DP83266 datasheet(17/152 Pages) NSC

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40 FDDI MAC Facilities (Continued)

435 Frames Not Copied Count (FNCT)

The Frames Not Copied Count is specified in the FDDI MAC

standard and is the count of frames intended for this station

that were not successfully copied by this station The count

is incremented when an internal or external (when

OptionEMIND is enabled) Destination Address match oc-

curs no errors were detected in the frame and the frame

was not successfully copied (VCOPY e 0) MAC frames

Void frames and NSA frames received with the A indicator

set are not included in this count

436 Frames Transmitted Count (FTCT)

The Frames Transmitted Count is specified in the FDDI

MAC standard and is incremented every time a complete

frame is transmitted from the MAC Request Interface Void

and MAC frames generated by the Ring Engine are not in-

cluded in the count

44 TIMERS

441 Token Rotation Timer (TRT)

The Token Rotation Timer (TRT) times token rotations from

arrival to arrival TRT is used to control ring scheduling dur-

ing normal operation and to detect and recover from serious

ring error situations

TRT is loaded with the maximum token rotation time TMAX

when the ring is not operational TRT is loaded with the

negotiated Target Token Rotation Time TNEG when the

ring is operational

442 Token Holding Timer (THT)

The Token Holding Timer 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 cap-

tured token is (still) usable for asynchronous transmission A

token is usable for asynchronous traffic if THT has not

reached the selected threshold Two asynchronous thresh-

olds are supported one that is fixed at the Negotiated Tar-

get Token Rotation Time (TNEG) and one that is program-

mable at one of 16 Asynchronous Priority Thresholds Re-

quests 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 (LTCT)

The Late Count is implemented differently than suggested

by the MAC standard but provides similar information The

function of the Late Count is divided between the Late

Flag that is equivalent to the MAC standard Late Count with

a non-zero value and a separate counter Late

Flag is

maintained 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 became non-operational When the ring is

non-operational 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 became non-operational (with TMAX resolution) in

order to determine if it is necessary to invoke recovery pro-

cedures When the ring becomes non-operational there is

no way to know how long it will stay non-operational There-

fore a timer is necessary If the Late Count were not provid-

ed SMT would be forced to start a timer every time the ring

becomes non-operational even though it may seldom be

used By using the provided Late Count an SMT implemen-

tation may be able to alleviate this additional overhead

444 Valid Transmission Timer (TVX)

The Valid Transmission Timer (TVX) is reset every time a

valid frame or token is received TVX is used to increase the

responsiveness of the ring to errors Expiration of the TVX

indicates that no frame or token has been received within

the timeout period and causes the Transmitter to invoke the

recovery (Claim) process

The Value of TVX is also used as the Duplicate MAC frame

detection delay DM

MIN This is the time after which a

MAC frame will be suspected as being generated by anoth-

er station with this station’s address when the ring is non-

operational

445 Token Received Count (TKCT)

The Token Received Count is incremented every time a val-

id token arrives The Token Count can be used with the

Ring Latency Count to calculate the average network load

over a period of time The frequency of token arrival is in-

versely related to the network load

446 Ring Latency Count (RLCT)

The Ring Latency Count is a measurement of time for

frames to propagate around the ring This counter contains

the last measured ring latency whenever the Ring Latency

Valid bit of the Token Event Register (TELR0RLVLD) 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 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 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

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