able for input to a standard telephone. The gateway is placed between the voice codec and the digital data transport circuit. An identical device will also be found at the far end of the link. This equipment carries out the signaling role on a telephone call among other functions. Moving from left to right in Figure 15.3, we have the spurty analog signal deriving from a standard telephone set. The signal is then converted to a digital counterpart using one of seven or SO codecs [coder-decoder(s)] that the VoIP system designer has to select from. Some of the more popular codecs for this application are listed in Table 15.1. The binary output of the codec is then applied to a conversion device (i.e., a "packetizer") that loads these binary 1s and Os into an IP payload of from 20 to 40 octets in length. The output of the converter consists of IP packets that are transmitted on the web or other data circuit for delivery to the distant end. At the far end the IP packets or frames are input to a converter (i.e., depacketizer) that strips off the IP header, stores the payload, and then releases it in a constant bit stream to a codec (i.e., a D-A converter). Of course this codec must be compatible with its Depacketizer conversion router router packetizer conversion Figure 15.3 Elements of basic operation of VolP where the input signal derives from a conventional analog telephone. This figure follows Figure 15.1 in sequence of complexity. The output may be ATM cells (see Chapter 20) if the intervening network is an ATM network. VOICE-OVER PACKETS IN A PACKET NETWORK Table 15.1 Characteristics of Speech Codecs Used in Packet Networks Voice Bit Voice Frame Header Packets per Packet Bit Coding Algorithm Rate (kbits/sec) Size (bytes) (bytes) Second Rate (kbits/sec) G.711 8-bit PCM [1] G.723.1 MPMLQa [4] G.723.1 ACELPb [5] G.726 ADPCMc [6] G.728 LD-CELPd [7] G.729a CS-ACELPe [8] aMPMLQ, multipulse maximum likelihood quantization. bacelp, algebraic code-excited linear prediction. CADPCM, adaptive differential PCM. "LD-CELP, low