ARCNET
From Wikipedia, the free encyclopedia
ARCNET (also CamelCased 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.
[edit] History
ARCNET was developed by principal development engineer John Murphy at Datapoint Corporation in 1976 and announced 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. This incremental approach broke new ground and by the end of the 1970s (before the first cassette-based IBM PC was announced in 1981) over ten thousand ARCnet LAN installations were in commercial use around the world, and Datapoint had become a Fortune 500 company. As microcomputers took over the industry, well-proven and reliable ARCNET was also offered as an inexpensive LAN for these machines.
ARCNET remained proprietary until the early-to-mid 1980s. This did not cause concern at the time, as most network architectures were proprietary. The move to non-proprietary, open systems began as a response to the dominance of International Business Machines (IBM) and its Systems Network Architecture (SNA). In 1979, the Open Systems Interconnection Reference Model (OSI Model) was published. Then, in 1980, Digital, Intel and Xerox (the DIX consortium) published an open standard for Ethernet that was soon adopted as the basis of standardization by the IEEE and the ISO. IBM responded by proposing Token Ring as an alternative to Ethernet but kept such tight control over standardization that competitors were wary of using it. ARCNET was less expensive than either, often much less, more reliable, more flexible, and by the late 1980s it had a market share about equal to that of Ethernet.
After Ethernet abandoned their original clumsy thick-wire and somewhat-less-clumsy thin-coax version and adopted ARCnet's innovative and more maintainable "interconnected stars" cabling topology based on active hubs, Ethernet became more attractive than before, thus Ethernet volumes increased. With more companies entering the market, the price of Ethernet started to fall, and ARCNET volumes tapered off. 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 (originally developed as custom LSI for Datapoint, but later) made available by Standard Microsystems to other customers. Datapoint eventually found itself in financial trouble and eventually moved into video conferencing and (later) custom programming in the embedded market.
[edit] Description
Original ARCNET used RG-62/U coax cable and either passive or active hubs in a star-wired bus topology, a layout eventually copied by 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 (and was more readily maintainable) than the clumsy linear bus Ethernet of the time. Another was cable distance – ARCNET coax cable runs could extend 2000 feet (610 meters) 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 (183 meters) 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. More importantly, the "interconnected stars" cabling topology made it easy to add and remove nodes without taking the whole network down, and much easier to diagnose and isolate failures within a complex LAN.
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 (viewed by a single stream), but is much more predictable. ARCnet also has the advantage that it achieves its best aggregate performance under the highest loading, approaching asymptotically its maximum throughput. While the best case performance may be less than Ethernet, the general case is equivalent and the worst case is dramatically better. An Ethernet network can collapse when too busy due to excessive collisions. An ARCNET will keep on going at normal (or even better) throughput. Throughput on a multinode Ethernet is limited to between 40% and 60% of bandwidth usage (depending on source). ARCNET appeared to give way to Ethernet due to the marketing difficulty of showing a 2.5 Mbit/s ARCNET as being better than a 10 Mbit/s Ethernet in a busy office (although their actual performance usually being nearly the same).[citation needed]
Another signficant difference is that ARCnet provides the sender with a concrete acknowledgement (or not) of successful delivery at the receiving end before the token passes on to the next node, permitting much faster fault recovery within the higher level protocols (rather than having to wait for a timeout on the expected replies). ARCnet also doesn't waste network time transmitting to a node not ready to receive the message, since an initial inquiry (done at hardware level) establishes that the recipient is able and ready to receive the larger message before it is sent across the bus.
One further advantage to this system is that it guarantees equitable 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. (Indeed, ARCNET has been demonstrated running successfully across coat hanger wire!)
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 lower-cost 100 Mbit/s Ethernet put an end to the general deployment of ARCNET.
