Cabletron Systems BRIM-F6 manual Appendix a Basic Fddi Networks

APPENDIX A: BASIC FDDI NETWORKS

When a station has a frame waiting to transmit, the station captures the token at the next opportunity, transmits the data frame, and then reissues the token. (A Token Holding Timer (THT) controls the maximum length of time that any station may retain the token.) Each station receives and repeats the data frame as it circulates around the ring. When the frame arrives at the station defined by the destination address, the receiving station copies the frame into its buffer and forwards with information reflecting the receipt of the frame and related frame status. After the data frame circulates completely around the ring, the source station strips the data frame from the ring.

FDDI networks use duplex fiber optic cable for point-to-point connections between a number of stations to form two closed loops. The two rings serve as redundant (primary and secondary) data paths that operate as counter-rotating rings. Redundant rings facilitate recovery procedures in the event of a ring segment failure. This recovery resembles that in Token Ring/IEEE 802.5 networks. This Appendix discusses this in greater detail later in this section.

The FDDI standard defines two ring access methods, single attachment and dual attachment (see Figure A-1). Dual attached stations (DAS) and dual attached concentrators (DAC) connect to both primary and secondary rings and can restore ring continuity in the event of a segment failure. Single attached stations (SAS) and single attached concentrators (SAC) cannot restore ring continuity and therefore cannot reside on the main ring path.

Single attached devices access the main ring through a DAC and duplex fiber optic cable connections. These connections form branches that extend from the DAC out to each of the attached SASs, creating a Ring of Trees topology. The DAC controls main ring access to the attached SASs, restoring the continuity of the ring whenever an SAS fails, becomes disconnected, or is turned-off.

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