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<h1>Abis Transmission by HDSL</h1>

<p>HDSL (High bit rate Digital Subscriber Line) is a transmission
interface using 2- or 4-wire twisted pair copper connections in the Abis interface.
Two HDSL modems are integrated in the BTS's transmission unit, TRU. There
is also one 2M PCM interface in this HDSL version of TRU.</p>


<p>The maximum speed of the HDSL modem is 2Mbit/s. Smaller
speeds (1Mbit/s, 512 kbit/s) can be used if full transmission capacity is
not required. Smaller speeds can also be used to extend the range of operation.
The lower the speed, the longer the distance of copper line can be. A typical
transmission distance can be 5 km with 2Mbit/s speed. This is, however, highly
dependent on the type of wire used.</p>


<p>The benefit in using HDSL modems is that normal telephone
lines can be used efficiently for the BSS transmission. There is, however,
a restriction that a galvanic contact is needed between modems. In practise,
this may result in a combination of HDSL and PCM lines in the Abis transmission.</p>
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