ADSL Testing Moves Out of the Lab | EDN
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`Martin Rowe -April 01, 1999
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`Recently, my cousin asked me to send an e-mail to her daughter
`at her junior high school. To my surprise, I discovered that Gina’s
`school has no ordinary Internet connection. The school has an
`asymmetric digital subscriber line (ADSL) connection to a local
`Internet service provider (ISP).1 ADSL has moved out of the lab
`and the field trials and now is available to homes, businesses,
`and even schools. ADSL currently makes up about 6% of all
`broadband connections. By 2004, ADSL is expected to take the
`lead in broadband subscribers with 37% market share compared
`to cable modems at 26%.2 Even though ADSL installations have
`begun, several versions of the technology still exist. And the PC
`industry is pushing for a scaled-down version of ADSL to sell to
`the mass market. As a result, testing of ADSL products must
`occur at several stages, ranging from the lab to the production
`floor to the telco that test products for deployment to the
`technician who installs ADSL service.
`Two years ago, ADSL testing consisted almost entirely of
`physical-layer tests on simulated local loops.3 Engineers tested
`new designs for conformance to ANSI T1.413, which defines
`discrete multitone (DMT) communications between a subscriber’s
`ADSL modem and a central office digital subscriber line access
`multiplexer (DSLAM).4 The DSLAM equipment located at a telco
`contains an ADSL transceiver unit-central office (ATU-C) while
`the modem at the subscriber’s premises is often called an ADSL
`transceiver unit-remote (ATU-R).
`New ADSL chip set and full transceiver designs still require
`rigorous physical-layer testing, but in production, manufacturers
`may perform a few parametric tests. Manufacturers more often
`test for bit-error rate and other functional attributes.
`Manufacturers also test their ADSL products for how well they
`connect to Ethernet and asynchronous transfer mode (ATM)
`networks.
`ADSL chip sets—like analog modems—have built-in test
`features. Chip sets have loopback features that let you test the
`devices’ transmitter and receiver circuits. The loopback tests can
`take place within the chips or over a simulated local loop to a
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`ADSL Testing Moves Out of the Lab | EDN
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`http://www.edn.com/electronics-news/4381946/ADSL-Testing-Moves-O...
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`remote transceiver. Manufacturers often start testing by using the
`chip set’s local-loopback feature.
`Figure 1 shows a typical setup for remote loopback testing.
`Loopback tests let manufacturers know that the chip set’s
`transmitter and receiver function properly. Because ADSL is
`asymmetric, upstream and downstream directions use different
`data rates. The upstream’s slower speed limits testing to about
`600 kbps. Loopback tests can’t test an ATU-R chip set at the
`receivers full speed, nor can they test an ATU-C at full
`downstream speed. Manufacturers could test their chip sets at
`full speed by using an ATU-C chip set to test an ATU-R chip set
`and vice versa. Because they often perform functional tests only,
`manufacturers typically test at the maximum upstream rate only
`and use the same type of device at both ends of a loop.
`
`Figure 1. Remote loopback in an ADSL chip set lets one ADSL
`transceiver test another.
`Once they know that their chip sets function properly,
`manufacturers such as Alcatel Microelectronics (Zaventem,
`Belgium) and Texas Instruments (Dallas, TX) perform tests for
`parameters such as:
`
`Spectral power. Manufacturers measure the power in each
`downstream transmit tone used in an ATU-R and in each
`upstream transmit tone of an ATU-C.
`
`Cross modulation. Manufacturers shut off some tones and
`measure any leakage from adjacent tones.
`
`Frequency response. Using a spectrum analyzer,
`manufacturers can test for bits per tone.
`
`Noise ratio. In this test, manufacturers measure the noise on
`the line with all tones turned on, then with all tones turned off.
`
`Control interface. Manufacturers must verify that the ATU-R’s
`or ATU-C’s control interface properly responds to commands
`from a host computer.
`After completing these tests, manufacturers test the device’s
`ability to communicate with a device over a simulated local loop.
`Manufacturers often simulate local loops to 18,000 feet and test
`for transmission errors.
`ADSL chip set makers also test their devices for how well they
`transport data. Typically, the DSLAM is connected to an ATM
`network at the CO. Subscriber modems typically transport data
`within ATM cells or Ethernet packets. Therefore, chip makers
`often test for the passing of data using both protocols.
`Once ADSL chip sets are assembled into modems, modem
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`ADSL Testing Moves Out of the Lab | EDN
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`http://www.edn.com/electronics-news/4381946/ADSL-Testing-Moves-O...
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`makers perform similar tests to those performed by chip makers.
`This time, however, the modem builder tests not only the chip set
`but also the phone line interface circuits, which include a
`transformer and a line driver IC. Because ADSL uses up to 256
`tones for transporting data, an ADSL modem—particularly the
`ATU-C—must drive significant current into the local loop. An
`ADSL driver IC can supply as much as 400 mA of peak current.5
`Production ATE
`Test Engineers at Alcatel USA (Plano, TX) use an ATE system to
`test both ATU-Cs and ATU-Rs. The system tests for both
`physical parameters and for data transport. Tests begin through
`the chip set’s loopback feature, which verifies modem
`functionality. After the loopback tests, Alcatel performs several
`parametric tests. One such test is for multitone power ratio. In
`this test, a manufacturer compares the power that the UUT
`produced with all tones off to the power produced with all tones
`on.
`In an idle channel noise test. Alcatel’s system measures the
`noise levels that an ADSL transmitter puts out if it has an idle
`channel (a tone not in use). Each ADSL product manufacturer
`has to decide how much noise is acceptable.
`Alcatel USA also produces DSLAMs. Engineers test ATU-Cs in
`the DSLAMS for power levels in the tones at frequencies up to
`1.1 MHz. In a production test, a test system measures dynamic
`range and SNR in transmitted signals. To perform these tests,
`the tester turns on all 256 tones at once. After taking 32
`ksamples of measurements in the time domain, the tester
`performs an FFT on the data. In the frequency domain, the FFT
`lets Alcatel engineers see noise (spectrum spreading of tones)
`and power levels in the tones.
`When testing ATU-Rs, manufacturers perform more tests on the
`receiver than on the transmitter because of ADSL’s higher
`downstream data rates. Alcatel tests receiver circuits by
`simulating transmitted tones with an arbitrary waveform
`generator. The test system then measures the ATU-R’s
`sensitivity to each tone.
`Alcatel then tests the ATU-C’s and ATU-R’s abilities to send and
`receive data over simulated local loops. Production tests require
`measurement of Ethernet and ATM traffic, which includes
`bit-error rate (BER) tests. To perform these measurements,
`Alcatel uses an ATM traffic analyzer and a protocol analyzer for
`measuring errors in Ethernet traffic.
`Two years ago, engineers at the telcos were performing
`physical-layer tests: bit-error rate (BER) measurements in the
`presence of simulated RF noise, crosstalk, and impulse noise.
`Today, telcos such as U S West (Minneapolis, MN) test the ADSL
`products through higher-layer tests. Dan Edeen, senior engineer
`at U S West points out he used to measure the physical layer
`performance of ADSL modems. Now, he puts his time into
`testing new features and functions of the products that his
`company installs at its COs and at customer sites.
`Edeen tests ATU-Rs and ATU-Cs for BER and for traffic
`throughput. ATU-Cs use Ethernet connections to the subscriber’s
`computer. DSLAMs connect to the digital network and transfer
`data to ISPs. Figure 2 shows how the company’s equipment
`works in an installation. A DSLAM contains a plain-old telephone
`service (POTS) splitter that isolates voice and data signals at the
`central office, then combines them for transmission on the local
`loop. For data, the DSLAM connects to an ATM switch, which
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`ADSL Testing Moves Out of the Lab | EDN
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`http://www.edn.com/electronics-news/4381946/ADSL-Testing-Moves-O...
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`distributes data to ISPs and corporate networks, typically over
`leased lines.
`
`Figure 2. One version of splitterless ADSL service requires
`microfilters to separate ADSL signals from POTS signals.
`(Courtesy of U S West.)
`The ADSL products that U S West installs use rate adaptive DSL
`(RADSL) technology; the subscriber can choose among several
`levels of service (data rates). Service ranges from 256 kbps
`downstream/256 kbps upstream for $40 per month to 7 Mbps
`downstream/1 Mbps upstream for $840 per month—plus ISP
`charges. The data rate that a subscriber can buy may be limited
`by the characteristics of the subscriber’s local loop. Therefore,
`some customers may not be able to subscribe to the service they
`want because their local lines won’t support their preferred data
`rates.
`For the telco to know how each local loop performs, it must test
`each loop. Here’s another place where ADSL testing has moved
`out of the lab. Technicians use portable equipment to test the
`local loops for throughput and noise. Bill Moten, product
`marketing manager at TTC (Germantown, MD) explains that
`these tests are important to ensure that the subscriber gets the
`quality of service he or she ordered. The local-loop
`measurements also help set the maximum and minimum rates of
`service that the ADSL modem can use. Unfortunately, if the
`minimum rate is set too high, then the customer’s modem will
`never synchronize to the ATU-C at the telco—the customer’s
`connection won’t work.
`If noise appears at certain frequencies during the tests on a local
`loop, the technician can find the source of the noise and
`crosstalk in the bundle of wires that contains the subscriber’s
`loop. For example, noise at 772 kHz indicates that noise is
`coming from a T1 line in the same bundle. That frequency is the
`Nyquist frequency of the T1’s 1.544 MHz. Noise at 196 kHz
`comes from a high-bit-rate digital subscriber line (HDSL) service.
`Splitters and Filters
`Right now, ADSL service that U S West sells requires
`subscribers to use the ATU-R equipment that the company
`provides. In addition, subscribers must install a small filter (called
`a microfilter) at each POTS device (each phone, analog modem,
`or fax machine) connected to the same phone line as the ADSL
`service. Some ADSL services require that the telco install a
`splitter on the subscriber side of the network interface device
`(NID) box. A splitter contains high-pass and low-pass filters that
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`ADSL Testing Moves Out of the Lab | EDN
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`http://www.edn.com/electronics-news/4381946/ADSL-Testing-Moves-O...
`
`separate the voiceband POTS signals from the ADSL signals.
`These installations always require a visit from a technician.
`Sending a technician to a subscriber’s premises is expensive. U
`S West’s distributed splitter system (Fig. 2) uses a splitter at the
`CO to separate POTS and ADSL signals. This eliminates the
`need for a technician to install a splitter, but still requires the
`subscriber to install the filters.
`A splitterless version of ADSL is vying to become a standard. For
`now, it’s called DSL-Lite or G.Lite. G.Lite modems are based on
`ANSI T1.413, the standard for full-rate ADSL.6 Now sanctioned
`by the International Telecommunications Union (ITU), G.Lite has
`begun appearing in modems built into new PCs, most notably
`those from Compaq. With G.Lite, subscribers should be able to
`buy any ADSL modem and subscribe to a service.
`G.lite modems use only 96 tones for data rather than the 256
`tones used by full-rate ADSL modems. (Fig. 3). The fewer tones
`limit the bandwidth to 420 kHz, a data rate of about 1 Mbps
`downstream and 512 kbps upstream. G.lite modems are
`designed to be compatible with the full-rate ATU-C systems at
`the telco side, but without letting the ATU-C assign data channels
`to tones greater than 96.7 G.Lite modems use less bandwidth,
`and hence less power, than full-rate ADSL modems. Because the
`bandwidth of G.Lite modems is about half that of full-rate ADSL,
`their total power usage drops by about 3 dB.8
`
`Figure 3. G.Lite ADSL uses less bandwidth than full-rate ADSL.
`POTS Interference
`ADSL subscribers should be able to use an analog dial-up
`modem or fax machine concurrently with an ADSL modem on the
`same phone line. Users might, for example, have an ADSL
`subscription for Internet service and still need to use a dial-up
`modem to get their office e-mail.
`Splitterless ADSL equipment has testing issues that don’t appear
`with full-rate, splittered ADSL systems. Testing of splitterless
`ADSL modems requires testing with POTS signals:
`on-hook/off-hook impulses, dial tones, DTMF dialing tones,
`pulses from pulse dialers, voice tones, analog-modem signals,
`and caller-ID data. Splitterless ADSL must also work with
`impairments caused by home wiring. Typical home telephone
`wires use a flat design, not a twisted pair. In benchmark tests,
`Aware (Bedford, MA) used 500 ft of flat 24 AWG wire, then
`added bridged taps and telephones at 50-ft. intervals.9 When you
`test any ADSL design, be sure to test using the flat phone wire
`that is available in any hardware store.
`According to Bill Timm, G.Lite development manager at Texas
`Instruments, manufacturers of splitterless ADSL products should
`test their designs with at least five POTS devices connected to
`the same line. You should test for any and all phones going
`off-hook during an ADSL downstream transmission. Even though
`G.Lite devices have high-pass filters, those filters work only for
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`http://www.edn.com/electronics-news/4381946/ADSL-Testing-Moves-O...
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`voiceband signals. When a phone goes off-hook, it generates an
`impulse with frequency components well into the high ADSL
`frequencies.
`The problem emerges because some phones have an off-hook
`impedance as low as 10 V. Such a low impedance draws more
`current from the ADSL driver IC at the other end of the line. The
`change in impedance can create echoes that reflect back to the
`CO equipment. That causes the ATU-C and ATU-R equipment to
`reestablish their connection, called retraining. Because no data
`can transfer in either direction during a retraining, subscribers will
`suffer a reduced quality of service.10 Texas Instruments has
`found that the interference from POTS equipment varies greatly
`among different phones. So, make sure you test your product
`with as many different phones as possible.
`Most POTS equipment has some sort of circuit (often with
`diodes) that protects the equipment from voltage surges on the
`line. These devices can clip some of the ADSL tones, which
`creates noise in the POTS band. Figure 4 shows a time-domain
`representation of an ADSL signal. The peak that exceeds the clip
`level will produce noise in POTS equipment.
`
`Figure 4. G.Lite splitterless ADSL can produce noise in the
`POTS band if tones have too much power, causing the receiver’s
`telecom line protection circuits to clip. (Courtesy of Texas
`Instruments.)
`While transmitting to the ATU-R, splitterless ADSL modems such
`as G.Lite modems can also produce interference in POTS
`equipment. That interference can disrupt communication with an
`analog modem or fax machine sharing the same phone line.
`Splitterless ADSL modems can’t transmit with as much power as
`splittered ADSL modems. The upstream power levels must be 6
`dB to 9 dB lower than the levels used by full-rate ADSL modems.
`T&MW
`FOOTNOTES
`1. "ADSL Internet Access and LAN Connectivity in a Campus
`Environment," Aware, Bedford, MA,
`support.aware.com/technology/whitepapers/index.html.
`2. Broadband Delivery in the Local Loop: 1999, Allied Business
`Intelligence, Oyster Bay, NY, February 1999.
`3. Rowe, Martin, "ADSL Products Must Conform and Perform,
`’’ Test & Measurement World, February 1997.
`4. ANSI T1.413-1995. Telecommunications Network and
`Customer Installation Interfaces–Asymmetric Digital Subscriber
`Line (ADSL) Metallic Interface. American National Standards
`Institute (ANSI), New York, NY, www.ansi.org.
`5. Schweber, Bill, "ADSL driver/receiver pumps up the current
`with fast-slewing drive,’’ EDN, February 6, 1998, p. 24.
`6. Fares, Nancy, "Universal DSL Deployment of G.Lite,’’ 1998
`Fall DSLcon Conference Proceedings, DSLcon, Morgan Hill, CA,
`www.dslcon.com
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`ADSL Testing Moves Out of the Lab | EDN
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`http://www.edn.com/electronics-news/4381946/ADSL-Testing-Moves-O...
`
`7. Splitterless G.Lite interoperability with ANSI T1.413 and/or
`G.DMT, Aware, Bedford, MA, support.aware.com/technology
`/ITU/ITUinterop.PDF
`8,9. Splittlerless DMT System Design and Measurements, Aware,
`Bedford, MA, support.aware.com/technology/ITU/itulite.PDF
`10. Fares, Nancy, op. cit.
`FOR FURTHER READING
`Midcom, a manufacturer of telecom transformers, has some
`definitions of critical parameters used in ADSL testing.
`www.midcom-inc.com/technology
`Technical Note: "ADSL Basics," TTC, Germantown, MD, May
`1998.
`The Universal ADSL Working Group (UAWG) has a Web site that
`contains the latest information about UADSL. (This Web site has
`been discontinued.—Eds., 8/15/01.)
`Ziemann, Peter, "Prove that ADSL modems have the right stuff to
`vanquish interference,’’ EDN, June 18, 1998.
`
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