How the Emergency Location Identification Number (ELIN) SPL Works

When a Lync client places a 911 emergency call through a mediation server to a Oracle® Enterprise Session Border Controller (E-SBC), the server indicates the emergency status in the priority field and provides a list of ELIN numbers. When the ELIN gateway module is enabled, the E-SBC intelligently selects a particular ELIN number and uses it as the ANI in the “From” field SIP URI in the outgoing INVITE.

The E-SBC preserves the mapping of used ELIN numbers in an internal table. This table includes the ELIN number, the caller (VoIP extension), the “in-use” count, and a timer field. The E-SBC retains these mappings for a configurable time period ranging from 30 to 60 minutes after the call is terminated. The default is 30 minutes. When the timer expires, the entry is purged from the table. The timer field shows the time of day that the timer started.

You can view the current ELIN table at any time using the ACLI command spl show sip elins.

After the Lync client call is disconnected, the 911 service may call back using the number provided in the “From” field of the original INVITE. This presence of this number in its ELIN number table allows the E-SBC to route the call back to the original caller.

Number Reuse

The E-SBC can use an ELIN number for multiple calls. When a call that requires an ELIN mapping arrives at the E-SBC, it checks to see if the numbers presented by the mediation server are in use. If a number is not in use, it simply uses that number. A number is not in use if it is not in the table or its “used count” is 0. An entry’s used count is zero when its not in use and its purge timer has not yet expired.

If all numbers are in use, the E-SBC employs a means of reusing a number, incrementing its used count for each additional call. The selection process proceeds in the following order:

If the “caller” is in the ELIN table, the E-SBC selects that mapping.
The E-SBCselects the number with the lowest “ELIN count”.

If an ELIN number is used by multiple calls, it maps callback attempts to that ELIN number to the client that was last associated with the number.

Error Handling

Lync mediation servers always expect 503 “Service Unavailable” as an error message to a failed ELIN call. There is a variety of error messages that the network may send back when a call fails. For the purposes of Lync support, the E-SBC sends 503 “Service Unavailable” to indicate call failure to a mediation server, regardless of the error it receives.