Voice over 802.11
This guide for telecommunications professionals describes in detail how to engineer each component of 802.11 and VoIP for reliable, high-quality voice transmission.
TABLE OF CONTENTS Voice over 802.11
Chapter 4: Switching TDM and VoIP Networks
As explained earlier in this book, a telephone network is comprised of three elements: access, switching, and transport. This chapter will introduce the reader to switching both for legacy TDM technologies and for VoIP. Vo802.11 replaces PSTN Class 4 and Class 5 switches with softswitch platforms. This potentially spares an enterprise high fees for Centrex services. In addition, a service provider utilizing softswitch technologies need not purchase multimillion-dollar Class 4 or Class 5 switches. In bypassing Class 4 and Class 5 switches, a Vo802.11 service provider can greatly lower the barriers to entry to the telecommunications market.
TDM Switching
To fully understand VoIP switching and subsequently Vo802.11, we must first understand the physics of voice switching. Much of this technology has evolved during the century of telephony. Many aspects of TDM voice technology have been incorporated by VoIP.
Multiplexing
The earliest approach to enabling multiple conversations over one circuit was frequency-division multiplexing (FDM). FDM was made possible by the vacuum tube, in which the range of frequencies was divided into parcels that were distributed among subscribers. In the first FDM architectures, the overall system bandwidth was 96 kHz. This 96 kHz could be divided among a number of subscribers into, for example, 5 kHz per subscriber, meaning almost 20 subscribers could use this circuit.
FDM, however, is an analog technology and suffers from a number of shortcomings. It is susceptible to picking up noise along the transmission path. This FDM signal loses its power over the length of the...
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