Digital Subscriber Line (xDSL)

 DSL or xDSL is a family of technologies that provides digital data transmission over the wires of a local telephone network. DSL originally stood for digital subscriber loop, but as of 2009 the term digital subscriber line has been widely adopted as a more marketing-friendly term for ADSL, the most popular version of consumer-ready DSL. DSL can be used at the same time and on the same telephone line with regular telephone, as it uses high frequency bands, while regular telephone uses low frequency.

The DSL family includes several variations of what is known as digital subscriber line. The lower case x in front of the xDSL stands for the many variations.

ADSL is the new modem technology to converge the existing twisted pair telephone lines into the high−speed communications access capability for various services. Most people consider ADSL as a transmission system instead of a modification to the existing transmission facilities. the IDSL technique is all digital operating at two channels of 64 Kbps for voice or non voice operation and a 16 Kbps data channel for signaling, control, and data packets. ISDN was very slow to catch on, but the movement to the Internet created a whole new set of demands for the carriers to deal with. HDSL was developed as a more efficient way of transmitting over the existing copper wires. HDSL does not require the repeaters on a local loop of up to 12K. Bridge taps will not bother the service, and the splices are left in place. This means that the provider can offer HDSL as a more efficient delivery of 1.544 Mbps. The modulation rate on the

HDSL service is more advanced. The goal of the DSL family was to continue to support and use the local copper cable plant. It was developed to provide high−speed communications on that single cable pair but at distances no greater than 10K. SDSL uses only one pair of wires, but is limited in its distance to provide duplex, high−speed communications. Not all users require symmetrical speeds at the same time. ADSL was, therefore, designed to support differing speeds in both directions over a single cable pair at distances of up to 18K. Because the speeds requested are typically for access to the Internet, most users look for higher speeds in a download direction and the lower speed for an upward direction. If the line conditions vary, the speed will be dependent on the sensitivity of the equipment. In order to achieve variations in the throughput and be sensitive to the line conditions, RADSL was developed. This gives the flexibility to adapt to the changing conditions and adjust the speeds in each direction to potentially maximize the throughput on each line.

CDSL does not use, nor need, a splitter on the line. Moreover, speeds of up to 1 Mbps in the download direction and 160 Kbps in the upward direction are provided. One of the most significant improvements SHDSL brings to the business market is increased reach — at least 30 percent greater than any earlier symmetric DSL technology. Furthermore, SHDSL supports repeaters, which further increase the reach capability of this technology. Clearly, changes will always occur as we demand faster and more reliable communications capabilities. It was only a matter of time until some users demanded higher−speed communications than was offered by the current DSL technologies. As a result, VDSL was introduced to achieve the higher speeds.

The download speed of consumer DSL services typically ranges from 256 kilobits per second (kbit/s) t o24,000 kbit/s, depending on DSL technology. Line conditions and service-level implementation, typically, upload speed is lower than download speed for Asymmetric Digital Subscriber Line and equal to download speed for the rarer Symmetric Digital Subscriber Line

Voice and data

Comparing DSL & Dial-Up

DSL (VDSL) typically works by dividing the frequencies used in a single phone-line into two primary "bands". The ISP data uses the high-frequency band (25 kHz and above) whereas the voice utilizes the lower-frequency band (4 kHz and below). (See the ADSL article for information on the subdivision of the high-frequency band.) The user typically installs a DSL filter on each phone outlet. This filters out the high frequencies from the phone line, so that the phone only sends or receives the lower frequencies and the user hears only the human voice. The DSL modem and the normal telephone equipment can be used simultaneously on the line without interference from each other provided filters are used for all voice devices.

History and science

DSL, like many other forms of communication, stems directly from Claude Shannon's seminal 1948 scientific paper: A Mathematical Theory of Communication. Employees at Bellcore (now Telcordia Technologies) developed ADSL in 1988 by placing wideband digital signals above the existing baseband analog voice signal carried between telephone-company central offices and customers on conventional twisted pair cabling.

U.S. telephone companies promote DSL to compete with cable Internet. the first DSL service ran over a dedicated "dry loop", but when the FCC required the incumbent local exchange carriers (ILECs) to lease their lines to competing providers such as Earthlink, shared-line DSL became common. Also known as DSL over Unbundled Network Element, this allows a single pair to carry data (via a digital subscriber line access multiplexer [DSLAM]) and analog voice (via a circuit switched telephone switch) at the same time.

Operation

Regular DSL

Telephone engineers initially developed the local loop of the public switched telephone network (PSTN) to carry POTS voice communication and signaling: no requirement for data communication as we know it today existed. For reasons of economy, the phone system nominally passes audio between 300 and 3,400 Hz, which is regarded as the range required for human speech to be clearly intelligible. This is known as voice band or commercial bandwidth.

The local telephone exchange or central office generally digitizes speech signals into a 64 kbit/s data stream in the form of an 8 bit signal using a sampling rate of 8,000 Hz, therefore, according to the Nyquist theorem, any signal above 4,000 Hz is not passed by the phone network (and has to be blocked by a filter to prevent aliasing effects).

Because DSL operates above the 3.4 kHz voice limit, it cannot pass through a load coil. Load coils are, in essence, filters that block out any non-voice frequency. They are commonly set at regular intervals in lines placed only for POTS service. A DSL signal cannot pass through a properly installed and working load coil, while voice service cannot be maintained past a certain distance without such coils. Therefore, some areas that are within range for DSL service are disqualified from eligibility because of load coil placement. Because of this, phone companies are endeavoring to remove load coils on copper loops that can operate without them, and conditioning lines to avoid them through the use of fiber to the neighborhood or node FTTN.

The commercial success of DSL and similar technologies largely reflects the advances made in electronics that, over the past few decades, have been getting faster and cheaper even while digging trenches in the ground for new cables (copper or fiber optic) remains expensive.

Naked DSL

Dry-loop DSL or "naked DSL," which does not require the subscriber to have traditional land-line telephone service, started making a comeback in the US in 2004 when Qwest started offering it, closely followed by Speakeasy. As a result of AT&T's merger with SBC, and Verizon's merger with MCI, those telephone companies have an obligation to offer naked DSL to consumers.

Even without the regulatory mandate, however, many ILECs offer naked DSL to consumers. The number of telephone landlines in the US dropped from 188 million in 2000 to 172 million in 2005, while the number of cellular subscribers has grown to 195 million.. This lack of demand for landline service has resulted in the expansion of naked DSL availability.

Typical setup and connection procedures

Physical connection must come first. On the customer side, the DSL Transceiver, or ATU-R, or more commonly known as a DSL modem, is hooked up to a phone line. The telephone company connects the other end of the line to a DSLAM, which concentrates a large number of individual DSL connections into a single box. The location of the DSLAM depends on the telephone company but it cannot be located too far from the user because of attenuation, the loss of data due to the large amount of electrical resistance encountered as the data moves between the DSLAM and the user's DSL modem. It is common for a few residential blocks to be connected to one DSLAM.

When the DSL modem powers up it goes through a sync procedure. The actual process varies from modem to modem but generally involves the following steps:

1.     The DSL Transceiver does a self-test.
2.   The DSL Transceiver checks the connection between the DSL Transceiver and the computer.
3.    The DSL Transceiver then attempts to synchronize with the DSLAM. Data can only come into the computer when the DSLAM and the modem are synchronized.

Modern DSL gateways have more functionality and usually go through an initialization procedure very similar to a PC boot up.

Equipment

The customer end of the connection consists of a Terminal Adaptor or in layman's terms “DSL Modem” This converts data from the digital signals used by computers into a voltage signal of a suitable frequency range which is then applied to the phone line.

In some DSL variations (for example, HDSL), the terminal adapter connects directly to the computer via a serial interface, using protocols such as RS-232 or V.35. In other cases (particularly ADSL), it is common for the customer equipment to be integrated with higher level functionality, such as routing, firewalling, or other application-specific hardware and software. In this case, the entire equipment is usually referred to as a DSL router or DSL gateway.

Some kinds of DSL technology require installation of appropriate filters to separate, or "split", the DSL signal from the low frequency voice signal. The separation can take place either at the demarcation point, or with filters installed at the telephone outlets inside the customer premises.

At the exchange, a digital subscriber line access multiplexer (DSLAM) terminates the DSL circuits and aggregates them, where they are handed off onto other networking transports. In the case of ADSL, the voice component is also separated at this step, either by a filter integrated in the DSLAM or by a specialized filtering equipment installed before it. The DSLAM terminates all connections and recovers the original digital information.

Protocols and configurations

Many DSL technologies implement an ATM layer over the low-level bitstream layer to enable the adaptation of a number of different technologies over the same link.

DSL implementations may create bridged or routed networks. In a bridged configuration, the group of subscriber computers effectively connect into a single subnet. The earliest implementations used DHCP to provide network details such as the IP address to the subscriber equipment, with authentication via MAC address or an assigned host name. Later implementations often use PPP over Ethernet or ATM (PPPoE or PPPoA), while authenticating with a userid and password and using PPP mechanisms to provide network details.

DSL technologies

The line-length limitations from telephone exchange to subscriber impose more restrictions on higher data-transmission rates. Technologies such as VDSL provide very high speed, short-range links as a method of delivering "triple play" services (typically implemented in fiber to the curb network architectures). Technologies likes GDSL can further increase the data rate of DSL. Fiber Optic technologies exist today that allow the conversion of copper based IDSN, ADSL and DSL over fiber optics.

Example DSL technologies (sometimes called xDSL) include:

1.    ISDN Digital Subscriber Line (IDSL), uses ISDN based technology to provide data flow that is slightly higher than dual channel ISDN.

2.    High Data Rate Digital Subscriber Line (HDSL / HDSL2), was the first DSL technology that uses a higher frequency spectrum of copper, twisted pair cables.

3.    Symmetric Digital Subscriber Line (SDSL / SHDSL), the volume of data flow is equal in both directions..

4.    Asymmetric Digital Subscriber Line (ADSL), the volume of data flow is greater in one direction than the other.

5.    Rate-Adaptive Digital Subscriber Line (RADSL), designed to increase range and noise tolerance by sacrificing up stream speed

6.    Very High Speed Digital Subscriber Line (VDSL)

7.    Etherloop Ethernet Local Loop

8.    Gigabit Digital Subscriber Line (GDSL), based on binder MIMO technologies.

Transmission methods

Transmission methods vary by market, region, carrier, and equipment.

1.      2B1Q: Two-binary, one-quaternary, used for IDSL and HDSL
2.     CAP: Carrierless Amplitude Phase Modulation - deprecated in 1996 for ADSL, used for HDSL
3.    DMT: Discrete multitone modulation, the most numerous kind, also known as OFDM (Orthogonal frequency-division multiplexing)

 Sources:

Broadband Communications Handbook

www.aaxnet.com/topics/cblmdm.html

www.jawin.com/protocolxDSL.html

www.business.com

www.dsl-direct.com/