Note: This Howto is still incomplete!
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Abstract
In state of the art telephone network everything is syncronized by a central, very high accuracy clock. The BTS also makes uses this clock for various things like channel allocation, frame clock, bitclock ect. If the clock gets a little bit incorrect the BTS will transmit on a wrong frequency and there will be lots of other effects that destabilize radio transmission.

The BS11 is shipped with a precalibrated internal oscillator. If you are lucky your the calibration (that is made over 10 years ago) in your BTS is still correct. If not your setup will not work unless you recalibrate the oscillator.

This document illustrates how you can use a ceap HFC-S card with the public telephone network as frequency standard for your E1 line.

Modifieing the HFC-S Card
All in all we need 2 Signals from the HFC-S Card: F0IO and C4IO. This signals are provided by the HFC-S Chip on the card and can be taken from pin 55 (F0IO) and 54 (C4IO) at the HFC-S Chip. (See also Datasheet page 8).