DP83261AVF datasheet(18/132 Pages) NSC

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50 Functional Description (Continued)

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 by asserting signal SAIGT (Pin 11)

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 utilize Source Routing In these applica-

tions the SA can still be generated by the Ring Engine

even when routing information is inserted into the data

stream

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 frames 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

If any frame of a Service Opportunity requests this option

then all frames on that service opportunity will be stripped

using this method Void Stripping is invoked upon the asser-

tion of the STRIP signal (Pin 13) at the beginning of a frame

transmission

Void Stripping is also automatically invoked by this station if

it wins the Claim Process before the initial token is issued

This removes all fragments and ownerless frames from the

ring when the ring becomes operational

FCS Transparency

Normally the BMAC device generates and transmits the

FCS When the Frame Check Sequence Transparency op-

tion is selected the Ring Engine device does not append

the FCS to the end of the Information field This option is

selected by asserting signal FCST (Pin 14)

The receiving stations treat the last four bytes of the data

stream as the FCS

This option may be useful for end to end FCS coverage

when crossing FDDI bridges for diagnostic purposes or in

Implementer frames

54 FRAME VALIDITY PROCESSING

A valid frame is a frame that meets the minimum length

criteria and contains an integral number of data symbol

pairs between the Starting and Ending Delimiters as shown

in Table 5-4

On the Transmit side frames are checked to see that they

are of a minimum length If the end of a frame is reached

before a valid length is transmitted the frame will be abort-

ed and a Void frame will be transmitted (as with all aborted

frames) A MAC frame with a zero length INFO field will not

be aborted even though the Receiver will not recognize it as

a valid frame Frame lengths are not checked for the maxi-

mum allowable length (4500 bytes)

Also on the Transmit side the L bit in the FC field is

checked against the ESA and ELA bits in the Option Regis-

ter (if the SA Transparency option is not selected) to insure

that a frame of that address length can be transmitted If the

selected address length is not enabled the frame is aborted

at the beginning of the SA field If SA Transparency is se-

lected the frame is not aborted

TABLE 5-4 Valid Frame Length

Frame Types

Short Address

Long Address

Notes

(Minimum Number of Bytes)

Void

MAC

Including a 4 Byte

INFO Field

Non-MAC

Including a 0 Byte

INFO Field

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