Cisco 7000 Hardware Installation And Maintenance page 99

Figure 2-20 Token Ring Network Physical Connections
MAU or MSAU
Ring
in 1 2 3 4
Stations
MAU or MSAU
Ring
in 1 2 3 4
Stations
Token Ring and IEEE 802.5 are token-passing networks, which move a small frame, called a token,
around the network. Possession of the token grants the right to transmit; a station with information
to transmit must wait until it detects a free token passing by.
When all stations on the ring are idle, the token packet is labeled as a free token. If the station
receiving the token has no information to send, it simply passes the free token to the next station. A
station that does have information to transmit seizes the token, alters a bit of the token frame (which
changes the free token to a busy token), appends the information it wishes to transmit, and then sends
this information to the next station on the ring. The busy token, with the information frame,
circulates the ring until it reaches the intended destination station, which copies the information for
further processing and passes the busy token and information frame back out to the ring. The
information frame continues to circle the ring until it reaches the original sending station, which
checks the returned frame to ensure that the destination station received the information. When the
original sending station determines that the receiving station accepted the information, it purges the
token and information frame, and transmits a new free token out to the ring.
While the information frame is circling the ring, there is no token on the network (unless early token
release is enabled, which is described later in this section), so other stations wishing to transmit must
wait. Therefore, collisions cannot occur in Token Ring networks.
A priority scheme allows the user to designate any station as high priority, which allows the station
to use the ring more frequently than lower-priority stations. Stations cannot seize or reserve a passing
token with a higher priority than its own designated priority, which allows highest-priority stations
to seize the token as soon as possible. The token frame contains a priority field, which is set by the
station that generates the token, and a reservation field, which can be set by a higher-priority station
as it receives and passes the token.
Ring
5 6 7 8 out
Patch
cables
Ring
5 6 7 8 out
Lobe
cables
Preparing Network Connections
MAU or MSAU
Ring
in 1 2 3 4 5
Stations
MAU or MSAU
Ring
in 1 2 3 4 5
Stations
Preparing for Installation 2-95
Ring
6 7 8 out
Ring
6 7 8 out