Operation of ADSL – what equipment do I need and how must it be connected together?
When ADSL is operated on a 2-wire analogue telephone (standard telephone) or ISDN line (e.g. BRI or ISDN2), the telephone service operates normally in the ‘baseband’ of the connection. The high speed data connection provided by ADSL uses only the high frequency signal transmission capabilities of the connection. To keep the the telephone/ISDN and ADSL services apart, high frequency filters or splitters are used at both ends of the user’s connection. At the customer’s premises, the splitting device is called simply a DSL splitter or DSL filter. This may take a number of different forms, but is generally a small device:
DSL splitter DSL filter
A DSL splitter or DSL filter must be provided at the ADSL customer’s premises in order for the service to work. Sometimes this device (e.g. DSL splitter) is despatched in the post to the customer by the service provider at the time when the customer signs up for ADSL service. The customer need only plug the device in according to the installation instructions. Alternatively, in some countries, it is the responsibility of the customer to purchase his own filter or splitter. In these countries, the splitter may be provided as an ‘in-cable device’ (like that illustrated on the right above) and be provided with a DSL modem, when purchased from a computer store.
In addition to the DSL splitter or DSL filter, the ADSL customer must provide a DSL modem (most devices sold as DSL modems are actually a combination of DSL modem, router and firewall) to plug into the DSL spliter or filter. Typically, national telecommunications law requires that the DSL modem be owned by the customer, who may purchase such devices either from the DSL service provider or from a computer store. The customer’s computer or LAN (local area network) is then connected to the DSL modem using either standard ethernet (10/100baseT with an 8-pin RJ-45 connector) or using WLAN (wireless LAN). DSL modems are available with a wide range of different capabilities. The ADSL user needs to consider carefully which features are necessary. more on DSL modem choice...
DSL modem (simple)
Meanwhile, at the public network operator’s ‘exchange premises’ the equivalent splitting device is called a DSLAM (digital subscriber line access multiplexor). This is a much larger, rack mounted device shared by several hundred of the operator’s DSL customers. The overall ADSL network connection is something like:
Typical ADSL line configuration
The fact that an ADSL service typically shares the telephone line with an analogue telephone (standard telephone) or ISDN telephone line connection makes for better overall economics in cases where the ADSL customer also requires a telephone line. Indeed, most ADSL service providers offer relatively cheap ADSL line subscription tariffs because they assume that a telephone service will be provided over the same line to the same customer. Indeed: it is usually a commercial pre-condition of ADSL service, that the customer also subscribes to a telephone or ISDN line service as well: even if this telephone or ISDN service is provided by a different service provider. The maximum bitrate of a standard ADSL line is 8 Mbit/s. ADSL is defined in ITU-T recommendations G.992.1 and G.992.2. More on ADSL connection types...