DP83266 datasheet(21/152 Pages) NSC

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50 Functional Description (Ring Engine) (Continued)

transmission is aborted due to an external error (and

OptionIRPT is not set) a Void frame is transmitted to reset

the TVX timers in all stations in the ring When a frame is

aborted due to a transmission error the service opportunity

is not automatically ended

53 REQUEST SERVICE PARAMETERS

531 Request Service Class

The Requested Service corresponds to the Request Serv-

ice Class and the Token Class parameters of the

(SM

)MA

DATArequest and (SM

)MA

Tokenrequest

primitives as specified in the Standard

TABLE 5-2 Token Transmission Type

Service Class

Token

Captured

Issued

Non-restricted

Non-restricted Non-restricted

Begin Restricted

Non-restricted Restricted

Continue Restricted

Restricted

End Restricted

Restricted

Non-restricted

Immediate

None

Immediate Non-restricted None

Non-restricted

Immediate Restricted

None

Restricted

14 useful combinations of the requested Service Class

(Non-Restricted Asynchronous Restricted Asynchronous

Synchronous Immediate) the Token Capture and Issue

Class and THT Enable are supported by the Ring Engine as

shown in Table 5-3

Requests are serviced on a Service Opportunity meeting

the requested criteria

A Token Capture Class of non-rstr indicates that the Trans-

mitter Token Class must be Non-Restricted to begin servic-

ing the request A Token Capture Class of rstr indicates

that the Transmitter Token Class must be Restricted to be-

gin servicing the Request A Token Issue Class of non-rstr

means that the Transmitter Token Class will be Non-restrict-

ed upon completion of the request A Token Issue Class of

rstr

means that the Transmitter Token Class will be Re-

stricted upon completion of the request

532 Request Options

The Request options provide the ability for Source Address

Transparency (SAT) and FCS Transparency (FCST) In both

cases data from the request stream is transmitted in place

of data from the Ring Engine The use of Source Address

Transparency has no effect on the sequencing of the inter-

face When Source Address transparency is not used the

SA from the internal parameter RAM is substituted for the

SA bytes in the request stream which must still be present

Since the FCS is appended to the frame when FCS trans-

parency is not used no FCS bytes are present in the re-

quest stream

Source Address Transparency (SAT)

Normally the SA field in a frame is generated by the Ring

Engine using either the MSA or MLA values programmed in

the parameter RAM When the SA Transparency option is

selected the SA from the data stream is transmitted in

place of the MSA or MLA The SAT option can be invoked

on a Request Object via the Request Configuration parame-

ters of the System Interface

When the SA Transparency option is selected it is neces-

sary to rely on an alternate stripping mechanism since strip-

ping based on the returning SA only guarantees that frames

with MSA or MLA will be stripped Either the Void Stripping

option (described below) may be invoked or external hard-

ware that forces stripping using the EM (External M

Flag)

signal is required

The MSB of the SA is not controlled by this option It is

normally forced to Zero It can be controlled using the

Source Address MSB Transparency option described be-

low

SA Transparency is possible for all frames (including MAC

frames) External support is required to limit the use of SA

Transparency to certain MAC Users SA Transparency

should not be used with externally generated MAC Frames

in order to maintain accountability but this is not enforced

by the Ring Engine When SA Transparency is used with

externally generated Beacon Frames that are transmitted

from the Beacon state the first 4 bytes of the INFO field are

passed transparently from the data stream instead of being

generated by the Ring Engine

SA Transparency also overrides the Long and Short Ad-

dressing enables For example if Long Addressing is not

enabled it is still possible to transmit frames with Long Ad-

dresses Similarly if Short Addressing is not enabled it is

still possible to transmit Frames with Short Addresses This

may be useful in full duplex point to point applications and

for diagnostic purposes

Source Address Most Significant Bit Transparency

With the Source Address MSB Transparency option the

MSB of the SA is sourced from the data stream as opposed

to being transmitted as Zero The SA MSB Transparency

option is selected through the Request Channel Configura-

tion Registers in the Service Engine

Unless the Source Address Transparency option is also se-

lected the rest of the SA is generated by the Ring Engine

The MSB of the SA is used to denote the presence of the

Routing Information Field used in Source Routing algo-

rithms (as in the IEEE 8025 protocol) This option is useful

for stations that use Source Routing In these applications

the SA can still be generated by the Ring Engine even

when routing information is inserted into the data stream

This allows the normal stripping to be accomplished in end

stations implementing Source Routing (without relying on

external software to not create no-owner frames)

Void Stripping

This option is useful for removing bridged and ownerless

frames and remnants (fragments) from the ring

In the Void Stripping protocol two My

Void frames are

transmitted at the end of a service opportunity Stripping

continues until one of the following conditions occur

One My Void frame returns (The Second My Void will

be stripped on the basis of the SA)

A Token is received

An Other Void is received

A MAC frame other than My Claim is received

A MAC Reset occurs

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