IMS and TISPAN, a bit of a mystery combination. In part one I've taken a look at what TISPAN is and how it uses IMS in it's architecture. This part focuses on the PSTN/ISDN Emulation Sub-System and how the IMS can be used to simulate an analogue telephony switching center.
Besides the IMS subsystem the PSTN/ISDN (Public Switched Telephone
Network / Integrated Services Digital Network) Emulation Sub-System
(PES) is another important element of TISPAN. Its aim is to enable
legacy analog and ISDN telephones to be connected to an IP based next
generation network (NGN) via a media gateway which is either part of
the access modem or a standalone device. An IMS independent
implementation for PES is described in ETSI ES 282 002 while ETSI TS
182 012 defines how a PES can be implemented with IMS. Both standards documents are available via ETSI's web site for free but one must register first.
As legacy devices can not be modified a gateway has to be deployed on the user’s location. On the one side of the gateway the analog or digital telephone is connected to a legacy connector. In case of a standalone device the other side of the device usually features an Ethernet connector with which the device can be connected to the DSL or cable modem. The PES specification in ETSI TS 182 012 knows two types of devices. Voice Gateways (VGWs) emulate a SIP User Agent on the behalf of the legacy device and communicate with SIP commands with the P-CSCF of the IMS. The second approach is to deploy a media gateway (GW) which communicates via the H.248 (Media Gateway Protocol, MEGACO) to the Access Gateway Control Function (AGCF). In this approach the SIP User Agent functionality is included in the AGCF, i.e. not on the customer device but in the network itself. Additionally, the AGCF includes the P-CSCF functionality. During call establishment, the P-CSCF or the AGCF then communicate with the RACS to reserve the required transport resources to ensure the quality of service for a call. In addition, PES requires an IMS Application Server (AS) to emulate the PSTN or ISDN service logic when the PES User Agent sends SIP messages with embedded ISUP messages.
And a final note: The TISPAN standard also aims to standardize non IMS sub-systems. While out of scope of the current version of the specification it is likely that TISPAN will specify a standardized IPTV and Video on Demand (VoD) system in the next version.