ARCNET is a local area network
Local area network
A local area network is a computer network that interconnects computers in a limited area such as a home, school, computer laboratory, or office building...

 (LAN) protocol, similar in purpose to Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

 or Token Ring
IBM token ring
thumb|Two examples of token ring networks: a) Using a single [[Media Access Unit|MAU]] b) Using several MAUs connected to each otherthumb|Token ring networkthumb|IBM hermaphroditic connector with locking clipthumb|An IBM 8228 MAU...

. ARCNET was the first widely available networking system for microcomputer
A microcomputer is a computer with a microprocessor as its central processing unit. They are physically small compared to mainframe and minicomputers...

s and became popular in the 1980s for office automation tasks. It has since gained a following in the embedded system
Embedded system
An embedded system is a computer system designed for specific control functions within a larger system. often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. By contrast, a general-purpose computer, such as a personal...

s market, where certain features of the protocol are especially useful.


ARCNET was developed by principal development engineer John Murphy at Datapoint
Datapoint Corporation, originally known as Computer Terminal Corporation , was a computer company based in San Antonio, Texas, United States. Founded in 1967 by Phil Ray and Gus Roche, its first products were, as the company's initial name suggests, computer terminals...

 Corporation in 1976 and announced in 1977. It was the first loosely-coupled LAN-based clustering solution, making no assumptions about the type of computers that would be connected. This was in contrast to contemporary larger and more expensive computer systems such as DECnet
DECnet is a suite of network protocols created by Digital Equipment Corporation, originally released in 1975 in order to connect two PDP-11 minicomputers. It evolved into one of the first peer-to-peer network architectures, thus transforming DEC into a networking powerhouse in the 1980s...

 or SNA, where a homogeneous group of similar or proprietary computers were connected as a cluster.

The token-passing bus protocol of that I/O device-sharing network was subsequently applied to allowing processing nodes to communicate with each other for file-serving and computing scalability purposes. An application could be developed in DATABUS, Datapoint's proprietary COBOL
COBOL is one of the oldest programming languages. Its name is an acronym for COmmon Business-Oriented Language, defining its primary domain in business, finance, and administrative systems for companies and governments....

-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
The IBM Personal Computer, commonly known as the IBM PC, is the original version and progenitor of the IBM PC compatible hardware platform. It is IBM model number 5150, and was introduced on August 12, 1981...

 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
Systems Network Architecture
Systems Network Architecture is IBM's proprietary networking architecture created in 1974. It is a complete protocol stack for interconnecting computers and their resources. SNA describes the protocol and is, in itself, not actually a program...

 (SNA). In 1979, the Open Systems Interconnection Reference Model (OSI Model
OSI model
The Open Systems Interconnection model is a product of the Open Systems Interconnection effort at the International Organization for Standardization. It is a prescription of characterizing and standardizing the functions of a communications system in terms of abstraction layers. Similar...

) was published. Then, in 1980, Digital, Intel and Xerox (the DIX consortium) published an open standard for Ethernet
Ethernet is a family of computer networking technologies for local area networks commercially introduced in 1980. Standardized in IEEE 802.3, Ethernet has largely replaced competing wired LAN technologies....

 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, more reliable, more flexible, and by the late 1980s it had a market share about equal to that of Ethernet.

When Ethernet moved from co-axial cable to twisted pair
Ethernet over twisted pair
Ethernet over twisted pair technologies use twisted-pair cables for the physical layer of an Ethernet computer network. Other Ethernet cable standards employ coaxial cable or optical fiber. Early versions developed in the 1980s included StarLAN followed by 10BASE-T. By the 1990s, fast, inexpensive...

 and an "interconnected stars" cabling topology based on active hubs, it became much more attractive. Easier cabling, combined with the greater raw speed of Ethernet helped to increase Ethernet demand, and as more companies entered the market the price of Ethernet started to fall—and ARCNET (and Token Ring) volumes tapered off.

In response to greater bandwidth needs, and the challenge of Ethernet, a new standard called ARCnet Plus was developed by Datapoint, and introduced in 1992. ARCnet Plus ran at , and was backward compatible with original ARCnet equipment. However, by the time ARCnet Plus products were ready for the market, Ethernet had captured the majority of the network market, and there was little incentive for users to move back to ARCnet. As a result, very few ARCnet Plus products were ever produced. Those that were built, mainly by Datapoint, were expensive, and hard to find.

ARCNET was eventually standardized as ANSI
American National Standards Institute
The American National Standards Institute is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organization also coordinates U.S. standards with international...

 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
Very-large-scale integration
Very-large-scale integration is the process of creating integrated circuits by combining thousands of transistors into a single chip. VLSI began in the 1970s when complex semiconductor and communication technologies were being developed. The microprocessor is a VLSI device.The first semiconductor...

 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.

Even though ARCNET is now rarely used for new general networks, the diminishing installed base still requires support - and it retains a niche in industrial control..


Original ARCNET used RG-62/U coax cable of impedance
Characteristic impedance
The characteristic impedance or surge impedance of a uniform transmission line, usually written Z_0, is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm...

 and either passive or active hubs in a star-wired bus
Computer bus
In computer architecture, a bus is a subsystem that transfers data between components inside a computer, or between computers.Early computer buses were literally parallel electrical wires with multiple connections, but the term is now used for any physical arrangement that provides the same...

 topology. 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 (2000ft) between active hubs or between an active hub and an end node, while the RG-58
RG-58/U is a type of coaxial cable often used for low-power signal and RF connections. The cable has a characteristic impedance of either 50 or 52 Ω. "RG" was originally a unit indicator for bulk RF cable in the U.S. military's Joint Electronics Type Designation System...

  ‘thin’ Ethernet most widely used at that time was limited to a maximum run of (600ft) 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. Passive hubs limited the distance between node and active hub to (100ft). 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
Media Access Control
The media access control data communication protocol sub-layer, also known as the medium access control, is a sublayer of the data link layer specified in the seven-layer OSI model , and in the four-layer TCP/IP model...

, ARCNET, like Token Ring, uses a token passing
Token passing
In telecommunication, token passing is a channel access method where a signal called a token is passed between nodes that authorizes the node to communicate. The most well-known examples are token ring and ARCNET....

 scheme, rather than the carrier sense multiple access
Carrier Sense Multiple Access
Carrier Sense Multiple Access is a probabilistic Media Access Control protocol in which a node verifies the absence of other traffic before transmitting on a shared transmission medium, such as an electrical bus, or a band of the electromagnetic spectrum."Carrier Sense" describes the fact that a...

 approach of Ethernet. 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. Because ARCNET is implemented as a distributed star, the token cannot be passed machine to machine around a ring. Instead, each node is assigned an 8 bit address (usually via DIP switches), and when a new node joins the network a "reconfig" occurs, wherein each node learns the address of the node immediately above it. The token is then passed directly from one node to the next.

Historically, each approach had its advantages: ARCNET added a small delay on an inactive network as a sending station waited to receive the token, but Ethernet's performance degraded drastically if too many peers attempted to broadcast at the same time, due to the time required for the slower processors of the day to process and recover from collisions. ARCNET had slightly lower best-case performance (viewed by a single stream), but was much more predictable. ARCNET also has the advantage that it achieved its best aggregate performance under the highest loading, approaching asymptotically its maximum throughput. While the best case performance was less than Ethernet, the general case was equivalent and the worst case was dramatically better. An Ethernet network could collapse when too busy due to excessive collisions. An ARCNET would keep on going at normal (or even better) throughput. Throughput on a multi-node collision-based Ethernet was limited to between 40% and 60% of bandwidth usage (depending on source). Although ARCNET could at one time outperform a Ethernet in a busy office on slow processors, ARCNET ultimately gave way to Ethernet as improved processor speeds reduced the impact of collisions on overall throughput, and Ethernet costs dropped.

In the early 1980s ARCNET was much cheaper than Ethernet, in particular for PCs. For example in 1985 SMC
SMC Networks
SMC Networks designs, develops and manufactures customer-premises equipment such as gateways, routers, modems and home security, automation, and monitoring products for residential and business customers....

 sold ARCNET cards for around whilst an Ungermann-Bass Ethernet card plus transceiver could cost .

Another significant difference is that ARCNET provides the sender with a concrete acknowledgment (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 that ARCNET enjoyed over collision-based Ethernet 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 made ARCNET an ideal real-time
Real-time computing
In computer science, real-time computing , or reactive computing, is the study of hardware and software systems that are subject to a "real-time constraint"— e.g. operational deadlines from event to system response. Real-time programs must guarantee response within strict time constraints...

 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 historic suitability for real-time environments such as process control, the general availability of switched
Network switch
A network switch or switching hub is a computer networking device that connects network segments.The term commonly refers to a multi-port network bridge that processes and routes data at the data link layer of the OSI model...

 gigabit Ethernet
Gigabit Ethernet
Gigabit Ethernet is a term describing various technologies for transmitting Ethernet frames at a rate of a gigabit per second , as defined by the IEEE 802.3-2008 standard. It came into use beginning in 1999, gradually supplanting Fast Ethernet in wired local networks where it performed...

 and Quality of service
Quality of service
The quality of service refers to several related aspects of telephony and computer networks that allow the transport of traffic with special requirements...

 capabilities in Ethernet switches has all but eliminated ARCNET today.

At first the system was deployed using RG-62/U coax cable (commonly used in IBM mainframe
IBM mainframe
IBM mainframes are large computer systems produced by IBM from 1952 to the present. During the 1960s and 1970s, the term mainframe computer was almost synonymous with IBM products due to their marketshare...

 environments to connect 3270
IBM 3270
The IBM 3270 is a class of block oriented terminals made by IBM since 1972 normally used to communicate with IBM mainframes. As such, it was the successor to the IBM 2260 display terminal. Due to the text colour on the original models, these terminals are informally known as green screen terminals...

 terminals and controllers), but later added support for twisted-pair and fibre media. At ARCNET's lower speeds , 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 90s, Thomas-Conrad Corporation developed a topology called TCNS based on the ARCNET protocol, which also supported RG-62, twisted-pair, and fiber optic media. TCNS enjoyed some success until the availability of lower-cost Ethernet put an end to the general deployment of ARCNET.

External links

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