ISDN Line Simulator User’s Manual
40-400-00012, Rev. E
14
Important: When the Simulator configuration is changed, always
reinitialize any connected devices. All devices must be compatible with
the selected switch protocol.
D Channel X.25
Packet Handling Function (PHF)
ISDN-BRI permits the use of the 16Kbs D Channel, usually reserved for
ISDN signaling, to exchange X.25 LAP-B user data with a host system.
While few applications for personal communications devices exploit this
service, it is available on some terminal adapters and integrated into some
telephone sets.
X.25 packet data is offered as an ISDN-BRI service, because the telephone
company 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 packets and other X.25 traffic on a multi-point line. For this reason,
the telephone company 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-2000 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.
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