Megaco to bridge PSTN, IP networks

Megaco/H.248 is an emerging standard that will enable voice, fax and multimedia calls to be switched between the public switched telephone network and emerging IP networks. Megaco stands for Media Gateway Control.

A joint activity of the International Telecommunication Union and the Internet Engineering Task Force (IETF), the Megaco framework could potentially enable service providers to offer a wide variety of converged telephone and data services.

Media gateways will be the junctions that provide a path between switched and packet networks for such media as voice and fax. Media gateways can either reside at the very edges of the public network at the customer premises or in a central office. Media gateway controllers (MGC) will serve as the brains of the operation because they will control many media gateways.

Another component, a signaling gateway, will play an analogous role in capturing requests for address and enhanced service information by enabling retrieval via IP from existing Signaling System 7 (SS7)-enabled networks and their databases of subscriber-related information. In turn, the relevant address information is passed from a signaling gateway to an MGC.

Megaco implementations can also be enhanced using extension methods called packages. Packages are sets of commands, related events and statistics that can be added to a basic Megaco device.

A future Megaco-enabled phone network could support customer premises equipment in an enterprise. For example, consider a fax device that sits behind a PBX in a company. In this case, a call is placed by the fax device and routed to the local central office by the PBX. The central office then sends an SS7 message to a signaling gateway in order to get access to calling and called address information. This information is sent as a setup message by the signaling gateway back to the MGC. The MGC then identifies a second media gateway (MG), which can be used to provide an end-to-end connection via the long-distance service provider's IP network to the end destination.

At this point, the MGC is aware of both a local MG and a remote MG. When the media gateways are initially set up for communication, a voice encoding (vocoder) approach will typically be used. In our example, each media gateway is set up with two terminations, a switched termination on one side and a packetized IP termination on the other.

To provide the flexibility to check for calls other than voice, the MGC may set up the media gateways to do call discrimination. This enables an MGC to gather information needed to identify the media type for a call (voice, fax or data) and then modify the media gateway configuration in real time as needed.

Suppose that a calling fax device issues a fax calling tone. Because it is possible that the receiving endpoint is not a fax device, it would not yet be safe to switch into a fax mode. To convey the tone information, the local MG issues a notify event, which tells the MGC about the fax tone detection. The MGC then issues a command to the remote MG requesting that it replay the fax tone.

The MGC will monitor the fax negotiation tones sent between the local and remote MGs. Once the evidence is clear that the call is indeed a fax, the MGC can swiftly take action, place both media gateways into fax-compatible modes, and a normal fax session can take place. The call discrimination package will then be deactivated and the fax session can proceed, delivering a document across the hybrid network. The Megaco call discrimination package is also sufficiently robust to enable detection of text telephone calls.

The emerging Megaco model and related standards will enable support of existing and new applications of phone service over hybrid phone networks that will contain a mix of switched, IP and ATM technology.

Rafferty is senior product manager for IP telephony at Brooktrout Technology. He can be reached at jraff@brooktrout.com.

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