FDDI Connection Types

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:

• Single-attached stations (SAS)

• Dual-attached stations (DAS)

This means that FDDI network interface cards can 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 FDDI's two rings. Figure 6-1 illustrates the way that a dual-attached station connects to the LAN. Each DAS device has two sets of media interface ports, each containing both A and B ports. Each port contains physical connections for two physical media: one for transmitting and the other for receiving data. Thus, a DAS device on a fiber-optic FDDI network actually has four fibers connected to it.

Figure 6-1: Dual-attached stations in a FDDI ring.

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, it would be correct to refer to both concentrators and stations with the phrase dual attached (DA) without specifically identifying the devices.

Figure 6-1: Dual-attached stations in a FDDI ring.

As shown in Figure 6-1, the physical device actually becomes an integral part of the two rings because the NIC provides physical continuity for the two rings in the interval between the A and B ports. Each of the illustrated physical rings actually consists of two strands of fiber-optic cabling: one for transmit and one for receive.

DAS connections can form a repeaterless, peer-to-peer LAN. This is accomplished by connecting the A port of one device's interface to the B port of another device, and vice versa. The drawback to this is that each DAS device must be powered on and 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, multiple stations are simultaneously powered down or otherwise out of service; the net result might be two or more smaller ring pairs.

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, which provides the connectivity to both rings. Figure 6-2 illustrates a single-attached station with its concentrator.

As is the case in Figure 6-1, the physical connections indicated in Figure 6-2 actually consist of two strands of fiber-optic cabling.

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