This page explains the differences between DSL splitters and DSL microfilters, aiding selection and understanding. Microfilters are fitted to every voice telephone handset, passing voice band and rejecting DSL data. A splitter is fitted at the demarcation point where data and voice lines diverge and separates voice from data.
Isomatic supplies ADSL, SDSL, SHDSL and VDSL filters and splitters, both customer premises (RT) types and central office (CO) types, plus a range of racks and cables.
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To view or download price list in Excel format click on DSL products price List. Please note that it is uneconomic for us to supply less than 100 RT units or 5 CO units.
For abbreviations and acronyms: Go to Glossary of terms page
Summary: In all our units a low-pass filter separates voice signals from the composite band of DSL data and voice. Splitters are three port devices with connections for Line, Modem and telephone. They protect the PSTN from interference caused by the ATU-R modem and protect the ATU-R modem from signals related to the PSTN. Both filters and splitters prevent data signals interfering with telephone conversations. They maintain their stable properties with the 100 mA DC of the RT line, have excellent inter-line (tip to ring) balance and meet ANSI and ITU standard specifications.
Microfilters (distributed filters, line filters, 'phone line filters) refer to implementations where the filter function has been moved from the demarcation point in the customer premises (CP), as in a Remote Terminal (RT) POTS splitter implementation, to locations inside the CP.
Initially, distributed implementations were proposed due to the difficulty and cost involved with installing the POTS splitter at the customer premises, the so-called 'truck-roll'. Then an alternative idea was proposed to install a small low pass filter at each connected Terminal Equipment of the customer, thus distributing the splitter function. This concept was originally developed for the G-Lite version of ADSL (ITU-T G992.2) and the filters were termed 'microfilters'. They are characterised by their small size and low cost.
After testing it was soon realised that the distributed filter implementations could be used not only with G-Lite but also with full-rate ADSL (ITU-T G992.1) and later with other versions of DSL.
A microfilter design capable of achieving this result requires more isolation between the ADSL devices and the POTS terminal equipment to minimise impedance changes and the non-linear and intermodulation effects that appear at the POTS connection.
Thus ETSI has included a 'distributed filter' section in its ADSL/POTS splitter
specification document, TR 101 728(ITU-T G992.1). The goal application of the filters described is clearly defined as full rate ADSL. This document refers to 'distributed filters' rather than microfilters due to the association of microfilters with G-Lite. The T1E1.4 committee upcoming specification refers
to 'In-line filters' with similar motivation.
The basic functionality of a CP POTS splitter is to separate the voice band and ADSL band data so that the relevant customer terminal equipment can access the required data. The POTS splitter is located at the demarcation point of the customer premises. A low pass filter prevents high frequency data traffic from entering the POTS terminal equipment. It provides sufficient attenuation of the local upstream ADSL signals to eliminate interference to the POTS equipment from the subscribers own modem. This could be harmful to the POTS equipment as it frequently contains many non-linear devices which can act as demodulators when presented with high frequency signals, resulting in voice band noise.
The high pass filter, which can be wholly or partially incorporated in the user's ADSL modem, shields the sensitive ADSL receiver from normal POTS voice band signals and other POTS services signals (e.g. ringing voltage). The excellent performance of our CP POTS splitter design allows this separation
of data to be performed very effectively. The splitter has negligible impact on existing POTS services. In addition, the splitter prevents the ADSL data stream signal being impacted by changes of impedance during on/off hook operation of the POTS terminal.
The filters in the splitter also isolate the ADSL transceiver from the customer premise copper wiring. Customer premises wiring typically comprises a mixture of bridge taps, wire gauge changes and changes in type of wire from twisted pair to flat cable. In addition, the net impedance of the in-house wiring changes every time a telephone changes between its on-hook and off hook states. Thus the POTS splitter provides a more controlled impedance to the modem.
Links to rest of Isomatic UK site:
Go to DSL splitters and filters starting page
Go to ADSL products page
Go to SDSL products page
Go to SHDSL products page
Go to VDSL products page
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This page was updated and its HTML syntax checked on 24 July 2005 using BBEdit on an iMac 
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