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`This article was posted on 10/01/1997
`
`Selecting an ADSL transceiver: ANSI standard
`offers two levels of modem performance
`
`Selecting an ADSL transceiver:
`ANSI standard offers
`two levels of
`modem performance
`T1.413 Category II defines an advanced feature set for higher
`data throughput and backward compatibility with Category I
`
`BY RICK HALL
`Motorola Semiconductor
`Austin, TX
`
`Asymmetric digital subscriber line (ADSL) is a modem technology for
`a new generation of highspeed modems. ADSL uses an advanced signal modulation
`technique called discrete multitone (DMT) to transmit data over normal
`copper telephone wires at rates 200 times faster than current V.34 modems.
`
`With a data rate of 6 Mbits/s, ADSL (see Fig. 1) offers users blazing Internet browsing speeds while simultaneously
`preserving Plain Old Telephone Service (POTS). Consequently, ADSL provides a quick, costeffective way for network
`service providers to offer multimegabit services, such as distance learning or telecommuting, using the 650 million
`telephone lines already in place throughout the world.
`
`Fig. 1. ADSL modem technology delivers highspeed Internet access over existing telephone lines,
`while simultaneously preserving Plain Old Telephone Service (POTS).
`
`Intensive standards development, spearheaded by the T1E1.4 working group of the American National Standards
`Institute (ANSI), led to the publication of the T1.413 ADSL interface standard in 1995. This document details the
`standard functions of an ADSL modem and enables the design of interoperable, multivendor products.
`
`More than 50 equipment and semiconductor vendors are now developing T1.413compliant ADSL products. Other
`standards bodies such as the European Telecommunications Standards Institute and the International
`Telecommunication Union point to T1.413 as a reference for their ADSL standards.
`
`For modem designers, a careful review of T1.413 is essential when evaluating ADSL chipsets. In particular,
`understanding the different categories of the T1.413 standard is crucial in determining how an ADSL modem will
`actually perform in the field.
`
`Two levels of performance
`
`While specifying a basic set of features common to any ADSL terminal unit (ATU) modem, T1.413 calls out additional
`functions that enhance performance, especially over longer and noisier lines. The basic ATU is called a Category I
`
`Dish
`Exhibit 1019, Page 1
`
`
`
`modem, while the enhanced ATU is called a Category II modem.
`
`Category II adds two key features that can increase data throughput significantly: trellis coding and echo cancellation.
`A third option, called power boosta brute force method of increasing performance by cranking up the transmit signal
`strengthis no longer under consideration because of concerns about excessive crossta k with other wires in the same
`binder group.
`
`To ensure compatibility between Category I and II modems, various device options and setup information are
`transmitted at startup. This procedure allows the device at each end of the wire to be aware of the other's capabilities
`and to adjust itself accordingly.
`
`A Category II modem falls back to basic Category I operation when necessary, similar to the way different Vseries
`modems fall back to lower data rates (28.8 to 14.4 kbits/s, for example). However, when acting in this mode, Category
`II features are lost since both modems must be Category IIcompliant in order to take advantage of the enhanced
`features.
`
`Category II benefits
`
`The first feature that aids Category II performance is trelliscoded modulation. Trellis coding was first introduced in the
`early 1980s as a novel way for modems to overcome the effects of noise found on satellite links and telephone lines.
`
`In trellis coding, biterrorrate (BER) performance is improved by mapping groups of data bits to specially designed
`symbol sequences that can then be decoded more accurately in the presence of noise at the receiver. By storing the
`incoming sequences of symbols at the receiver, clever decoding schemes such as the Viterbi algorithm can be
`employed to make better decisions about the actual transmitted data. Trellis coding can boost performance by 4 dB,
`an improvement that can transform a marginal BER of 1 error in 1,000 bits into a much more palatable BER of 1 error
`in 100,000 bits.
`
`The second performanceboosting feature is echo cancellation, a bandwidthsharing scheme commonly used in high
`speed dialup modems. To understand the benefit of echo canceling, it's necessary to first understand how ADSL uses
`bandwidth.
`
`ADSL employs fullduplex data transmission of up to 6 Mbits/s in the downstream direction to the end user and up to
`640 kbits/s in the upstream direction back to the network. In Category I, the downstream and upstream signals are
`able to coexist on the same line via frequency division multiplexing (FDM) as shown in Fig. 2. The downstream signal
`is isolated in the 138 to 1,100kHz frequency band while the upstream signal uses the 26 to 134kHz band. The lower
`4kHz band is reserved for existing POTS voice transmissions.
`
`Fig. 2. Frequency division multiplexing allows upstream and downstream
`data to exist on the same line.
`
`To fully maximize the available bandwidth, Category II employs an echo cancellation scheme so that the downstream
`and upstream signals can overlap in frequency (see Fig. 3). The chief benefit is that the downstream signal now has
`more bandwidth to work with, theoretically boosting data throughput by about 10%.
`
`Fig. 3. Echo cancellation enables overlapping of upstream and downstream
`data, reusing the highquality, lowfrequency spectrum for increased throughput.
`
`Since the downstream/upstream signals cannot be separated at the receiver using normal bandpass filtering, an echo
`cancellation circuit must be used to subtract out the unwanted transmit signal as well as its return echoes (caused by
`impedance mismatches down the line). Echo cancellation is clearly a more complex operation than FDM, but one
`easily implemented with today's VLSI technology.
`
`The extra bandwidth used by the downstream signal is in a very desirable location since signal distortion caused by
`the telephone line decreases dramatically at lower frequencies. In the DMT modulation scheme used in ADSL, data is
`transmitted over 256 different subcarriers, each with its own frequency band and bitloading (that is, the number of bits
`transmitted on a subcarrier during each symbol time).
`
`Dish
`Exhibit 1019, Page 2
`
`
`
`Typically, the lowerfrequency subcarriers can be assigned higher bitloads, since these signals encounter less
`distortion, which means cleaner signals and higher data throughput. For example, a subcarrier near 1 MHz might have
`a bitload of only 6 bits/Hz, while a lowerfrequency subcarrier near 100 kHz could have twice that amount. Since echo
`cancellation switches on extra downstream subcarriers in the lower 26 to 138kHz band, the resulting bitloading gains
`can improve data throughput well in excess of the expected 10%.
`
`Extending coverage
`
`Both trellis coding and echo cancellation are common techniques used to squeeze the maximum amount of data from
`a transmission link, while preserving an acceptable BER (typically 1 error in 10 million bits). However, increasing data
`throughput is not the only reason for their usage. Often the goal of improving performance is to allow a modem to
`operate over longer distances, whether for a microwave relay station or a deep space probe.
`
`Operating reach is a formidable challenge to the widespread deployment of ADSL, since the telephone network
`presents such a staggering array of different wire gauges, cable types, and loop lengths. The system is replete with
`nasty impairments such as attenuation, phase distortion, crosstalk, and bridged taps (unterminated wire pairs hanging
`off the main line that are particularly onerous to highfrequency signals like ADSL). As a result, the telephone line
`network is a particularly demanding environment for highspeed data transmission, particularly over long distances.
`
`With its arsenal of enhanced features, a Category II modem can more effectively combat line impairment gremlins to
`help ensure that the maximum data rate of 6 Mbits/s can be realized over the target range of 0 to 12,000 ft. To reach
`beyond this range, the data rate can be reduced, which improves the ability of the modem's signal to travel further
`down the line.
`
`Category II modems can decrease the data rate to 1.5 Mbits/s or less so as to operate over longer distancesup to
`18,000 ftwhile ensuring optimized performance. Since it is estimated that 80% of telephone lines are less than this
`distance, the vast majority of homes can now be reached by ADSL service.
`
`Implementing ADSL
`
`One approach to ADSL modem design is provided by Motorola's CopperGold chipset, which consists of the
`MC145650 ADSL transceiver and the MC03AX1456CO/RT line driver. The two devices integrate all the signal
`processing and analog line interface circuitry necessary for developing a T1.413compliant ADSL modem (see Fig. 4).
`In addition, the transceiver features a DSPbased echo canceller and a trellis encoder/ Viterbi decoder to allow either
`Category I or Category II operation in the end system.
`
`Fig. 4. The CopperGold ADSL chipset from Motorola is designed to offer Categories I and II performance.
`
`The line driver comes in two versions: one optimized for the central office (CO) side of an ADSL connection and the
`other for the remote terminal (RT) at the end user's site. Separate devices are desirable due to the asymmetric nature
`of ADSL.
`
`The CO version has the tougher job of transmitting the downstream signal with frequencies up to 1,100 kHz, while the
`RT version transmits the upstream signal at frequencies only up to 134 kHz. Included in both devices is an onchip
`electronic hybrid that aids echo cancellation by attenuating the transmit signal to the receiver by about 10 dB.
`
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