7/12/2016
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`EE Times - ADSL Technology Explained, Part 1: The Physical Layer
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`ADSL Technology Explained, Part 1: The Physical
`Layer
`Louis Litwin, Michael Pugel, Rob
`Rhodes, and John Richardson
`3/1/2001 04:35 PM EST
`Posta comment
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`ADSL Technology Explained, Part 1: The Physical Layer
`For residential and commercial users with an ongoing need for broadband data access, but
`who do not send out correspondingly large data streams, asymmetric digital subscriber line
`(ADSL) services work well. This service is so named because the data rate sent to the user
`(downstream) is much greater than the data rate sent from the user (upstream). This
`asymmetric model is based on typical Internet usage patterns.
`
`For example, a user sends a Web page request (small amount of upstream data) and
`receives the HTML for the Web page with graphics and sound (large amount of
`downstream data). Various services, such as cable modems, satellite services, and DSL
`exist to provide such access.
`
`An ADSL system uses existing telephone wire to allow bidirectional data communications
`between a user and the telephone company's central office (CO). Some other popular
`services, such as an ISDN line or a standard dial-up modem, also use the phone lines to
`communicate. However, those services prevent the simultaneous operation of standard
`analog phone service on the same phone line. An important advantage of ADSL is that it
`allows the plain old telephone system (POTS) signal to co-exist with the ADSL data signal.
`
`We begin our tour of the ADSL system architecture with a look at the physical layer (PHY).
`Topics covered at the PHY will include ADSL's multicarrier modulation technique, common
`impairments, and phone-line characteristics.
`
`Spectrum allocation
`
`The ADSL PHY was designed so that it could peacefully co-exist with the standard POTS
`spectrum. The two services can co-exist because the ADSL spectrum only uses the
`frequencies above POTS. The POTS spectrum goes from near DC to approximately 4 kHz.
`A frequency guard band is placed between the POTS spectrum and the ADSL spectrum to
`help avoid interference. The ADSL spectrum starts above the POTS band and extends up
`to approximately 1.1 MHz. The lower part of the ADSL spectrum is for upstream
`transmission (from the customer to the CO) and the upper part of the spectrum is
`fordownstream transmission. There are actually two different ways that the upstream and
`downstream spectra can be arranged (see Figure 1).
`
`In a frequency division multiplexed (FDM) system, the upstream and downstream spectra
`use separate frequency ranges. They can vary for different implementations, but typically
`the upstream band is from 25 to 200 kHz and the downstream band is from 200 kHz to 1.1
`
`http://www.eeti m es.com/docum ent.asp?doc_id= 1277268&pri nt=yes
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`Exhibit 1035, Page 1
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`

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`7/12/2016
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`EE Times - ADSL Technology Explained, Part 1: The Physical Layer
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`MHz. Other divisions are also permitted within the ADSL standard. This system is free from
`the occurrence of a type of interference called self-crosstalk. One drawback, however, is
`that the downstream bandwidth is reduced in comparison to an echo-cancelled system.
`
`An echo-cancelled system allows the downstream band to overlap with the upstream band.
`The upstream band still uses the frequencies from 25 to 200kHz, but the downstream band
`can now extend over the upstream band. The main advantage of this system is that it
`significantly extends the available downstream bandwidth. However, it does require echo-
`canceling circuitry due to the full-duplex transmission. In addition, the presence of self-
`crosstalk causes additional interference.
`
`The DMT approach
`
`The PHY of ADSL uses a multicarrier modulation technique known as discrete multitone
`(DMT). A DMT system transmits data on multiple subcarriers in a manner very similar to
`the orthogonal FDM (OFDM) technique that is used in many wireless applications. A DMT
`modulator takes in N data symbols in parallel and transmits the symbols on N subcarriers.
`
`The data rate on each subcarrier is 1/N the original data rate.
`
`Reducing the data rate results in a DMT symbol period that is N times as long as the
`original symbol period. Increasing the symbol period can make the symbol longer than the
`time span of the channel. This situation can make it easier to combat the effects of
`intersymbol interference.
`
`The DMT signal is formed by using an Inverse Fast Fourier Trans-form (IFFT) to generate
`orthogonal subcarriers at the transmitter. The data symbols at the transmitter are treated
`as being in the frequency domain and act as complex weights for the basis functions
`(orthogonal sinusoids at different frequencies) of the IFFT. The IFFT then converts the
`data symbols into a time-domain "sum of sinusoids" signal.
`
`The block of IFFT output samples is known as a DMT symbol. This time-domain signal is
`transmitted across the channel, and an FFT is used at the receiver to bring the signal back
`into the frequency domain. A block diagram of a typical ADSL transmitter/receiver pair is
`shown in Figure 2.
`
`A 2N-point IFFT is used to generate the DMT symbol, and the N negative-frequency IFFT
`bins are the complex conjugate of the N positive-frequency bins. This symmetric spectrum
`results in a real time-domain signal. The DMT signal is centered at DC with the subcarriers
`around DC zeroed out (not used) to create a hole in the DMT spectrum in order to make
`room for the POTS spectrum. DMT is thus a true baseband system.
`
`DMT supports inclusion of a cyclic prefix. A cyclic prefix is a block of samples with a length,
`Lp, that is a replica of the last Lp samples of the DMT symbol. The prefix is then
`
`transmitted first, followed by the 2N samples of the DMT symbol. The length Lp is chosen
`
`such that it will be longer than the length of the channel response. The cyclic prefix
`contains redundant information. However, the DMT receiver exploits the presence of the
`prefix in order to mitigate the effects of the channel. The use of the cyclic prefix will be
`described in further detail in the Impairments section.
`
`The dynamic bit allocation technique allows DMT to make efficient use of the available
`channel capacity. This technique enables the system to vary the number of bits per symbol
`for each subcarrier based on the subcarrier's signal-to-noise ratio (SNR). Subcarriers with
`a low SNR transmit binary phase-shift keying (BPSK) or quadrature PSK (QPSK) because
`they are robust modulation formats. If the subcarrier's SNR is very low, that subcarrier will
`not be used to transmit data at all. Subcarriers with a higher SNR transmit higher-order
`quadrature amplitude modulation (QAM) in order to achieve an increased throughput.
`
`Impairments
`
`http:/lwww.eetimes.com/document.asp?docJd=1277268&print=yes
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`Dish
`Exhibit 1035, Page 2
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`

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`7/12/2016
`
`EE Times - ADSL Technology Explained, Part 1: The Physical Layer
`
`There are several significant types of impairments encountered in an ADSL system:
`additive white Gaussian noise (AWGN), crosstalk, impulse noise, bridged taps, and radio
`noise.
`
`AWGN is the thermal noise that is common to all communication systems. In a digital
`system such as ADSL, AWGN can cause symbol errors to occur at the receiver when noise
`pushes the received sample beyond a decision boundary. Like many other digital
`communication systems, ADSL employs error-control coding to help mitigate the effect of
`AWGN. Coding adds redundancy to the transmitted signal and exploits the redundancy at
`the receiver to detect and correct errors.
`
`ADSL uses three layers of coding. The innermost code in the PHY is a convolutional code.
`These codes get their name because the encoding process can be viewed as the
`convolution of the message with the code's impulse response. The Viterbi algorithm is used
`at the receiver to decode the received sequence.
`
`Convolutional codes
`
`Convolutional codes are good at correcting random errors. However, the nature of the
`decoding algorithm is such that the decoder can cause burst errors to occur if errors are
`made during the decoding process.
`
`A Reed-Solomon block code is used on top of the convolutional code. Reed-Solomon
`codes are powerful codes that are good at detecting and correcting burst errors, such as
`those generated by the Viterbi decoder. The ADSL specification allows Reed-Solomon
`code-word lengths of up to 255 bytes with the addition of up to 16 parity bytes for each
`code word. The outermost code is a cyclic redundancy check (CRC) code. The CRC can
`detect errors, but it cannot correct them. The CRC code is used as a top-level error-
`detection mechanism in order to detect any errors that remain after Viterbi and Reed-
`Solomon decoding.
`
`Because bundled telephone cable contains many wires for many different users, crosstalk
`is a common impairment. These wires radiate electromagnetically and can induce currents
`in other wires in the cable. This interference effect is known as crosstalk. There are two
`
`basic types of crosstalk and they both appear at the receiver as additive noise.
`
`Near-end crosstalk (NEXT) occurs when a transmitter interferes with a receiver located on
`the same end of the cable. Far-end crosstalk (FEXT) occurs when the transmitter
`interferes with a receiver on the opposite end of the cable. The effect of NEXT is more
`severe than FEXT since the FEXT interference travels the entire length of the cable and is
`attenuated by the time it reaches the receiver.
`
`Crosstalk can be further subdivided into self-crosstalk and foreign crosstalk. Self-crosstalk
`is interference from another ADSL system using the same spectrum allocation. Foreign
`crosstalk is interference from an ADSL system using a different spectrum allocation or from
`a completely different type of system (such as ISDN). One way to reduce the effects of
`crosstalk is with spectrum allocation.
`
`In echo-cancelled ADSL, the upstream and downstream channels overlap (see Figure 1).
`Since the same frequency band is being used for transmission and reception, the system
`will suffer from self- and foreign crosstalk. However, in FDM ADSL the upstream and
`downstream channels use separate frequency bands. This system will not suffer from self-
`crosstalk, although foreign crosstalk will still be present.
`
`Interference that is short in duration but of a large magnitude is known as impulse noise.
`Impulse noise can be caused by lightning or by a motor turning on and creating a power
`surge. ADSL systems use a combination of interleaving and coding to correct the errors
`caused by impulse noise. The interleaving process rearranges data so that those samples
`that were located contiguously in time are spaced far apart. Impulse noise can cause a
`
`http:/lwww.eetimes.com/document.asp?docJd=1277268&print=yes
`
`Dish
`Exhibit 1035, Page 3
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`

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`7/12/2016
`
`EE Times - ADSL Technology Explained, Part 1: The Physical Layer
`
`burst of errors that is hard for the decoders to correct. The use of interleaving combined
`with coding spreads out these errors in time to improve decoding performance.
`
`An additional impairment
`
`Bridged taps are an additional impairment type found in ADSL systems. A bridged tap is a
`section of wire connected to the loop at one end and unterminated at the other end.
`Examples of bridged taps are unterminated wires that are laid out in areas where housing
`is still being built. When a transmitted signal arrives at a bridged tap, the signal divides.
`Part of the energy continues to the receiver and the rest of the energy reflects off of the
`unterminated end. This reflection causes delayed versions of the signal to arrive at the
`receiver, and these reflections distort the received signal. The effect is very similar to the
`interference that occurs with a multipath channel in a wireless communication system.
`
`Bridged taps cause two problems. The first is intersymbol interference where a received
`DMT symbol is distorted due to delayed versions of the previous DMT symbol. The effect
`of intersymbol interference is removed by discarding the cyclic prefix at the receiver. The
`cyclic prefix contains redundant information, and so it is not needed at the receiver. The
`length of the prefix is chosen such that the delayed versions of the previous symbol only
`distort the cyclic prefix and not the actual data part of the DMT symbol. The ability to
`remove the effect of intersymbol interference by discarding the cyclic prefix is one of the
`advantages of having a long symbol period.
`
`The second problem is the intrasymbol interference that is caused when delayed versions
`of a DMT symbol cause the symbol to interfere with itself. The effect of intrasymbol
`interference in the frequency domain is a shaping of the received-signal spectrum. The
`received spectrum is essentially a multiplication of the transmitted spectrum and the
`channel's frequency response. The distortion due to intrasymbol interference is removed
`by using a frequency-domain equalizer.
`
`And finally, radio noise is interference due to a wireless source. The copper phone lines act
`as antennae and pick up this interference. The most common source of radio noise comes
`from AM radio since its spectrum overlaps with the ADSL spectrum. Coding can help
`correct the errors caused by radio noise, and adaptive RF cancellation filters can also be
`used. The previously mentioned technique of dynamic bit allocation can be used to turn off
`subcarriers near frequencies of interference.
`
`ADSL in the physical plant
`
`ADSL operates within the existing POTS plant structure. The phone company has been in
`existence for nearly 100 years and despite modernization, the structure today remains
`much as it did years ago. There are more than 700 million phone lines in the world with
`two-thirds in the US alone. Figure 3 illustrates the basic structure that is utilized in many US
`phone-service deployments.
`
`Mainline routes shown on the left side of Figure 3 are connected by fiber between toll
`offices (TOs). These TOs provide the interface between local exchanges and long-haul
`fiber runs for long-distance service. The TOs also connect COs that control local exchange
`service. The CO houses the main switching equipment for the home and also serves as the
`location for the head-end DSL equipment.
`
`Multiple interconnects are often made between several COS and TOs for operational
`reliability. From the CO, the network is further subdivided into customer service areas
`(CSAs) that are serviced by remote terminals (RTs). This region represents the last several
`miles before the home, such as a large neighborhood or small city.
`
`The CSA is then further subdivided into a distribution area (DA) that is initiated at a feeder
`distribution interface (FDI) serving up to 500 phone lines. The FDI represents the last point
`where the phone lines are still bundled together. The phone wires take separate routes to
`
`http:/lwww.eetimes.com/document.asp?docJd=1277268&print=yes
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`Dish
`Exhibit 1035, Page 4
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`

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`7/12/2016
`
`EE Times - ADSL Technology Explained, Part 1: The Physical Layer
`
`each home and typically cover the last mile of service routing.
`
`Standard copper twisted pair is the primary medium for routing between the CO and RT,
`and then on to the home residence. Fiber deployment through these regions is technically
`possible but economically unjustifiable, especially for points past the FDI. Fiber routing will
`eventually be used for routes between the CO and RT. This upgrade will support
`improvements in ADSL service as well.
`
`The ability to supply phone service over a very long distance of wire, often called reach, is
`limited by the ability of the phone switching equipment at the CO to function correctly. The
`network switch can operate with a maximum DC load resistance of approximately 1,500 9.
`Most of this resistance is found in the copper lines running between the CO and home.
`
`The distance at which this resistance is reached is known as the revised resistance design
`(RRD) distance. Voice quality over the phone often suffers prior to reaching the RRD
`distance, which can extend to more than 3 miles. The phone company employs tricks to
`improve quality, including thicker wire and load coils. ADSL performance is also a function
`of the distance from the CO and service is often extremely limited or impossible on these
`long reaches.
`
`Plain old copper
`
`The copper pair wire that runs between the CO and the home residence has the greatest
`impact on ADSL system performance. The wire provides simultaneous signal routing of
`bidirectional voice communications, which will occupy the spectrum up to approximately 4
`kHz. Above this frequency, the ADSL signals are inserted. The wire is best modeled as a
`lossy transmission line. Most plants employ one of four copper wire gauges ranging from
`19 to 26 gauge.
`
`Thinner wire (higher gauge number) is typically used close to the CO to allow smaller
`bundles to be formed. Thicker wire is used close to the residence to extend the reach from
`
`the CO to home while keeping within the resistance limits required for proper voice
`switching.
`
`The transmission-line model contains a shunt capacitance in parallel with a shunt
`resistance (due to dielectric losses) along with a series resistance and series inductance.
`The cable specifications dictate that the capacitance (per mile) is constant for all wire
`gauges. As a result, the nominal line impedance is typically around 120 SE. The series
`resistance changes by approximately 20% over the possible wire gauges and increases
`logarithmically with frequency. Typical loss through the wire is 3 to 6 dB per mile in the
`voice band, depending on wire gauge, and also increases logarithmically with frequency.
`
`Most system impairments are created by, or enter the system through, the copper wire.
`The wire, through its lossy nature, accounts for degradation in SNR. Reflections are
`created in the system due to unterminated lines such as bridged taps and wire splices.
`These reflections create self-interference. Reflections cause energy cancellation at certain
`frequencies as discussed in the Impairments section. The wire is typically unshielded from
`the FDI (point of fanout) to the home location and is susceptible to ingress (pickup of
`external signals in the frequency range of interest).
`
`Part two of this article will complete the discussion of PHY equipment and move on to
`discuss the link layer. Other elements of the PHY plant will be introduced with a focus on
`the equipment at the home. Link layer topics include ADSL modem initialization and packet
`framing. ATM and IP will be covered in the network-layer section. Finally, the application
`layer will address some new services such as voice over DSL (VoDSL) and video
`streaming.
`
`http://www.eetimes.com/document.asp?docJd=1277268&print=yes
`
`Dish
`Exhibit 1035, Page 5
`
`

`
`7/12/2016
`
`EE Times - ADSL Technology Explained, Part 1: The Physical Layer
`
`Rob Rhodes is manager of the Communications Design group at Thomson Multimedia
`(Indianapolis, IN). The group is responsible for the development of the PHYinterfaces for
`satellite, cable, and DTV communications. He can be reached at rhodesr@tce.com.
`
`Mike Pugel is a principal member of the technical staff at Thomson Multimedia
`(Indianapolis, IN). He is currently working on advanced communication receiver concepts.
`He can be reached at pugelm@tce.com.
`
`Louis Litwin is a member of the technical staff at Thomson Multimedia (Princeton, NJ).
`His focus is the development of wireless communication devices used for digital home
`networking and mobility applications. He can be reached at litwinl@tce.com.
`
`John Richardson is a member of the technical staff at Thomson Multimedia (Princeton,
`NJ). He is primarily involved in the areas of digital home networking and ADSL system
`development. He can be reached at richardsonj@tce.com.
`
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