Emergency Location Identification Number (ELIN) Gateway Support
An ELIN-capable gateway supports connection to a qualified E911 service provider. The connection supports PSTN-based E911 functions, including user callback when there is a disconnect. Enterprises often deploy ELIN numbers based on physical location to locate the physical source of a 911 call. By using multiple ELINs, an enterprise can support multiple, simultaneous E911 calls.
Typically, an enterprise purchases multiple ELIN numbers. An ELIN gateway replaces VoIP extension URIs with ELIN numbers and maintains the mapping. For example, if an emergency service replied to a VoIP URI without using an ELIN gateway, the reply would be delayed or fail. An ELIN gateway can use its mapping to translate the ELIN number back to the VoIP extension from within the enterprise session network. The gateway can immediately forward the call back to the original client.
The Oracle® Enterprise Session Border Controller supports E911 ELIN for Lync-enabled Enterprises using the ELIN_Gateway SPL option. Enable this option in the global SPL configuration. The Oracle® Enterprise Session Border Controller supports up to 300 ELIN numbers simultaneously and it can reuse numbers allowing a greater number of emergency calls.
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 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.
Configure the ELIN Gateway Option
To enable an Emergency Location Identification Number (ELIN) gateway to support connections to an emergency service provider, you must configure the ELIN gateway option on the Oracle® Enterprise Session Border Controller (E-SBC). Oracle delivers the E-SBC pre-configured with the 911 and 112 Public Safety Answering Point (PSAP) callback numbers. You can add more PSAP numbers, as needed. You can also specify the length of time that you want the E-SBC to retain ELIN mappings.
- Determine the preferred length of time, in minutes, that you want the E-SBC to retain ELIN mappings.
- Determine whether or not you want to add more PSAP callback numbers.
The E-SBC requires ELIN configuration at the global level, rather than at the session-agent, realm-config, or sip-interface level. Select the spl-config option under system for this ELIN configuration. In the following configuration you can set the time limit for retaining ELIN mappings and you can add more PSAP callback numbers.
To configure the ELIN Gateway option:
