TOKEN BUS

TOKEN BUS

A token bus network is much like a token-ring network except that the ends of the network do not meet to form the ring. Instead, the network is still terminated at both ends.

A token is still required before a node can use the network. Like in a token-ring, it needs to include the address of the destination along with the data it needs to send. Although in the token bus, it implements a virtual ring on the coaxial cable.

Though both topologies use tokens, the similarities end there, as token bus uses a different topology and the token-passing protocol is different. In a token-ring network, the token and data is passed to the next physical node along the line, but in a token bus network, it does not matter where the nodes are physically located since token-passing is done via a numeric sequence of node addresses. The token or data is passed to the next sequential node address no matter if the physical location of that node is at the very end of the bus network. This is the virtual ring; the physical layout of the network will not change it.

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.

A Token Ring network is a local area network (LAN) in which all computers are connected in a ring or star topology and a bit- or token-passing scheme is used in order to prevent the collision of data between two computers that want to send messages at the same time. The Token Ring protocol is the second most widely-used protocol on local area networks afterEthernet. The IBM Token Ring protocol led to a standard version, specified as IEEE 802.5. Both protocols are used and are very similar. The IEEE 802.5 Token Ring technology provides for data transfer rates of either 4 or 16 megabits per second.