Order of Operation in Digit Manipulation

As you have seen, some digit manipulation techniques accomplish the same result. You can use all the various methods of digit manipulation on the same gateway, but it is not recommended and usually not needed. Choosing which technique to use is sometimes more of an art than a science. For instance, an emergency call to 911 routed out a POTS interface would have all of its digits stripped before tranmission, so the call would not go through. To remedy this, you could configure no digit-strip, or forward-digits 3, or prefix 911 on the dial peer. You could even configure a translation profile or number expansion to change the called number so that the digits 911 would be sent, although that would be the hardest way to do it.

The point at which you must perform the manipulation is also is a factor. If the digit manipulation should apply to all calls, or be done before a dial peer is matched, you can use one of the global techniques. If the manipulation will apply only to certain calls, you can configure it under the dial peers for those calls. Some techniques are specific to POTS or VoIP dial peers, so that might aid your decision.

Consistency is a major consideration if you have many gateways needing similar digit manipulations to manage. Pick a technique (or techniques, if necessary), and standardize on it. In general, stick to the simplest way of accomplishing what you want. This consistency makes the gateways easier to configure, easier to manage, easier to troubleshoot, and probably easier on the router.

A good understanding of how the different methods interact will help your decision making, and in troubleshooting problems, also. Figure 10-1 shows how a call flows through the router, and where each technique is applied.

Figure 10-1. Order of Operation in Digit Manipulation

A POTS call first comes in a voice port, is matched to an incoming dial peer, and then is matched to an outgoing dial peer. Rules that are assigned to the voice port are applied first, next any global number expansion, then those on the incoming dial peer, and lastly those on the outgoing dial peer. If a prefix command is under the dial peer, the translation is done first.

A VoIP call matches an incoming dial peer, then an outgoing dial peer, and possibly a POTS voice port. Globally assigned rules such as VoIP incoming translation profile and number expansion are executed first, then any rules on the incoming dial peer, and finally any rules on the outgoing dial peer. If a Tcl script is associated with a dial peer to manipulate the ANI or DNIS, the script runs after the dial peer is matched but before any translation profile associated with that dial peer.

Table 10-3 summarizes the order of operation for techniques discussed in this chapter.