View source for ARCNET

== Description ==
Original ARCNET used RG-62/U [[coaxial cable]] of {{nowrap|93 [[Ohm|Ω]]}} [[Characteristic impedance|impedance]] and either passive or active [[Ethernet hub|hub]]s in a star-wired [[bus (computing)|bus]] topology. At the time of its greatest popularity, this was a significant advantage of ARCNET over thin Ethernet ([[10BASE2]]). A star-wired topology was easier to build, expand and service than the linear Ethernet bus of the time. The "interconnected stars" cabling topology made it easy to add and remove nodes without taking down the whole network, and much easier to diagnose and isolate failures within a complex LAN.

Another significant advantage ARCNET had over Ethernet was cable distance. ARCNET coax cable runs could extend {{convert|2000|ft|m|abbr=on|disp=flip|0}} between active hubs or between an active hub and an end node, while the [[RG-58]] (50&nbsp;Ω) thin Ethernet was limited to a maximum run of {{convert|185|m|ft|abbr=on}} from end to end.<ref>IEEE 802.3 Clause 10.1.1.1</ref>

ARCNET had the disadvantage of requiring either an active or passive hub between nodes if there were more than two nodes in the network, while Ethernet allowed nodes to be placed anywhere along the linear coax cable. However, ARCNET passive hubs were very inexpensive, being composed of a simple, small, unpowered box with four BNC ports, wired together with just a star of four 47&nbsp;Ω resistors<ref>{{cite web |url=https://apenwarr.ca/arcnet/howto/cabling.html |title=Cabling an arcnet network |first=Vojtech |last=Pavlik |website=apenwarr |location=Canada |access-date=2025-11-22}}</ref>, so the disadvantage was not significant. This disadvantage can also be seen as an advantage: often the cost of a 4 port ARCNET passive hub was less than the 4 [[BNC connector|BNC Tee]] connectors and 2 terminators that thin Ethernet requires to connect 4 computers. Unlike BNC Tee connectors that could sometimes be hard to obtain in the early days of Ethernet, an ARCNET passive hub could be easily manufactured in the field with 9 readily available parts: 4 connectors, 4 resistors and a box to put them in.

Passive hubs limited the distance between a node and an active hub to {{convert|100|ft|m|abbr=on|disp=flip|-1}}. A passive hub could not be connected directly to another passive hub. Unused ports on both types of hubs had to be terminated with a special connector. This special connector, called a terminator, is just a BNC connector with a 93&nbsp;Ω resistor in it. Thin Ethernet also requires nearly identical terminators at the two terminal ends, the only difference being Ethernet uses a 50&nbsp;Ω resistor.

To reduce costs while still allowing wide area coverage, a common practice was to use one or more interconnected active hubs, each of which provided coverage for nodes no more than {{convert|200|ft|m|abbr=on|disp=flip|-1}} away. Cable was run from each port of the active hubs to a different location no more than {{convert|100|ft|m|abbr=on|disp=flip|-1}} away. A passive hub would then be attached to the end of the cable, and cables would be run locally from the passive hub, allowing connection of up to three nodes. In this way, a single 8-port active hub could be used to connect 24 networked devices over an area not exceeding {{convert|400|ft|m|abbr=on|disp=flip|-1}} in diameter.

ARCNET allowed only 255 nodes per network. Node IDs for LAN workstations were typically set by DIP switches on the network interface card. Larger networks would have to be split into smaller networks, and bridged. The small number of possible nodes and the need to manually configure IDs was a disadvantage compared with Ethernet, particularly as large enterprise networks became common.

To [[media access control|mediate access to the bus]], ARCNET, like Token Ring, uses a [[token passing]] scheme, rather than the [[carrier sense multiple access]] 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 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 original, shared-medium performance with [[CSMA/CD]] 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.<ref name=csmacd>{{cite web|url=http://www.sce.carleton.ca/faculty/lambadaris/courses/462/csma_cd.pdf|title=Efficiency of CSMA/CD|author=Carleton University}}</ref> 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 {{nowrap|2.5 Mbit/s}} ARCNET could at one time outperform a {{nowrap|10 Mbit/s}} 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. {{Citation needed|date=August 2010}}

In the early 1980s, ARCNET was much cheaper than Ethernet, in particular for PCs. For example, in 1985 [[SMC Networks|SMC]] sold ARCNET cards for around {{USD|300}} whilst an Ungermann-Bass Ethernet card plus transceiver could cost {{USD|500}}.

Another significant difference is that ARCNET provides the sender with a definite success/failure status of delivery at the receiver before the token passes on to the next node. This permits 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 the initial hardware-level inquiry 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 takes a time to get the token depending on the number of nodes and the size of the messages currently being sent, a node will always receive it within a predictable maximum time. It is therefore ''deterministic''. This made ARCNET an ideal [[real-time computing|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 historic suitability for real-time environments such as process control, the general availability of [[Network switch|switched]] [[gigabit Ethernet]] and [[Quality of service]] capabilities in Ethernet switches has all but eliminated ARCNET today.

At first the system was deployed using the RG-62/U [[coaxial cable]] commonly used in [[IBM mainframe]] environments to connect [[IBM 3270|3270]] terminals and controllers, but later added support for [[twisted pair]] and [[optical fiber|fibre]] media. At ARCNET's lower speeds ({{nowrap|2.5 Mbit/s}}), [[Category 3 cable|Cat-3]] cable is good enough to run ARCNET. Some ARCNET twisted-pair products supported cable runs over {{convert|2000|ft|m|abbr=on}} on standard Cat-3 cable, far beyond anything Ethernet could do on any kind of copper cable.

In the early 1990s, [[Thomas-Conrad|Thomas-Conrad Corporation]] developed a {{nowrap|100 Mbit/s}} topology called TCNS based on the ARCNET protocol, which also supported RG-62, twisted-pair, and fiber optic media.<ref>[https://archive.org/stream/ARCNET-DocumentManagement/ARCNET-DocumentManagement_djvu.txt "The Rodney Dangerfield of Network Computing"], archive.org</ref> TCNS enjoyed some success until the availability of lower-cost {{nowrap|100 Mbit/s}} Ethernet put an end to the general deployment of ARCNET as a LAN protocol.

However, because of its simple and robust nature, ARCNET controllers are still sold and used in industrial, embedded, and automotive applications.