3Com CoreBuilder 3500, Implementation Manual

Page: 100 / 592

100 C
HAPTER
6: F
IBER
D
ISTRIBUTED
D
ATA
I
NTERFACE
(FDDI)
MAC Services
Some of the services that the MAC performs include:
■
Frame repetition and reception
■
Frame removal
■
Frame validity criteria checking
■
Token capture
■
Token rotation
■
Ring initialization
■
Beacon process
MAC services are provided by all conforming stations that are attached to
the FDDI network.
MAC Operation
The MAC controls access to the physical medium by passing a token
around the ring. When a station receives the token, the station may
transmit a frame or a sequence of frames. When a station wants to
transmit, it removes the token from the ring and transmits the queued
frames. After transmission, the station issues a new token, which the
downstream station uses.
Stations that are not transmitting only repeat the incoming symbol
stream. When repeating, the station determines whether the information
was destined for it by comparing the destination address to its own
address. If it sees a match, the MAC processes subsequent received
symbols or sends them to the Logical Link Control (LLC) in the data-link
layer for translation.
Ports As parts of the Physical Layer, the PHY and PMD entities work together to
support each link between FDDI stations. These entities provide the
protocols that support the transmission and reception of signals between
stations, as well as the optical fiber hardware components that link FDDI
stations together. Within an FDDI station, the PHY and PMD entities make
up a
port . Together, they create a PHY/PMD pair that connects to the
fiber-optic media and that provides one end of a physical connection with
another station.