Digital Subscriber Line, or DSL service, is broadband because it uses a shared pair of wires to transmit digital information on one channel while another is used to transmit sound. DSL lines are used in the customer interface to a local telephone network. The download speed of DSL can vary from 128 kilobits per second (kbit/s) to 24,000 kbit/s depending on DSL technology and service implemented. Downloading is faster than uploading for ADSL, and identical for SDSL.



DSL is the most commonly used alternative to cable modems in Europe and North America. The older ADSL standards have a data speed of approximately 8 Mbits/s over about a 2 kilometer radius (1-1/4 miles), whereas the new ADSL2+ can transmit up to 24 Mbits/s. Of course, actual performance depends how far away customers are located from the central office, since the further away they are, the less bandwidth will be available.

DSL was first used in 1988 by Bellcore to carry a digital signal over unused frequencies on twisted pair of cables connecting the central phone company office with customer premises. Their engineers discovered that digital data could be sent along phone lines without interfering with normal voice telephone services. Existing local phone companies did not promote DSL aggressively, because is was not as profitable as installing extra phone lines for customers who wanted simultaneous voice phone and internet access. However, this changed in the 1990s when cable television companies began to offer broadband internet access, and the telephone companies realized that most of their customers would prefer the new faster service. Phone companies quickly began to implement DSL service in an effort to keep their customer base.

When the public telephone system was designed, its inventors were not planning for future internet access; they simply wanted to transmit voice messages. The phone system passes audio over the lines at 300 to 3,400 Hz, the range required for human speech to be clearly intelligible. Any signal above 4,000 Hz must be filtered by a voice phone to prevent distortion of the audio message. Frequency used to transmit telephone calls is known as commercial bandwidth.

However, the system is capable of sending transmissions over the existing equipment both below and above the 300 to 3,400 range. Depending on the length and quality of the phone cable, the upper limit can be tens of megahertz.

Data transmission channels are created as 4312.5 Hz wide channels starting between 10 and 100 kHz, depending on system configuration, skipping over the audio phone transmission range and then continuing up to frequencies as high as 1.1 MHz for ADSL. The available channels are then bonded into a pair of virtual circuits, one for downstream traffic and another for upstream. The quality of transmission is constantly checked by DSL transceivers, and channels are added or removed from service depending on how much functional bandwidth is available on a given line. It is cheaper to develop new technologies to exploit existing cables than it is to install extra lines, and all forms of DSL employ highly complex digital signal processing to maximize the usage of available wiring.

The subscriber end of the connection consists of a DSL modem which converts the digital output from a computer into a voltage signal and vice versa.

Wireless Internet is accessed over cellular towers. This is a contentious subject because it creates more electromagnetic pollution in an environment already overburdened with electronic devices and their associated EM output. The cities of Toronto, San Fransisco and Philedelphia have all adopted ambitious plans to have publicly-funded Wireless access throughout the municipal area; meanwhile, Ontario's Lakehead University has banned the technology on its campus (at least in areas where cable connections are available) due to fears of radiation-induced cancer. However, to date no definitive study exists to prove or disprove a correlation between low-level radiation from EMF sources and illness.

Analog modems which operate at speeds over 600 bit/s are often also called broadband. broadband. They reach high effective transmission rates by employing multiple channels, though the rate of each channel is limited to 600 baud. For example, a 2400 bit/s modem is made up of four 600 baud channels. This is fundamentally different from the operation of a baseband transmission such as Ethernet, which fills its entire bandwidth with one type of signal.

Each individual channel taking part in a multiplexing function is by definition narrowband, whereas the entire set of channels used simultaneously for the same communication are defined as broadband.

Broadband is usually faster than narrowband, and therefor the term is often used to refer to any fast mode of transmission, regardless of its technical method of data transmission. This is not accurate, because many slower devices, such as analog modems above 600 bit/s, are technically broadband because of the way in which it is constructed; while Ethernet, which is much faster, is actually narrowband.

To avoid this confusion, the International Telecommunication Union Standardization Sector has recommended that broadband be defined as a transmission capacity that is faster than primary rate ISDN, at 1.5 to 2 Mbit/s. Currently, devices and services with speeds of 256 kbit/s and higher are sold as "broadband."