Inter-Networking, E9-1-1 Tandem to Tandem

This Technical Reference is a guide for designers and manufacturers of selective routing tandems and similar equipment. It may also be of value to purchasers, maintainers and users of such equipment.

This document describes the use of an ISUP Signaling to allow appropriate information to be passed between E911 Tandem Network Elements. An E911 Tandem Network Element is the switching element within the telephone company network which receives 9-1-1 calls originated within the Public Telephone Network, routes the call to the appropriate Public Safety Answering Point (PSAP)^(?), and provides the signaling interface to the PSAP itself.

The purpose of utilizing ISUP Signaling is to facilitate the delivery of all of the information necessary for an E911 Tandem to provide the functions described above. That is, when an E911 Tandem decides that a 9-1-1 call should be routed to another E9-1-1 Tandem, all of the information necessary for the routing and delivery of that 9-1-1 call at another E9-1-1 Tandem should be passed in ISUP parameters in an outgoing message to the second E9-1-1 Tandem. The information necessary for properly routing and delivering a 9-1-1 call in this context is:

• Calling Party Directory Number information

• Called Party Directory Number information (typically the digits “9-1-1”, but may have other values as described below)

• Location Information (e.g. for wireless 9-1-1 calls, location information must be passed for correct routing decisions to be made)

• Emergency Call Indicators (depending on the context of the call, it may be necessary to use an indicator in the call to mark it as a 9-1-1 call).

This technical reference will allow for two types of interfaces between E9-1-1 Tandems. First, a dedicated, direct ISUP trunk interface may be used between E9-1-1 Tandems. On this type of trunk interface, all calls processed on the trunks are assumed to be 9-1-1 calls. As an alternative, this reference will prescribe the necessary signaling for non-dedicated, non-direct trunks to be used for processing 9-1-1 calls. On these types of trunks, it cannot be assumed that calls originating or terminating on the trunks are 9-1-1 calls.