Phone lines pick up SPEED

DN Staff

October 20, 1997

10 Min Read
Phone lines pick up SPEED

Sure, the Internet's great, but what about the World Wide Wait? I can almost feel myself age waiting for graphics-intensive home pages to grace my computer screen.

Homes and small businesses without high-speed digital lines have it worst. Even with 56-kbps analog modems, you'd think Internet service providers live by the warning "Speed kills."

Solutions to this log jam include using satellite communications, cable modems, or wiring the world with coaxial or fiber-optic cable. Each would support megabit data rates.

But what if you could get that performance from the omnipresent public switched telephone network (PSTN)? Well, guess what: You can, using an 8-year-old technology called asynchronous digital subscriber loop, or ADSL.

Since ANSI and ETSI standardized on the discrete multitone (DMT) modulation scheme in May, several companies--among them Analog Devices, Motorola, and Texas Instruments--have announced lower-cost, lower-power integrated ADSL chip sets. In addition, many of these semiconductor companies will make available ADSL modem reference designs, so companies without previous design experience can get products to market quickly.

Many firms will likely do just that. As one TI engineer put it: "It's a pull, not a push, for the telcos--they're clamoring for this equipment be-cause it gives them the throughput to compete with cable modems."

How it works. ADSL can achieve downstream speeds of up to 8 Mbps because it takes advantage of asymmetrical bandwidth requirements. Typically, the network sends much more data to a subscriber than a subscriber sends to the network. ADSL reserves most of the phone line's 1.1-MHz bandwidth, therefore, for downstream data.

An ADSL circuit connects an ADSL modem on each end of a twisted-pair telephone line, creating three information channels: a high-speed downstream channel, a medium-speed duplex channel, and a plain old telephone service (POTS) channel. (Conventional analog modems use the 4-kHz POTS channel.) Filters split off the POTS channel from the digital modems, thus guaranteeing uninterrupted POTS, no matter what other services are utilizing the line.

Infrastructure changes needed to implement ADSL technology include adding ADSL modems and POTS splitters at a telephone company's central office and at a customer's premises.

The high-speed downstream channel ranges from 1.5 to 8 Mbps; the duplex channel ranges from 64 to 640 kbps. Both can be broken down into a total of six subchannels. For example, your spouse, son, daughter, and dog could each watch separate videos being simultaneously downloaded via an ADSL set-top box; you could hold a videoconference with a coworker; your brother could surf the Internet; and your mother could call a neighbor or a distant relative. All at the same time. All using the same twisted-pair wire.

Competing with cable. The throughput of ADSL lets telcos compete with cable companies' push for cable modems. While cable modems have greater downstream bandwidth (up to 30 Mbps), that bandwidth is shared among all users on a line and therefore varies dramatically with traffic. Cable modem upstream traffic will in many cases be slower than ADSL, either because the particular cable modem is inherently slower or because of rate reductions caused by contention for upstream bandwidth slots.

The big difference between ADSL and cable modems, however, is the number of lines available to each. Only about 12 million homes today can support cable modems, although the number grows every day. There are more than 600 million twisted-pair loops capable of connecting ADSL modems.

Another plus for telephone companies is that ADSL modem lines won't tie up expensive switching equipment. Telephone companies transport virtually all local Internet data using local loops and switches that were designed for short-duration voice traffic. The average voice call lasts about four minutes (if you don't factor in my sister's marathon calls); switches accommodate approximately nine minutes per line during peak hours.

However, the telephone industry reports that more than 35% of all on-line data service connections last more than an hour. And subscribers are often paying only $19.95 per month for unlimited access.

Clearly, something has to give. By adding ADSL modems, phone companies can switch off high-speed digital data directly to the network's fiber-optic backbone and save the switching equipment for voice calls. It also gives phone companies an easy and relatively inexpensive way to compete with cable and other services while taking their business into the twenty-first century.

Twisted-pair copper has a bandwidth of approximately 1.1.MHz. POTS takes up 4kHz at the dc end of the band; then upstream data from a client; then high-speed downstream data.

Chip-set tradeoffs. One decision ADSL chip-set designers have had to make is just how many chips to design. The fewer chips they do design, the lower the power dissipation, the lower the parts count, and the smaller the required PC-board real estate.

However, if standards change or speeds increase, a designer would have to do a major redesign of a highly integrated solution. If a chip set comprised five chips, perhaps only one chip would have to be redesigned, thus saving cost and inventory.

Motorola's ADSL solution is essentially one highly integrated chip--the CopperGold Transceiver--plus a line driver and your own personal choice of processor.

The functional blocks for implementing an ADSL modem include a multiplexer with error control, demultiplexer with error control, receiver, transmitter, D/A and A/D converters, line coupler, and a channel separator. DSPs handle many of these compute-intensive tasks. The POTS splitter can be incorporated in the modem or separate, and the customer premise end is the mirror image of the network end shown.

Texas Instruments' chip set comprises four chips: the new C6X DSP, an ASIC digital interface, a codec, and a line driver. "As standards change and speeds increase, we can reprogram the DSP with new software so the modem keeps up with technology," says Fred Reuter, member, group technical staff, at TI's Networking Business Unit Semiconductor Group. In fact, the residential-side modems can be programmed from the telco's central office.

Analog Devices' AD20msp910 chip set includes five chips: the modem interface, DMT coprocessor, analog front end, line driver, and DSP.

Market-research firm Dataquest projects DSL (digital subscriber line) modems to outsell cable and ISDN modems combined in 1998.

The chip sets are becoming available now and as demand for ADSL service ramps up, the price will go down. Research shows that most consumers wouldn't pay more than $100 per month for an ADSL-like service--no matter how fast the data rate. It's up to engineers to create equipment to reach that sweet spot.

At a glance

  • ADSL enables data rates of up to 8 Mbps to a client and 640 kbps from a client to the service provider over regular copper phone lines.

  • Since ANSI and ETSI standardized on a modulation scheme for ADSL, semiconductor companies have announced lower-cost, lower-power integrated ADSL chip sets.

  • Many of these semiconductor companies will make available ADSL modem reference designs, so companies without previous design experience can get products to market quickly.

  • Applications include Internet access, video on demand, video teleconferencing, remote LAN access, and CD-ROM database links.


Access node--Concentration point for broadband and narrowband data located at a central office or a remote site.

ADSL (asymmetric digital subscriber line)--ADSL is one of a family of digital subscriber line technologies that deliver high-speed digital data over the existing telephone line. The ANSI ADSL standard defines a high-speed modem capable of transmitting more than 6 Mbps over twisted-pair copper. Propelling data down existing telephone lines at rates 300 times the rate of an analog modem, ADSL is quick enough to bring remote multimedia--video, audio, graphics, and text--to millions of users.

ANSI (American National Standards Institute)--The U.S. telecommunications standards-setting organization.

ASIC (application-specific integrated circuit)--A chip designed from the ground up for a specific task.

Broadband network--Switching system for data rates above 1.5/2.0 Mbps.

CO (central office)--Telephone company equipment premises.

DMT (discrete multitone)--A modulation scheme in which available bandwidth is divided into many subchannels.

Noisy subchannels carry few or zero bits; clear channels carry a maximum number of bits. DMT essentially guarantees the telephone companies and consumers that they are achieving the greatest throughput possible over any given copper line.

Downstream--Data flow direction from the service provider, such as a telephone company, to the client.

DSP (digital signal processor)--A processor optimized to compute mathematical algorithms on a sequence of numbers.

ETSI (European Telecommunications Standards Institute)--The Europe-based telecommunications standards-setting organization.

ISDN (Integrated Services Digital Network)--A digital network with circuit and packet switching for voice and data communications at data rates up to 1.544 or 2.048 Mbps and 144 kbps for Basic Rate Access (BRA). BRA provides two voice channels at 64 kbps each and a data channel at 16 kbps.

kbps (kilobits per second)--A measure of digital transmission speed.

Loop--Twisted-pair copper telephone line. Loops may differ in distance, diameter, and transmission characteristics, depending on the network.

Mbps (megabits per second)--A measure of digital transmission speed.

Narrowband network--Switching system for data rates at or below 1.5/2.0 Mbps.

POTS (plain old telephone service)--Basic analog telephone service. POTS takes the lowest 4 kHz of bandwidth on twisted-pair wiring.

PSTN (public switched telephone network)--Network that lets telephone users call almost any point on the planet wired for telephone service.

Splitter--Filters that separate high-frequency (ADSL) and low-frequency (POTS) data at the client end and the telephone company end.

Upstream--Data flow direction from a client to a service provider, such as a telephone company.


ADSL Forum

Analog Devices

Motorola Corp.

Texas Instruments

Texas Instruments Network Technology Center

For more information on ADSL, click here.

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