ginally said. Echo cancelers can be tuned or can tune themselves to the echo delay on any particular connection. Each echo canceler design has a limit as to the maximum delay of echo that it can identify. Echo cancelers are bypassed if a fax signal or modem data is on the line. 15.5 MEDIA GATEWAY CONTROLLER AND ITS PROTOCOLS The gateway controller or media gateway controller (MGC) carries out the signaling function on VoIP circuits. Some texts call an MGC a soft switch even though they are not truly switches but are servers that control gateways. This function is illustrated in Figure 15.7. An MGC can control numerous gateways. However, to improve reliability and avail- ability several MGCs may be employed in separate locations with function duplication on the gateways they control. Thus if one MGC fails others can take over its functions. We must keep in mind that the basic topic of Section 15.4 is signaling. That is the establishing (setting up) of telephone connectivities, maintaining that connectivity and the taking down of the circuit when the users are finished with conversation. There is a basic discussion of signaling in Chapter 4 of this text. There are four possible signaling protocol options between an MGC and gateways. These are: ITU-T Rec. H.323. This is employed where all network elements (NEs) have software intelligence. SIP (session initiation protocol [2]) is used when the end devices have software intelligence and the network itself is without such intelligence. Signaling Signaling IP Network Figure 15.7 A media gateway controller (MGC) provides a signaling interface for media gateways (MG) and thence to the IP network. VOICE-OVER PACKETS IN A PACKET NETWORK MGCP (media gateway control protocol) is another gateway control protocol. Megaco (ITU-T Rec. H.248 [13]) is a gateway control protocol applicable when end devices are without software intelligence and the network has software intelligence. 15.5.1 Overview of the ITU-T Rec. H.323 Standard In May 1996 the ITU ratified the H.323 specification,