Information Technology Reference
In-Depth Information
Class B or single-attachment stations (SASs) attach to one ring; Class A or dual-attachment stations
(DASs) attach to both rings. SASs are attached to the primary ring through a concentrator, which
provides connections for multiple SASs. The concentrator ensures that failure or power down of any
given SAS does not interrupt the ring. This is particularly useful when PCs, or similar devices that
frequently power on and off, connect to the ring.
A typical FDDI configuration with both DASs and SASs is shown in Figure 5-2.
Figure5-2
FDDI Nodes: DAS, SASs, and Concentrator
FDDI
DAS
Concentrator
SAS
SAS
SAS
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.
Figure5-3
FDDI DAS Ports
Primary
Primary
Port A
Port B
Secondary
Secondary
FDDI DAS
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.
