Before micro filters, the filter was located inside an engineer only install master socket adapter. This persisted for some time after ADSL was launched. Micro filter was just a buzz word as the new plug in filters were small and easy to install. I guess they actually are smaller than the original faceplate filters.Here's a random question. The first time I encountered a microfilter was when ADSL came along, and I can't find any reference to them before this time. If there was never a larger version of a filter, why are microfilters called 'micro'?