dp83261 National Semiconductor Corporation, dp83261 Datasheet - Page 14

• Manufacturer Part Number: dp83261
• Description: Bmac Device Fddi Media Access Controller
• Manufacturer: National Semiconductor Corporation

4 0 FDDI MAC Facilities
4 5 3 Restricted Asynchronous Service Class
The Restricted Asynchronous service class is useful for
large transfers requiring all of the available Asynchronous
bandwidth The Restricted Token service is useful for large
transfers requiring all of the available (remaining) asynchro-
nous bandwidth
The Restricted Token service may also be used for opera-
tions requiring instantaneous allocation of the remaining
synchronous bandwidth when Restricted Requests are
serviced with THT disabled This is useful when it is neces-
sary to guarantee atomicity i e that a multi-frame request
will be serviced on a single token opportunity
A Restricted dialogue consists of three phases
1 Initiation of a Restricted dialogue
2 Continuation of a Restricted dialogue
3 Termination of a Restricted dialogue
Initiation of a Restricted dialogue will prevent all Non-re-
stricted Asynchronous traffic throughout the ring for the du-
ration of the dialogue but will not affect Synchronous traffic
To ensure that the Restricted traffic is operating properly it
is possible to monitor the use of Restricted Tokens on the
ring When a Restricted Token is received the event is
latched and under program control may generate an inter-
rupt In addition a request to begin a Restricted dialogue
will only be honored if both the previous transmitted Token
and the current received Token were Non-restricted tokens
This is to ensure that the upper bound on the presence of a
Restricted dialogue in the ring is limited to a single dialogue
As suggested by the MAC-2 Draft standard to help ensure
that only one Restricted dialogue will be in progress at any
given time Restricted Requests are not serviced after a
MAC frame is received until Restricted Requests are explic-
itly enabled by management software Since the Claim Pro-
cess results in the generation of a Non-restricted Token
this prevents stations from initiating another restricted dia-
logue without the intervention of management software
4 5 4 Immediate Service Class
The Immediate service class facilitates several non-stan-
dard applications and is useful in ring failure recovery (e g
Transmission of Directed Beacons) Certain ring failures
may cause the ring to be unusable for normal traffic until
the failure is remedied









Capture a Non-Restricted Token
Transmit zero or more frames to establish a Restricted
dialogue with other stations
Issue a Restricted Token to allow other stations in the
dialogue to transmit frames
Capture a Restricted Token
Transmit zero or more frames to continue the Restrict-
ed dialogue
Issue a Restricted Token to allow other stations in the
dialogue to transmit frames
Capture a Restricted Token
Transmit zero or more frames to continue the Restrict-
ed dialogue
Issue a Non-restricted Token to return to the Non-re-
stricted service class
(Continued)
14
Immediate requests are only serviced when the ring is non-
operational Immediate requests may be serviced from the
Transmitter Data Claim and Beacon states Options are
available to force the Ring Engine to enter the Claim or
Beacon state to prohibit it from entering the Claim state or
to remain in the Claim state when receiving My Claim
On the completion of an Immediate request a Token (Non-
restricted or Restricted) may optionally be issued Immedi-
ate requests may also be used in non-standard applications
such as a full duplex point to point link
5 0 Functional Description
5 1 TOKEN HANDLING
5 1 1 Token Timing Logic
The FDDI Ring operates based on the Timed Token Rota-
tion protocol where all stations on the ring negotiate on the
maximum time that the stations have to wait before being
able to transmit frames This value is termed the Negotiated
Target Token Rotation Time (TTRT) The TTRT value is
stored in the TNEG Register
Stations negotiate for TTRT based on their TREQ that is
assigned to them upon initialization
Each station keeps track of the token arrival by setting the
Token Rotation Timer (TRT) to the TTRT value If the token
is not received within TTRT (the token is late) the event is
recorded by setting the Late Flag If the token is not re-
ceived within twice TTRT (TRT expires and Late Flag is
set) there is a potential problem in the ring and the recovery
process is invoked
Furthermore the Token Holding Timer (THT) is used to limit
the amount of ring bandwidth used by a station for Asyn-
chronous traffic once the token is captured Asynchronous
traffic is prioritized based on the Late Flag which denotes
a threshold at TTRT and an additional Asynchronous Priori-
ty Threshold (THSH) The Asynchronous Threshold com-
parison (Apri 1) is pipelined so a threshold crossing may not
be detected immediately however the possible error is a
fraction of the precision of the threshold values
The Token Timing Logic consists of two Timers TRT and
THT in addition to the TMAX and TNEG values loaded into
these counters (See Figure 5-1 )
The Timers are implemented as count-up counters that in-
crement every 80 ns The Timers are reset by loading TNEG
or TMAX into the counters where TNEG and TMAX are un-
signed twos complement numbers This allows a Carry flag
to denote timer expiration
On an early token arrival (Late Flag is not set) TRT is
loaded with TNEG and counts up On a late token arrival
(Late Flag is set) Late Flag is cleared and TRT contin-
ues to count When TRT expires and Late Flag is not set
Late Flag is set and TRT is loaded with TNEG
THT follows the value of TRT until a token is captured
When a token is captured TRT may be reloaded with TNEG
while THT continues to count from its previous value (THT
does not wrap around) THT increments when enabled THT
is disabled during synchronous transmission and a special
class of asynchronous transmission THT is used to deter-
mine if the token is usable for asynchronous requests