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APPENDIX A: BASIC FDDI NETWORKS
Page A-6 BRIM-F6 User’s Guide
Other functions of the PHY include generation of a 125 Mhz transmit
clock, synchronization of the receive clock with an upstream transmitter,
encoding and decoding for media control symbols, and in some
applications, buffering for the incoming bit stream.
The MAC entity resides on the lower sublayer of the Data Link layer. The
upper sublayer, Logical Link Control (LLC) serves as an interface
between the OSI model and the FDDI network. The MAC element, under
control of Station Management, performs many of the tasks associated
with frame preparation and media access: ring scheduling, token
generation, timers that monitor ring activity, ring initialization, and
beaconing. Other tasks for the MAC entity include assembling data
frames, maintaining medium addressing, and generating and checking
data check bytes.
The MAC generates two basic message formats, tokens and frames.
Figure A-4 shows the layout for each message. Control and format bits
within the header define specific types of frames and classes of tokens.
The MAC sublayer receives transmittable Data frames from the LLC as
Service Data Units (SDUs). MAC uses these SDUs to construct Protocol
Data Units (PDUs) that it passes on to PHY. The PDUs consist of a MAC
header, the encapsulated SDU, and a Frame Check Sequence (FCS). The
MAC generates FCS during transmission.
Figure A-4. Token and Frame Formats
Frame
Control
2 Symbols
Preamble
≥ 16 Symbols
Starting
Delimiter
2 Symbols
Destination Address
4 or 12 Symbols
Source Address
4 or 12 Symbols
Information
≥ 0 Symbol Pairs
Frame Check
Sequence
8 Symbols
Ending
Delimiter
1 Symbols
Frame Status
≥ 3 Symbols
Frame
Control
2 Symbols
Ending
Delimiter
2 Symbols
Preamble
≥ 16 Symbols
Starting
Delimiter
2 Symbols
JK TT
Frame Check
Sequence Coverage
T
Maximum - 9000 Symbols
TOKEN
FRAME
BRIM-F Book Page 6 Monday, January 29, 1996 9:26 AM