General

These Operational Directions for PSTN fallback for VoIP media, Inter Gateway Routing, describe how to create a media connection between two gateways using existing ISDN or TL30 CAS routes.

The connection is programmed with first igw_route commands then transport commands and is used in the same way as RTP and Group switch connections.

There are two ways to distinguish this function from normal PSTN calls. The first way is to use a special identifier in UUS 1. This can be used only with ISDN.

The second way is to use a designated number type for called number and calling number used as call id. This can be used with both ISDN and TL30 CAS.

The call signaling would still be used and require TCP/IP between the servers, but voice media could be routed via public PSTN.

The following figure depicts the basic scenario, where Call Server 1 (LIM1) and Call Server n (LIMn) both belong to the same MX-ONE system, but are geographically dispersed, and normally use TCP/IP connection between the servers (LIMs) but also have public PSTN connections.