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			Home &gt; Graph Theory: Networking
					&gt; Telephone Networks
					&gt; DNHR</div>
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    <div id="content"><h1>DNHR</h1>
				<h2>Telephone Networks: Dynamic Non-Hierarchical Routing (DNHR)</h2>
				<p>
				A major advance in telephone routing was the development of
				<i>dynamic non-hierarchical routing (DNHR)</i>.  DNHR
				makes use of the <i>statistical predictability</i> of aggregated
				telephone traffic and the fact that switches and links are usually
				available to select two-hop paths when the shortest one-hop path is
				blocked.
				</p>
				<p>
				DNHR divides the day into a number of periods (usually 10 or so), and
				for each period, each toll switch is assigned:
				</p>
				<ol><li>
					A <b>primary</b> one-hop path to another toll switch,
					and
					</li><li>
					An ordered set of <b>alternative</b>, two-hop paths.
					</li></ol>
				<h2>The Routing Algorithm</h2>
				<ol><li>
					Incoming calls are forwarded on the primary path.
					</li><li>
					If the resources are already in use and cannot be delegated to the current call 
					(this situation is termed <i>spilling</i> or 
					<i>overflow</i>), try the alternative two-hop path according to the 
					pre-computed order for the current period 
					(known as the <i>crank-back</i>).
					</li><li>
					If all alternative paths are busy, the call is rejected.
					</li></ol>
				<div class="box">
					<b><i>Warning!</i></b>
					
					<p>
					DNHR's performance may suffer if the traffic changes suddenly, so that
					the list of alternative paths ought to be changed (but is not).  Any
					unexpected traffic change may increase the load on some trunk groups
					while leaving other trunks underutilized.
					</p>
					
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