DP83261AVF datasheet(15/132 Pages) NSC

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50 Functional Description (Continued)

TLF10387 – 3

FIGURE 5-1 Token Timing Logic

If TRT expires while Late

Flag is set TRT is loaded with

TMAX and the recovery process (Claim) is invoked When

TRT expires and the ring is not operational TRT is loaded

with TMAX TRT is also loaded with TMAX on a MAC Reset

512 Token Recovery

While the ring is operational every station in the ring uses

the Negotiated Target Token Rotation Time TNEG The

MAC implements the protocol for negotiation of this target

token rotation time (TTRT) through the Claim Process The

shortest requested Token Rotation Time is used by all of

the stations in the ring as the TNEG

If TRT expires with Late

Flag set a token has not been

received within twice TTRT (Target Token Rotation Time) If

TVX (Valid Transmission Timer) expires the station has not

received a valid token within TVX Max Both these events

require token recovery and cause the Ring Engine to enter

the Claim Process

In the Claim Process a MAC continuously transmits Claim

frames containing TREQ Should the MAC receive a Claim

frame with a shorter TREQ (larger valueHigher

Claim) it

leaves the Claim State A station that receives its own Claim

frame gains the right to send the first token and make the

ring operational again If the Claim Process does not com-

plete successfully TRT will expire and the Beacon Process

is invoked

The Beacon Process is used for fault isolation A station

may invoke the Beacon Process through an SM

Con-

trolrequest(Beacon) When a station enters the Beacon

Process it continuously sends out Beacon frames The

Beacon Process is complete when a station receives its

own Beacon frame That station then enters the Claim Pro-

cess to re-initialize the ring

52 SERVICING TRANSMISSION REQUESTS

A Request to transmit one or more frames is serviced by the

Ring Engine After a Request is submitted to the Ring En-

gine the Ring Engine awaits an appropriate Service Oppor-

tunity in which to service the Request Frames associated

with the Request are transmitted during the Service Oppor-

tunity The definition of a Service Opportunity is different

depending on the operational state of the ring

A Service Opportunity begins when the criteria presented to

the Ring Engine are met This criteria contains the request-

ed service class (synch asynch asynch priority immediate)

and the type of token to capture (restricted non-restricted

any none)

During a service opportunity the Ring Engine guarantees

that a valid frame is sent with at most 40 bytes of preamble

When data is not ready to be transmitted Void frames are

transmitted to reset the TVX timers in all stations During an

immediate request while in the Claim or Beacon States

when no Claim or Beacon frames are ready to be transmit-

ted the internally generated Claim or Beacon frames are

transmitted

521 Service Opportunity While Ring Operational

Beginning of Service Opportunity

Table 5-1 shows the conditions that must be true when a

valid token is received in order to begin a Service Opportu-

nity when the ring is operational

TABLE 5-1 Beginning of Service Opportunity

Requested

Requested Token

Criteria

Received

Service Class

Capture Class

Token Class

Asynchronous Priority

non-restricted

THT l THSH

non-restricted

Late

Flag e 0

Ring

Op e 1

Asynchronous

non-restricted

Late

Flag e 0

non-restricted

Ring

Op e 1

Asynchronous

restricted

Late

Flag e 0

restricted

Ring

Op e 1

Synchronous

any

Ring

Op e 1

any

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