<?xml version="1.0" encoding="UTF-8"?>

<section xml:id="introTech" 
         xmlns="http://docbook.org/ns/docbook" 
         xmlns:xlink="http://www.w3.org/1999/xlink" 
         xmlns:xi="http://www.w3.org/2001/XInclude">

<info>
  <title>Technical background</title>
</info>

<para>
  In the previous sections, I tried to explain the basic concepts of the DMR operation. That is, the
  repeater independent private and group calls. This section concerns the more technical 
  details of the DMR mode. In particular the <glossterm>Time Slot</glossterm> and <glossterm>Color 
  Code</glossterm>.
</para>


<section xml:id="introTimeSlot"> 
<info><title>Time Slots</title></info>
<para>
  As mentioned before, DMR is a digital mode. The speech signal is first digitized and compressed 
  with a lossy compression codec. The latter is also called <glossterm>VOCODER</glossterm>. Modern 
  codecs are very efficient and allow to transfer two independent speech signals within a single 
  12.5kHz wide channel. This is exploited in DMR using a technique called 
  <glossterm>TDMA</glossterm>.
</para>

<informalfigure>
  <mediaobject>
    <imageobject><imagedata fileref="fig/timeslot.png" width="7cm" align="center"/></imageobject>
  </mediaobject>
</informalfigure>

<para>
  <glossterm>TDMA</glossterm> means <quote>time-division media access</quote> and describes that two 
  independent calls can happen simultaneously on one physical channel. To achieve this, each call
  is assigned a <glossterm>Time Slot</glossterm> (time slot 1 &amp; 2) and both are transmitting 
  and receiving only within their assigned time slot. These time slots are very short. For DMR they
  are only 30ms long. This short time, however, is sufficient to transfer audio for at least 60ms.
  Therefore, DMR allows for two independent and simultaneous calls on a single physical channel.
</para>

<para>
  When time slot 1 or 2 <quote>happens</quote>, is determined by the repeater. The repeater defines
  the beat. This also implies that time slots are irrelevant for the simplex operation. Thus you 
  can ignore the time slot settings when programming simplex channels. (see 
  <xref linkend="introCodeplug"/>).
</para>

<note>
  <para>
    What happens on each time slot, is a convention defied by the repeater. A general suggestion is
    that regional communication happens on the time slot 2 while trans-regional communication 
    should happen on time slot 1.
  </para>
</note>
</section>


<section xml:id="introColorCode">
<info><title>Color Codes</title></info>

<para>
  <glossterm>Color Code</glossterm>s are a technical tool to avoid conflicts between repeaters 
  operating on the same frequency. This usually happens in commercial applications of DMR. One 
  company usually gets only a small number of frequencies assigned. To cover the entire campus, 
  much more repeaters are needed than frequencies are available. Consequently, overlapping 
  repeater ranges on the same frequency will occur and two repeaters may receive the call of a 
  participant. Then, the color code allows the repeater to detect whether a call was intended 
  for it. Only if the color code of a call matches the color code of the repeater, the repeater
  will react on that call. 
</para>

<para>
  This issue usually does not arise in amateur radio applications. We just have enough channels.
  Hence the color code is usually set to 1. In very densely populated areas, however, overlapping 
  repeater ranges may still occur and different color codes might be used there.
</para>

<note>
  <para>
    To use a repeater, you not only need to know the input and output frequencies but also the 
    color code of the repeater.
  </para>
</note>
</section>

</section>