Editing IP Multimedia Subsystem (section)

=== IMS-based PES architecture ===
IMS-based PES (PSTN Emulation System) provides IP networks services to analog devices. IMS-based PES allows non-IMS devices to appear to IMS as normal SIP users. Analog terminal using standard analog interfaces can connect to IMS-based PES in two ways:
* Via A-MGW (Access Media Gateway) that is linked and controlled by AGCF. AGCF is placed within the Operators network and controls multiple A-MGW. A-MGW and AGCF communicate using [[H.248]].1 ([[Megaco]]) over the P1 reference point. POTS phone connect to A-MGW over the z interface. The signalling is converted to H.248 in the A-MGW and passed to AGCF. AGCF interprets the H.248 signal and other inputs from the A-MGW to format H.248 messages into appropriate SIP messages. AGCF presents itself as P-CSCF to the S-CSCF and passes generated SIP messages to S-CSCF or to IP border via IBCF (Interconnection Border Control Function). Service presented to S-CSCF in SIP messages trigger PES AS. AGCF has also certain service independent logic, for example on receipt of off-hook event from A-MGW, the AGCF requests the A-MGW to play dial tone.
* Via VGW (VoIP-Gateway) or SIP Gateway/Adapter on customer premises. POTS phones via VOIP Gateway connect to P-CSCF  directly. Operators mostly use [[session border controller]]s between VoIP gateways and P-CSCFs for security and to hide network topology. VoIP gateway link to IMS using [[Session Initiation Protocol|SIP]] over Gm reference point. The conversion from POTS service over the z interface to SIP occurs in the customer premises VoIP gateway. POTS signaling is converted to SIP and passed on to P-CSCF. VGW acts as SIP user agent and appears to P-CSCF as SIP terminal.

Both A-MGW and VGW are unaware of the services. They only relay call control signalling to and from the PSTN terminal. Session control and handling is done by IMS components.