System information

Token Passing
Book Title
6-80
Figure 6-1 IBM Token Ring Network/IEEE 802.5 Comparison
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.
IBM Token
Ring Network
4 or 16
Mbps
280 (shielded
twisted pair)
–2 (unshielded
twisted pair)
Star
Twisted pair
Baseband
Token passing
Differential
Manchester
IEEE 802.5
4 or 16
Mbps
250
Not specified
Not specified
Baseband
Token passing
Differential
Manchester
Data rates
Stations/segment
Topology
Media
Signaling
Access method
Encoding