Access devices on the IP Multimedia Subsystem (IMS) provide various access modes and convert various services into a uniform format that can be transmitted. On the IMS, an access device is called access gateway (AG). AGs play an important role on the IMS and are connected to users.
AGs and media gateway controllers (MGCs) use Session Initiation Protocol (SIP). SIP-based AGs are called SIP AGs.
A traditional private branch exchange (PBX) manages the incoming and outgoing calls of an enterprise. It connects the enterprise to the Public Switched Telephone Network (PSTN) and provides services for devices such as telephones, fax machines, and modems. It allows users in the enterprise to call each other using extension phones and routes inter-office calls to the PSTN through a trunk line. Traditional PBXs cannot meet the requirements of computer telephony integration (CTI) and voice over IP (VoIP). In addition, these PBXs are expensive and do not use standard and open platforms, creating difficulties in interconnecting PBXs of different vendors. IP PBXs overcome the limitations of traditional PBXs. IP PBXs are based on the IP protocol and provide both local exchange and IP user access functionality. IP PBXs integrate the voice communications system of an enterprise into the enterprise's data network so that the enterprise can build a uniform voice and data network connecting branches, offices, and staff around the world.
The AR1200/2200/3200 can function as a PBX to provide traditional PBX functions and IP PBX functions.
SIP AGs and PBXs have the following differences:
PBXs have two functions. PBXs can serve as switching devices on a private network to locate voice users' addresses and process other services. PBXs also allow users on the private network to access the IMS or PSTN through SIP trunks.
SIP AGs provide only the second function of PBXs. SIP AGs can be considered to be a plain old telephone service (POTS) that can register with the IMS using SIP trunks. PBXs use SIP trunks to register with other SIP servers. SIP AGs register with the IMS using POTS user numbers. PBXs and SIP AGs provides the same services. However, PBXs provide the services by themselves.