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

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

<info>
  <title>DMR Introduction &amp; Origin</title>
</info>

<para>
  <acronym>DMR</acronym> (digital mobile radio) is a digital radio standard to transmit speech 
  and data. That is, the speech is not directly modulated on the carrier but is first digitalized and 
  compressed using a lossy compression codec (VOCODER). The compressed speech is then transmitted as 
  data packets. The latter allows to attach meta-information to the data packet like source and
  destination of the packet. 
</para> 

<para>
  <acronym>DMR</acronym> was designed to be the digital replacement for analog trunking 
  networks in commercial applications. A classic example for such a commercial application
  of DMR would be a civil air port. With this, I do not mean the air-traffic radio but all 
  the communication of the ground staff in and around the actual air port buildings.
</para>

<para>
  At such an imaginary air port, there is a huge staff with a wide variety of tasks. For example 
  (without any claim of completeness):
</para>

<itemizedlist>
  <listitem><para>The cleaning crew,</para></listitem>
  <listitem><para>technicians,</para></listitem>
  <listitem><para>security staff,</para></listitem>
  <listitem><para>apron staff for refueling, luggage and catering,</para></listitem>
  <listitem><para>the fire brigade and </para></listitem>
  <listitem><para>the headquarters.</para></listitem>
</itemizedlist>

<para>
  All these people carry a radio and should be able to 
</para>
<itemizedlist>
  <listitem><para>Directly call the headquarters. All staff should be able to call the headquarters
    directly.</para></listitem>
  <listitem><para>Direct communication between members of the same group, without interfering with
    other groups. For example, the cleaning staff should be able to communicate with each other, 
    without interfering with the fire brigade.</para></listitem>
  <listitem><para>Each person should be able to call a complete group. For example, the headquarters 
    may call the entire fire brigade or one member of the security staff may call the entire security
    for help.</para></listitem>
</itemizedlist>

<para>
  An air port, however, is a rather large area. Consequently, not all staff members are able to 
  reach all others. Therefore, some repeaters must be installed to cover the entire air port 
  including all interiors.
</para>

<para>
  If you compare this scenario with the classic FM repeater networks (see 
  <xref linkend="introForeknowledge"/>), it gets clear that is hard to implement these concepts 
  using analog FM repeaters. Especially, if several repeaters are connected though a network. In 
  this case, a single call on one repeater may block the entire network<footnote><para>
    There are means to implement this concept on analog repeater networks using tone-signaling 
    techniques (e.g., DTMF, five-tone etc.).</para></footnote>.
</para>

<para>
  Certainly, it would be much better if only those repeaters get activated that are actually 
  required for the communication between two parties. Then, the remaining repeaters are still 
  available for the rest of the staff. This routing, however, should happen automatically. An 
  operator may not know, which repeater to use to reach a particular person.
</para>

<para> 
  DMR was designed to implements such complex communication networks without requiring from every
  participant to have detailed knowledge about the structure of the network. That is, the knowledge 
  about where every repeater is installed and which participants are reachable on which repeater.
</para>

<note>
  <para>
    DMR is more similar to a phone network that to classic FM repeater networks.
  </para>
</note>

<para>
  Speaking of phone networks: Each participant and thus his/her radio is uniquely identified by a 
  number. The DMR-ID. This is a number between 1 and 16777215. And like for any other phone network,
  each participant may call any other directly using this number. This call is called 
  <glossterm>Private Call</glossterm>. 
</para>

<para>
  And there are groups. Each of these so-called <glossterm>Talk Group</glossterm>s is also assigned 
  a unique number. A talk group can be used to group all staff with a specific task (e.g., the 
  security, fire brigade, etc.). It is then possible to reach all members of this group at once, 
  by performing a <glossterm>Group Call</glossterm> to that talk group. The network, however, 
  does not know which participant is member of which group. Consequently, the radio of the participant 
  needs to know which group calls to accept and which to ignore.
</para>

<note>
  <para>
    This point is important to remember: The DMR network does not know which participant is member of 
    which group. The radio needs to be configured to react on specific group calls.
  </para>
</note>

<example xml:id="figIntroExNet1">
  <title>Simplified air-port network</title>
  <mediaobject>
    <imageobject><imagedata fileref="fig/trunk_net_ex1.png" width="10cm" align="center"/></imageobject>
  </mediaobject>
</example>

<para>
  There are 3 cleaning staff, two security and one headquarters. To cover the entire area, three 
  repeaters are required. One in terminal 1, one in Terminal 2 and one on the apron.
</para>

<para>
  <xref linkend="figIntroExNet1"/> is a simplified air port network (in reality, it is much larger 
  and way more complex). Consider the situation, where a cleaning stuff 1 &amp; 3 want to communicate.
  At the same time, the headquarters want to talk to security 1. In an simple analog network, the 
  call between cleaning 1 &amp; 3 would block the entire network and therefore the call between 
  the headquarters and security 1. 
</para>

<example xml:id="figIntroExNet2">
  <title>Simultaneous calls</title>
  <mediaobject>
    <imageobject><imagedata fileref="fig/trunk_net_ex2.png" width="10cm" align="center"/></imageobject>
  </mediaobject>
</example>
<para>
  Two Simultaneous private calls in the example network between cleaning 1 &amp; 3 as well as between 
  headquarters and security 1.
</para>

<para>
  Private calls in DMR networks only use those repeaters, that are actually required to establish 
  the communication. This is shown in <xref linkend="figIntroExNet2"/>: Cleaning 1 starts a private
  call to cleaning 3. As the DMR network knows that cleaning 3 was last active on the apron repeater, 
  this call gets routed only though repeaters terminal 1 and apron. The repeater in terminal 2, however, 
  is not affected. Consequently, this repeater remains available for the call between the headquarters
  and security 1.  
</para>  

<note>
  <para>
    The network only knows at which repeater each participant was last active. The network will therefore
    try to establish a connection though that repeater to the participant.
  </para>
</note>

<para>
  During the call between cleaning 1 &amp; 3, the repeaters in terminal 1 and apron are blocked. 
  This means, that the headquarters may not be able to reach cleaning 2 and security 2 immediately. 
  This sounds worse than it actually is. In contrast to classic phone networks, a direct call is 
  considered interrupted, once the calling participant releases the PTT button. To this end, the 
  headquarters may use the pauses between calls to reach the other participants. 
</para>

<para>
  In the next <xref linkend="figIntroExNet3"/>, the headquarters want to reach all cleaning staff. 
  Therefore, they start a group call to the talk group <quote>cleaning</quote> (<emphasis>C</emphasis> 
  for <quote>cleaning</quote> and <emphasis>S</emphasis> for <quote>security</quote>). With this 
  call, it can reach cleaning 1 &amp; 2 immediately. However, cleaning 3 does not receive that call.
</para>

<para>
  This is due to the fact, that the DMR network does not know which participants are members of 
  which groups. As the cleaning crew is usually not on the apron, the apron repeater has not 
  subscribed the talk group <quote>cleaning</quote>. Therefore, it does not forward group calls 
  to that talk group.
</para>

<example xml:id="figIntroExNet3">
  <title>Temporary subscription of talk groups.</title>
  <mediaobject>
    <imageobject><imagedata fileref="fig/trunk_net_ex3.png" width="10cm" align="center"/></imageobject>
  </mediaobject>
  <mediaobject>
    <imageobject><imagedata fileref="fig/trunk_net_ex4a.png" width="10cm" align="center"/></imageobject>
  </mediaobject>
  <mediaobject>
    <imageobject><imagedata fileref="fig/trunk_net_ex4b.png" width="10cm" align="center"/></imageobject>
  </mediaobject>
</example>

<para>
  To remain reachable for group calls, cleaning 3 needs to temporarily subscribe the apron repeater
  to the <quote>cleaning</quote> talk group. This can be done by starting a group call to that talk
  group on the apron repeater. Then the repeater will temporarily subscribe to that talk group for
  a limited amount of time (usually between 10-30min). During that time, the repeater will forward 
  group calls to that talk group and cleaning 3 remains reachable via that repeater.
</para>

<para>
  This temporary subscription will be renewed every time a participant starts a group call to that 
  talk group on this repeater. 
</para>

<para>
  With these examples, the most basic terms of DMR (DMR-ID, talk group, private and group calls as 
  well as talk group subscriptions where introduced and explained on an example network. The 
  following sections will concern the use DMR in ham radio.
</para>
</section>
