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... nd widths that can be greater than 1 MHz, much greater than the 3000 Hz or so allocated for voice transmission. There are several types of xDSL signal in commercial use today. Each signal type is implemented in circuitry with accompanying software, called a transceiver. The transceiver design includes the encoding or modulation scheme along with decoding or demodulation applied to convert serial binary data streams into a form suitable for transmission through twisted wire pairs.
The transceivers may also employ various signal processing, equalization, amplification, and shaping techniques to adapt transmission for physical attenuation and phase distortions experienced by signals transmitted through twisted wire pairs. The transceiver software and circuitry may also use coding techniques to detect and correct noise that is present on a twisted wire pair. A variety of signal processing techniques have been developed over the past 10 years to increase the bit rate of digital transmission through telephone loop twisted pairs. The following sections will describe these technologies.
The DSL acronym was first used as shorthand to refer to the line code designed to support basic rate integrated services digital network (ISDN) transmission through twisted wire pair loops. The ISDN basic rate signal is required to carry an information payload of 144 kbps, consisting of two "B" channels of 64 kbps each and one packet data or "D" channel of 16 kbps added for framing, error detection, and other overhead functions. The ISDN line of "U" interface operates at a raw data rate of 160 kbps. In the mid 1980 's the T 1 committee in the United States created a standard U interface using a four-level line code referred to as 2 B 1 Q for two binary bits per symbol carried by a quaternary symbol design. 2 B 1 Q line code was designed to support ISDN transmission through loops of 18000 ft or less, meeting voltage pulses of +/- 875 V and +/- 2. 625 V. The symbol rate is 80000 baud and the energy spectrum used by ISDN peaks at 40000 Hz. The ISDN signal is transmitted in full duplex mode, bidirectionally on the same pair of wires.
In order to accomplish this, transceivers must contain a hybrid function to separate the two directions of transmission. To help the receiver differentiate between far-end transmission and reflections of near-end transmission from irregularities in the twisted pair transmission line due to wire gauge changes and bridged taps, echo cancellation techniques are used. The range of operation of ISDN is dictated by both attenuation and self near-end crosstalk (NEXT) from adjacent 2 B 1 Q ISDN signals. The 2 B 1 Q line code is sometimes referred to as a baseband signal because it uses energy in frequencies down to zero, overlapping with the voice frequency band. In order to carry voice through a DSL, the voice signal is digitized using PCM techniques and carried in one of the B channels.
In ISDN applications the D channel is reserved for data packets that are primarily used for call processing. In carrying simultaneous voice and data the ISDN basic rate line carries a maximum of 64 kbps of data. In the absence of voice, both B channels may be bonded together to increase the data capacity to 128 kbps. Both ends of an ISDN connection must use the same bonding protocol. ISDN connections are made by dialed access though a local digital switch that also terminates voice lines. Quadrature Amplitude Modulation (QAM) utilizes amplitude and phase modulation to transmit multiple bits per baud.
Unmodulated signal exhibits only two possible states allowing us only to transmit a zero or a one. With QAM, it is possible to transmit many more bits per state as there are many more states. This scheme utilizes a signal that can be synthesized by summing amplitude modulated cosine and sine waves. These two components, being 90 deg out of phase, are called quadrature, hence the name Quadrature Amplitude Modulation. By combining amplitude and phase modulation of a carrier signal, we can increase the number of states and thereby transmit more bits per every state change. Carrier less amplitude and phase (CAP) modulation technique is closely related to QAM in that amplitude and phase are used to represent the binary signal.
The difference between CAP and QAM lies in the state representation of the constellation pattern. CAP does not use a carrier signal to represent the phase and amplitude changes. Rather, two waveforms are used to encode the bits. The encoder replaces a stream of digital data with a complex equation that symbolizes a point on the constellation diagram. Thus, for a 32 -CAP, there would be 32 possible locations on the diagram, all of which can be represented as a vector consisting of real and imaginary coordinates.
Consequently, 32 -CAP would result in 32 distinct equations of the type, each one representing five bits of data. CAP modulation is very suitable for use with ADSL. The spectrum from 0 to 4 kHz, voice band, is designated for plain old telephone services (POTS). Downstream (ATU-C to ATU-R), the spectrum from 26 kHz to 1. 1 MHz is further divided into 249 discrete channels. Upstream (ATU-R to ATU-C), the spectrum above the POTS band consists of 25 channels between 26 kHz and 138 kHz.
Echo canceling between the downstream and upstream signals permits reuse of these sub-channels. With the exception of carriers used for timing, each carrier is capable of carrying data. However, only those carriers with sufficient signal to noise ratio (SNR) are allocated payload for transmission. Each transmitting carrier is allotted a bit count and transmit power, based on the characteristics of the sub-channel. This results in an optimized data transfer rate for the current line conditions.
DMT allocates bits and transmission power away from the induced noise. The advantages of this process are an optimized data rate and less interference with other services existing in the same sheath, due to the symmetrical nature of induced crosstalk. The DMT technique exhibits a high degree of spectral compatibility based on power spectral density, rather than absolute transmit power. DMT has a substantial advantage over single carrier modulation systems in the presence of impulse noise.
DMT spreads impulses over a large number of bits, averaging peaks. Only if the average exceeds the margin does DMT produce an error -- single carrier systems will error every time a peak exceeds the margin. Discrete wavelet multitude (DWMT) technology increases the usable capacity of telephone wires and coaxial cable, allowing telephone companies and cable operators to deliver two-way broadband telecommunications services over their existing networks. DWMT uses Multi carrier Modulation. A multi carrier system uses a transmission band efficiently by dividing it into hundreds of subchannel's that are totally independent and spectrally isolated.
In practice, implementations of multi carrier systems use orthogonal digital transformations on blocks of data, a process called sub channelization, in an attempt to achieve the frequency partitioning shown in the figure below. By keeping the signal sub channel power contained in a narrow bandwidth, each sub channel occupies only a small fraction of the total transmission band and overlaps only with immediately adjacent subchannel's. When a signal is transmitted over a long copper loop (e. g.
several miles), the higher frequency components of the signal attenuate significantly more (tens of dB) than the lower frequency components. Narrowband interferer's from AM or amateur radio signals also affect the transmission by destroying the signal in parts of the band. Multi carrier technology, called Discrete Wavelet Multitone (DWMT), provides sub channel isolation that is superior to DMT. DWMT uses an advanced digital wavelet transform instead of the Fourier transform used in DMT. The T 1. 413 standard for ADSL defines two categories of modems: frequency division multiplex (FDM) modems (Category I) and echo cancellation modems (Category II). FDM systems allocate separate frequency bands for upstream and downstream transmissions.
Echo canceled systems send upstream and downstream signal over the same frequencies. Since the attenuation of a signal over a copper line increases with frequency, it is desirable to transmit data using a frequency band that is as low as possible. In an ADSL system, the lowest attenuated frequencies begin right after the POTS band. In FDM system, the lower frequency band is used for upstream transmission while the downstream transmissions are allocated to the higher attenuated frequencies.
Some xDSL transceivers use echo cancellation (similar to the echo cancellation utilized in the standard V. 34 28. 8 kbps duplex modem) to exploit the lower attenuated frequencies and increase its downstream performance. By utilizing the lower frequencies for both upstream and downstream performance, the transceiver can deliver higher downstream performance, particularly on the longer loops where the higher frequencies become severely attenuated. In an effort to promote interoperability among FDM and EC systems, the echo-canceled transceivers can be configured to operate in an FDM mode in order to communicate with a category I (FDM) modem. The twisted pair wire between the telephone central office and end users of telecommunication services has a great deal more information capacity than used for the regular voice services. Several baseband and passband transmission systems collectively referred to as xDSL, have been developed over the last ten years that enable up to several megabits per second of data to be carried over the regular telephone twisted pair line. The xDSL family of technologies provides a wide variety of line driving schemes to accomplish and satisfy different market needs over todays infrastructure.
xDSL has application in both the corporate and residential environments as well as flexibility to meet the market challenges. Since xDSL operates at the physical layer of OSI seven layer standard, it can be used in conjunction with ATM and Frame Relay technology. The most promising of the xDSL technologies for integrated Internet access, intranet access, remote LAN access, video-on-demand, and lifeline POTS applications in the near term is ADSL or R-ADSL (a rate-adaptive version of ADSL). During the past year, ADSL has concluded trials by more than 40 network service providers throughout the world, primarily in North America and northern Europe.
Service introduction began in 1997, but ADSL service is still being rolled out in many areas. In the meantime, xDSL technologies and standards will continue to evolve, as will user demand for these emerging services relative to other local access service alternatives. The ability to utilize the existing telephone copper wire infrastructure as well as interoperability with ATM and Frame Relay technology, position xDSL as the most promising of the broadband access technology options for both residential and business users 1. Marlis Humphrey and John Freeman, "How XDSL Supports Broadband Services to the Home", IEEE Network. , vol. 11, no. 1, Jan-Feb 1997, p. 14 - 23. 2. George T. Hawley, "Systems Considerations for the use of XDSL Technology for Data Access", IEEE Communication, vol. 35, no. 3, Mar 1997, p. 56 - 60. 3.
Bhumip Khasnabish, "Broadband to the Home (BTTH): Architectures, Access Methods, and the Appetite for it", IEEE Communication, vol. 35, no. 3, Mar 1997, p. 58 - 69 4. ADSL Forum website, web 5. Analog Devices website, web Bibliography: 1. Marlis Humphrey and John Freeman, "How XDSL Supports Broadband Services to the Home", IEEE Network. , vol. 11, no. 1, Jan-Feb 1997, p. 14 - 23. 2. George T. Hawley, "Systems Considerations for the use of XDSL Technology for Data Access", IEEE Communication, vol. 35, no. 3, Mar 1997, p. 56 - 60. 3.
Bhumip Khasnabish, "Broadband to the Home (BTTH): Architectures, Access Methods, and the Appetite for it", IEEE Communication, vol. 35, no. 3, Mar 1997, p. 58 - 69 4. ADSL Forum website, web 5. Analog Devices website, web 6. Kimo website, web 7. Western website, web
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