6.0 Encoding and modulation
[6.1] What is QAM? [top]
Quadrature amplitude modulation (QAM) is a method for encoding data on a single carrier frequency. The modulation encodes data (or bits) as discrete phase plus amplitude changes of a carrier tone. The phase vectors are arranged in a pattern of points called a constellation from which the transmitted point is selected based on the data to be sent. The modem sends the symbols as abrupt changes in phase and amplitude, but only as what emerges from a sharp cutoff filter which carefully limits the bandwidth. The transmitted signal occupies slightly more than ±1/2 the modulation rate either side of the carrier frequency. The excess bandwidth, perhaps as much as 10%, is required for recovering symbol timing within the remote receiver. The receiver has to pick which point was transmitted with great reliability. It may employ adaptive equalization or other methods to reduce inter-symbol interference to levels which are acceptable for discriminating the received point. The background noise level of the receiver limits the number of distinct constellation points which may be reliably determined, and hence limits the data rate for a given symbol rate. QAM has become the dominate modulation for high speed voice band modems. Examples are V.22bis, V.27, V.29, V.32bis, V.34. About every 2/3 of a carrier cycle the phase or amplitude is changed to a new value. This signaling rate is known as the baud (or symbol) rate. The highest QAM baud rate in use today for telephone line modems is 10/7 of 2400Hz or about 3429 baud on a 1920Hz carrier in V.34. By encoding something between 9 to 10 bits per baud a final data rate of 33.6Kbps is developed. To encode this number of bits, over 1000 different phase/amplitude values must be resolved by the receiver. This is a nontrivial process involving adaptive equalizers, trellis coding, and other highly sophisticated signal processing. Transmit path: scrambler -> symbol generator -> 3x upsample (S1,0,0,S2,0,0,S3,...) -> complex transmit baseband FIR filter -> e^jwt carrier modulation -> scale real signal output -> DAC converter The baseband filter is about 3dB down at ±1/2 symbol rate, so for 3429 baud the signal out of the filter extends from -1715Hz to +1715Hz. This is shifted by the positive 1920Hz carrier to +205Hz to +3635Hz. One can see that this just fits in the frequency spectrum of the voice band telephone network. This filter, the analog electronics and the phone channel smear any given symbol over a 10msec period of the signal (about 32 symbols). The scrambler is very important. It randomizes the signal so an adaptive equalizer in the remote modem can build the inverse channel response (including the transmit filter). The smearing (or inter-symbol interference) is largely eliminated by dynamically adjusting adaptive equalizer coefficients with the goal of minimizing least square error in the received points. The major adaptation is done during the training phase, although the feedback loops remain active throughout the connection. Other impairments to be solved are gain normalization, timing recovery, carrier offset frequency, phase jitter removal, and echo cancellation.
[6.2] What is PCM? [top]
Pulse code modulation (PCM) is used in the phone network to reduce the data rate required for voice grade audio to less than 64Kbps. It uses either u-law (North America) or A-law (Europe) as the compression method. Any given 8KHz analog audio sample is converted to 4 bits of mantissa, 3 bits of exponent, and a sign bit. This code has a characteristic that quantization noise is proportional to signal amplitude and does not become objectionable to the average telephone user. For a conventional modem this noise floor limits the available dynamic range to about 36dB which sets the maximum data rate. The least significant bit of the mantissa may be periodically stolen for signaling within the phone network (called robbed-bit signaling) further increasing the noise. The 8-bit codes are processed through the telephone switching network in fixed time slots. There exists an ever increasing hierarchy of data rates to support this. A DS0 is a 64Kbps time slot. 24 DS0s become a DS1. 4 DS1s become a DS2 (now obsolete). 7 DS2s become a DS3, etc. The physical layer of a DS1 (T1) may be re-modulated as alternate mark inversion for passing over a wire pair as a method to concentrate local loops. Repeaters regenerate the signal every 6000-9000'. These signals may coexist with xDSL in the same wire bundle.
[6.3] What is PAM? [top]
Pulse amplitude modulation (PAM) is the physical layer of an ISDN or HDSL connection. The modulation consists of sending discrete amplitude levels (symmetric about 0 volts) at a regular rate. Both use the two binary one quaternary (2B1Q) line code. Four analog voltages (called quaternary symbols) are used to represent the four possible combinations of two bits. These symbols are assigned the names +3, +1, -1, and -3. So each amplitude level being held for one symbol time communicates two bits. The following diagram is typical of the 2B1Q waveform at the transmitter: +3 = 2.64V + .--. .--. .-- + | | | | | +1 = 0.88V + | `--. .--' | .--. | ++++|+++++|++|+++++|++++++++|++|++++++++|++++ -1 = -0.88V + --' | | | .-----' `--. | + | | | | | | -3 = -2.64V + `--' `--' `-----' One might assume this is a digital signal relative to the definition in [4.2] , but by the time the signal has reached the receiver these discrete levels have diffused into each other because of phone line induced amplitude and phase distortion. This is called inter-symbol interference. Therefore an adaptive equalizer must be used to restore the levels to values which may be discriminated for recovering the data. The symbol timing is recovered by examining the squared signal energy for a tone at the modulation rate. Transitions between levels cause the instantaneous power to dip on average provided there is adequate excess bandwidth. PAM differs from the other modulations in that it is baseband modulation and does not use a carrier. Some versions of HDSL increase the number of levels to 16 which communicates four bits per symbol in the same bandwidth.
[6.4] What is V.90? [top]
V.90 is actually a variant of PAM. It has 256 PCM levels from which to choose a more limited set. The spacing between levels is set by the u-law or A-law characteristic described in [6.2] . The inner levels become more closely spaced so some of these must be excluded for reasons of limited signal-to-noise ratio. In addition, outer codes are excluded to keep transmit power on the local loop below -12dBm, a formal limit established by the FCC. V.90 includes a spectral shaping algorithm to prevent sending signal at DC. V.90 bypasses the problems associated with a conventional modem. It recognizes that with enough signal processing the original PCM samples sent by the phone company may be resolved as individual levels using a 16-bit A/D converter on the receiving end. Audio is sent through the digital network as 8-bit u-law or A-law samples. Of course, the telco D/A converter, reconstruction filter, and phone network blur the levels into one continuous signal, so it's up to the receiver to reconstruct what was sent. An additional problem is recovering symbol (i.e. PCM sample) timing information which must be inferred from the residue of modulation at a frequency around 4KHz. By just selecting a limited set of codes with say 64 levels, 6 bits per 8KHz symbol may be sent for a data rate of 48Kbps. More levels, more data, but a maximum of about 53.3Kbps is a practical limit.
[6.5] What is CAP? [top]
Carrierless amplitude and phase (CAP) modulation is a proprietary standard implemented by Globespan Semiconductor. While the name specifies that the modulation is "carrierless" an actual carrier is imposed by the transmit band shaping filter through which the outbound symbols are filtered. Hence CAP is algorithmically identical to QAM. The upstream symbol rate is 136K baud on a 113.2KHz carrier, while the downstream symbol rate is 340K baud on a 435.5KHz carrier, 680K baud on a 631KHz carrier, or 952K baud on a 787.5KHz carrier. This allows the modem to be symbol rate adaptive to varying line conditions (see RADSL). The QAM modulation is also rate adaptive by varying the number of bits per symbol. One advantage CAP claims to have is a lower peak-to-average signal power ratio relative to DMT. This means that the drivers and receivers may operate at lower power than DMT because they are not required to have the peak signal capacity that is required in the DMT circuitry. This is mitigated by the infrequency of the really high signal peaks in DMT which may be just considered to be another form of noise if they happen to clip. CAP's principle advantage is its installed base of modems. It is actively being deployed in many trial markets and is available from several manufacturers.
[6.6] What is DMT? [top]
Discrete multitone (DMT) modulation is a method by which the usable frequency range is separated into 256 frequency bands (or channels) of 4.3125KHz each. These are intimately connected to the FFT (fast Fourier transform) algorithm which DMT uses as its modulator and demodulator. The FFT is not perfect in separating the frequencies into individual bands, but it does well enough, and it generates spectra which are fully separable on the receiving end. By dividing the frequency spectrum into multiple channels DMT is thought to perform better in the presence of interference sources such as AM radio transmitters. It is also better able to focus its transmit power on those portions of the spectrum in which it is profitable to send data. The assignment of channels is left flexible, but typical settings might be channels 6-31 for upstream (24KHz-136KHz), 32-250 for downstream (136KHz-1.1MHz). The modulation used on any given frequency channel is QAM. Channels 16 and 64 are reserved for pilot tones which are used to recover timing. The number of bits per symbol within each channel may be independently selected allowing the modem to be rate adaptive. The use of the FFT is considered to be somewhat substandard to other orthogonal transformations such as the discrete wavelet transform which do a better job of isolating the individual frequency spectra. The FFT is chosen for its computational efficiency. While DMT is off to a slow start in the marketplace, it is expected to dominate for two reasons: it is thought to perform better for technical reasons and there is an ANSI standard behind it (not to mention Intel/Microsoft support).
7.0 Setup and Configuration
[7.1] What hardware does my home computer need? [top]
Although it depends on your provider and the equipment they use, typically you will need a 10Base-T adapter with which to connect to the external DSL device. Typically the customer DSL device is implemented as a bridge, router or both.
[7.2]  <reserved> [top]
[7.3] Can I use my 28.8K/56K modem with my xDSL line? [top]
Theoretically yes. However, most DSL providers have been installing separate DSL circuits to the remote user without using a splitter to separate out the voiceband bandwidth. If a splitter was used, you could use a traditional POTS modem over the the voiceband frequency spectrum of your phone line as you always did. In most cases however, the line is dedicated for DSL.

Appendix A - Acronym List  [top]
ADSL - Asymmetric Digital Subscriber Line ANSI - American National Standards Institute ATM - Asynchronous Transfer Mode ATU-C - ADSL Termination Unit - Central Office ATU-R - ADSL Termination Unit - Remote AWG - American Wire Gauge BERT - Bit Error Rate Test bps - Bits Per Second BRI - Basic Rate Interface CAP - Carrierless Amplitude and Phase CATV - Cable TV CBR - Constant Bit Rate CCITT - Consultative Committee for International Telegraph and Telephone CLEC - Competitive Local Exchange Carrier CO - Central Office CODEC - Coder/Decoder CPE - Customer Premise (or Provided) Equipment CSU - Channel Service Unit DCE - Data Communication (or Circuit-Terminating) Equipment DLC - Digital Loop Carrier DMT - Discrete Multi-tone DSL - Digital Subscriber Line DSLAM - Digital Subscriber Line Access Multiplexer DSP - Digital Signal Processor DSU - Data Service Unit DTE - Data Terminal (or Termination) Equipment EMI - Electromagnetic Induction ETSI - European Telecommunications Standards Institute FCC - Federal Communications Commission FDM - Frequency Division Multiplexing FEXT - Far-end crosstalk FTTC - Fiber To The Curb FTTH - Fiber To The Home HDSL - High bit-rate Digital Subscriber Line HFC - Hybrid Fiber-Coax IEC - Inter-Exchange Carrier IEEE - Institute of Electrical and Electronics Engineers IETF - Internet Engineering Task Force ILEC - Incumbent Local Exchange Carrier IP - Internet Protocol ISDL - ISDN Digital Subscriber Line ISDN - Intergrated Services Digital Network ISO - International Organization for Standards ISP - Internet Service Provider ITU - International Telecommunications Union IXC - Inter-exchange Carrier Kbps - Kilobits Per Second LADC - Local Area Data Circuit LADS - Local Area Data Service LAN - Local Area Network LATA - Local Access and Transport Area LEC - Local Exchange Carrier Mbps - Megabits Per Second MDF - Main Distribution Frame MUX - Multiplexer MVL - Multiple Virtual Line NAP - Network Access Provider NEBS - Network Equipment Building Standards NEXT - Near-end Crosstalk NIC - Network Interface Card NID - Network Interface Device PBX - Public Branch Exchange PCM - Pulse Code Modulation POP - Point of Presence POTS - Plain Old Telephone Service PPP - Point to Point Protocol PRI - Primary Rate Interface PSTN - Public Switched Telephone Network PTT - Postal, Telegraph and Telephone PVC - Permanant Virtual Circuit QAM - Quadrature Amplitude Modulation QoS - Quality of Service RADSL - Rate Adaptive Digital Subscriber Line RBOC - Regional Bell Operating Company SDSL - Symmetric Digital Subscriber Line SNR - Signal-to-Noise Ratio SOHO - Small Office/Home Office SVC - Switched Virtual Circuit TCP - Transport Control Protocol TELCO - Telephone Company TDM - Time Division Multiplexing UBR - Unspecified Bit Rate UDSL - Unidirectional Digital Subscriber Line UTP - Unshielded Twisted Pair VBR - Variable Bit Rate VDSL - Very high bit-rate Digital Subscriber Line VoIP - Voice over Internet Protocol VPN - Virtual Private Network WAN - Wide Area Network xDSL - (generic) Digital Subscriber Line - [top]