Interworking ISDN Overlap Dialing with SIP According to RFC 3578

With overlap dialing disabled, the device expects to receive the digits all at once (enbloc) or with very little delay between digits and then sends the complete number in a single message. Overlap signaling sends portions of the number in separate messages as it collects the digits from the sender. The interval between receiving the digits (time between digits) is relatively long. However, overlap dialing allows the device to begin call setup (routing) even before all digits have been collected. For example, if the dialled (destination) number is "3312418", the device first receives the digits "331" and then routes the call based on these digits. It then delivers the remaining 4 digits "2418" in overlap mode. The device supports the interworking of ISDN overlap dialing to SIP and vice versa, according to RFC 3578.

■

Interworking ISDN overlap dialing to SIP (Tel to IP): The device sends the first digits (e.g., "331") received from the ISDN Setup message to the IP side in the initial SIP INVITE message. Each time it receives additional (collected) digits, which are received from subsequent Q.931 Information messages, it sends them to the IP side in SIP re-INVITE or SIP INFO messages. You can use the following parameters to configure overlap dialing for Tel-to-IP calls:

●

ISDNRxOverlap: Enables Tel-to-IP overlap dialing and defines how the device sends the collected digits to the IP side - in SIP re-INVITE [2] or INFO messages [3].

●

MinOverlapDigitsForRouting: Defines the minimum number of overlap digits to collect from the Tel side before the device can send the first SIP message (INVITE) for routing the call to the IP side.

●

MaxDigits: Defines the maximum number of collected digits that can be received from the Tel side (if ISDN Sending Complete IE is not received). When the number of collected digits reaches the maximum, the device uses these digits for the called destination number.

●

TimeBetweenDigits: Defines the maximum time (in seconds) that the device waits between digits received from the Tel side. When the time expires, the device uses the collected digits to dial the called destination number.

●

MuteDTMFInOverlap: Enables the device to ignore in-band DTMF digits received during overlap dialing.

■

Interworking SIP to ISDN overlap dialing (IP to Tel): The device sends the first digits (e.g., "331") received from the initial SIP INVITE message to the Tel side in an ISDN Setup message. Each time it receives additional (collected) digits for the same dialog, which are received from subsequent SIP re-INVITE messages or SIP INFO messages, it sends them to the Tel side in SIP Q.931 Information messages. For each subsequent re-INVITE or SIP INFO message received, the device sends a SIP 484 "Address Incomplete" response to the IP side to maintain the current dialog session and to receive additional digits from subsequent re-INVITE or INFO messages. You can use the following parameters to configure overlap dialing for IP-to-Tel calls:

●

ISDNTxOverlap: Enables IP-to-Tel overlap dialing and defines how the device receives the collected digits from the IP side - in SIP re-INVITE [1] or INFO messages [2].

●

TimeBetweenDigits: Defines the maximum time (in seconds) that the device waits between digits received from the IP side. When the time expires, the device uses the collected digits to dial the called destination number.

For more information on the above mentioned parameters, see PSTN Parameters. To configure ISDN overlap dialing using the Web interface, see Configuring Trunk Settings.