DP83266 datasheet(18/152 Pages) NSC

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40 FDDI MAC Facilities (Continued)

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

453 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 ie 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

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

2 Continuation of a Restricted dialogue

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

3 Termination of a Restricted dialogue

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

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 to-

kens This is to ensure that the upper bound on the pres-

ence of a Restricted dialogue in the ring is limited to a single

dialogue

As suggested by the MAC-2 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 explicitly en-

abled by management software Since the Claim process

results in the generation of a Non-restricted Token this pre-

vents stations from initiating another restricted dialogue

without the intervention of management software

454 Immediate Service Class

The Immediate Service Class facilitates several non-stan-

dard applications and is useful in ring failure recovery (eg

Transmission of Directed Beacons) Certain ring failures

may cause the ring to be unusable for normal traffic until

the failure is remedied

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

50 Functional Description

(Ring Engine)

51 TOKEN HANDLING

511 Token Timing Logic

The FDDI Ring operates based on the Timed Token Rota-

tion protocol where all stations on the ring negotiate for 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

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