ach packet, and the packet can now be sent over the data packet network. This type of operation is most commonly called voice over IP (VoIP). "IP" is the Internet protocol, which we discuss in Chapter 12. Others prefer the broader term voice over packet. This would also encompass "voice over ATM (asynchronous transfer mode" (Chapter 20). Still another approach is to place voice (in packets) over frame relay (Chapter 12). The concept of VoIP is illustrated in Figure 15.1. 15.2 DATA TRANSMISSION VERSUS CONVENTIONAL DIGITAL TELEPHONY Conventional voice telephony is transported in a full duplex mode on PSTN circuits optimized for voice. By the full-duplex mode we mean that there are actually two circuits, one for "send" and one for "receive" to support a normal telephone conversation between two parties. With a few exceptions in the local area, all these circuits are digital. The descriptive word digital may seem ambiguous to some. When we say digital in this context, we mean that all circuits would carry 8-bit "words" (timeslots), where each "word" represents an 8-bit voltage sample ostensibly of an analog voice conversation in a PCM format. This is often characterized in the literature as G.711 service (i.e., ITU-T Rec. G.711). Data are also commonly transported in 8-bit sets called bytes, but more properly called octets from our vantage point. It is comparatively easy to replace 8-bit voltage samples of voice with 8-bit octets of data. However, there remained essential philosophical differences between voice in 8-bit octets and data transmission. A voice circuit is established when a subscriber desires to converse by telephone with some other telephone subscriber. The circuit between the two Voice Packetizer Depacketizer Voice A/D conv. V/A conv. Figure 15.1 Rough conceptualization of VolP. Fundamentals of Telecommunications, Second Edition, by Roger L. Freeman ISBN 0-471-71045-8 Copyright © 2005 by Roger L. Freeman VOICE-OVER PACKETS IN A PACKET NETWORK Direction of transmission Frame check Information Control Ad