System information

Troubleshooting Fiber Distributed Data Interface 5-67
Traffic Types
Figure 5-2 FDDI Nodes: DAS, SASs, and Concentrator
Each FDDI DAS has two ports, designated A and B. These ports connect the station to the dual FDDI
ring. Therefore, each port provides a connection for both the primary and the secondary ring, as
shown in Figure 5-3.
Figure 5-3 FDDI DAS Ports
Traffic Types
FDDI supports real-time allocation of network bandwidth, making it ideal for a variety of different
application types. FDDI provides this support by defining two types of traffic: synchronous and
asynchronous. Synchronous traffic can consume a portion of the 100-Mbps total bandwidth of an
FDDI network, and asynchronous traffic can consume the rest. Synchronous bandwidth is allocated
to those stations requiring continuous transmission capability. Such capability is useful for
transmitting voice and video information, for example. Other stations use the remaining bandwidth
asynchronously. The FDDI SMT specification defines a distributed bidding scheme to allocate FDDI
bandwidth.
Asynchronous bandwidth is allocated using an eight-level priority scheme. Each station is assigned
an asynchronous priority level. FDDI also permits extended dialogues, where stations may
temporarily use all asynchronous bandwidth. The FDDI priority mechanism can essentially lock out
stations that cannot use synchronous bandwidth and have too low an asynchronous priority.
Fault-Tolerant Features
FDDI provides a number of fault-tolerant features, the most important of which is the dual ring. If
a station on the dual ring fails or is powered down or if the cable is damaged, the dual ring is
automatically “wrapped” (doubled back onto itself) into a single ring, as shown in Figure 5-4. In this
FDDI
ConcentratorDAS
SAS SAS SAS
Primary
Secondary
Primary
Secondary
Port A Port B
FDDI DAS