ity. It must provide interface between media gateway control device and the media gateway. This involves one of four protocols: SIP [2], H.323 [3], MGCP, and Megaco (H.248). Can handle switching and media processing based on standard network PCM, ATM, and traditional IP. Transport of voice. There are four transmission categories involved: 1. Standard PCM (E0/E1 or DS0/DS1) 2. ATM over AAL1/AAL2 3. IP-based RTP/RTCP 4. Frame relay The most powerful gateway supports the public network or PSTN requiring a high reliability device to meet the PSTN availability requirements. It will be required to process many thousands of digital circuits. As shown in Figure 15.4, it has a network management capability most often based on SNMP (see Chapter 21). A somewhat less formidable gateway is employed to provide VoIP for small and medium-sized business. Some texts call this type of gateway an integrated access device (IAD) if it can handle data and video products as well. An IAD will probably be remotely configurable. The least powerful and most economic gateways are residential. They can be deployed in at least five settings: POTS (telephony) Set-top box (CATV), which provides telephony as well PC/modem XDSL termination Broadband last mile connectivity (to the digital network) Figure 15.5 shows gateway interface functions via a block diagram. On the left are time slots of a PCM bit stream (T1, in this case). The various signal functions are shown to develop a stream of data packets carrying voice or data. The output on the right consists of IP packets. The first functional block of the gateway analyzes the content on a time-slot basis. The time slot may contain an 8-bit data sequence where we must be hands-off regarding VOICE-OVER PACKETS IN A PACKET NETWORK Convert 2100 Hz to data Time slots Demodulate modem format signal 12345--24 (data) compress Transmit Time slot dialed digit Compress either/or type decision signaling packet decision circuit tones Outputs to Internet (1) data packets Speech (2) signaling packets Speech acti