DSL Filter-You Need To Have It
If you have DSL coming in through your phone lines you will need to have either DSL filters or a splitter installed. A splitter will divide the line up into two lines - one for DSL, and one for everything else. If you don't use a splitter, then a DSL filter will have to be attached to every phone device in the building. Theoretically, the DSL and the phone frequencies should be well separated without use of a filter, but in the real world, the filter (or a splitter) is usually necessary.
Phone devices that need a DSL filter include answering machines, fax machines, and phones - basically everything connected to the phone system except for the DSL modem. The filter will strip the DSL signal from the wires going to these devices. Without the filter, interference from DSL signals can cause noise on the phone line, and interference from phone signals can cause poor performance and dropped signals on the DSL line. DSL filters are classified as 'low pass' filters because they allow the lower frequencies (the DSL signal) to pass through while greatly reducing or eliminating frequencies above a certain level.
If it's possible to install a splitter instead, that's usually seen as a better solution. Then there will be no DSL signals coming anywhere near telephone devices and vice versa. The splitter is installed near the phone entry junction box, sometimes called the NID (for Network Interface Device), and a new line is run from there to the location of the DSL modem. The reason why most companies use filters instead of splitters is ease of installation. The splitter requires a visit by a technician to install it, while filters are easily installed by the customer.
If you have more than six phone devices in your home or business, you'll probably need to use a splitter rather than filters. Six is considered to be the cutoff point where the filters begin to degrade the DSL signal excessively. Be aware that the regular DSL filters used for phones will not work on some other devices that may be hooked into the phone line, such as monitored alarm systems. These will require special filters specifically made for alarms. Unfortunately, some alarms will not work with any filters at all, and will require a splitter and their own dedicated line. Additionally, DSL filters won't work with Telephone Devices for the Deaf (TDDs). They also require their own line.
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