On a
local area network
A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, school, laboratory, university campus or office building. By contrast, a wide area network (WAN) not only covers a larger ...
, token passing is a
channel access method
In telecommunications and computer networks, a channel access method or multiple access method allows more than two terminals connected to the same transmission medium to transmit over it and to share its capacity. Examples of shared physical ...
where a packet called a ''token'' is passed between nodes to authorize that node to communicate. In contrast to
polling access methods, there is no pre-defined "master" node. The most well-known examples are
IBM Token Ring
Token Ring network
IBM hermaphroditic connector with locking clip. Screen contacts are prominently visible, gold-plated signal contacts less so.
Token Ring is a computer networking technology used to build local area networks. It was introduc ...
and
ARCNET, but there were a range of others, including
FDDI
Fiber Distributed Data Interface (FDDI) is a standard for data transmission in a local area network.
It uses optical fiber as its standard underlying physical medium, although it was also later specified to use copper cable, in which case i ...
(Fiber Distributed Data Interface), which was popular in the early to mid 1990s.
Token passing schemes degrade deterministically under load, which is a key reason why they were popular for industrial control LANs such as
MAP, (Manufacturing Automation Protocol). The advantage over
contention based channel access (such as the
CSMA/CD
Carrier-sense multiple access with collision detection (CSMA/CD) is a medium access control (MAC) method used most notably in early Ethernet technology for local area networking. It uses carrier-sensing to defer transmissions until no other stati ...
of early Ethernet), is that collisions are eliminated, and that the channel
bandwidth can be fully utilized without idle time when demand is heavy.
The disadvantage is that even when demand is light, a station wishing to transmit must wait for the token, increasing
latency.
Some types of token passing schemes do not need to explicitly send a token between systems because the process of "passing the token" is implicit. An example is the channel access method used during "Contention Free Time Slots" in the
ITU-T
The ITU Telecommunication Standardization Sector (ITU-T) is one of the three sectors (divisions or units) of the International Telecommunication Union (ITU). It is responsible for coordinating standards for telecommunications and Information Commu ...
G.hn standard for high-speed
local area network
A local area network (LAN) is a computer network that interconnects computers within a limited area such as a residence, school, laboratory, university campus or office building. By contrast, a wide area network (WAN) not only covers a larger ...
ing using existing home wires (power lines, phone lines and coaxial cable).
See also
*
Cambridge Ring The term Cambridge Ring could refer to:
* The Cambridge Ring (computer network) technology developed at the university of Cambridge, England
* The Cambridge Five
The Cambridge Spy Ring was a ring of spies in the United Kingdom that passed inform ...
*
HP Interface Loop (HP-IL)
*
Ring network
A ring network is a network topology in which each node connects to exactly two other nodes, forming a single continuous pathway for signals through each node – a ring. Data travels from node to node, with each node along the way handling ever ...
References
Channel access methods
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