AudioCodes Mediant 500 E-SBC, User Manual

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User's Manual 428 Document #: LTRT-10437
Mediant 500 E-SBC
In the ELIN table of the ELIN Gateway, the temporarily stored From header value of the
SIP INVITE message originally received from the E9-1-1 caller is used for PSAP callback.
When the PSAP makes a callback to the E9-1-1 caller, the ELIN Gateway translates the
called number (i.e., ELIN) received from the PSAP to the corresponding E9-1-1 caller's
extension number as matched in the ELIN table.
The handling of PSAP callbacks by the ELIN Gateway is as follows:
1. When the ELIN Gateway receives any call from the PSTN, it searches the ELIN table
for an ELIN that corresponds to the received Called Party Number in the incoming
PSTN call.
2. If a match is found in the ELIN table, it routes the call to the Mediation Sever by
sending a SIP INVITE, where the values of the
To and Request-URI are taken from
the value of the original From header that is stored in the ELIN table (in the Call From
column).
3. The ELIN Gateway updates the Time in the ELIN table. (The Count is not affected).
The PSAP callback can be done only within a user-defined timeout (see ''Configuring the
E9-1-1 Callback Timeout'' on page
429 ) started from after the original E9-1-1 call
established with the PSAP is terminated. After this time expires, the table entry with its
ELIN is disregarded and no longer used (for PSAP callback). Therefore, table entries of
only the most recently terminated
E9-1-1 callers are considered in the ELIN table. If the PSAP callback is done after this
timeout expires, the ELIN Gateway is unable to route the call to the E9-1-1 caller and
instead, either sends it as a regular call or most likely, rejects it if there are no matching
routing rules. However, if another E9-1-1 caller has subsequently been processed with the
same ELIN number, then the PSAP callback is routed to this new E9-1-1 caller.
In scenarios where the same ELIN number is being used by multiple E9-1-1 callers, upon
receipt of a PSAP callback, the ELIN Gateway sends the call to the most recent E9-1-1
caller. For example, if the ELIN number "4257275678" is being used by three E9-1-1
callers, as shown in the table below, then when a PSAP callback is received, the ELIN
Gateway sends it to the E9-1-1 caller with phone number "4258359555".
Table 26-3: Choosing Caller of ELIN
ELIN Time Call From
4257275678 11:00 4258359333
4257275678 11:01 4258359444
4257275678 11:03 4258359555
26.5.2.3.4 Selecting ELIN for Multiple Calls within Same ERL
The ELIN Gateway supports the receipt of up to five ELIN numbers in the XML message of
each incoming SIP INVITE message. As discussed in the preceding sections, the ELIN
Gateway sends the ELIN number as the E9-1-1 calling number to the PSTN-based
emergency provider. If the XML message contains more than one ELIN number, the ELIN
Gateway chooses the ELIN according to the following logic:

If the first ELIN in the list is not being used by other active calls, it chooses this ELIN.

If the first ELIN in the list is being used by another active call, the ELIN Gateway skips
to the next ELIN in the list, and so on until it finds an ELIN that is not being used and
sends this ELIN.

If all the ELINs in the list are in use by active calls, the ELIN Gateway selects the ELIN
number as follows:
1. The ELIN with the lowest count (i.e., lowest number of active calls currently using
this ELIN).