{{Short description|Channel access method}} On a local area network, '''token passing''' is a channel access method where a packet called a ''token'' is passed between nodes to authorize that node to communicate.<ref>{{cite web|last1=Sheldon|first1=Tom|title=Token and Token-Passing Access Methods|url=http://www.linktionary.com/t/token_access.html|website=Linktionary|accessdate=8 July 2017}}</ref><ref>{{cite book|last1=Littlejohn Shinder|first1=Debra|title=Computer Networking Essentials|date=2001|publisher=Cisco Press|isbn=978-1587130380|page=123|url=https://books.google.com/books?id=ZMizSUW7XF4C&q=token+passing&pg=PA123|accessdate=8 July 2017}}</ref><ref>{{cite web|last1=Savage|first1=Stefan|title=Lecture 6: Media Access Control|url=https://cseweb.ucsd.edu/classes/fa11/cse123-a/123f11_Lec6.pdf|website=Computer Science and Engineering|publisher=UC San Diego|accessdate=8 July 2017}}</ref>
==Channel access method== In contrast to polling access methods, there is no pre-defined "master" node.<ref>{{cite web|last1=DiNicolo|first1=Dan|title=Essential Network Concepts Part 3|url=http://www.securitypronews.com/essential-network-concepts-part-3-2004-03|website=Security Pro News|accessdate=9 July 2017|archive-date=16 July 2013|archive-url=https://web.archive.org/web/20130716120055/http://www.securitypronews.com/essential-network-concepts-part-3-2004-03|url-status=dead}}</ref> The most well-known examples are IBM Token Ring and ARCNET, but there were a range of others, including FDDI (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 Manufacturing Automation Protocol (MAP).<ref>{{cite web|title=Token Bus (IEEE 802.4)|url=https://www.technologyuk.net/telecommunications/networks/token-bus.shtml|website=www.technologyuk.net|accessdate=8 July 2017|archive-date=4 July 2019|archive-url=https://web.archive.org/web/20190704173558/http://www.technologyuk.net/telecommunications/networks/token-bus.shtml|url-status=dead}}</ref> The advantage over contention based channel access (such as the CSMA/CD of early Ethernet), is that collisions are eliminated, and that the channel bandwidth can be fully utilized without idle time when demand is heavy.<ref>{{cite book|last1=Hura|first1=Gurdeep|title=Data and Computer Communications: Networking and Internetworking|date=2001|publisher=CRC Press|isbn=978-0849309281|page=327|url=https://books.google.com/books?id=c0z7d9S82EsC&q=token+deterministic&pg=PA327}}</ref> 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 G.hn standard for high-speed local area networking using existing home wires (power lines, phone lines and coaxial cable).{{cn|date=July 2017}}
==See also== *Cambridge Ring *HP-IL (HP Interface Loop) *Ring network
==References== {{reflist}}
{{DEFAULTSORT:Token Passing}} Category:Channel access methods