Understanding the network.pdf - Back to Home

have to transmit against the priority of the free token. This is set in the access
control field. If the token's control field priority is less than or equal to their traffic's
priority, they can transmit. If the token's priority is greater than the station's traffic
priority, they pass the token on without transmitting. The token priority is set using
the token priority reservation mechanism. If any station has high priority traffic to
send, when it gets the token it can set its frame's priority using the reservation bits
in the access control field, provided the reservation bits are not already set with a
priority higher than the traffic the station wants to send. When the reservation bits
are set and the currently transmitting station receives the transmitted frame back,
it reads the reservation bits, sets the new token's priority value to the reserved
value, and sends the token on to the ring.
Ring Management and Operation
The IBM and IEEE 802.5 Token Ring architectures both define a set of management
agents that operate on the ring station to provide facilities that maintain the ring's
proper operation. These ring management facilities include token generation and
traffic monitoring, ring error detection, ring error data collection, and ring recovery
functions. Each of these defined agents or "server" functions is performed (in most
cases) in addition to the station's primary function, which is to transmit and receive
tokens containing ULP application data. Any Token Ring-defined management
facility can be performed by any properly functioning ring station connected to the
ring. In this respect, like access to the transmission medium, each station is equal.
Each management server function is essentially a finite state machine, which is to
say that any station can change its operational "state" dynamically in response to
changes in the ring's larger operational state.
This dynamic state-based approach to implementing the ring management
functions reflects the underlying operational basis of Token Ring's design. The
design provides a fault-tolerant, self-recovering data transmission environment
that provides contentionless access to the transmission medium for all stations.
Each of these qualities is essential to provide data transmission services for the
transaction-oriented processing applications that operated on IBM's mainframe
computers (for which Token Ring was originally developed). With this background in
mind, it is easier to understand why the IEEE and IBM Token Ring specifications
differ in some areas, particularly when it comes to the defined ring management
functions.
In the case of the IEEE 802.5 specification, its focus is on defining a transmission
protocol and the basic management services needed for proper ring operation to
take place. Therefore, the IEEE specification focuses on the management server
functions of the active monitor and the standby monitor. The IBM Token Ring
architecture, on the other hand, is focused on providing a data transmission
protocol for transaction processing, which requires not only ring operation