3Com LANPLEX 2500, Руководство по эксплуатации

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9-4 C
HAPTER
9: FDDI O
VERVIEW AND
I
MPLEMENTATION
Each port is one of four types: A, B, M, and S.
■
A port — Connects to the primary ring on the incoming fiber and the
secondary ring on the outgoing fiber. A properly formed FDDI dual ring is
composed of a set of stations with the A port of one station connected to
the B port of the neighboring station.
■
B port — Connects to the incoming fiber of the secondary ring and the
outgoing fiber of the primary ring.
■
M port — Also referred to as Master port; used by a concentrator station to
provide connections within a concentrator tree.
■
S port — Also referred to as Slave port; used by a single attachment station
to provide attachment to an M port within a concentrator tree.
MACs
The Media Access Control (MAC) uses a token-passing protocol to
determine which station has control of the physical medium (the ring). The
primary purpose of the MAC is to deliver frames to their destinations by
scheduling and performing all data transfers.
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, and the beacon process. MAC services are
provided by all conforming stations attached to the FDDI network.
MAC Operation The MAC controls access to the physical medium by passing a token around
the ring. When the token is received by a station, 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 transmis-
sion, the station issues a new token, which is used by the downstream sta-
tion.
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.