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xDSL FAQ 1.0 What is xDSL?
6.0 How much does xDSL cost? xDSL is a generic abbreviation for the many flavors of DSL or Digital Subscriber Line technology. DSL refers to the technology used between a customer's premises and the telephone company, enabling more bandwidth over the already installed copper cabling than users have traditionally had. 2.0 How do other residential and broadband technologies compare? 2.1 Cable Modems Cable modems are devices that attach to the cable TV network connection
in a home. This broadband technology is being driven by the cable companies
to provide services beyond traditional broadcast cable TV such as Internet
access. Along with xDSL, it is still in the early stages of development.
There are a number of challenges faced by this industry, including return
path capabilities, customer service issues and standards. However, potential
bandwidth estimates range upwards of 30Mbps from the service provider
to subscriber. Cable networks are inherently different in design than
telephone networks. Cable networks are broadcast oriented, with each
subscriber in an area receiving the same signals as all others in that
area. xDSL is circuit oriented so that each connection is independent
of all others. Cable networks are inherently hierarchical in nature
and thus require two paths, one for downstream and one for upstream.
This requires 2.2 Wireless There are a number of different wireless schemes proposed, planned, and implemented throughout the world. Wireless access technology takes shape in a number of different forms such as via a satellite TV service provider or a cellular phone network. Wireless systems can provide ubiquitous access to a large number of subscribers in a relatively large area. Bandwidth can range from a few kilobits a second to many megabits and be either symmetrical or asymmetrical. Like all other technologies, there can be deployment issues which may include spectrum licensing, interference and noise problems, or bandwidth limitations. 2.3 Analog Modems Analog modems use a telephone network as is. That is, there are no special provisions that are required to use analog modems in today's telephone networks. Analog modems simply allow digital data to flow over the telephone company's already analog network by performing a digital to analog conversion for transmission onto the network and vice versa on the receiving end. The only necessity for analog modems is that each end of the call must have a compatible modem. In essence, this makes analog modem connections the most ubiquitous form of data communications available today. However, analog modems are thus limited by the telephone company's voice bandwidth service. Current analog modems are struggling to achieve rates of only 56Kbps. With only a bandwidth of about 3,000 Hz, there is a extremely small finite limit on the amount of data that may be encoded and sent reliably through this network. User requirements far outstrip what analog modems can obtain today. 2.4 ISDN ISDN is a telephone company technology that provides digital service typically in increments of 64Kbps channels. ISDN has been around for many years, but it's popularity is now only beginning to increase due to the limitations of analog modems and the rise of Internet usage. ISDN requires the phone company to install services within their phone switches to support this digitally switched connection service. Roll out of this service initially got off to a slow start and was stalled by high costs, lack of standards and low acceptance rate by consumers.
xDSL is technology backed by telephone companies to provide next generation high bandwidth services to the home and business using the existing telephone cabling infrastructure. xDSL to the home over existing phone lines promises bandwidths up to 9Mbps, but distance limitations and line quality conditions can reduce what will actually be achievable. xDSL technologies will use a greater range of frequencies over the cable than traditional telephone services which in turn allow for greater bandwidth with which to send and receive information. This technology is still in the early stages of roll out with standards and products just getting under way. Driving this market is the competition from competing access providers and the pursuit of your Internet access dollar. 3.0 Where are the xDSL standards? American National Standards Institute <http://www.ansi.org> Standards Committee T1-Telecommunications <http://www.t1.org> European Telecommunications Standards Institute <http://www.etsi.org> That depends on a number of answers to questions which you'll need to ask yourself. First and foremost you need to determine if DSL is even available in your area. You may not have a choice. By reading this FAQ, you can hopefully learn enough about xDSL and how to get more information to make an informed decision. Although there are merits to all competing technologies, we make no recommendation in this FAQ to specify which one is right for you. 5.1 How does xDSL work? xDSL utilizes more of the bandwidth on copper phone lines than what is currently used for plain old telephone service (POTS). By utilizing frequencies above the telephone bandwidth (300Hz to 3,200Hz), xDSL can encode more data to achieve higher data rates than would otherwise be possible in the restricted frequency range of a POTS network. In order to utilize the frequencies above the voice audio spectrum, xDSL equipment must be installed on both ends and the copper wire in between must be able to sustain the higher frequencies for the entire route. This means that bandwidth limiting devices such as loading coils must be removed or avoided. 5.2 What are the various types of xDSL? There are several forms of xDSL, each designed around specific goals and needs of the marketplace. Some forms of xDSL are proprietary, some are simply theoretical models and some are widely used standards. They may best be categorized within the modulation methods used to encode data. Below is a brief summary of some of the known types of xDSL technologies. 5.2.1 ADSL 5.2.2 ADSL Lite A lower data rate version of Asymmetric Digital Subscriber Line (ADSL) has also been proposed as an extension to ANSI standard T1.413 by the UAWG (Universal ADSL Working Group) led by Microsoft, Intel, and Compaq. This is known as G.lite in the ITU standards committee. It uses the same modulation scheme as ADSL (DMT), but eliminates the POTS splitter at the customer premises. As a result, the ADSL signal is carried over all of the house wiring which results in lower available bandwidth due to greater noise impairments. 5.2.3 CDSL 5.2.4 EtherLoop 5.2.5 HDSL High Bit-rate Digital Subscriber Line (HDSL) is generally used as a substitute for T1/E1. HDSL is becoming popular as a way to provide full-duplex symmetric data communication at rates up to 1.544 Mbps (2.048 Mbps in Europe) over moderate distances via conventional telephone twisted-pair wires. Traditional T1 (E1 in Europe) requires repeaters every 6000 ft. to boost the signal strength. HDSL has a longer range than T1/E1 without the use of repeaters to allow transmission over distances up to 12,000 feet. It uses pulse amplitude modulation (PAM) on a 4-wire loop. 5.2.6 IDSL 5.2.7 RADSL Rate Adaptive Digital Subscriber Line (RADSL) is any rate adaptive xDSL modem, but may specifically refer to a proprietary modulation standard designed by GlobeSpan Semiconductor. It uses carrierless amplitude and phase modulation (CAP). T1.413 standard DMT modems are also technically RADSL, but generally not referred to as such. The up-link rate depends on the down-link rate, which is a function of line conditions and signal to noise ratio (SNR). 5.2.8 SDSL 5.2.9 VDSL It varies. xDSL service availability still in the early stages, but pricing in some areas has been very aggressive. Prices can change over night and differ significantly depending on the service provider and surrounding area. Local tariffs and government regulations may also play a role in determining end user cost. 7.0 Is xDSL available in my area? To find out, you can check a number of sources. First, you can check with your local telephone company to see if they are providing xDSL services. Second, check around with your local Internet Service Providers (ISPs). Thirdly, try the competitive local exchange companies (CLEC's) in your area. A good resource for CLECs is at <http://www.clec.com>. Lastly, ask around in the xDSL newsgroup(s) or mailing list(s). Lastly, you can try a few of these links, although you are cautioned that their accuracy or completeness can not be vouched for by the maintainers of this FAQ. 8.0 Why are some variations of xDSL asymmetric? It is primarily due to near-end cross-talk (NEXT). The large bundle of wire at the CO is heavily susceptible to cross-talk when the data is traversing from the far end (the end user). At the far end, there are fewer problems with NEXT so bandwidth is greater from the CO to the user. High bit rates, or in this case, higher frequencies suffer a greater amount of attenuation. The reason that the upstream speed in ADSL is generally much less than the downstream rate is due to this fact. When the high frequencies have attenuated at the CO end, they are very susceptible to all the other signals in the binder group due to EMI. In the downstream direction, the high frequencies still attenuate, but at the customer end, they have a better chance of avoiding cross-talk since most subscribers will not have large bundles of cables running into their premises. 9.0 What does a POTS splitter do and when do I need one? A POTS splitter uses a low pass filter to separate the low-end frequencies of the telephone audio spectrum from the higher frequencies of the xDSL signals. The splitter should be a passive device, not requiring power so that "life-line" voice service can be provided as has been in the past. This splitter allows for the traditional voice service that consumers are accustomed to. A splitter is required at both the customer premises and at the far end (CO). xDSL that does not use a POTS splitter is termed "splitter-less xDSL". Whether a POTS splitter is required or not depends on the xDSL service being provided. Cross-talk 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 cross-talk. Near-end cross-talk (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 cross-talk (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. A pair of wires, moderately twisted for the entire length between the telephone company's end office and the user premises (the common telephone set) form a loop, so it is referred to as the local loop. This loop provides a user with access to the global telecommunications infrastructure that is installed all over the world. The local loop has been historically designed to provide voice grade audio service. The circuit is powered from the central office with 48V (open circuit voltage) limited in current to a value somewhat higher than 20mA. This current is used for signaling phone access, burning off moisture, breaking through metallic oxides caused by corrosion, and powering a carbon microphone. The original telephone equipment contained no active electronics. The actual wiring of the local loop may be considered to be a poor transmission line. xDSL uses whatever frequencies will propagate on this line for purposes of digital data transmission. T1 modulation (alternate mark inversion) has been doing this for years. xDSL extends the capability by using modern technology to increase the data rates and distances spanned. 12.0 How do I determine how far I am from my CO? You can call your service provider and ask them for the address of your local CO. Using a map, you get an approximate distance from your residence to the CO. However, these are very rudimentary measurements because you can never be sure exactly what route your line takes between the two points. It may not be a direct route. If you're interested in whether you will qualify for high-speed broadband service or if you're just wondering what the potential speed you could attain may be, there are other factors to consider (i.e. wire gauge, element continuity, environments, etc.) In a nutshell, just knowing the where the CO is, may not tell you much at all. 13.0 What do people mean by a "truck roll"? Anytime a service technician needs to be dispatched in order to install, configure or troubleshoot a line installation, it is referred to as a "truck roll". The significance of this term implies a real cost to the service provider whenever a technician's time is required. The term derives from the scene of a technician driving the familiar "company truck" and pulling up to the curb of your premises with the intention to install, configure or troubleshoot a line. 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. 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. 16.0 Can I use my 28.8K/56K modem with my xDSL line? 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. 17.0 Appendix A - Acronym List ADSL - Asymmetric Digital Subscriber Line |
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