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CROS.NET DSL DSL TECH


Getting familiar with DSL means looking at the basics. It all started with "digital subscriber line," which doesn't refer to a "line" at all; it's the modem specification for ISDN. On a single wire pair, DSL modems use two binary one quaternary (2B1Q) encoding along with time-division multiplexing (TDM) to create the familiar 2B+D basic ISDN channels. ISDN transmission can be viewed as a form of DSL, with one important difference: ISDN integrates (the "I" in ISDN) voice and data communications on the phone network, while DSL transmits voice and data on separate networks. 

Next came high-speed DSL (HDSL). In the late 1980s, Bellcore engineers realized the superior spectral and distance characteristics of 2B1Q could be used to replace the alternate-mark inversion (AMI) encoding commonly used on T1 (1.544-Mbit/s) lines. HDSL reduces the need for repeaters, and it causes less interference in cable bundles. It uses two wire pairs, and it's not a consumer access line technology; it is deployed only within the (copper) networks of carriers (and private campuses). But Pairgain Technologies Inc. (Tustin, Calif.) has developed a single-pair version (S-HDSL) for access line apps (up to 768 kbit/s). 

In the 1990s, vendors developing video-on-demand products started using ADSL (asymmetric DSL) technology, with different speeds for sending and receiving channels. Although the video idea didn't pan out (at least not yet), the Internet did—and ADSL emerged as a hot product for residential Web surfing. The only thing missing was the telephone line, so frequency-division multiplexing (FDM) was added to reserve a separate 4-kHz POTS (plain old telephone system) channel below the higher frequency data band. ADSL uses a single wire pair. Speeds on the receiving channel can climb to 6 Mbit/s, but actual performance depends on line conditions. 

Many modem vendors also offer symmetric DSL (SDSL) equipment. SDSL stands as an attractive alternative to symmetrical links already used in such business apps as leased lines or frame relay service. It can handle branch-office applications needing symmetrical send/receive channels (such as transaction processing). SDSL transmission over a single wire pair is technically challenging: Because SDSL generates more near-end crosstalk (Next) than ADSL, maximum speeds will be 2 Mbit/s or less. 
   
 
Rate-adaptive DSL (RADSL) is the latest DSL modem technology to make it into production. It's also the most promising, since it measures the performance characteristics of t he local loop and dynamically adjusts to the highest speed possible. RADSL can operate with symmetrical or asymmetrical send/receive channels, but asymmetrical configurations yield higher bandwidths (because of Next reduction). RADSL makes service deployment easier because it can operate on a wider range of loop conditions. 

In the future, look for very high-speed DSL (VDSL). Basically, it's DSL with a shorter copper wire—and it achieves transmission rates of up to 51 Mbit/s. When it arrives, VDSL will be expensive because carriers have to use fiber links to replace most of the local loop. The copper-wire DSL part of the circuit might only include the drop line to a residence or business. 
In residential markets, some carriers plan to offer low-speed ISDN DSL (IDSL) and consumer DSL (CDSL). IDSL is ISDN without the telephone switch: The two B channels of an ISDN BRI (basic-rate interface) are multiplexed to offer a dedicated 128 kbit/s of bandwidth for data only. (Some might argue that IDSL is not really a form of DSL in that it doesn't offer voice.) CDSL, on the other hand, is a low-cost 1-Mbit/s version of DSL based on chip sets developed by Rockwell Corp. (Costa Mesa, Calif.). The technology was announced last year, and products aren't available yet. 

But don't think DSL is all about acronyms. The market is shaping up to be huge, and everyone has the best idea about how to do it. As a result, today's products employ different encoding schemes and upper-layer protocol stacks. In other words, they're proprietary. 

The most popular DSL encoding schemes are carrierless amplitude and phase modulation (CAP) and discrete multitone (DMT). Both achieve high bandwidth by packing more bits into kilohertz signaling rates, but they use different (and incompatible) approaches. DMT offers more granular increments for RADSL rate adaptation, but CAP is a more proven technology (and appears favored by more equipment makers). Both have been endorsed by standards bodie s, and the adoption of two parallel standards appears likely. As for upper-layer protocols, some vendors use point-to-point protocol (PPP), others use ATM, and some offer a combination of the two. Some modems have IP routing built in, while others don't. The upshot for network managers? Be sure end-users get the modem brand specified by the LEC that furnishes the DSL service. 

DSL is a completely different twist on twisted pair. Unlike private line or frame relay or anything else before, it's not an end-to-end WAN service or protocol. It's an access line technology used only on a LEC's copper wire local loop connecting a remote site to the nearest central office (or remote terminal location). From there, the DSL data channel is converted to some other transmission method—probably ATM—which the LEC connects to an ISP (Internet service provider) or to a central site serving a private DSL network. 

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DSL replaces the local loop to a business or residence. In the carrier's central office, the DSL line first passes through splitters (passband filters) to split off the POTS voice channel, which is routed into the phone network equipment. The data channel is then terminated on a DSL access multiplexer (DSLAM) which, depending on the vendor, links DSL access lines to ATM networks, Ethernets, T1 lines, serial lines, or frame relay.

Digital Subscriber Line Multiplexer (DSLaM) equipment being installed in a Subscriber Line Concentrator (SLiCk) enclosure. Having a router in the neighborhood for ADSL IP packets reduces the number of circuits to the Central Office. Here Lucent technicians were installing Philips ADSL equipment for BellSouth.

 

There are two problems: load coils and bridge taps. Inductive load coils are used on longer POTS lines to extend distance and limit bandwidth to a 4-kHz passband for standard telephone service. Bridge taps are created when wire pairs are reassigned over the years and the existing copper wiring is simply tapped with a branch instead of rerouted to the new location. 

DSL modems are not regulate yet. Most DSL modems handle loop lengths of about 18,000 feet, which includes 90 percent of the world's phone lines (maximum lengths in the U.S. tend to be a bit greater). Typically, ADSL modems can receive 6 Mbit/s and transmit 640 kbit/s, but that's with a 12,000-foot loop length. ADSL receiving speed drops to about T1 rates on an 18,000-foot line, and T1 or less is the top speed most DSL customers will see today (however, some vendor specs claim slightly longer lengths and higher speeds). RADSL is the best compromise, offering the maximum performance possible under varying loop conditions and customer requirements (some may choose slower, cheaper service). 

Don't expect to see any dramatic speed improvements in the years ahead; today's maximum speeds are determined mostly by the spectral limits of typical copper wire. In time, as carriers expand fiber infrastructures and shorten copper loops, more customers will gain access to the higher DSL rates (including VDSL for some). Vendors also have developed modems capable of DSL inverse multiplexing over multiple local loops. 

Remote access via DSL is much easier than using a dial-up modem over the phone network. Remember, there's no more dialing—and the familiar squawk of the analog modem may become a distant memory. With DSL, telecommuters can plug into a 10Base-T port at home and get instant access to services exactly as they do in the office (just a bit slower). 
Remote users will need a DSL modem. External varieties have at least three connection ports: a modular socket or wire terminals to connect to the carrier network termination, a socket to connect POTS devices, and a LAN port (probably 10Base-T Ethernet). DSL internal modem cards also are available. Some vendors offer weatherproof outdoor modules to replace a residential phone termination, while others offer indoor units or a separate box to split off the phone channel. Modems with multiple POTS lines are also in the works. 

With DSL, every remote site gets an Ethernet LAN (unless attaching a single PC via an internal modem card). Wire must be run and Ethernet cards installed. As a low-cost residential wiring option, Category 3 copper can be used with the send/receive pairs rolled for a single connection without a hub (unless, of course, the user is the sort of person who already has Ethernet in the bedroom). Business locations will use Category 5 wiring. 

As noted, a DSL link will include phone service—and all current phones will continue to work fine. And unlike ISDN, the phone line really is POTS, so the service is line-powered and works even when the electricity goes out.

 
 
 

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