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What is ADSL? Detail about ADSL

What is ADSL

The ADSL is a distance-sensitive technology. This is because the quality of the signal and the internet speed depends on the user’s distance from NSP Central Office. The ADSL distance limit is 18,000 feet (5,460 meters), but the lesser the distance better the service.

ADSL can provide maximum downstream (Internet Provider to customer) speeds of 8 Mbps at a distance of about 6.000 feet (1,820 meters), and upstream speeds of up to 640Kbps. The speeds which are available today are 1.5 Mbps downstream, with upstream speeds varying between 64 and 640 Kbps.

The copper wire telephone connections do not have a distance limitation. But when these lines are conditioned to use DSL, the DSL service has a distance limitation. Since DSL cannot work with loading coils the distance limitation cannot be prevented.

Equipment Used in DSL

The DSL basically uses two types of equipment. One which is fixed at the user end and the other at the Internet Service Provider or DSL provider. A Transceiver is fixed at the customer’s location, which can be used to take other internet services. The service provider of DSL has to provide connections to all the customers. For this purpose, they use a DSLAM (Digital Subscriber Line Access Multiplexer).


The transceiver is called a “DSL modem” by most of the residential customers. This DSL modem is the actual point where the DSL line is connected to the customer’s computer or the customer’s local network.

The transceiver or modem can connect to a customer’s equipment in many ways, though most residential installation uses USB or 10 base-T Ethernet connections. While most of the ADSL transceivers given by telephone companies are simple transceivers, some advanced devices which are used by businesses may combine network routers or switches.



A DSLAM takes connections from all customers and puts them onto a single, high-capacity connection to the Internet. DSLAM is quite flexible and they support multiple types of DSL in a common central office. They also handle various protocols and modulations. For example: In the same DSL it can have CAP and DMT. The DSLAM can also provide functions like routing or dynamic IP address assignment for the customer connections.

Very-High-Data-Rate Digital Subscriber Line (VDSL)

The fiber to the neighborhood (FTTN) is a better topology than fiber to the curb(FTTC) with short drops and fiber to the basement (FTTB), serving tall buildings. One of the enabling technologies for FTTN is VDSL.

VDSL transmits high-speed data, it transmits this data for short lengths of twisted-pair of copper telephone lines. The speed of data depends on the line length of the twisted copper telephone lines. Around 51Mbps-55Mbps is the maximum downstream rate. Upstream rates in early models are asymmetric, just like ADSL, at speeds from 1.6- Mbps. Both data channels will be separated in frequency from bands that are used for ISDN and telephone services. Currently, based on the frequency the two high-speed channels are separated.

ISDN Digital Subscriber Line (IDSL)

ISDN digital subscriber line (IDSL) has some features of ISDN and some features of XDSL. Like ISDN, it uses a single-wire pair to transmit full-duplex data. This data is transmitted at around 128 kbps and a distance of RRD range. As mentioned IDSL has the feature of ISDN and uses a 2B1Q line code to enable transparent operation through the ISDN “U” interface. As result, the user continues to use existing CPE (ISDN BRI terminal adapters, bridges, and routers) to make the Network Service Provider Central Office connections.

The major difference is from the carrier’s point of view. IDSL does not connect through the voice switch, which is unlike ISDN. New equipment for data communications equipment terminates the IDSL connection and shuts it off to a router or data switch. This is an important feature. because the overloading of central office voice switches by data users is a growing problem for telecom companies.

The limitation of IDSL is that the customer does not have access to ISDN signaling or voice services. But for Internet service providers, who do not provide public voice service, IDSL is an interesting way of using POTS dial service.

Symmetric Digital Subscriber Line (SDSL)

this is a rate-adaptive version of HDSL and is it symmetric. Symmetric means it allows equal bandwidth downstream from a Network Service Provider Central Office to the customer site as upstream from the subscriber to the central office. this is not other high-speed Internet technologies, SDSL does not require a sophisticated optical or coaxial cable network to provide broadband Internet connectivity.

SDSL supports data only on one line and it does not support analog calls. SDSL uses 2B1Q line coding and can transmit up to 1.54 Mbps to and from a subscriber. SDSL can transfer data at speeds of up to 3 Mbps (megabits per second) in either direction. SDSL can also undergo a configuration and provide bandwidth up to 1.45 Mbps. It is used primarily by small and medium-sized enterprises (SMEs) to transmit and/or receive large volumes of data across the Internet at very high speeds.

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


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