X25AM Programming Manual

Introduction
X25AM Programming Manual—527201-001
1-3
Port Numbers and Subaddressing
During the call setup phase, a Call Request packet is sent by the calling network, and
an Incoming Call packet is received by the called network. These packets contain the
called and calling addresses and can contain information about optional network
facilities. Once a Call Connected packet is received or a Call Accepted packet is sent,
a virtual circuit (SVC) is established.
During the data-transfer phase, data is exchanged in both directions simultaneously
when either the PTP or the ITI protocol is used.
During the call clearing phase in an SVC connection, a Clear Request packet is sent
and a Clear Indication packet is received. Then a Clear Confirmation packet is sent
and received.
For a PVC connection no Call Setup or Call Clearing information is required.
Port Numbers and Subaddressing
A network routes calls to a DTE using the called address contained in the Call Request
or Incoming Call. X25AM implements port numbers (also referred to as subaddressing
by some networks) so that incoming calls can be directed to specific subdevices or
applications on the X.25 link.
Port numbers can be used as the link from the numeric address to the subdevice. The
port number is derived from the two or more digits following the incoming address. Not
all subdevices have port numbers.
Figure 1-2 illustrates the association between subdevices and port numbers.
Figure 1-1. Call Setup, Data Transfer, and Call Clearing
File-system Procedure Local DTE or DCE Remote DCE or DTE
Call Setup Phase
CONTROL 17 (SVC only) Call Request Incoming Call
Call Connected Call Accepted
CONTROL 11 (SVC only) Incoming Call Call Request
Call Accepted Call Connected
Data Transfer Phase
WRITE, Data Data
READ, Data Data
or WRITEREAD
Call Clearing Phase
CONTROL 12 (SVC only) Clear Request Clear
Indication
Clear Clear
Confirmation Confirmation
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