Digit Signaling, aka Digit Addressing, is the method by which digits are sent from a dialing endpoint (e.g. phone) to a CUCM server, or by a voice gateway or trunk. There are three known digit signaling methods:
- En Bloc dialing
- SIP Dial Rules
- Digit-by-digit dialing
1. En Bloc digit dialing
In the En Bloc digit signaling method, digits are sent as a whole block. Think of it like a brick that you send to UCM. This is the digit addressing method used on Type A Cisco phones, whether the endpoint signaling protocol is SIP or SCCP.
En Bloc dialing is the default method used with the famous gateway protocols -MGCP, H323 and SIP- and also on H323 trunks and SIP trunks. Besides, the resulting pattern of a Translation Pattern operation is sent as En bloc to the call control system.
2. SIP Dial Rules
It is important to know that all SIP phones support SIP dial rules, whether they are Type A or Type B. SIP dial rules are created on CUCM and downloaded by phones at registration time. The concept is simple: whenever a user dials a string of digits, it is matched against the SIP dial rules, digit by digit. If the match is positive (i.e there is a SIP dial rule that has that pattern), the string of digits is sent to CUCM… en bloc!
However, if there is no match against the SIP dial rules, the call is aborted. This mechanism saves CPU time for the call processing control system on CUCM. It’s like saying “Hey CUCM, I’m taking care of these bunch of strings. I won’t bother you until I find a positive match”.
3. Digit-by-digit dialing
In this type of digit signaling, I’d like to point that there are three variants:
- Gateway Overlap Sending&Receiving,
- Cisco’s implementation of the digit-by-digit method,
3.1 Digit-by-digit dialing: Gateway Overlap Sending&Receiving
When we have a digital voice gateway that is connected to ISDN PRI circuits, and Overlap Sending&Receiving (OSR) is configured, digits are sent (or received) one by one to/from the ISDN provider. Bare in mind that some countries such as Germany use OSR a lot with their customers’ PRI networks.
Enabling overlap receiving on PRI service parameter enables overlap receiving on all PRI links that use PRI backhaul.
3.2 Digit-by-digit dialing: Cisco’s implementation
This is the Cisco touch, as always. Cisco implements its own variation of digit-by-digit dialing on its SCCP phones.
3.3 Digit-by-digit dialing: KPML
KPML is enabled by default on all Type B Cisco SIP phones. One of the reasons is that KPML is an IETF standard. However, we can find it on some Type A phones too.