he MSC switch and the PSTN in a conceptual model and usually resides phys- ically at the MSC. The cdmaOne and cdma2000 systems use Qualcomm code excited linear prediction (QCELP) speech coding, whereas Wideband CDMA (W-CDMA) uses regular pulse excitation-long-term prediction (RPE-LTP) from the Global System for Mobility (GSM) and lower bit rates when it can. Both these speech coding systems are aggressive in conserving bits over the air interface. The backhaul communication link between the base station and MSC enjoys that lower bit rate efficiency as well by using the low bit rate of the speech coding. Some systems use IP-based packet communications to send the speech coded data be- tween base station and MSC. A packet carries a set of bits, a fragment of the entire transmission. In this way, packets are similar to the frames transmitted over digital ra- dio links or in the time slots of time division multiple access (TDMA) systems. How- ever, packets are different from frames in an important way. Frames do not necessar- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. The PSTN and Telephone Switching The PSTN and Telephone Switching ily contain information that identifies the call of which they are a part. Frames are car- ried in sequence over a particular fixed channel, and as a result, their location in the system identifies the conversation of which they are a part and the frame's place in the sequence in the call. Packets contain a header with information defining the source lo- cation, target location, position within the transmission, and other signal information along with their user data. As a result, packets from a single conversation can travel over different pathways or out of order and still be reassembled correctly at their des- tination. This form of packet transmission is the basis of the Internet. In telephony, however, most