dp83261 National Semiconductor Corporation, dp83261 Datasheet - Page 13

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

4 0 FDDI MAC Facilities
4 4 2 Token Holding Timer
The Token Holding timer (THT) is used to limit the amount
of ring bandwidth used by a station for asynchronous traffic
once the token is captured THT is used to determine if the
captured token is (still) usable for asynchronous transmis-
sion A token is usable for asynchronous traffic if THT has
not reached the selected threshold Two asynchronous
thresholds are supported one that is fixed at the Negotiated
Target Token Rotation Time (TNEG) and one that is pro-
grammable at one of 16 Asynchronous Priority Thresholds
Requests to transmit frames at one of the priority thresholds
are serviced when the Token Holding Timer (THT) has not
reached the selected threshold
4 4 3 Late Count
The Late Count (LTCT) is implemented differently than sug-
gested by the Standard but provides similar information
The function of the Late Count is divided beween the Late
Flag that is equivalent to the standard Late Count with a
non-zero value and a separate counter Late Flag is main-
tained by the Ring Engine to indicate if it is possible to send
asynchronous traffic When the ring is operational Late
Count indicates the time it took the ring to recover the last
time the ring went non-operational When the ring is non-op-
erational Late Count indicates the time it has taken (so far)
to recover the ring
The Late Count is incremented every time TRT expires
while the ring is non-operational and Late Flag is set (once
every TMAX)
The Late Count is provided to assist Station Management
SMT in the isolation of serious ring errors In many situa-
tions the ring will recover very quickly and late count will be
of marginal utility However in the case of serious ring er-
rors it is helpful for SMT to know how long it has been since
the ring went non-operational (with TMAX resolution) in or-
der to determine if it is necessary to invoke recovery proce-
dures When the ring goes no operational there is no way to
know how long it will stay non-operational therefore a timer
is necessary If the Late Count were not provided SMT
would be forced to start a timer every time the ring goes
non-operational even though it may seldom be used By us-
ing the provided Late Count an SMT implementation may
be able to alleviate this additional overhead
4 4 4 Valid Transmission Timer
The Valid Transmission Timer (TVX) is reset every time a
valid PDU is received TVX is used to increase the respon-
siveness of the ring to errors Expiration of the TVX indi-
cates that no PDU has been received within the timeout
period and causes the Transmitter to invoke the recovery
Claim Process
4 4 5 Token Received Count
The Token Received Count (TKCT) is incremented every
time a valid token arrives The Token Count can be used
with the Ring Latency Count to calculate the average net-
work load over a period of time The frequency of token
arrival is inversely related to the network load
4 4 6 Ring Latency Count
The Ring Latency Count (RLCT) is a measurement of time
for PDUs to propagate around the ring This counter con-
tains the last measured ring latency whenever the Ring La-
tency Valid bit of the Token Event Register (TELR RLVLD)
is one
(Continued)
13
The Latency Counter increments every 16 byte times
(1 28 ms) and is used to measure ring latencies up to
1 3421772 seconds directly with an accuracy of 1 2 ms No
overflow or increment event is provided with this counter
4 5 RING SCHEDULING
FDDI uses a timed token protocol to schedule the use of the
ring The protocol measures load on the network by timing
the rotation of the token The longer the token rotation time
the greater the instantaneous load on the network By limit-
ing the transmission of data when the token rotation time
exceeds a target rotation time a maximum average token
rotation time is realized The protocol is used to provide
different classes of service
Multiple classes of service can be accommodated by setting
different target token rotation times for each class of serv-
ice
The Ring Engine supports Synchronous Non-Restricted
Asynchronous Restricted Asynchronous and Immediate
service classes The Immediate service class is supported
when the ring is non-operational the other classes are sup-
ported when the ring is operational
4 5 1 Synchronous Service Class
The Synchronous service class may be used to guarantee a
maximum response time (2 times TTRT) minimum band-
width or both
Each time the token arrives a station is permitted to trans-
mit one or more frames in accordance with its synchronous
bandwidth allocation regardless of the status of the token
(late or early Restricted or Non-Restricted)
Since the Ring Engine does not provide a mechanism for
monitoring a station’s synchronous bandwidth utilization
the user must insure that no synchronous request requires
more than the allocated bandwidth
To help ensure that synchronous bandwidth is properly allo-
cated after ring configuration synchronous requests are not
serviced after a Beacon frame is received After a major
reconfiguration has occurred management software must
intervene to verify or modify the current synchronous band-
width allocation
4 5 2 Non-Restricted Asynchronous Service Class
The Non-Restricted Asynchronous service class is typically
used with interactive and background traffic Non-restricted
Asynchronous requests are serviced only if the token is ear-
ly and the Token Holding Timer has not reached the select-
ed threshold
Asynchronous service is available at two priority thresholds
the Negotiated Target Token Rotation Time plus one pro-
grammable threshold Management software may use the
priority thresholds to discriminate additional classes of traf-
fic based on current loading characteristics of the ring The
priority thresholds may be determined using the current
TTRT and the Ring Latency In this case application soft-
ware is only concerned with the priority level of a request
As an option Asynchronous Requests may be serviced with
THT disabled This is useful when it is necessary to guaran-
tee that a multi-frame request will be serviced on a single
token opportunity Because of the possibility of causing late
tokens this capability should be used with caution and
should only be allowed when absolutely necessary