Selecting Elin For Multiple Calls Within Same Erl - AudioCodes Mediant 4000 SBC User Manual

CHAPTER 16 Services
In the ELIN table of the device, 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 device 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 device is as follows:
1. When the device receives a call from the emergency service provider, it searches the ELIN
table for an ELIN that corresponds to the received called party number in the incoming
message.
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 device updates the Time in the ELIN table. (The Count is not affected).
The PSAP callback can be done only within a user-defined period (see
Callback Timeout
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 device 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, the PSAP callback is routed to this
new E9-1-1 caller.
In scenarios where the same ELIN number is used by multiple E9-1-1 callers, upon receipt of a
PSAP callback, the device 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 device sends it to the E9-1-1 caller with phone number
"4258359555".

Selecting ELIN for Multiple Calls within Same ERL

The device 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 device sends the ELIN number
as the E9-1-1 calling number to the emergency service provider. If the XML message contains more
than one ELIN number, the device 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 device 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 device selects the ELIN number as
follows:
a. The ELIN with the lowest count (i.e., lowest number of active calls currently using this
ELIN).
), started from after the original E9- 1- 1 call established with the PSAP is
Table 16-29:Choosing Caller of ELIN
ELIN
4257275678
4257275678
4257275678
Time
11:00
11:01
11:03
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Mediant 4000 SBC | User's Manual
Configuring the E9-1-1
Call From
4258359333
4258359444
4258359555