DP83261AVF datasheet(14/132 Pages) NSC

14 / 132 page

40 FDDI MAC Facilities (Continued)

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 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

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

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 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

DP83261AVF датащи(PDF) 14 Page - National Semiconductor (TI)

DP83261AVF датащи(HTML) 14 Page - National Semiconductor (TI)

Html Pages

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60 61 62 63 64 65 66 67 68 69 70 71 72 73 74 75 76 77 78 79 80 81 82 83 84 85 86 87 88 89 90 91 92 93 94 95 96 97 98 99 100 ...More

датащи скачать

ссылки URL