 delay code-excited linear prediction. eCS-ACELP, conjugate structure algebraic code-excited linear prediction. near-end counterpart. The codec converts the digital bit stream back to an analog signal that is then input to a standard telephone subset. The insightful reader will comment that many steps of translation and interface have been left out. Most of these considerations will be covered below in Section 15.4.1 in our discussion of the gateway. 15.4.1 VolP Gateway Gateways are defined in different ways by different people. In our context here a gateway is a server; it may also be called a media gateway. Figure 15.4 illustrates a typical gateway. It sits on the edge of the network and carries out a switching function of a local, tandem, or toll-connecting PSTN switch described in Chapters 3 and 6. Media gateways are part Gateway Server Authorization Maintenance Identification Voice Prompts Records Administration Control Billing Records H.323 API Hardware H.323 Gateway Hardware Protocol Stack T1/E1, DSPs, etc. Figure 15.4 A media gateway from one perspective. API, application programs interface. From IEC on-line: www.iec.org/online/tutorials/int_tele/topic03.html (Jan. 2003). 15.4 VOIP, INTRODUCTORY TECHNICAL DESCRIPTION of the physical transport layer. They are controlled by a call control function housed in a media gateway controller. A media gateway with its associated gateway controller is at the heart of the network transformation to packetized voice. Several of the media gateway functions are listed here: Carries out A/D conversion of the analog voice channel (called compression in many texts). Converts a DSO or E0 to a binary signal compatible with IP or ATM. Supports several types of access networks including media such as copper (including various DSL regimes), fiber, radio (wireless), and CATV cable. It is also able to support various formats found in PDH and SDH hierarchies. Competitive availability (99.999%). Capable of handling several voice and data interface protocols. Multivendor interoperabil