Voice over DSL Tutorial

by Ray Horak
CommWeb.com
11/05/01

For those of you who regularly grace this space (i.e., read this column), you know that we've spent a lot of time working through many of the variations on the xDSL (generic Digital Subscriber Line, with x being the whatever) theme.

Specifically, we've worked through HDSL (High-bit-rate DSL), ADSL (Asymmetric DSL), and ADSL Lite. We've also worked through a number of Vox (Voice over whatever) technologies. Specifically, we've talked about VoFR (Voice over Frame Relay), VoATM (Voice over ATM), and VoIP (Voice over IP).

At this point, let's pull it together and wrap it up. Then we can move onto something else.

VoDSL may not seem like much of a trick, as we discussed the fact that voice commonly is transmitted over ADSL and ADSL Lite. As you may recall in these two cases, voice travels in its native analog format over a 4 KHz channel, while data travels in digital format in higher frequency channels.

The voice and data signals coexist quite nicely on the same local loop, as they are widely separated by frequency. VoDSL means something quite different, however.

The Basic Concept

VoDSL is a non-standard technique for supporting voice and data over a single DSL circuit. It is, however, based on a number of standards.

The idea is to support multiple voice calls, either inbound or outbound, over a single DSL circuit, while simultaneously supporting high speed Internet access. All transmissions are in digital format, unlike the hybrid analog/digital approach we examined in ADSL and ADSL Lite.

Now, mixing voice and data over a digital access line isn't exactly news. We've been doing that over channelized T1 circuits for a very long time. VoDSL, however, doesn't make use of T1 which, as you'll recall, is based on TDM (Time Division Multiplexing).

DSL technologies are packet-based, and make use of STDM (Statistical Time Division Multiplexing). Further, the bandwidth allocation is dynamic, shifting between voice and data while always prioritizing voice.

The Specifics

There are several DSL options over which VoDSL runs. SDSL (Symmetric DSL) is a perfectly symmetrical version, with equal amounts of bandwidth both downstream and upstream. SDSL runs at signaling rates as high as 1.544 Mbps, which is equivalent to T1 speed, over distances up to 2 miles or so. SDSL also can be adjusted downward to run at rates of 384 Kbps and 128 Kbps, for example.

S-HDSL (Single-pair High-data-rate DSL), recently (February 2000) standardized as G.991.2, runs at rates up to 2.3 Mbps over distances well beyond the typical DSL limit of 18,000 feet, and it does so over a single UTP copper pair.

ATM is used as the Layer 2 protocol in VoDSL implementations. At the customer premises, both voice and data equipment connect to the DSL circuit through a modem, bridge, or router, which is known in this case as an IAD (Integrated Access Device). At that point, outbound analog voice is converted to digital PCM (Pulse Code Modulation) format through a codec (coder/decoder).

The PCM samples are formed into ATM cells using AAL1 or AAL2, depending on the vendor's specific implementation. In either case, the ATM voice cells take precedence over the IP-based packet data transmissions, which are segmented into AAL3/4 or AAL5 cells.

At the CO (Central Office) or other POP (Point Of Presence), the service provider's DSLAM (DSL Access Multiplexer) typically demultiplexes the voice cells and runs them through a voice gateway, where they are put back into pure PCM byte-interleaved format, presented to a Class 5 circuit switch, and sent over the PSTN (Public Switched Telephone Network). The data cells are carried as packet or cell traffic to their ultimate destinations, which could be the Internet or a corporate Intranet, through the ISP.

VoDSL implementations typically support as many as 16 voice conversations over a full-rate SDSL link running at 1.544 Mbps, although some manufacturers support as many as 24 conversations. The balance of the bandwidth is reserved for data communications purposes.

Thereby, voice demands cannot stall data traffic. Should the entire 1.544 Mbps be in use for data communications at a given moment, that traffic will be throttled back in favor of voice, up to the maximum of 16/24 conversations.

Note: The exact amount of bandwidth required for a voice conversation is not specified, as the specific encoding techniques and compression algorithms employed are selected by the manufacturer. Assuming that the approach is AAL1 and PCM at 64 Kbps per voice conversation, 16 voice conversations would still leave 512 Kbps available for data. This approach is typical, if any implementation of such a new technology can be characterized as being typical.

The Applications

VoDSL is intended for application in small- and medium-size businesses that require both voice services and high speed Internet access. It also has been trialed in Centrex applications, where it allows up to 24 voice conversations to be multiplexed over a single DSL circuit. The conventional approach, of course, requires a local loop per Centrex station, which is extremely loop-intensive.

The Benefits

VoDSL offers a number of benefits. Here's a short list:

• First, a single DSL-equipped local loop can support integrated voice/data access, with up to 16 (or even 24) voice conversations supported simultaneously.
• Second, dynamic bandwidth allocation ensures that circuit usage is maximized at all times, with voice precedence being honored. The cost advantages of a single local loop (comprising a single pair) for both voice and high speed data are clear.
• Third, voice features and feature access remain intact, with none of the technical problems associated with signaling and control issues between IP networks and the PSTN that plague pure VoIP.