Gateway Decomposition

The different types of gateways described iare specific instances of a generic gateway notion. It is useful to decompose the generic gateway into several functional components. Figure 1 depicts a common view of such a decomposed gateway. Three components are identified: (1) the media gateway (MG) function, (2) the signaling gateway (SG) function, and (3) the media gateway control (MGC) function. We describe these functions next.

Figure 1: Components of a decomposed PSTN-Internet gateway.

Physically, the media gateway function terminates PSTN circuits and connections to IP routers (in relation to which it is a host). It also performs all the necessary transformation to convert bit streams received from the sending network into bit streams particular to the receiving network. The transformation occurs at two levels: transmission and application.

At the transmission level, the MG function converts the bit streams between two different framing schemes. This usually involves multiplexing of bit streams of distinct communication sessions and the reverse operation—demultiplexing. In the PSTN, fixed-size digital channels (each typically carrying a voice conversation) are multiplexed based on the time division multiplexing (TDM) scheme at various hierarchical levels (for example, T1 and E3) and packed into frames for transmission over high-capacity facilities. In the IP networks, bits representing a voice conversation are packetized according to the Real-time Transport Protocol (RTP) profile for audio and video payloads (RFC 1890).

At the application level, the transformation takes place between different media-encoding schemes (see the section on codecs for more information) and is commonly known as transcoding. In IP telephony, two prevalent speech encoding schemes are G.711 and G.729. Operating at a bit rate of 64 kbps, G.711 is used ubiquitously in the digital backbone of the PSTN and sets what is known as the toll-quality voice standard. G.729 operates at a much lower bit rate of 8 kbps but still supports near-toll-quality voice service. For this reason, it is widely used in IP networks where bandwidth is constrained. Note that transcoding is computationally intensive and thus causes delays. In addition, transcoding results in degradation of voice fidelity, in particular when a speech coder uses compression.

Another important task of the MG function is to support the use of the QoS facilities of the IP network. Other tasks include echo cancellation (if required), event detection, signaling generation, usage recording, and support of specialized resources such as conference bridges, fax machines, modem pools, and interactive voice response units.

The SG function receives and sends PSTN signaling (such as SS No. 7 or ISDN access) messages. Depending on the arrangement, it may relay, translate, or terminate the PSTN signaling. It exchanges signaling information with the MGC function over IP, and with the PSTN using the SS No. 7.

The MGC function provides control of the media gateway function, including call and connection control and resource management. To this end, it terminates and originates all the relevant signaling. In addition, the MGC function keeps an inventory of the MG resources (for example, bearer circuits and RTP streams) and instructs the MG to reserve or release resources as required. (Naturally, some sort of local policy will govern the use of resources.) With its central role in call and connection control, the MG function logically also provides support for Internet offloading or advanced services and features (such as freephone or call-forwarding). It has the ability to detect data calls from the PSTN and to direct the data traffic straight to a network access server as well as to launch queries to SCPs for instructions for further call processing.