Next Generation Architecture

One NGN architecture for VoIP with centralized call processing decomposes the functional elements of a traditional circuit switch into specialized components with open interfaces. Following are the key logical elements of this reconstruction are the following:

• The MG handles the media, or bearer, interface. It converts media from the format used in one network to the format required in another network. For example, it can terminate the TDM trunks from the PSTN, packetize and optionally compress the audio signals, and then deliver the packets to the IP network using the Real Time Protocol (RTP) [120].

• The MGC (also known as a Call Agent) contains the call processing. In addition, it manages the resources of the MGs that it controls. The MGC controls the MG using a control protocol to set up the RTP connections and control the analog or TDM endpoint in the MG.

• The SG sits at the edge of an IP network and terminates circuit-switched network signaling, such as SS7 or ISDN, from the circuit-switched network. It transports, or backhauls, this signaling to the MGC or other IP-based application endpoint.

Figure 14-1 shows an example of these logical elements and their connections.

Figure 14-1. NGNs—Sample Architecture

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As Figure 14-1 shows, the evolution of specialized components provided open interfaces between these logical elements. The Internet Engineering Task Forces (IETF) created two working groups to address these open interfaces at the same time that ITU-T SG16 began to study the MGC to MG interface. Thus, the definition of the bearer control protocol between the MG and the MGC became a joint effort by the IETF MeGaCo (MGC) Working Group and the ITU-T SG16. The output from these groups is known as the Megaco [RFC 3015] [121] protocol in the IETF, and the H.248 [122] protocol in the ITU-T.

Also worth mentioning is a precursor to Megaco protocol: the Media Gateway Control Protocol (MGCP) [RFC 3435] [123].

MGCP can also be used as a control protocol between an MGC MGCU (TG) and an MG. While MGCP is defined by an Informational (versus standards track) RFC, it is commonly used in many products today because the specification was available before Megaco and H.248 were finished. Both MGCP and Megaco/H.248 assume that the call control intelligence is outside the MGs and that the MGC handles it.

Closely related to the MGCP protocol are the PacketCable protocols, Network-Based Call Signaling (NCS) and PSTN Gateway Call Signaling Protocol (TGCP). These protocols provide functionality similar to MGCP for cable-based networks.

The IETF SigTran Working Group focused on the SG to MGC open interface. The Working Group produced a set of standard protocols to address the needs and requirements of this interface.