3.2: DSL
DSL stands for Digital Subscriber Line, a broadband Internet technology offered over the existing Public Switched Telephone Network. DSL is actually short for ADSL (Asymmetric DSL), but this form of DSL is, by far, the predominant version, so the “A” is usually dropped. While the twisted pair cabling used in the local loop running to a house or building can physically 
handle a wide bandwidth, it is artificially throttled to 3100 Hz using a filter at the central office (CO) for reasons of economy. Essentially, anything on that local loop below 300 Hz and above 3400 Hz is cut off [72: 132]. This narrow bandwidth is all that is needed for regular voice communication and 56 Kbps dial-up connections, but doesn't provide a wide enough frequency band for DSL. The full potential of the local loop can be taken advantage of by moving it to a filterless switch at the CO, thereby opening up a bandwidth of 1.1 MHz. That spectrum is then divided up into 256 channels, 4312.5 Hz wide each. Channel 0 is used for regular voice communication, or POTS (Plain Old Telephone Service). Channels 1-5 are unused to prevent interference between the voice channel and the data channels. Of the remaining 250 channels, two are used for upstream and downstream control, and the rest are available for data [72: 132]. Some ratio is chosen for splitting up the data channels between upstream and downstream data, with the downstream part often having several times the amount of channels. Line conditions and the length of the local loop will sometimes mean that not all of these channels are usable, however, so DSL transceivers (often called DSL modems) constantly monitor the quality of these channels and will determine which ones to use [23].
In order for DSL to actually be offered on the Public Switched Telephone ![Figure 7: PSTN Equipment Cabinets [66]](figures/7.jpg)
Network, a device called a DSLAM needs to be installed at the CO. A DSLAM, short for DSL Access Multiplexer, receives signals from every DSL customer whose local loop is connected to that CO. It passes traffic back and forth between the customers and the backbone connection, which is usually a fibre optic pipeline owned by the telephone company or leased from another operator. On the other end of the local loop, a customer needs to have a DSL transceiver installed. These are small devices, about the same size as a home router, which hook up directly to a computer or a router. The transceiver has a built-in filter to filter out the
POTS channel, while an in-line filter is installed on every telephone to filter out the data channels. A DSL transceiver is a very inexpensive device and is easy to install, usually less than $100, or they can be rented from the DSL provider. A DSLAM, however, will cost a telephone company about $200,000 to install [52], assuming that a high-speed pipeline is nearby.
The speed of a DSL connection is very fast if you're located close to the CO – about 24 Mbps up to a kilometre away [72: 131]. It decreases rapidly with increasing distance however, and becomes unusable about 5.5 km away from the CO. A DSL provider will usually cap the speed at somewhere between 1 Mbps and 5 Mbps to provide a fair level of service to all customers, regardless of their proximity to the CO. High speed DSL service costs between $30 and $50 a month, with lower speed options available for less. Given that many rural areas have a low population density and larger spaces between COs, DSL is probably not feasible physically nor economically. Installing a $200,000+ DSLAM will not reach that many customers in rural areas, and such an installation would take years to be profitable.
© Jake Cormier, 2006 [jake (at) stormcloudstudios.com]
Completed as a partial requirement for the degree of Bachelor of Science (specialized)
Department of Computer Science :: Algoma University College :: Sault Ste. Marie, Ontario :: Spring 2006