Generally speaking the terms "micro-filter", "splitter" and "ADSL filter" mean the same thing.

When your telephone line is ADSL enabled it is able to carry information on different frequency ranges:
ADSL uses the higher frequencies to transmit data, whilst voice uses the lower frequency band.

Although you will still be able to use your telephone as normal once your line is activated, the transmission of adsl data on the line will cause interference.

In order that the adsl data does not interfere with the normal voice data (and vice-versa), you need to ensure you install a filter on each device that uses the lower frequencies. These devices are telephones, fax machines, answer phones, sky boxes.

An ADSL filter (low pass filter) strips out any high frequency data signals which are being carried on your phone line from the telephone side of the filter.

There are different types of splitter available for purchase, but they all essentially work in a similar way.  However, it should be pointed out that some of the cheaper filters available are not as reliable as others, and are not able to filter or terminate the data signals as effectively as they should. NTE5 adsl filtered faceplates are generally the most effective since they filter your connection at the point of entry.

Some common types of adsl filters
Type 1 (T-Filter)	Type 2 (Dongle)	adsl NTE5 Faceplate

Filters will also "terminate" your ADSL line properly by ensuring that data is not reflected back down the phone line which could cause connection problems.

A maximum of four microfilters can be used on a BT phone line. This corresponds to the recommended maximum number of items of telephony equipment you can plug into a standard voice line - known as the maximum Ringing Equivalent Number (REN).

See the next page on how to install adsl micro-filters.