Softswitches�Part III: The Media Gateway Controller

In the first tutorial in this sub-series regarding softswitches, we briefly discussed the concepts of telephone switching and three key functions, routing, transmission, and billing, which are involved in connecting a call from the source to the destination. We also considered the distinction between the switching fabric which makes the physical connection, and the switching logic, which handles routing, billing, and associated functions. One of the key concepts behind the softswitch is the separation between the hardware (physical) and software (logical) functions, which allow different devices, in perhaps different geographical locations, to handle these separate functions. This provides significant flexibility advantages over monolithic legacy switches, which can be challenging to upgrade—and even harder to replace with newer technology.

In the second softswitch tutorial we looked at the architecture of a softswitch, as defined by the International Packet Communications Consortium (IPCC) in their IPCC Reference Architecture Document. This document details the architecture of a softswitch, and describes four major functional areas, called functional planes. The Transport Plane is responsible for transporting call media (audio/video information), call setup, and signaling information across the network. The Call Control and Signaling Plane establishes and tears down media connections. The Service and Application Plane provides the functions to control applications that are offered on the network. Finally, the Management Plane provides for subscriber and service provisioning, support and billing functions. This tutorial will focus on the Call Control and Signaling Plane, and the Media Gateway Controller that operates within that plane. (In contrast, the Media Gateway operates within the Transport Plane, and provides the physical network connection. We will investigate the Media Gateway in our next tutorial.)

Telephone calls transported by either a VoIP network or the Public Switched Telephone Network (PSTN) share a common requirement for end-to-end reliability, and with that, the ability to establish and disconnect the end-to-end connection. The Media Gateway Controller, or MGC (which is also known as the Call Agent or the Call Controller) is the device that provides those functions, and therefore contains a substantial amount of the network intelligence. Thus, call routing, control of the connection, and the control of network resources are all functions that the MGC provides. These specific operations include:

• Originating and terminating signaling messages to and from end user stations, other MGCs, or networks (such as the PSTN) that are external to the VoIP network.
• Maintaining call state information for every call on the Media Gateway.
• Acting as a conduit for the negotiation of media parameters. For example, the selection of a particular audio or video codec may impact the bandwidth consumption for that particular call, and in the broader scale, the bandwidth consumption of the network as a whole. These parameters are typically negotiated between the end stations when the call is established, which may require the involvement of the MGC.
• Managing network resources, such as ports on the Media Gateway, the available network bandwidth, and so on.
• Providing policy functions, such as access capabilities and permissions for endpoints.
• Interfacing with other services and/or servers, such as the Application, Accounting or Routing functions that are required in support of the call.
• Participation in network management tasks, as required.

The IPCC Architecture defines two additional functions that are considered subsets of the MGC. The first is called the Call Agent Function, which provides the call control and call state maintenance operations. This function may involve a number of protocols, including the Session Initiation Protocol (SIP), defined by the Internet Engineering Task Force, or the H.323 protocol, defined by the International Telecommunications Union—Telecommunications Standard Sector (ITU-T). The second is called the Interworking Function, which is used when a connection is needed to/from a network that deploys another type of networking protocol. Examples of interworking functions would include H.323/SIP or Internet Protocol/Asynchronous Transfer Mode (IP/ATM) network connections.

Our next tutorial will describe the Media Gateway, another significant part of the softswitch reference architecture.

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