Hop Counter / Max Forward Interworking

The Hop Counter / Max Forwards Interworking field can enable or disable the mapping of the Hop Counter Parameter in the ISUP IAM message and the Max Forward Header in the SIP INVITE. When enabled, the IMG will map the Hop Counter parameter and the Max Forwards Header so there is a correlation between the two. This feature prevents a loop from happening when call is made between the PSTN and SIP domain. The Hop Counter / Max Forward feature is consistent with RFC 3261 and ITU Q.1912.5  specifications and is supported in both ANSI and ITU protocols. See information below.

 

Call Flow Diagrams

cf_f5084_hopcount_maxforward.png

 

Examples

ISUP to SIP (Example 1)

Hop counter is received in the ingress IAM: The Max-Forwards header value on the SIP side will be the value of the Hop counter less 1.

cf_f5084_isup_sip_example1.png

 

ISUP to SIP (Example 2)

No Hop counter received in the ingress IAM: The Max-Forwards header value on the SIP side will use the default value of 70 (as recommended by RFC 3261).

cf_f5084_isup_sip_example2.png

 

SIP to ISUP (Example 3)

The Max-Forwards header will always be received since it is a mandatory header. The value of the Hop Counter parameter on the SS7 side will be created with the value in the Max-Forwards less 1.

cf_f5084_sip_isup_example3.png

 

Configuration

See Interworking between SIP and SS7/ISUP topic

 

Additional Information