Media Access Control MAC

Defines token-passing protocol for FDDI networks, as are packet formation, addressing and recovery mechanisms.

MAC controls the flow of Data on the ring.
Acting like a switch, MAC normally sources IDLE(IDL) control symbols for Transmission on the ring. When a start delimiter arrives on the ring, MAC will monitor each packet. If the packet is destined for another station, MAC will simply repeat the packet on the ring, only noting if a transmission error has occurred. If the packet is addressed to MAC’s station, the packet will be copied into its buffers while simultaneously repeated on the ring. If the packet was sourced by MAC, the packet will be absorbed and not retransmitted. A fragment of the packet (about 6 bytes) is repeated by MAC. When MAC receives a token and has data to transmit, the token is absorbed. SWITCHING into a sourcing mode, MAC encapsulates its data into proper control symbols for an FDDI packet, and inserts the packet into the ring. MAC continues to Insert packets into the ring until it has completed its data transmission or token holding time has expired. At the completion of the transmission, MAC will issue a token that allows multiple packets from different stations to be on the ring concurrently, which increases the effective bandwidth utilization.

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