DP83261 datasheet(11/132 Pages) NSC

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40 FDDI MAC Facilities (Continued)

Frames Generated Externally

The Ring Engine transmits frames passed to it from the Sys-

tem Interface The data portion of the frame is created by

the System Interface This begins with the FC field and ends

with the last byte of the INFO field The FC field is passed

transparently to the ring The length bit in the FC field is

used to determine the length of the transmitted addresses

The data is passed as a byte stream across the MAC Re-

quest Interface as shown in Table 4-5

Before the frame is transmitted the Ring Engine inserts the

Start of Frame Sequence with at least 8 bytes of Preamble

but no more than 40 bytes of Preamble The starting delimi-

ter is transmitted as a JK symbol pair The Source Address

is normally transmitted by the Ring Engine since it uses the

Source Address to strip the frame from the ring This can be

overridden by using the Source Address transparency capa-

bility Similarly the Frame Check Sequence (4 bytes) is nor-

mally transmitted by the Ring Engine This can be overrid-

den with the FCS transparency capability With FCS trans-

parency the FCS is transmitted from the data stream The

End of Frame Sequence is always transmitted by the Ring

Engine as TR RR

Frames transmitted by the Ring Engine must have a valid

DA and SA field If the end of a frame is reached before a

valid length is transmitted the frame will be aborted and a

Void frame will be transmitted

TABLE 4-5 Frame Formats

Field

Size

Request

8 s40

Idle Pairs

2 or 6

MSA MLA

or SA

INFO

FCS

4 if Present

FCS

Frames Generated by the Ring Engine

The Ring Engine generates and detects several frames in

order to attain and maintain an operational ring

Void Frames

Void frames are used during normal operation The Ring

Engine generates two types of void frames regular Void

frames and My

Void frames See Table 4-6

If short addressing is enabled Void frames with the short

address are transmitted otherwise Void frames with the

long address are transmitted

Void frames are transmitted in order to reset the Valid

Transmission timers (TVX) in other stations in order to elimi-

nate an unnecessary entry to the Claim state Stations are

not required to copy Void frames Void frames are transmit-

ted by the Ring Engine in two situations

1 While holding a token when no data is ready to be trans-

mitted

2 After a frame transmission is aborted

Void frames are transmitted by the Ring Engine in

three situations

1 After a request to measure the Ring Latency has been

made when the next early token is captured

2 After this station wins the Claim Process before the token

is issued

3 After a frame has been transmitted with the STRIP option

before the token for that service opportunity is issued

Void frames are also detected by the Ring Engine A Void

frame with a Source Address other than MSA or MLA is

considered an Other

Void frame

Claim Frames

Claim frames are generated continuously with minimum pre-

amble while the Ring Engine is in the Transmit Claim state

The format of Claim frames generated by the Ring Engine is

shown in Table 4-7 When long addressing is enabled

frames with the long address are transmitted otherwise

frames with the short address are transmitted

The Ring Engine detects reception of valid Claim frames A

comparison is performed between the (first) four bytes of

the received INFO field and TREQ in order to distinguish

Higher

Claim Lower

Claim and My

Claim Details are

given in Appendix A

TABLE 4-6 Void Frames

Type

Enable

Size

SFS

FCS

EFS

Void

ESA

Short

Null

MSA

FCS

TRRR

Void

Not ESA

Long

Null

MLA

FCS

TRRR

Void

ESA

Short

MSA

FCS

TRRR

Void

Not ESA

Long

MLA

FCS

TRRR

TABLE 4-7 Claim Frames

Type

Enable

Size

SFS

INFO

FCS

EFS

Claim

Not ELA

Short

MSA

TREQ

FCS

TRRR

Claim

ELA

Long

MLA

TREQ

FCS

TRRR

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