DSL Internet(DIGITAL SUBSCRIBER LINE)

DSL Internet

Digital Subscriber Line(DSL Internet) technology is a modern technology that uses existing twisted pair telephone lines to transport high-bandwidth data, such as multimedia and video, to service subscribers. The term xDSL covers a number of similar yet competing forms of DSK technologies, including ADSL, SDSL,HDSL,IDSL and VDSL. xDSL is drawing significant attention from implementers and service providers because it promised to deliver high-bandwidth data rates to dispersed locations with relatively small changes to the existing telephone infrastructure.

Telephone companies have installed high-speed digital wide area networks to handle communications between their central offices. The link between the user and the network, however, is still an analogue line(local loop). The challenge is to make these links digital ? a digital subscriber line-without changing the existing local loops. The local loop is a twisted pair cable with a potential bandwidth of 1 MHz or more.

ADSL

Asymmetric digital subscriber line(ADSL Internet) is asymmetrical, which means it provides higher bit rates in the downstream direction(from the telephone central office to the subscribers site) than the upstream direction(from the subscriber site to the telephone central office). This is what subscribers usually want. They want to receive higher volume file quickly from the internet, but they usually have small files, such as a short message, to send.

ADSL uses two pieces of equipment: One on the customer end and one at the provider end.

?  Transceiver : At the customer's location, there is a DSL transceiver, which may also provide other service. In the user group it is called a ?DSL Modem? while at the telephone company call it an ATU-R. It can connect to a customer's equipment in several ways, though in most case uses USP or 10base T Ethernet connection..

?  DSL access multiplexer(DSLAM): The DSL Internet service provider has a DSLAM to receive customer connections. The DSLAM at the access provider is the equipment that really allows DSL to happen. A DSLM takes connections from many customers and aggregates them onto a single, high-capacity connection to the internet. DSLAMs are generally flexible and able to support multiple types of DSL in a single central office, and different varieties of protocol and modulation both CAP and DMT, for example in the same type of DSL. In addition it DSLAM may provide additional functions including routing or dynamic IP address assignment for the customer.

The DSLAM provides one of the main differences between user service through ADSL and through cable modems. Because cable modem users generally share a network loop that runs through a neighbourhood, adding users means lowering performance in many instances. ADSL provides a dedicated connection from each user back to the DSLAM, meaning that users won't see a performance decrease as new users are added-until the total number of users begins to saturate the single, high-speed connection to the internet.

Standard of ADSL Internet

There are two competing and incompatible standards for ADSL Internet. The official ANSI standard for ADSL is a system called discrete multi tone or DMT. According to equipment manufacturers, most of the ADSL equipment installed today uses DMT. An earlier and more easily implemented standard was the carrier less amplitude/phase(CAP) system, which was used on many of the early installations of ADSL.

CAP operates by dividing the signals on the telephone line into three distinct bands: Voice conversations are carried in the 0 to 4KHz band, as they are in all POTS(plain Old telephone service) circuits. The upstream channel(from the user back to the server) is carried in a band between 25 and 160 KHz. The downstream channel begins at 240 KHz and goes up to a point that varies depending on a number of conditions (line length, line noise, number of users in a particular telephone company switch) but has a maximum of about 1.5 MHz. This system, with the three channels widely separated, minimizes the possibility of interference between the channels on one line, or between the signals on different lines. DMT also divides signals into separate channels, but doesn't use two fairly broad channels for upstream and downstream data. Instead, DMT divides the data into 247 separate channels, each 4 KHz wide. One way to think about it is to imagine that the phone company divides your copper line into 247 different 4KHz lines and then attaches a modem to each one. You get the equivalent of 247 modem connected with the computer at once. Each channel is monitored and, if the quality is too impaired, the signal is shifted to other channels. This system constantly shifts signals between different channels, searching for the best channels for transmission and reception. In addition, some of the lower channels(those starting at about 8KHz), are used as bidirectional channels, for upstream and downstream information. Monitoring and sorting out the information on the bidirectional channels, and keeping up with the quality of all 247 channels, make DMT more complex to implement than CAP, but gives it more flexibility on lines of differing quality.

CAP and DMT are similar in one way that we can see as a DSL user. If we have ADSL installed, we were almost certainly given small filters to attach to the outlets that don't provide the signal to the ADSL modem. These filters are low-pass filters-dimple filters that block all signals above a certain frequency. Since all voice conversations take place below 4 KHz, the low-pass filters are built to block everything above 4 KHz, preventing the data signals from interfering with standard telephone calls.

Distance Sensitivity

ADSL is a distance-sensitive technology. As the connection's length increases, the signal quality decreases and the connection speed goes down. The limit for ADSL service is 18,000 feet(5460 meters), though for speed and quality of service reasons many ADSL providers place a lower limit on the distances for the service. At the extremes of the distance limits, ADSL customers may see speeds far below the promised maximums, while customers nearer the central office have faster connections and may see extremely high speeds in the future. ADSL technology can provide maximum downstream(internet to customer) speeds of up to 8 Mbps at a distance of about 6000 feet(1820 meters), and upstream speed of up to 640 Kbps. In practice, the best speeds widely offered today are 1.5 Mbps downstream, with upstream speeds varying between 64 and 640 Kbps.

VDSL

The very high bit rate digital subscriber line(VDSL), an alternative approach that is similar to ADSL, uses, coaxial, fibre-optic, or twisted pair cable for short distance(300 to 1800 meters). The modulating technique is DMT with a bit rate of 50 to 55 Mbps downstream and 1.5 to 2.5 Mbps upstream.

VDSL operates over the copper wires in the phone line in much the same way that ADSL does, but there are a coupe of distinctions. VDSL can achieve incredible speeds, as high as 52Mbps(from home) and 16 Mbps upstream which is much faster than ADSL. But its amazing performance comes at a price: it can only operate over the copper line for a short distance about 4000 feet.

The key to VDSL is that the telephone companies are replacing may of their feeds with fibre-optic cable. In fact, many phone companies are planning fibre to the cube(FTTC), which means that they will replace all existing copper lines right up to the point where your phone line branches off at your house. At the least many companies expect to implement Fibre to the Neighbourhood(FTTN). Instead of installing fibre-optic cable along each street, FTTn has fibre going to the main junction box for a particular neighbourhood.

By placing a VDSL transceiver in the home and VDSL gateway in the junction box, the distance limitation is neatly overcome. The gateway takes care of the analogue -digital-analogue conversion problem that disables ADSL over fibre-optic lines. It converts the data received from the transceiver into pulses of light that can be transmitted over the fibre-optic system to the central office, where the data is routed to the appropriate network to reach its final destination. When data is sent back to the computer, the VDSL gateway converts the signal from the fibre-optic cable and sends it to the transceiver. All of this happens millions of time each second.

HDSL

Originally developed by Bell core, high bit-rate DSL(HDSL)/T1/E1 technologies have been standardized by ETSI in Europe and ANSI in US. The ANSI standard covers two-pair T1 transmission, with a data rate of 786 Kbps on each twisted pair. ETSI standards exist both for a two-pair E1 system, with each pair carrying 1168Kbps and a three-pair 786Kbps on each twisted pair.

HDSL became popular because it is a better way to provisioning T1 or E1 over twisted-pair copper lines than the long-used technique known as Alternative Mark Inversion(AMI). HDSL uses less bandwidth and requires no repeater up to the CSA range. By using adaptive line equalization and 2B1Q modulation, HDSL transmits 1.544Mbps or 2.048Mpbls in bandwidth ranging from 80ntrast to the 1.5 MHz require by AMI. HDSL is heavily used in cellular telephone build outs. Traffic from the base station is backhauled to the CO using HDSL in more than 50 percent of installations. Currently the vast majority of new T1 lines are provisioned HDSL. HDSL does have drawbacks. First, no provision exists for analogue voice because it uses the voice band. Second, ADSL achieves better speeds than HDSL because ADSL's asymmetry deliberately keeps the crosstalk at one end of the line. Symmetric systems such as HDSL have crosstalk at both ends.

HDSL-2

HDSL-2 is an emerging standard and a promising alternative to HDSL. The intention is to offer a symmetric service at T1 speeds using a single-wire pair rather than two pairs. This will enable it to operate for a larger potential audience. It will require more aggressive modulation, shorter distances (about 10,000 feet), and better phone lines.

Much of the SDSL equipment in the market today uses the 2B1Q line code developed for Integrated Services Digital Network. The Bell companies have insisted that using this SDSL at speeds higher than 768 kbps can cause interference with voice and other services that are offered on copper wire within the same wire bundle.

The biggest advantage of HDSL-2, which was developed to serve as a standard by which different vendors' equipment could interoperate, is that it is designed not to interfere with other services. However, HDSL-2 is full rate only, offering services only at 1.5 Mbps.

G.SHDSL

G.SHDSL is a standards-based, MultiMate version of HDSL-2 and offers symmetrical service. The advantage of HDSL-2, which was developed to serve as a standard by which different vendors' equipment could interoperate, is that it is designed not to interfere with other services. However, the HDSL-2 standard addresses only service at 1.5 Mbps. Multi rates HDSL-2 is part of Issue 2 of the standard known as G.SHDSL, and is ratified by the ITU. G.SHDSL builds upon the benefits of HDSL-2 by offering symmetrical rates of 2.3 Mbps.

ISDN Digital Subscriber Line

ISDN digital subscriber line ( IDSL) is a cross between ISDN and xDSL. It is like ISDN in that it uses a single-wire pair to transmit full-duplex data at 128 kbps and at distances of up to RRD range. Like ISDN, IDSL uses a 2B1Q line code to enable transparent operation through the ISDN "U" interface. Finally, the user continues to use existing CPE (ISDN BRI terminal adapters, bridges, and routers) to make the CO connections.

The big difference is from the carrier's point-of-view. Unlike ISDN, ISDL does not connect through the voice switch. A new piece of data communications equipment terminates the ISDL connection and shuts it off to a router or data switch.

The limitation of ISDL is that the customer no longer has access to ISDN signalling or voice services. But for Internet service providers, who do not provide a public voice service, ISDL is an interesting way of using POTS dial service to offer higher-speed Internet access, targeting the embedded base of more than five million ISDN users as an initial market.

SDSL

Symmetric Digital Subscriber Line (SDSL) is a rate-adaptive version of HDSL and, like HDSL, is symmetric. It allows equal bandwidth downstream from an NSP's central office to the customer site as upstream from the subscriber to the central office. SDSL supports data only on a single line and does not support analogue calls. SDSL uses 2B1Q line coding and can transmit up to 1.54 Mbps to and from a subscriber, or can be configured to offer a variable range of bandwidth up to 1.45 Mbps.

The symmetry that SDSL offers, combined with always-on access (which eliminates call setup), makes it a favourable WAN technology for small to medium businesses and branch offices, and can be an affordable alternative to dedicated leased lines and Frame Relay services. Because traffic is symmetrical, file transfer, web hosting, and distance-learning applications can effectively be implemented with SDSL.

Comparative analysis of different types of DSL

Decisions

ADSL is the best from the xDSl family for its cost, reliability and availability.

See Also