ADSL deployment on an existing plain old telephone service (POTS) telephone line presents some problems because the DSL is within a frequency band that might interact unfavourably with existing equipment connected to the line. Therefore, it is necessary to install appropriate frequency filters at the customer's premises to avoid interference between the DSL, voice services, and any other connections to the line, for example in support of intruder alarms "RedCARE" being an example in the UK. This is desirable for the voice service and essential for a reliable ADSL connection.
In the early days of DSL, installation required a technician to visit the premises. A splitter or microfilter was installed near the demarcation point, from which a dedicated data line was installed. This way, the DSL signal is separated as close as possible to the central office and is not attenuated inside the customer's premises. However, this procedure was costly, and also caused problems with customers complaining about having to wait for the technician to perform the installation. So, many DSL providers started offering a "self-install" option, in which the provider provided equipment and instructions to the customer. Instead of separating the DSL signal at the demarcation point, the DSL signal is filtered at each telephone outlet by use of a low-pass filter for voice and a high-pass filter for data, usually enclosed in what is known as a microfilter. This microfilter can be plugged by an end user into any 'phone jack: it does not require any rewiring at the customer's premises.
Commonly, microfilters are only low-pass filters, so beyond them only low frequencies (voice signals) can pass. In the data section, a microfilter is not used because digital devices that are intended to extract data from the DSL signal will, themselves, filter out low frequencies. Voice telephone devices will pick up the entire spectrum so high frequencies, including the ADSL signal, will be "heard" as noise in telephone terminals, and will affect and often degrade the service in fax, dataphones and modems. From the point of view of DSL devices, any acceptance of their signal by POTS devices mean that there is a degradation of the DSL signal to the devices, and this is the central reason why these filters are required.
A side effect of the move to the self-install model is that the DSL signal can be degraded, especially if more than 5 voiceband (that is, POTS telephone-like) devices are connected to the line. Once a line has had DSL enabled, the DSL signal is present on all telephone wiring in the building, causing attenuation and echo. A way to circumvent this is to go back to the original model, and install one filter upstream from all telephone jacks in the building, except for the jack to which the DSL modem will be connected. Since this requires wiring changes by the customer, and may not work on some household telephone wiring, it is rarely done. It is usually much easier to install filters at each telephone jack that is in use.
DSL signals may be degraded by older telephone lines, surge protectors, poorly-designed microfilters, Repetitive Electrical Impulse Noise, and by long telephone extension cords. Telephone extension cords are typically made with small-gauge, multi-strand copper conductors which do not maintain a noise-reducing pair twist. Such cable is more susceptible to electromagnetic interference and has more attenuation than solid twisted-pair copper wires typically wired to telephone jacks. These effects are especially significant where the customer's phone line is more than 4 km from the DSLAM in the telephone exchange, which causes the signal levels to be lower relative to any local noise and attenuation. This will have the effect of reducing speeds or causing connection failures.
Read more about this topic: Asymmetric Digital Subscriber Line
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