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<html><!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 3.2 Final//EN"><!--Converted with jLaTeX2HTML 2002 (1.62) JA patch-1.4patched version by: Kenshi Muto, Debian Project.LaTeX2HTML 2002 (1.62),original version by: Nikos Drakos, CBLU, University of Leeds* revised and updated by: Marcus Hennecke, Ross Moore, Herb Swan* with significant contributions from: Jens Lippmann, Marek Rouchal, Martin Wilck and others --><HTML><HEAD><TITLE>17.2.3 Handoffs </TITLE><META NAME="description" CONTENT="17.2.3 Handoffs "><META NAME="keywords" CONTENT="everything"><META NAME="resource-type" CONTENT="document"><META NAME="distribution" CONTENT="global"><META HTTP-EQUIV="Content-Type" CONTENT="text/html; charset=iso-8859-1"><META NAME="Generator" CONTENT="jLaTeX2HTML v2002 JA patch-1.4"><META HTTP-EQUIV="Content-Style-Type" CONTENT="text/css"><LINK REL="STYLESHEET" HREF="everything.css" tppabs="http://www.isi.edu/nsnam/ns/doc/everything.css"><LINK REL="next" HREF="node203.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node203.html"><LINK REL="previous" HREF="node201.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node201.html"><LINK REL="up" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html"><LINK REL="next" HREF="node203.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node203.html"></HEAD><BODY ><!--Navigation Panel--><A NAME="tex2html4390" HREF="node203.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node203.html"><IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="file:/usr/share/latex2html/icons/next.png"></A> <A NAME="tex2html4384" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html"><IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="file:/usr/share/latex2html/icons/up.png"></A> <A NAME="tex2html4378" HREF="node201.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node201.html"><IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="file:/usr/share/latex2html/icons/prev.png"></A> <A NAME="tex2html4386" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html"><IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="file:/usr/share/latex2html/icons/contents.png"></A> <A NAME="tex2html4388" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html"><IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index" SRC="file:/usr/share/latex2html/icons/index.png"></A> <BR><B> Next:</B> <A NAME="tex2html4391" HREF="node203.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node203.html">17.2.4 Routing</A><B> Up:</B> <A NAME="tex2html4385" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html">17.2 Using the satellite</A><B> Previous:</B> <A NAME="tex2html4379" HREF="node201.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node201.html">17.2.2 Satellite links</A>   <B> <A NAME="tex2html4387" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html">Contents</A></B>   <B> <A NAME="tex2html4389" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html">Index</A></B> <BR><BR><!--End of Navigation Panel--><H2><A NAME="SECTION031423000000000000000"></A><A NAME="sec:satellite_usage_handoffs"></A><BR>17.2.3 Handoffs </H2><P>Satellite handoff modelling is a key component of LEO satellite network simulations. It is difficult to predict exactly how handoffs will occurin future LEO systems because the subject is not well treated in theliterature. In these satellite extensions, we establish certain criteria for handoffs, and allow nodes to independently monitor for situations that require a handoff. An alternative would be to have all handoff eventssynchronized across the entire simulation- it would not be difficult to change the simulator to work in such a manner.<P>There are no link handoffs involving geostationary satellites, but there are two types of links to polar orbiting satellitesthat must be handed off: GSLs to polar satellites, and crossseam ISLs. A third type of link, interplane ISLs, are not handed off but are deactivatedat high latitudes as we describe below.<P>Each terminal connected to a polar orbiting satellite runs a timer that,upon expiry, causes the <TT>HandoffManager</TT> to check whether the current satellite has fallen below theelevation mask of the terminal. If so, the handoff manager detaches theterminal from that satellite's up and down links, and searchesthrough the linked list of satellite nodes for another possible satellite.First, the ``next'' satellite in the current orbital plane is checked- a pointer to this satellite is stored in the Position object of eachpolar satellite node and is set during simulation configuration usingthe <TT>Node/SatNode</TT> instproc ``<TT>$node set_next $next_node</TT>.''If the next satellite is not suitable, the handoff manager searchesthrough the remaining satellites. If it finds a suitable polarsatelite, it connects its network interfaces to that satellite's uplink and downlink channels, and restarts the handoff timer. If it does not find a suitablesatellite, it restarts the timer and tries again later. If any linkchanges occur, the routing agent is notified.<P>The elevation mask and handoff timer interval are settable via OTcl: <PRE>HandoffManager/Term set elevation_mask_ 10; # degreesHandoffManager/Term set term_handoff_int_ 10; # seconds</PRE>In addition, handoffs may be randomized to avoid phase effects by settingthe following variable:<PRE>HandoffManager set handoff_randomization_ 0; # 0 is false, 1 is true</PRE>If <TT>handoff_randomization_</TT> is true, then the next handoff intervalis a random variate picked from a uniform distribution across<!-- MATH $(0.5 * term\_handoff\_int\_, 1.5 * term\_handoff\_int\_)$ -->. <P>Crossseam ISLs are the only type of ISLs that are handed off. The criteriafor handing off a crossseam ISL is whether or not there exists a satellitein the neighboring plane that is closer to the given satellite than theone to which it is currently connected. Again, a handoff timer runningwithin the handoff manager on the polar satellite determines when theconstellation is checked for handoff opportunities. Crossseam ISLhandoffs areinitiated by satellites in the lower-numbered plane of the two. It istherefore possible for a transient condition to arise in which a polarsatellite has two crossseam ISLs (to different satellites). Thesatellite handoff interval is again settable from OTcl and may also berandomized:<PRE>HandoffManager/Sat set sat_handoff_int_ 10; # seconds</PRE><P>Interplane and crossseam ISLs are deactivated near the poles, because the pointing requirements for the links are too severe as the satellitedraw close to one another. Shutdown of these links is governed by a parameter:<PRE>HandoffManager/Sat set latitude_threshold_ 70; # degrees</PRE>The values for this parameter in the example scripts are speculative;the exact value is dependent upon the satellite hardware. The handoffmanager checks the latitude of itself and its peer satellite upon a handofftimeout; if either or both of the satellites is above <TT>latitude_threshold_</TT> degrees latitude (north or south), the linkis deactivated until both satellites drop below this threshold.<P>Finally, if crossseam ISLs exist, there are certain situations in whichthe satellites draw too close to one another in the mid-latitudes (ifthe orbits are not close to being pure polar orbits). We check forthe occurence of this orbital overlap with the following parameter:<PRE>HandoffManager/Sat set longitude_threshold_ 10; # degrees</PRE>Again, the values for this parameter in the example scripts are speculative.If the two satellites are closer together in longitude than <TT>longitude_threshold_</TT> degrees, the link between them is deactivated.This parameter is disabled (set to ) by default- all defaults forsatellite-related bound variables can be found in tcl/lib/ns-sat.tcl.<P><HR><!--Navigation Panel--><A NAME="tex2html4390" HREF="node203.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node203.html"><IMG WIDTH="37" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="next" SRC="file:/usr/share/latex2html/icons/next.png"></A> <A NAME="tex2html4384" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html"><IMG WIDTH="26" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="up" SRC="file:/usr/share/latex2html/icons/up.png"></A> <A NAME="tex2html4378" HREF="node201.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node201.html"><IMG WIDTH="63" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="previous" SRC="file:/usr/share/latex2html/icons/prev.png"></A> <A NAME="tex2html4386" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html"><IMG WIDTH="65" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="contents" SRC="file:/usr/share/latex2html/icons/contents.png"></A> <A NAME="tex2html4388" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html"><IMG WIDTH="43" HEIGHT="24" ALIGN="BOTTOM" BORDER="0" ALT="index" SRC="file:/usr/share/latex2html/icons/index.png"></A> <BR><B> Next:</B> <A NAME="tex2html4391" HREF="node203.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node203.html">17.2.4 Routing</A><B> Up:</B> <A NAME="tex2html4385" HREF="node199.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node199.html">17.2 Using the satellite</A><B> Previous:</B> <A NAME="tex2html4379" HREF="node201.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node201.html">17.2.2 Satellite links</A>   <B> <A NAME="tex2html4387" HREF="node1.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node1.html">Contents</A></B>   <B> <A NAME="tex2html4389" HREF="node590.html" tppabs="http://www.isi.edu/nsnam/ns/doc/node590.html">Index</A></B> <!--End of Navigation Panel--><ADDRESS>2003-09-23</ADDRESS></BODY></HTML>⌨️ 快捷键说明
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