CHAPTER23 Digital PSTN
Mediant 1000 Gateway & E-SBC | User's Manual
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.
If the device receives SIP 4xx responses during the overlap dialing (while collecting
digits), it does not release the call.
■ 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.
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