Introduction to Tandem X.25 Capabilities

Flexible Call-Setup
Tandem X.25 Features and Capabilities
065307 Tandem Computers Incorporated 2–7
Reverse Charging Request and Acceptance
These facilities allow the destination DTE to accept the packet network charges (like a
collect call on a telephone network). The reverse charging request and acceptance
facilities can be used with either virtual calls or fast selects. Although these facilities
are closely related, they do not necessarily have to be used together.
Reverse charging requests the remote DTE to pay for the call. It is requested by the
calling DTE on a per-call basis.
Reverse charging acceptance authorizes the remote DCE to pass to the DTE the
incoming calls that request the reverse charging. If reverse charging acceptance is not
selected, the DCE does not pass the calls, and the originating DTE receives a clear
indication packet.
X25AM accepts an incoming Call Request packet with reverse charging requested and
assigns it to a subdevice configured to accept reverse charges (ACCEPTCHG
attribute). If there is no such subdevice, the call is cleared.
X25AM requests reverse charging in an outgoing Call Request packet when the
subdevice that issues the call is configured to request reverse charging (REVERSECHG
attribute).
Closed User Group (CUG) Selection
The CUG facility allows users to form groups of DTEs to which access is restricted (for
security and privacy). The CUG facility is established for a specified period of time.
When a DTE belongs to more than one CUG, the CUG selection must be specified
during call setup.
The X25AM attributes for CUG selection are CUG and CUGTYPE. These attributes
can be specified for either a line or a subdevice. If both the line and subdevice
attribute are specified, the subdevice attribute is used. The user application can also
specify a dynamic value for a subdevice. The dynamic value overrides the value set by
SCF and stays in effect until the subdevice is closed.
Bilateral Closed User Group (BCUG)
The BCUG facility allows access restrictions to be established between pairs of DTEs.
These restrictions can exclude access to and from other DTEs. The X25AM attribute,
BCUG, can be specified for either a line or a subdevice with SCF. If both the line and
subdevice attribute are specified, the subdevice attribute is used. The user application
can also specify a dynamic value for a subdevice after it is opened. The dynamic value
overrides the value set by SCF and stays in effect until the subdevice is closed.
X25AM ignores the BCUG in an Incoming Call Packet.
X25AM inserts the BCUG facility in a Call Request Packet if BCUG is specified for the
subdevice.