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82 Chapter 3: Switched/Campus LAN Solutions
Figure 3-3
Token Ring Network
Token Ring Operation
The Token Ring network specifies a star, with all end stations attached to a device called a
multistation access unit (MAU). Token Ring is a token-passing technology and is an alternative
to Ethernet's collision-detection method. Token-passing networks operate at 4 or 16 Mbps and
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 passes the token to the
next end station.
Each station can hold the token for a determined maximum period of time. If a station
possessing the token does have information to transmit, it seizes the token, alters one bit of the
token (which turns the token into a start-of-frame sequence), appends the information it wants
to transmit, and sends this information to the next station on the ring. While the information
frame is circling the ring, no token is on the network (unless the ring supports early token
release
), which means that other stations wanting to transmit must wait. As a result, collisions
cannot occur in Token Ring.
Token Ring networks generally have higher utilization of available bandwidth. Token Ring
networks are ideal for applications where delay must be predictable and robust network
operation is important. Individual Token Rings are interconnected with bridges. In recent
years, wire-speed Token Ring switches have replaced the original slow and expensive bridges,
making more rings with fewer stations attached to each ring an economical alternative for
Token Ring users.
Why Use Token Ring Switches?
The traditional method of connecting multiple Token Ring segments is to use source-route
bridging (SRB). Bridges can be used to link workgroup rings to the backbone ring. Bridges
introduce congestion and reduce performance at the user's workstation. To maintain
performance, the CCDP could use a collapsed backbone. Collapsed backbone routers can offer
greater throughput than bridges and can interconnect a larger number of rings without
becoming overloaded. However, routers have a relatively high price per port, and throughput
typically does not increase as ports are added. As a collapsed backbone device, a Token Ring
switch offers a lower per-port cost and can incur lower latency than a router. In addition, you
Token
Ring
Token
Ring
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