Token Bus

Token Bus was a 4 Mbps Local Area Networking technology created by IBM to connect their terminals to IBM mainframes. Token bus utilized a copper coaxial cable to connect multiple end stations (terminals, wokstations, shared printers etc.) to the mainframe. The coaxial cable served as a common communication bus and a token was created by the Token Bus protocol to manage or 'arbitrate' access to the bus. Any station that holds the token packet has permission to transmit data. The station releases the token when it is done communicating or when a higher priority device needs to transmit (such as the mainframe). This keeps two or more devices from transmitting information on the bus at the same time and accidentally destroying the transmitted data.

Token Bus suffered from two limitations. Any failure in the bus caused all the devices beyond the failure to be unable to communicate with the rest of the network. Second, adding more stations to the bus was somewhat difficult. Any new station that was improperly attached was unlikely to be able to communicate and all devices beyond it were also affected. Thus, token bus networks were seen as somewhat unreliable and difficult to expand and upgrade.

Token Ring

Token Ring was created by IBM to compete with what became known as the DIX Standard of Ethernet (DEC/Intel/Xerox) and to improve upon their previous Token Bus technology. Up until that time, IBM had produced solutions that started from the mainframe and ran all the way to the desktop (or dumb terminal), allowing them to extend their SNA protocol from the AS400's all the way down to the end user. Mainframes were so expensive that many large corporations that purchased a mainframe as far back as 30-40 years ago are still using these mainframe devices, so Token Ring is still out there and you will encounter it. Token Ring is also still in use where high reliability and redundancy are important--such as in large military craft.

Token Ring comes in standard 4 and 16 Mbsp and high-speed Token Ring at 100Mbps(IEEE 802.5t) and 1Gbps (IEEE 802.5v). Many mainframes (and until recently, ALL IBM mainframes) used a Front End Processor (FEP) with either a Line Interface Coupler (LIC) at 56kbps, or a Token-ring Interface Coupler (TIC) at 16 Mbps. Cisco still produces FEP cards for their routers (as of 2004).

Token Ring uses a ring based topology and passes a token around the network to control access to the network wiring. This token passing scheme makes conflicts in accessing the wire unlikely and therefore total throughput is as high as typical Ethernet and Fast Ethernet networks. The Token Ring protocol also provides features for allowing delay-sensitive traffic, to share the network with other data, which is key to a mainframe's operation. This feature is not available in any other LAN protocol, except Asynchronous Transfer Mode (ATM).

Token Ring does come with a higher price tag because token ring hardware is more complex and more expensive to manufacture. As a network technology, token ring is passing out of use because it has a maximum speed of 16 Mbps which is slow by today's gigabit Ethernet standards.

  • Token Ring
  • Token passing
  • Media Access Unit
  • Line Interface Coupler (LIC)
  • Token Ring Interface Coupler (TIC)

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