DSL and Networks Information

The local loop of the public switched telephone network (PSTN) was initially designed to carry POTS voice communication and signaling, since the concept of data communications as we know it today did not exist. 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 voiceband or commercial bandwidth.

At the local telephone exchange (United Kingdom) or central office (United States) the speech is generally digitized 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).

The laws of physics, specifically the Shannon limit, cap the speed of data transmission. For a long time, it was believed that a conventional phone line couldn’t be pushed beyond the low speed limits (typically under 9600 bit/s). In the 1950s, 4 MHz television signals were often carried between studios on ordinary twisted pair telephone cable, suggesting that the Shannon Limit would allow transmitting many megabits per second. However, these cables had other impairments besides Gaussian noise, preventing such rates from becoming practical in the field. In the 1980s techniques were developed for broadband communications that allowed the limit to be greatly extended.

The local loop connecting the telephone exchange to most subscribers is capable of carrying frequencies well beyond the 3.4 kHz upper limit of POTS. Depending on the length and quality of the loop, the upper limit can be tens of megahertz. DSL takes advantage of this unused bandwidth of the local loop by creating 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on how the system is configured. Allocation of channels continues at higher and higher frequencies (up to 1.1 MHz for ADSL) until new channels are deemed unusable. Each channel is evaluated for usability in much the same way an analog modem would on a POTS connection. More usable channels equates to more available bandwidth, which is why distance and line quality are a factor (the higher frequencies used by DSL travel only short distances). The pool of usable channels is then split into two different frequency bands for upstream and downstream traffic, based on a preconfigured ratio. This segregation reduces interference. Once the channel groups have been established, the individual channels are bonded into a pair of virtual circuits, one in each direction. Like analog modems, DSL transceivers constantly monitor the quality of each channel and will add or remove them from service depending on whether they are usable.

One of Lechlider’s greatest contributions to DSL was his insight that an asymmetric arrangement offered more than double the bandwidth capacity of synchronous DSL. This allowed Internet Service Providers to offer efficient service to consumers, who benefitted greatly from the ability to download large amounts of data but rarely needed to upload comparable amounts. ADSL supports two modes of transport: fast channel and interleaved channel. Fast channel is preferred for streaming multimedia, where an occasional dropped bit is acceptable, but lags are less so. Interleaved channel works better for file transfers, where transmission errors are impermissible, even though resending packets may increase latency.

Because DSL operates at above the 3.4 kHz voice limit, it cannot be passed 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, nor can voice service be maintained past a certain distance without such coils. 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 not to need 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. Several factors contributed to the popularization of DSL technology:

* Until the late 1990s, the cost of digital signal processors for DSL was prohibitive. All types of DSL employ highly complex digital signal processing algorithms to overcome the inherent limitations of the existing twisted pair wires. Due to the advancements of VLSI technology, the cost of the equipment associated with a DSL deployment (a DSLAM at one end and a DSL “modem” at the other end) lowered significantly.
* A DSL line can be deployed over existing cable. Such deployment, even including equipment, is much cheaper than installing a new, high-bandwidth fiber-optic cable over the same route and distance. This is true both for ADSL and SDSL variations.
* In the case of ADSL, competition in Internet access caused subscription fees to drop significantly over the years, thus making ADSL more economical when compared to dial up access. Telephone companies were pressured into moving to ADSL largely due to competition from cable companies, which use DOCSIS cable modem technology to achieve similar speeds. Demand for high bandwidth applications, such as video and file sharing, also contributed to popularize ADSL technology.

Most residential and small-office DSL implementations reserve low frequencies for POTS service, so that with suitable filters and/or splitters the existing voice service continues to operate independent of the DSL service. Thus POTS-based communications, including fax machines and analog modems, can share the wires with DSL. Only one DSL “modem” can use the subscriber line at a time. The standard way to let multiple computers share a DSL connection is to use a router that establishes a connection between the DSL modem and a local Ethernet, Powerline, or Wi-Fi network on the customer’s premises.

Once upstream and downstream channels are established, they are used to connect the subscriber to a service such as an Internet service provider.

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 are required 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 has 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.