Fibre Channel Primer

Chapter 4: Physical and Signaling Layer

Sequences) to be retransmitted. Each Sequence is uniquely identified

by the initiator of the Sequence via the Sequence Identifier (SEQ_ID)

field within the frame header. Additionally, each frame within the

Sequence is uniquely numbered with a Sequence Count (SEQ_CNT).

4. Exchange. An Exchange is composed of one or more

nonconcurrent Sequences for a single operation. For example, an

operation may consist of several phases: a command to read some

data, followed by the data, followed by the completion status of the

operation. Each phase of command, data, and status is a separate

Sequence, but they can form a single Exchange. Within a single

Exchange, only a single Sequence may be flowing at any one time,

although Sequences for different Exchanges may be concurrently

active. This is one form of multiplexing supported by Fibre Channel.

The Exchange is uniquely identified by each participating N_Port.

An Originator Exchange ID (OX_ID) is assigned, in an

implementation-dependent manner, by the originating N_Port

(initiator of the first Sequence), and a Responder Exchange ID

(RX_ID) is assigned, in an implementation-dependent manner, by

the responding N_Port (recipient of the first Sequence). The Exchange

IDs are contained within the frame header and are used locally by the

N_Ports to manage the Exchange.