to the bus, ARCNET uses a token-passing scheme, a bit different from 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 to receive the token, sends its message, and then passes the token on to the next station.

Each approach has its advantages: ARCNET adds a small delay on an inactive network as a sending station waits to receive the token, 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.

The advantage to this system is that it guarantees access to the bus by everyone on the network. Although it might take a short time to get the token depending on the number of nodes and the size of the messages currently being sent about, you will always receive it within a predictable maximum 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 RG-62/U coax cable (commonly used in IBM mainframe environments to connect 3270 terminals and controllers), but later added support for twisted-pair and fibre media. At ARCNET's lower speeds (2.5 Mbit/s), Cat-3 cable is good enough to run ARCNET. Some ARCNET twisted-pair products supported cable runs over 2000' on standard CAT-3 cable, far beyond anything Ethernet could do on any kind of copper cable.

In the early 90's, Thomas-Conrad Corporation developed a 100 Mbit/s topology called TCNS based on the ARCNET protocol, which also supported RG-62, twisted-pair, and fibre optic media. TCNS enjoyed some success until the availability of affordable 100 Mbit/s Ethernet put an end to the general deployment of ARCNET.

External link

es:ARCNET ja:アークネット pl:Arcnet pt:Arcnet


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