Understanding the network.pdf - Back to Home

the next station. When a station has data to transmit, it captures the token and
transmits its data. The receiving station copies the transmitted frames and then
retransmits them back on to the ring. The amount of time a station has to transmit
data is governed by the token holding rules. When its transmission is complete or
when the holding timer has expired, the sending station transmits a new token. It is
the responsibility of the transmitting station to strip its transmitted frames off the
ring when they return from the destination station.
The stations attached to the ring determine the token holding rules through the use
of the Token Rotation Protocol (TRP). The TRP defines three classes of data traffic:
two synchronous classes and one asynchronous.
• Synchronous (Class 1) supports guaranteed bandwidth and response times
• Synchronous (Class 2) is restricted token traffic that supports point-to-point
sessions between stations.
• Asynchronous supports multiple traffic priorities
The synchronous classes allocate a dedicated amount of bandwidth; the remaining
bandwidth is used for asynchronous transmissions. This async bandwidth is shared
by all the stations through the use of timers. The TRP is tunable, and some of these
parameters are Target Token Rotation Timer (TTRT), Token Rotation Timer (TRT),
and the Token Holding Timer (THT). These and other tuning options allow network
performance to be tuned to best meet higher level application access needs.
The token holding rules are negotiated by the station management interface. The
negotiation process occurs whenever a new station joins the ring. This process is in
essence a bidding war between the stations to decide who will initialize the ring and
to agree upon a guaranteed service time. The bidding is done by having each station
announce how often it needs access to the token, based on its synchronous service
requirement. The station with the lowest bit wins. The winning bid is stored as the
TTRT, and the guaranteed service time is two times the TTRT. The winning station
sends the first token, and on the second pass other synchronous devices might start
to transmit. When the synchronous "class" stations have transmitted their data, the
asynchronous stations can have access to the token.
The FDDI frame has a maximum size of 4,500 bytes. The token and frame are two
different frame entities. They are not a single frame appended with delivery
information and data, as with Token Ring. The FDDI MAC frames carry SMT control
information and use 802.2 LLC-PDUs to transport ULP data. Figure 4.25 illustrates
the FDDI MAC data frame and token formats.