9000

20 Chapter 1

HP FDDI/9000 Resources

FDDI Concepts

The Fiber Distributed Data Interface (FDDI) is a high speed local

area network which has been deﬁned as a standard by an American

National Standards Institute committee, ANSI X3T9.5 and by ISO.

FDDI is a 100 Mbps, token-passing, single or dual ring interface that can

be implemented with Fiber Optic or Unshielded Twisted-Pair (UTP)

media. A Timed Token Protocol (TTP) is used to control when a station

can transmit data to the network. A station can transmit a message on

the network only after it has received a token. Upon receiving the token,

a station begins transmitting data. The station can transmit until the

message is transmitted or until the TTP timer expires. This allows all

stations fair access to the ring. Once the message is sent or the timer

expires, the station generates a new token and releases it on the ring.

Any downstream station with data to send can capture the token and

repeat the timed-transmission cycle.

A dual ring conﬁguration for the network media provides a secondary

backup ring in case of a fault on the primary ring. It is typically

implemented as a campus backbone or within buildings where a failure

in the primary ring would have serious consequences. A break in the

primary ring causes the two stations on each side of the fault to

automatically wrap the data to the secondary ring. Stations in a single

ring conﬁguration can only attach to the primary ring. There is no

secondary backup path in the event of a failure. A typical FDDI network

layout is shown in Figure 1-1.