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. The short answer is "it depends". Typically speeds start at about 128Kb/s and go up to 1.5Mb/s for most home users. Some installations may go as fast as 50Mb/s or more depending primarily on the equipment used, distances involved, cabling quality, encoding techniques, frequency spectrum available and even to some degree, end system configurations. Be aware that some xDSL is sold as asymmetric or "rate-adaptive". It is best to consult the providers in your area as to the access rates available in your area. Speeds can vary from provider to provider even if they are all servicing your area from the same central office.
How xDSL Works?
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.
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.
ADSL
Asymmetric Digital Subscriber Line (ADSL) is the most popular form of xDSL technology. The key to ADSL is that the upstream and downstream bandwidth is asymmetric, or uneven. In practice, the bandwidth from the provider to the user (downstream) will be the higher speed path. This is in part due to the limitation of the telephone cabling system and the desire to accommodate the typical Internet usage pattern where the majority of data is being sent to the user (programs, graphics, sounds and video) with minimal upload capacity required (keystrokes and mouse clicks). Downstream speeds typically range from 768 Kb/s to 9 Mb/s Upstream speeds typically range from 64Kb/s to 1.5Mb/s.
CDSL
Consumer Digital Subscriber Line (CDSL) is a proprietary technology trademarked by Rockwell International.
CiDSL
Globespan's proprietary, splitter less Consumer-installable Digital Subscriber Line (CiDSL).
Ether Loop
Ether Loop is currently a proprietary technology from Nortel, short for Ethernet Local Loop. Ether Loop uses the advanced signal modulation techniques of DSL and combines them with the half-duplex "burst" packet nature of Ethernet. Ether Loop modems will only generate hi-frequency signals when there is something to send. The rest of the time, they will use only a low-frequency (ISDN-speed) management signal. Ether Loop can measure the ambient noise between packets. This will allow the ability to avoid interference on a packet-by-packet basis by shifting frequencies as necessary. Since Ether Loop will be half-duplex, it is capable of generating the same bandwidth rate in either the upstream or downstream direction, but not simultaneously. Nortel is initially planning for speeds ranging between 1.5Mb/s and 10Mb/s depending on line quality and distance limitations.
G.lite
A lower data rate version of Asymmetric Digital Subscriber Line ADSL) was 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.992.2 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. Often a misnomer, this technology is not splitter less per se. Instead of requiring a splitter at customer premises, the splitting of the signal is done at the local CO.
G.shdsl
G.shdsl is a ITU standard, which offers a rich set of features (e.g. rate adaptive) and offers greater reach than many current standards. G.shdsl also allows for the negotiation of a number of framing protocols including ATM, T1, E1, ISDN and IP. G.shdsl is touted as being able to replace T1, E1, HDSL, SDSL HDSL2, and ISDN and IDSL technologies.
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 Mb/s (2.048 Mb/s 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.
HDSL2
High Bit-rate Digital Subscriber Line 2 was designed to transport T1 signaling at 1.544 Mb/s over a single copper pair. HDSL2 uses overlapped phase Trellis-code interlocked spectrum (OPTIS).
IDSL
ISDN based DSL developed originally by Ascend Communications. IDSL uses 2B1Q line coding and typically supports data transfer rates of 128 Kb/s. Many end users have had to suffice with IDSL service when full speed ADSL was not available in their area. This technology is similar to ISDN, but uses the full bandwidth of two 64 Kb/s bearer channels plus one 16 Kb/s delta channel.
MDSL
Usually this stands for multi-rate Digital Subscriber Line (MDSL). It depends on the context of the acronym as to its meaning. It is either a proprietary scheme for SDSL or simply a generic alternative to the more common ADSL name. In the former case, you may see the acronym MSDSL. There is also another proprietary scheme, which stands for medium-bit-rate DSL. Confused yet?
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 carrier less amplitude and phase modulation (CAP). T1.413 standard DMT modems are also technically RADSL, but generally not referred to as such. The uplink rate depends on the downlink rate, which is a function of line conditions and signal to noise ratio (SNR).
SDSL
Symmetric Digital Subscriber Line (SDSL) is a 2-wire implementation of HDSL. Supports T1/E1 on a single pair to a distance of 11,000 ft. The name has become more generic over time to refer to symmetric service at a variety of rates over a single loop.
Cost of xDSL
It varies. xDSL service availability is still in the early stages, but pricing in some areas has been very aggressive. Prices can change overnight 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. To complicate matters further, some providers are claiming to offer free xDSL service. In many of these cases however, it requires you to be subjected to directed marketing or to make long-term commitments to their service. You should first determine what your needs and tolerances are. Do you want static IP addresses? How fast do you want to go? What level of service do you require? Do you need multiple email addresses? and so on. Your answers to these types of questions will help you narrow down your choices.
ADSL (Asymmetric Digital Subscriber Line)
ADSL Capabilities
An ADSL circuit connects an ADSL modem on each end of a twisted-pair telephone line, creating three information channels .a high-speed downstream channel, a medium-speed duplex channel, and a basic telephone service channel. The basic telephone service channel is split off from the digital modem by filters, thus guaranteeing uninterrupted basic telephone service, even if ADSL fails. The high-speed channel ranges from 1.5 to 6.1 Mbps, and duplex rates range from 16 to 640 kbps. Each channel can be sub multiplexed to form multiple lower-rate channels.
ADSL modems provide data rates consistent with North American T1 1.544 Mbps and European E1 2.048 Mbps digital hierarchies and can be purchased with various speed ranges and capabilities. The minimum configuration provides 1.5 or 2.0 Mbps downstream and a 16 kbps duplex channel; others provide rates of 6.1 Mbps and 64 kbps duplex. Products with downstream rates up to 8 Mbps and duplex rates up to 640 kbps are available today ADSL modems accommodate Asynchronous Transfer Mode (ATM) transport with variable rates and compensation for ATM overhead, as well as IP protocols. Downstream data rates depend on a number of factors, including the length of the copper line, its wire gauge, presence of bridged taps, and cross-coupled interference. Line attenuation increases with line length and frequency and decreases as wire diameter increases. Ignoring bridged taps ADSL performs as shown Although the measure varies from telco to telco, these capabilities can cover up to 95% of a loop plant, depending on the desired data rate. Customers beyond these distances can be reached with fiber-based digital loop carrier (DLC) systems. As these DLC systems become commercially available, telephone companies can offer virtually ubiquitous access in a relatively short time. Many applications envisioned for ADSL involve digital compressed video. As a real-time signal, digital video cannot use link- or network level error control procedures commonly found in data communications systems. ADSL modems therefore\ incorporate forward error correction that dramatically reduces errors caused by impulse noise. Error correction on a symbol-by-symbol basis also reduces errors caused by continuous noise coupled into a line
0 comments:
Post a Comment
Thanks for your Valuable comment