16.2 What is Voice-over DSL?

   


In voice-over DSL (VoDSL), the voice and the telephony signaling is digitized and carried over the DSL PHY along with the data traffic. Unlike the frequency division multiplex provided with ADSL over POTS (or over ISDN), the voice channels are multiplexed into the DSL path . Digitally multiplexed lines are sometimes known as "derived lines." Although it is true that earlier digital technologies (e.g., T1, HDSL, and ISDN) were used to support telephony, VoDSL is distinguished from these voice environments in several ways. ADSL and SDSL (symmetric DSLs, both proprietary and the standard ITU-T G.991.2) are PHYs that were initially developed primarily to carry data. Unlike the telephony-oriented digital technologies that were developed primarily to transport voice traffic, carrying of voice traffic over the DSL protocols involves solving the problems carrying high-quality full-duplex voice traffic over protocols that were originally optimized for another purpose.

The worldwide public switched telephone network (PSTN) typically transports digitized telephony in 64 Kbps connections that carry uncompressed digitized voice. The analog voice signal is digitized within the telephone network in the case of POTS, while the digitization occurs in the customer's telephone instrument in the case of ISDN services (Figure 16.2).

Figure 16.2. Overview of the PSTN carrying digitized telephony traffic.

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Thus, ISDN and T1 were designed from the onset for integration with the PSTN. They are easily channelized into 64 Kbps voice channels, have signaling channels and protocols defined which are compatible with those used in the PSTN, and are integrated with the existing tools used by carriers to manage and trouble shoot their networks. These telephony features are not inherently part of the features of the data-oriented DSLs. This is both a disadvantage and an advantage. Although features that support voice may need to be added to the DSL protocol stack either as enhancements to the PHY protocols or in the higher layers , the support for voice-over DSL is not linked to the legacy of existing PSTN protocols or architectures. Thus, VoDSL architectures can be integrated into both the legacy PSTN or into recently emerging telephony architectures. The PSTN is, and will remain for a considerable time, the main method worldwide for voice connectivity. However, new architectures are being developed that remove some of the existing limits on voice telephony. These architectural enhancements can include use of connectionless protocols that could reduce costs, use of digital compression to reduce bandwidth required to support voice, and novel control and management methods that could increase service flexibility and decrease time to market for new telephone services.

VoDSL supports voice services by multiplexing the voice traffic with the data traffic. Figure 16.3 illustrates this multiplex at the highest and most abstract level. Because the voice traffic is multiplexed within the data channel, it can be carried over the same network as the data traffic and separated from the data traffic at any of several appropriate points in the network. The voice traffic may be integrated with the PSTN at any of these places or may remain completely separated. The exact place where the voice traffic and the data traffic are demultiplexed is independent of the fact that DSL is used to carry both the voice and data traffic. There are several specific architectures proposed for VoDSL, which are discussed later in this chapter.

Figure 16.3. Generalized VoDSL architecture.

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DSL Advances
DSL Advances
ISBN: 0130938106
EAN: 2147483647
Year: 2002
Pages: 154

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