AIS AI2524 user manual ITU-T carries out the functions of the former, Telephone Ccitt

Chapter 12: AI2524 X.25 Configuration Steps

Set the Virtual Circuit Ranges

The X.25 protocol maintains multiple connections over one physical link between a DTE and a DCE. These connections are called virtual circuits or Logical Channels (LCs). X.25 can maintain up to 4095 vir- tual circuits numbered 1 through 4095. You identify an individual vir- tual circuit by giving its Logical Channel Identifier (LCI) or Virtual Circuit Number (VCN). Many documents use the terms virtual circuit and LC, VCN, LCN, and LCI interchangeably. Each of these terms re- fers to the virtual circuit number

An important part of X.25 operation is the range of VCNs. VCNs are broken into four ranges (listed here in numerically increasing order):

1.Permanent virtual circuits (PVCs)

2.Incoming-only circuits

3.Two-way circuits

4.Outgoing-only circuits

The incoming-only, two-way, and outgoing-only ranges define the VCNs over which a Switched Virtual Circuit (SVC) can be established by the placement of an X.25 call, much like a telephone network es- tablishes a switched voice circuit when a call is placed.

The rules about DCE and DTE devices initiating calls are:

zOnly the DCE device can initiate a call in the incoming-only range.

zOnly the DTE device can initiate a call in the outgoing-only range.

zBoth the DCE device and the DTE device can initiate a call in the two-way range.

The ITU-T Recommendation for X.25 defines incoming and outgoing in relation to the DTE or DCE interface role; this documentation uses the more intuitive sense. Unless the ITU-T sense is explicitly refer- enced, a call received from the interface is an incoming call and a call sent out the interface is an outgoing call.

Note:	The ITU-T carries out the functions of the former
	Consultative Committee for International Telegraph and
	Telephone (CCITT).

There is no difference in the operation of the SVCs ranges except th restrictions on which device can initiate a call. These ranges can be

August 1997	Page 12-3
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