DP83261 datasheet(12/132 Pages) NSC

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40 FDDI MAC Facilities (Continued)

Beacon Frames

Beacon frames are transmitted continuously with minimum

preamble when the Ring Engine is in the Transmit Beacon

state The format of Beacon frames generated by the Ring

Engine is shown in Table 4-8 When long addressing is en-

abled frames with the long address are transmitted other-

wise frames with the short address are transmitted

When the Transmit Beacon State is entered from the Trans-

mit Claim State the first byte of the 4 byte TBT Field is

transmitted as Zero

Beacon frames that require alternative formats such as Di-

rected Beacons must be generated externally

The Ring Engine detects reception of valid Beacon frames

and distinguishes between Beacon frames transmitted by

this MAC (My

Beacon) and Beacon frames transmitted by

other stations (Other

Beacon) Details are given in Appen-

dix A

43 FRAME COUNTS

To aid in fault isolation and to enhance the management

capabilities of a ring the Ring Engine maintains several

frame counts The Error and Isolated frame counts incre-

ment when a frame is received with one or more errors that

were previously undetected The Ring Engine then corrects

the error such that a downstream station will not increment

its count

The size of the counters has been chosen such that minimal

software intervention is required even under marginal oper-

ating conditions

The following counts are maintained by the Ring Engine

FRCT

Frame Received

EICT

Error Isolated

LFCT

Lost Frame

FCCT

Frames Copied

FNCT

Frames Not Copied

FTCT

Frames Transmitted

431 Frame Received Count

The Frame Received Count (FRCT) is specified in the FDDI

MAC Standard and is the count of all complete frames re-

ceived This count includes frames stripped by this station

432 Error Isolated Count

The Error Isolated Count (EICT) is specified in the FDDI

MAC Standard and is the count of error frames detected by

this station and no previous station It increments when

1 An FCS error is detected and the received Error Indicator

(Er) is not equal to S

2 A frame of invalid length (ie off boundary T) is received

and Er is not equal to S

3 Er is not R or S

433 Lost Frame Count

The Lost Frame Count (LFCT) is specified in the FDDI MAC

Standard and is the count of all instances where a format

error is detected in a frame or token such that the credibility

of PDU reception is placed in doubt The Lost Frame Count

is incremented when any symbol other than data or Idle

symbols are received between the Starting and Ending Del-

imiters of a PDU (this includes parity errors)

434 Frame Copied Count

The Frames Copied Count (FCCT) is specified in the FDDI

MAC-2 Standard and is the count of the number of frames

copied by this station The count is incremented when an

internal or external match occurs (when OptionEMIND is

enabled) on the Destination Address no errors were detect-

ed in the frame and the frame was successfully copied

(VCOPY e 1) This can be used to accumulate station per-

formance statistics Frames copied promiscuously MAC

frames Void frames and NSA frames received with the A

indicator set are not included in this count

435 Frames Not Copied Count

The Frames Not Copied Count (FNCT) is specified in the

FDDI MAC-2 Standard and is the count of frames intended

for this station that were not successfully copied by this

station The count is incremented when an internal or exter-

nal (when OptionEMIND is enabled) Destination Address

match occurs no errors were detected in the frame and the

frame was not successfully copied (VCOPY e 0) This

count is an indication of insufficient buffering or frame pro-

cessing capability for frames addressed to the station MAC

frames Void frames and NSA frames received with the A

indicator set are not included in this count

436 Frames Transmitted Count

The Frames Transmitted Count (FTCT) is specified in the

FDDI MAC-2 Standard and is incremented every time a

complete frame is transmitted from the MAC Request Inter-

face The count is provided as an aid to accumulate station

performance statistics Void and MAC frames generated by

the Ring Engine are not included in the count

44 TIMERS

441 Token Rotation Timer

The Token Rotation Timer (TRT) times token rotations from

arrival to arrival TRT is used to control ring scheduling dur-

ing normal operation and to detect and recover from serious

ring error situations

TRT is loaded with the maximum token rotation time TMAX

when the ring is not operational TRT is loaded with the

negotiated Target Token Rotation Time TNEG when the

ring is operational

TABLE 4-8 Beacon Frames

Type

Enable

Size

SFS

INFO

FCS

EFS

Beacon

Not ELA

Short

Null

MSA

TBT

FCS

TRRR

Beacon

ELA

Long

Null

MLA

TBT

FCS

TRRR

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