Welltech Dialogic SS7 Protocols ISUP - Manual (Page 94)

Section 6 Interface to Application
94
6.8.3 Hop Counter Procedure
The ITU-T Recommendation Q.764 (09/97) specifies a hop counter procedure
which is designed to detect routing errors introduced when configuration
changes are made for instance when new circuits are added. A hop counter
parameter may optionally be included in a Setup Request primitive and its
value is decremented at each exchange. This is a temporary problem and the
hop counter procedure is optional.
Configuration
To activate the hop counter procedure, the initial hop count value should be
configured and set in the ‘ihop’ per circuit group parameter in the Configure
Circuit Group Request message (0x7701) message (refer to
Section : 8.2
Configure Circuit Group Request on page 126 ).
Note that the hop counter procedure will not be activated if the initial hop
counter value is set to zero.
Procedure
If the hop counter procedure is activated (i.e. initial hop count value is
greater than zero):
• If the hop counter parameter (refer to Section : 6.7 Parameter Definitions
on page 80 ) is present in the Setup Request primitive, the value of the
hop counter parameter will be decremented by one. However, if the
value of the hop counter reaches zero signifying that a routing error has
been detected, ISUP will release the call by sending a Release indication
with cause #25 (exchange routing error) to the user application. In order
to return the circuit to the idle state the call clearing sequences as
described in
Section : 6.8.1 Call Clearing Procedure on page 90 shall
apply. A Maintenance Event Indication with status CCm_CC_Zero_hops
(0x3b) will also be reported indicating the circuit on which the routing
error was detected.
• If the Setup Request primitive does not contain a hop counter parameter,
a hop counter parameter will be added, by ISUP, to the Setup Request
primitive (before the it is sent to the network) and the value of the hop
counter will be set to the initial hop count value.
Note: If the hop counter parameter is to be included, an additional 3 bytes will be added
to the Setup Request primitive.
If the hop counter procedure is not activated (i.e. initial hop count value is
set to zero):
• If the hop counter parameter is present in the Setup Request primitive,
the value of the hop counter parameter will not be decremented but will
be included and conveyed in the Setup Request primitive.
• If the Setup Request primitive does not contain a hop counter parameter,
a hop counter parameter will not be added, by ISUP, to the Setup
Request primitive.