Digital Subscriber Line

DSL or Digital Subscriber Line is a group of technology that is used to transmit digital data over telephones.

DSL is a broadband high speed technology. That brings ample information via copper telephone lines to homes and offices. It provides network connections to small business and homes.

Although it uses the same cable which is used for normal telephones, it offers higher data rates by using digital modem technology. It is assumed digital data does not require changing digital into analogue form and back to original form.

DSL uses high frequency bands for data. To enable simultaneous use of the DSL service and voice, a DSL filter is used to block any high frequency interference. Typically a filter is needed for each analog device.

DSL service can also be simultaneously delivered with wired telephone service on one line. Wired telephone service is a voice grade telephone service. This service employs analog signal transmission over copper loops.

The bit rate ranges from 256kbit/s to 100Mbit/s depending on DSL technology, service level implementation and line conditions. Although bit rates have reached 1Gbit/s, most homes have a limit of 500-800Mbit/s.

While delivering 1Gbit/s symmetrical broadband services, researchers at Bell Lab have reached speeds of 10Gbit/s. But these are lab searches. However a survey shows that, with 364.1 million subscribers worldwide, DSL is still a dominant for broadband access.


John Cioffi is an American electrical engineer and an inventor who has made contributions in telecommunication system theory. He worked at Bell Laboratories. He is known as the Father of DSL.


The telephone network had its beginnings in the late 1800s. After traditional modems, another technology that is DSL was developed. Digital Subscriber Line was originally Digital Subscriber Loop. Loop refers to the connection between one ends of a user device to the local telephone company where DSL device resides. This supports the most high speed digital communication existing local loops.
Although there were several media that could be used to provide network access, none of them could match telephone network. DSL was developed to overcome the communication problems.



The term DSL is widely known as Asymmetric Digital Subscriber Line in telecommunication marketing. It is the most installed technology for internet access. ADSL is an adaptive technology. The system uses a data based on the condition of the local loop line.

ADSL is designed for residential users. This line is not suitable for business. Customers nearer the central office have faster connections and are able see extremely high speeds.

ASDL is a distance sensitive technology. It has different speeds. Downstream speed range from 1.59Mbps. While upstream speeds are up to 1.5Mbps. Transmission uses one pair of wires.

ADSL makes use of techniques such as error correcting codes. ADSL uses variable constellation sizes and sophisticated algorithms to determine where best to distribute energy in the usable bandwidth.
ADSL has the following Digital Subscriber Line:

  • ANSI T1.413 issue 2
  • Asymmetric Digital Subscriber line 2 or ADSL 2
  • Asymmetric Digital Line 2 plus or ADSL 2 +
  • Very high bit rate Digital Subscriber Line 2 or VDSL 2
  • G fast


Symmetric Digital Subscriber Line is the opposite of ASDL. SDSL is a technology that was never standardized. In SDSL rate bit of upstream is lower than downstream.

SDSL also have different speed limit. Upstream can reach from 1.5 to 2.5Mbps, while downstream can reach up to 50 to 55 Mbps. While using SDSL, phone lines are not usable.

SDLS is generally marketed at business customers.

The SDSL has the following Digital Subscriber Line:

  • Symmetric Digital Subscriber Line or SDSL.
  • Single pair high speed Digital Subscriber Line or G.SHDSL.
  • ISDN Digital Subscriber Line or IDSL.
  • HDSL or high bit rate Digital Subscriber Line with HDSL2 and HDSL 4/.


HSDSL is the most common of the digital subscriber line. While using this line, telephone calls cannot be taken. High speed digital subscriber line is intended for the use of the telephones company’s own feeder lines, Internet servers and private data networks.


A DSL modem is hooked up to a phone line. While the other end of the line is connected to a DSLAM. A DSLAM stands for Digital Subscriber Line access multiplexer. A DSLAM is a network device that connects multiple customer digital subscriber line interfaces to high speed digital communication channels using special techniques.

A DSLAM concentrates a large number of DSL individual connections into one box. It covers one part of the city. It is common for one residential block to have a single DSLAM. The DSL internet provider usually offers three services over the same phone. Having access to the internet, the phone and television over the internet on one telephone line is called triple play service. Television over the internet is known as IPTV. There are several services providers from which a customer can choose a modem from.



SHDSL standards have two types of transmission systems:

  • 2-wire SHDSL system – use a single unloaded twisted pair.
  • 4-wire (4W) SHDSL system – uses two unloaded twisted pairs, and distributes the payload among the two pairs using the same modulation methods as the 2W system. The 4 wire system has longer range, as it operates at lower line rates.

This section presents only the 2W system, however the information is also applied to 4 wire systems.


The basic SHDSL transmission subsystem includes two units that are interconnected by a single pair of wire, which is usually a regular unloaded twisted pair. Two different types are defined:

  • SHDSL central unit is usually located at the network site or at the central office (CO). This controls the operation of the whole subsystem and also provides the timing reference.
  • SHDSL remote unit (STU-R) is used to terminate the line at the user’s side, or the customer’s premises (CPE).

Because DSL uses higher frequency, one line can carry two different services at the same time. Speeds of 1Gbps have reached trials.


  • Electronic commerce
  • Emailing is enhanced
  • High-quality of face-to-face Video Conferencing
  • High-speed LAN-to-LAN or LAN-to-mainframe connections
  • Cost-effective telecommuting
  • Document sharing


Users should be aware about the method and issues associated to the modern technologies.
Digital subscriber Line has the following advantages.

1. The performance of a DSL is 100 times more than what a dial up modem can deliver. They are measured in mega bits per second.

2. Offers a reliable and a stable connection.

3. The charges of DSL are cheap.

4. High bandwidth.

5. A DSL connection is always on. Users do not have to dial up internet service provider each time.

6. In DSL each subscriber is configured so that other subscribers do not get mixed up.

7. DSL allow the users to have a simultaneous access to a telephone as well as a computer.

8. It offers file security.

9. Because the service relies on phones lines, there are few service issues.

10. Fixed monthly bills.


1. The speed is not always consistent.

2. Different providers have their own modems. This makes it quite costly.

3. Uploads are slower with DSL.

4. Often not available on remote areas.

5. It receives data faster as compared to sending it.

6. Speed can sometimes depend on encoding techniques, equipment used and even on end system encoding.


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