Information Technology Reference
In-Depth Information
Figure6-1
IBM Token Ring Network/IEEE 802.5 Comparison
IBM Token
Ring Network
IEEE 802.5
4 or 16
Mbps
4 or 16
Mbps
Data rates
280 (shielded
twisted pair)
-2 (unshielded
twisted pair)
Stations/segment
250
Topology
Star
Not specified
Media
Twisted pair
Not specified
Signaling
Baseband
Baseband
Access method
Token passing
Token passing
Differential
Manchester
Differential
Manchester
Encoding
Token Passing
Token Ring and IEEE 802.5 are the primary examples of token-passing networks. Token-passing
networks move a small frame, called a
token
, around the network. Possession of the token grants the right
to transmit. If a node receiving the token has no information to send, it simply passes the token to the
next end station. Each station can hold the token for a maximum period of time.
If a station possessing the token does have information to transmit, it seizes the token, alters 1 bit of the
token (which turns the token into a start-of-frame sequence), appends the information it wishes to
transmit, and finally sends this information to the next station on the ring. While the information frame
is circling the ring, there is no token on the network (unless the ring supports early token release), so
other stations wishing to transmit must wait. Therefore, collisions cannot occur in Token Ring networks.
If early token release is supported, a new token can be released when frame transmission is complete.
The information frame circulates the ring until it reaches the intended destination station, which copies
the information for further processing. The information frame continues to circle the ring and is finally
removed when it reaches the sending station. The sending station can check the returning frame to see
whether the frame was seen and subsequently copied by the destination.
Unlike carrier sense multiple access collision detect (CSMA/CD) networks—such as
Ethernet—token-passing networks are deterministic. In other words, it is possible to calculate the
maximum time that will pass before any end station will be able to transmit. This feature and several
reliability features, which are discussed in the section “Fault Management Mechanisms” later in this
chapter, make Token Ring networks ideal for applications where delay must be predictable and robust
network operation is important. Factory automation environments are examples of such applications.
