[...]... Since then, I have moved on and joined HP Labs, India The separation in distance (and in time zones) has been a challenge for both of us and for our editors I would like to thank Dave for his commitment and initiative, and our editors for being patient with us I would also like to thank my extended family in Saharanpur, Kanpur and Datia for their constant encouragement and support Finally, I would... in the expression of language) A voice frequency (VF) or voice band is one of the frequencies, within part of the audio range that is used for the transmission of speech In telephony, the usable voice frequency band ranges from approximately 300 Hz to 3400 Hz The bandwidth allocated for a single voice- frequency transmission channel is usually 4 kHz, including guard bands, allowing a sample rate of 8... Instruments and has worked on various aspects of Voice over IP for the past eight years Prior to that, he has worked on Cable Modem design and on weather satellite ground systems He lives with his family in Rockville, Maryland xx CHAPTE R 1 The Telephony World 1.1 The Basics This is a book about using wireless local area networks (LANs) to carry human speech and voice In this first chapter, we look at how voice. .. encouraged me to shoot for the stars —Praphul Chandra I’d like to thank my colleague, Praphul Chandra, for inviting me to join him in this project and for his leadership, despite the challenges of time and distance I’d also like to thank my family, especially my wife Nellie, for giving me the time to work on this project Finally, I’d like to thank all my colleagues at Texas Instruments for their dedication... trunk 1.5 Voice and Wireless Networks Thus far we have talked about wired networks being used to carry voice In this section we give a brief overview of how wireless networks are used to carry voice 1.5.1 First-Generation Wireless Networks The earliest wireless voice networks were deployed in 1980 and 1981 in Japan and Scandinavia In the following years, various cellular systems were developed and deployed... phone network where end user voice and signaling are converted to the digital domain in the customer premises (as opposed to being conveyed as analog over the local loop) When ISDN runs over T1 lines (known as the primary rate interface or PRI), it utilizes 23 out of the 24 timeslots for carrying voice (and possibly data) These are referred to as “B” channels, where “B” stands for bearer One channel (channel... formula: • P = a*S/ (1 + ln a) for S ≤ 1/a where a is a constant with value 87.6 • P = (1 + ln a*S) / (1 + ln a) for 1/a ≤ S ≤ 1 A-law is, in theory, easier for computers to implement In either case, the result is a 64-kbps data stream consisting of 8-bit values produced every 125 µs This stream is convenient for digital telephony to handle, and several communications standards have evolved to deal with... networks: the signaling network and the media network To understand the difference between signaling and media, 6 The Telephony World consider what happens when you pick up your telephone and make a call You get a dial tone, dial digits, hear a ring-back tone and are then connected to the called party if (s)he answers the call Notice that a whole lot of things happen before the voice actually starts flowing... such as dial tone, dialed digits, and ringing tones are all audio signals that travel over the same channel on the same pair of wires in the local loop When the call signaling is completed, voice is carried over the same path that was used for the signaling One question with in-band signaling is what happens to the analog signaling when the voice stream is digitized and converted to PCM With the 64-kbps... Logarithmic Quantization 4 0.8 1 The Telephony World Most implementations, however, do not use a linear quantization scheme (where the finite set of values to choose from is uniformly spaced) like PCM Instead, a process known as companding is used Companding (COMPression – expANDING) expands small values and compresses large values In other words, when a signal goes through a compander, small amplitudes are . of Congress Cataloging-in-Publication Data Chandra, Praphul. Wi-Fi telephony : challenges and solutions for voice over WLANs / by Praphul Chandra and David Lide. p. cm. Includes index. ISBN-13:. SINGAPORE • SYDNEY • TOKYO Newnes is an imprint of Elsevier Wi-Fi Telephony Challenges and Solutions for Voice over WLANs By Praphul Chandra and Lide Newnes is an imprint of Elsevier 30 Corporate Drive,. 0-7506-7971-9 (pbk. : alk. paper) 1. Internet telephony 2. Wireless LANs. I. Lide, David R., 1928- II. Title. III. Title: Challenges and solutions for voice over WLANs. TK5105.8865.C47 2007 004.69