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40-400-00001, Rev. A
X.25 packet data is offered as an ISDN-BRI service, because the telephone com-
pany configured their network with it before any protocol had become popular. It
seems unlikely that X.25 will become more popular now that it is available via ISDN-
BRI. In fact, most Terminal Adapters that transport packet data across ISDN-BRI
lines do so by sending their preferred packet protocol over a 64Kbs B Channel
during a Circuit Mode Data call.
One severe limitation of using X.25 LAP-B protocol on the D Channel is that it must
share the bandwidth, or available throughput rate, with Call Control signaling pack-
ets and other X.25 traffic on a multi-point line. For this reason, the telephone com-
pany does not assure X.25 LAP-B data throughput exceeding 9600 baud.
There are three methods for establishing a D Channel X.25 Packet Mode Data
(PMD) call. The first, but least used, is to negotiate the call using ISDN-BRI Call
Control signaling. The other two methods employ X.25 Virtual Circuit channel
negotiation with the telephone company’s Packet Handling Function (PHF), or host
computer.
The second method, referred to as Semi-permanent Virtual Circuit or SVC, employs
a precursor to ISDN Call Control. Specific X.25 packet formats are used to initiate
calls, confirm “connections”, and clear calls when completed. This method allows
dial up for outgoing calls and notification of incoming calls based on a Directory
Number.
The third method of X.25 call is Permanent Virtual Circuit or PVC. This method does
not employ any form of call control, but simply establishes a constant virtual circuit
between two fixed nodes whenever both nodes are active.
The ILS-1000 Line Simulator supports a single channel for a Semi-Permanent or
Permanent Virtual Circuit connection between Port 1 and Port 2. To demonstrate
these services,
STEP 1
Run the Windows based configuration software.
STEP 2
Select the D Channel X.25 setting.
STEP 3
Change the setting to SVC (for dial-up services) or PVC (for a perma-
nent connection). Select OFF if X.25 functions are not required.
STEP 4
Remove all communications devices from the Simulator and press the
DOWNLOAD button. An automatic RESTART is issued as part of the
DOWNLOAD.
STEP 5
Connect a terminal that supports X.25 data on the D Channel to Port
1 and another to Port 2. When using PVC, the terminals should
automatically negotiate an open X.25 channel with the ILS-1000’s
PHF simulator. When using the SVC setting, dial the Directory Num-
ber assigned to the X.25 terminal at the other port to establish the
connection.
Note: Some terminals require that you send an X.25 RESET command manu-
ally before the terminal will establish a PVC connection.