ARCNET
ARCNET (also camel cased as ARCnet) is a local area network (LAN) protocol, similar in purpose to Ethernet or TokenRing. ARCNET was the first widely available networking system for microcomputers and became popular in the 1980's for office automation tasks. It has since gained a following in the embedded systems market, where certain features of the protocol are useful in that role.
History
ARCNET was developed by Datapoint Corporation in 1977, originally intended to allow groups of their Datapoint 2200 terminals 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.
It remained proprietary until the late 1980's. This did not cause concern in the 1980's, as TokenRing 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 1980's it had a market share about the same as Ethernet.
However as more and more companies started producing Ethernet equipment the prices started to fall rapidly, and ARCNET disappeared over the course of a few short years. The same was largely true of TokenRing, 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. 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, but the peers only "listen" to messages for them by inspecting the address. This is different than Ethernet, where messages are broadcast to everyone on the network at the same time. It also means there is a delay for every peer as then inspect the packet and pass if off, and this delay grows as more peers are added to the network.
To mediate access to the bus, ARCNET uses a token-passing scheme, similar to that used by TokenRing. When the bus is inactive a single "token" message is passed around the network from machine to machine, and no one is allowed to use the bus unless they have the token. If a particular peer wishes to send a message, they wait for the token to appear (which will arrive in turn), send their message, and then places the token packet onto the end of the message slightly modified. Since the token is modified, no one else will "see" it. When the receiver sees the token at the end of the message, it modifies it back to the normal state and passes it off, thus freeing the bus.
The advantage to this system is that it guarentees 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, it is deterministic. This makes it an ideal real time networking system, which explains its use in the embedded systems market. TokenRing has similar qualities, but is much more expensive to implement than ARCNET.
At first the system was deployed using coax cabling, but has since added support for twisted-pair and fibre. Due to it's lower speeds (2.5Mbps and down), CAT3 is enough to run ARCNET on twisted-pair.
Links: