| 3.2 | Layer 2 Protocol Overview - LAN Protocols | ||
| 3.2.9 | FDDI access method |
FDDI actually has a much more complex
physical layer than most other LANs. Given that FDDI is frequently
used to interconnect LANs and premise edge routers, it is imperative
to examine this complexity in detail. FDDI supports two types of
connections:
This means that FDDI NICs for DASs have two sets of physical media interfaces. They are known as Ports A and B. Port A is the primary interface, and Port B is the secondary interface. DASs feature two sets of media
interfaces. This enables a DAS device to have a physical connection to
each of the two FDDI rings. Figure Note: A concentrator is a device that aggregates multiple LAN connections on to a common electrical backplane. The most common type of LAN concentrator is known as a hub. Concentrators can also be dual attached. Consequently the phrase dual attached (DA) can be used to describe either dual-attached stations or dual-attached concentrators. As shown in Figure DAS connections can form a repeater-less, peer-to-peer LAN. This is accomplished by connecting the A port of the interface of one device to the B port of another device, and vice versa. The drawback to this is that each DAS device must be powered on and must be functioning for the rings to be complete. FDDI can wrap around a break in the ring, but this directly impacts the performance of the entire ring. More significantly, if multiple stations are simultaneously powered down or otherwise out of service, the net result might be two or more smaller rings. SASs eliminate the potential
performance problems inherent in DAS by eliminating the wraparound
feature. Each SAS device has just a single communications interface,
S, with two media ports. The separate fibers are used to transmit and
receive. Both fibers terminate at the concentrator, providing the
connectivity to both rings. Figure
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