DP83261 datasheet(13/132 Pages) NSC

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40 FDDI MAC Facilities (Continued)

442 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

443 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

444 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

445 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

446 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 (TELRRLVLD)

is one

The Latency Counter increments every 16 byte times

(128 ms) and is used to measure ring latencies up to

13421772 seconds directly with an accuracy of 12 ms No

overflow or increment event is provided with this counter

45 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

451 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

452 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

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