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Some of the more well known forms of xDSL are listed below:
IDSL ISDN Digital Subscriber Line
HDSL High Bit-Rate Digital Subscriber Line
SDSL Symmetric Digital Subscriber Line
ADSL Asymmetric Digital Subscriber Line
VDSL Very High Bit-Rate Digital Subscriber Line
RADSL Rate Adaptive Asymmetric Digital Subscriber Line
All too often not considered within the scope of xDSL simply because ISDN is the commonly used terminology and has been internationally in use for many years. The two bearer channels of ISDN each deliver 64Kbp/s and may be bonded by a user application to provide a single delivery of 128Kbp/s. Again it is an application which “unpacks” the data. One advantage of ISDN is that the signalling protocol is entirely separate from the bearer channels and so “housekeeping” is an independent operation, hence basic rate is often referred to as 2B+D.
HDSL provides an improved method of transmitting T1 (1.55 Mbps) or E1 (2.048 Mbps) over twisted-pair copper lines. HDSL transmits 1.544 Mbps or 2.048 Mbps using bandwidths ranging from 80 kHz to 240 kHz instead of the 1.5 MHz used by AMI, the traditional method of transmitting T1/E1. Thislower frequency generates less corrup-tionof the cable spectrum enablingmore than one circuit in a multi-pairbundle. HDSL transmission uses typically 2 pairs for T1 and up to three pairs for E1. This may be seen as adisadvantage, however, it is highlylikely that these pairs will already be inplace and therefore will not require thecostly provision of new cables. A sec-ond version of HDSL, known somewhat unsurprisingly as HDSL II, can offer the ame performance as HDSL, but using a single twisted-pair. Deployment of HDSL II has a great deal of appeal to the large national carriers as it gives them all the benefits of using their installed base of copper pairs for high bandwidth applications yet minimises the number of pairs used. This tech-nique is now being deployed or tested in a number of countries.
SDSL is in essence a version of HDSL that can transmit T1 or E1 signals utilising a single twisted-pair, and typi-cally operating over POTS, giving the benefit of supporting POTS and T1/E1 simultaneously. There is, however, a distance limitation of around 10,000 feet. At face value SDSL would appear ideal for delivering high-speed access to individual subscriber premises for applications such as remote LAN or Internet access. Consideration, howev-er, should be given to the fact that ADSL can achieve higher data rates at the 10,000 feet limitation of SDSL andthat the typically usage of remote LAN and Internet access is by nature asymmetric.
ADSL Asymmetric Digital Subscriber Line
ADSL is one of the hottest topics around at the moment and it is easy to understand why. Depending on the form of ADSL deployed it is possible to achieve downstream (towards the user) speeds up to 8 Mbps and upstream (towards the service provider) speeds up to 1.5 Mbps. This asymmetric nature of ADSL makes it very attractive for applications that typically download much more data than is sent. The most obvious of these is Internet usage, however, video on demand is potentially the most exciting.
A variety of ADSL formats are being proposed at this time and they are all based on a trade-off between cost, complexity and performance. In vogue at the moment is G.Lite ADSL with downstream speed of 1.5Mbps and upstream of 384Kbps. This could well be the most user-friendly form of DSL and could become the implementation of choice for domestic users. The cost of equipment and service will almost certainly be lower than the other varieties of DSL.
VDSL is the highest speed DSL technology with data rates up to 52 Mbps downstream and up to 2.3 Mbps upstream. To achieve this speed the maximum distance from the exchange to the user is between 1,000 and 4,500 feet. VDSL is still very much at an experimental stage and no real world applications are in use.
RADSL Rate Adaptive Asymmetric Digital Subscriber Line
RADSL operates at the same band-widths as ADSL with the added ability of adjusting bandwidth to suit the quality of the line during the actual transmission.
Testing DSL technology
The advantages of xDSL technologies are self-evident, but what must we consider to be the potential problems?
In most cases we are dealing with a legacy infrastructure and topology which, because of bridge taps, mis-matching and the general degradation of the electrical characteristics of the line, may not be capable of supporting DSL services efficiently. The first step is to establish that the line is capable of supporting the chosen DSL technology and secondly, once operational, to be able to determine the fault should problems occur.
The procedures for this fall into two categories, analogue testing to determine the quality and appropriateness of the line and digital testing to determine the quality and correctness of the service. Quite simply, problems associated with quality of service can be caused by configuration issues or by the physical characteristics of the line, the latter requiring further analogue tests to determine the problem. The physical characteristics which determine DSL suitability include line length, cable suitability, bandwidth availability, environmental noise, thermal/mechanical effects, cross talk, impedance, terminations, bridge taps, insulation, and split pairs. Digital testing requires a tester with the capability to simulate a DSL modem or to replace the DSL modem, known in Trend as “Golden Modem” operation.
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Let’s get some of the terminology clear
What is attenuation?
Attenuation is signal loss due to the diminishing availability of signal energy, or signal power. As a analog or digital signal traverses across a medium, it fades. High attenuation may lead to the inability to recover the signal on the far end. Signal repeaters may be used on the transmission path to periodically boost the signal strength. Baseband transmission is extremely limited to attenuation. Broadband much less so. In addition, wireless communications is much less susceptible to attentuation that is wireline communications such as xDSL or cable modems.
What is crosstalk?
Crosstalk refers to the interference between channels. In the xDSL world, the interference between nearby cables can have a negative impact on the performance of the affected cable(s). Have you ever been on the phone and heard some other conversation, not yours, in the background? If so, you have experienced the effect of crosstalk.
Near-end crosstalk (NEXT) occurs when the transmitter sends a signal and a nearby transceiver at the same end of link, through capacitive and inductive coupling, “hears” the signal.
Far-end crosstalk (FEXT) occurs when the transmitter sends a signal and a transceiver at the far end of the link, through capacitive and inductive coupling, “hears” the signal. FEXT will be of more concern in an asymmetrical system such as ADSL than symmetrical systems like HDSL. This is because strong signals originating from the near end, can interfere with the weaker signals originating at the far end.
What is the effect of noise?
Noise may be defined as the combination of unwanted interfering signal sources whether it comes from crosstalk, radio frequency interference, distortion, or random signals created by thermal energy. Noise impairs the detection of the smallest analog levels which may be resolved within the demodulator. The noise level along with the maximum clip level of an analog signal path set the available amplitude dynamic range.
The maximum data rate of a modem is limited by the available frequency range (bandwidth) and signal-to-noise ratio (SNR) which is amplitude dynamic range. If more of either is available, more bits may be transferred per second. The information carrying limit was discussed theoretically by Claude Shannon and is known as Shannon’s limit, or information theory.
xDSL modems take advantage of the spectrum above the telephone audio channel. While operating with somewhat less amplitude dynamic range they increase data rates by greatly increasing the frequency range of the communication signal (from about 10KHz to over 1.0MHz). To do this they require the installation of special equipment at the central office and customer premise.
What is a bridge tap?
A bridge tap is an accidental connection of another local loop to the primary local loop. Generally it behaves as an open circuit at DC, but becomes a transmission line stub with adverse effects at high frequency. It is generally harmful to xDSL connections and should be removed.
Extra phone wiring within one’s house is a combination of short bridge taps. A POTS splitter isolates the house wiring and provides a direct path for the xDSL signal to pass unimpaired to the ATU-R modem.
What are loading coils?
Loading coils are used to extend the range of a local loop for voice grade communications. They are inductors added in series with the phone line which compensate for the parallel capacitance of the line. They benefit the frequencies in the high end of the voice spectrum at the expense of the frequencies above 3.6KHz. Thus, loading coils prevent xDSL connections.
Testing of DSL products using Trend products can take the form of field based installation or fault-finding assessment.
ADSL transmissions are affected through two key factors, external conditions and line properties.
External conditions which affect transmission include: Noise (conducted), Noise (induced/RFI), Cross-talk (Near/Far), Inter-operability, Line code (CAP v DMT), DSLAM/ATU-C settings.
Line properties cover: Balance, Load coils/taps, Hi Z/Lo Z, Non-uniform pairs, Pair length, Copper diameter.
How GOOD is the Copper?
It is vital that the DSL service provider knows the condition of the digital local loop in order to be able to provision a defined service level. When a subscriber requests an ADSL service, the provider needs to be in a position to state whether or not the service can be supported over the specific subscriber loop. This will depend upon the minimum service requirement (i.e. bandwidth) and the condition of the loop.
European ETSI specification ETR 328 is the only current ADSL European standard in force and only covers loop and noise models. Additional specifications such as TS 101 388 for ADSL and TS 101 270 for VDSL are currently being developed by the ETSI TM6 workgroup. These specifications will also include requirements for electrical characteristics of the transceivers as well as loop and noise models.
The ITU-T G.99X.X series of standards provide design rules for the xDSL models and stipulate loop tests and noise injection models. They also contain electrical requirements for the transceivers including PSD masks for ADSL over POTS/ISDN and for systems providing reduced NEXT. They also provide for splitter characteristics.
DSLAM – Remote Modem
In the absence of established access performance requirements, various DSL vendors market their own remote CPE and CO exchange equipment. It is highly likely the two ends from one manufacturer will operate well together, but it is far from certain that, once mixed installations are involved, various manufacturers modems will interoperate.
Power Spectral Density
Power spectral density is the power expressed in dBms per Hz bandwidth. Limitations are accorded to the spectral profile of the PSD in a defined bandwidth in order to minimise the effect of crosstalk and therefore any destructive influence of the transmission on other services carried in the same bundle.
PSD masks will vary dependant upon the type of service provided. For example ADSL over POTS or ISDN will offer different potential for interference to the other service. Likewise full rate ADSL, G.Lite or VDSL occupy different proportions of the bandwidth and will therefore need to comply with different PSD mask limits.
Similarly, in an asynchronous transmission such as ADSL the upstream and downstream transmission occupies different areas of the transmission band and consequently PSD masks will differ for upstream (from ATU-R to ATU-C) and downstream (from ATU-C to ATU-R).
Power Measurements (ITU-T G.991.2)
Power measurements measure the power in the voice band and the power in the out-of band regions from the ADSL port. Since there should be no DSL signals transmitted in these regions both these power levels should be low.
Power in the voice band is measured as an average over the region from 0 to 4 kHz with the maximum value required to be less than 15dBm. Power in the out-of band region is measured over a sliding 1MHz bandwidth and is required to be less than –50dBm.
Longitudinal Conversion Loss
LCL measures the ratio of an applied longitudinal voltage (with reference to earth) against the measured voltage across a 100 ohm termination placed across the ADSL transceiver tip and ring.
If the HPF function of the POTS splitter is incorporated into the ADSL modem then the test is applied to the ADSL port using a 100 termination.
If the LPF function of the POTS splitter is also built into the ADSL modem then the test is repeated with the LPF POTS port terminated with impedances ZTC (000) and ZTR (600).
Return Loss of POTS and ADSL Line Ports.
Return Loss is essentially a indication of the degree of impedance matching of a connection. This matching is essential to achieve the best possible power transfer from one piece of equipment to another. Typically impedance matching of an ADSL Port is taken to be 100ohms. The POTS port is typically 600ohms for the UK.
And finally -The Acronym List. In no way intended to be definitive, but may help
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
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
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
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