Access Session Border Controller (ASBC)

Session border controllers (SBCs) offer required functionalities at the border to the applications and subscribers who access the IMS services. The SBC is responsible for connecting mobile devices to the SIP-based IMS services and related applications, including IPTV, interactive gaming, voice, video, and messaging. In SAE, the access SBCs are responsible to connect all access networks to the IMS network, including the 3G RAN, the LTE RAN, the trusted non-3GPP IP access networks (such as DSL, FTT_x and WiMAX), and un-trusted non-3GPP IP access network (such as the Internet and Wi-Fi networks).

The SBCs are located at the border point of IMS and SAE networks and integrate two functional elements from the IMS Release 8 architecture. A few architectural entities are introduced below:

• Topology Hiding Internetwork Gateway (THIG): THIG can be used with I-CSCF, which hides the capacity, configuration, and topology of the network from the outside. The P-CSCF forwards the SIP messages received at the I-CSCF and/or the S-CSCF from the User Equipment, depending on the procedure and the type of message. The I-CSCF offers a contact point within an operator's network, which allows registration to the subscribers of that network operator and roaming subscribers. The S-CSCF will maintain session state for all IMS services, once registered. Other elements include: The Home Subscriber Server (HSS), or User Profile Server Function (UPSF), which is also called a Master User Database (MUD) that supports the IMS network entities, actually handling calls. The HSS contains the subscription-related information, which performs user-based authentication and authorization and provides user's physical location information.
• BGCF (Border Gateway Control Function): The BGCF is used to select the network where PSTN connection is going to be made. It is responsible to forwards to another BGCF or to a MGCF controlling the access in regards to the PSTN.
• MGCF (Media Gateway Control Function): The MGCF controls the media gateway call management, which is responsible of sending or receiving calls from or to PSTN or other circuit switched networks. MGCF uses SIP messages to or from the BGCF/CSCF and uses Media Gateway Control (MGWC) messages to or from the Media Gateway.
• MGW (Media Gateway): The MGW is responsible for call media processing to or from PSTN/Circuit Switched Network (CSN).
• MRF (Media Resource Function): The MRF provides media related functions such as media manipulation.
• MRFC (Media Resource Function Controller): The MRFC is responsible for signaling plane node, which acts as a SIP User Agent to the S-CSCF. The MRFC also controls the MRFP.
• MRFP (Media Resource Function Processor): The MRFP is a media plane node, which is responsible for the implementation of media-related functions.
• MSG (Multiservice Security Gateways): The MSG delivers service provider's voice and data services securely over un-trusted Wi-Fi access networks and Internet links to dual mode handsets and femtocells.
• SRP (Session Routing Proxy): The SRP selects the destination for incoming and outgoing SIP sessions and provides core session routing. It also deals with the traffic to or from media gateways and interconnects session border controllers.
• AGWF (Access Gateway Function): The transport boundary at layers 3 and 4 between the service provider's network and the subscribers is being controlled by the AGWF. It acts as a NAT device and a pin-hole firewall, which protects the service provider's IMS network. The AGWF also controls the access through IP address/port packet filtering and opening/closing gates into the network. It uses NAPT to hide the IP addresses/ports relate to the service elements in the IMS network. Other features of the AGWF include: bandwidth and signaling rate policing, QoS packet marking, usage metering and media flows QoS measurements.