%\documentstyle[11pt,fullpage]{article}%\setlength{\parindent}{0 in}%\setlength{\parskip}{.1in}%\setlength{\topmargin}{-0.5in}%\setlength{\textheight}{8.5in}%\begin{document}\chapter{Satellite Networking in \ns}\label{chap:satellite}This chapter describes extensions that enable the simulation of satellitenetworks in \ns.  In particular, these extensions enable \ns~to modelthe following:  i) traditional geostationary ``bent-pipe'' satellites with multiple users per uplink/downlink and asymmetric links, ii) geostationary satellites with processing payloads (either regenerative payloads or full packet switching), and iii) polar orbiting LEO constellations such as Iridium and Teledesic.  These satellite models are principally aimed at using \ns~to study networking aspects of satellite systems; in particular, MAC, link layer, routing, and transport protocols.  %\paragraph{Notice (caveat emptor)} %This code (including perhaps the APIs at OTcl level) is likely to change %over the next few months (as of this writing in June 1999) as the \ns~%developers work on integrating the structure of satellite nodes, %wireless nodes, hierarchical nodes, etc.  In particular, we plan on%modifying the code to support mixed-node topologies (e.g., simulations%consisting of traditional \ns~nodes and satellite nodes) and running existing %unicast and multicast OTcl-based routing protocols.  \nam~~is %not currently supported with these extensions.%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%\section{Overview of satellite models}\label{sec:satellite/overview}Exact simulation of satellite networks requires adetailed modelling of radio frequency characteristics (interference, fading),protocol interactions (e.g., interactions of residual burst errors on the link with error checking codes), and second-order orbital effects (precession,gravitational anomalies, etc.).  However, in order to study fundamentalcharacteristics of satellite networks from a {\em networking} perspective,certain features may be abstracted out.  For example, the performance ofTCP over satellite links is impacted little by using an approximate rather than detailed channel model-- performance can be characterized to first orderby the overall packet loss probability.  This is the approach taken in thissimulation model-- to create a framework for studying transport, routing, and MAC protocols in a satellite environment consisting ofgeostationary satellites or constellations of polar-orbiting low-earth-orbit (LEO) satellites.  Of course, users may extend these modelsto provide more detail at a given layer.   %%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%\subsection{Geostationary satellites}\label{sec:satellite/overview/geo}Geostationary satellites orbit the Earth at an altitude of 22,300 miles above the equator.  The position of the satellites is specified in termsof the longitude of the nadir point (subsatellite point on the Earth'ssurface).  In practice, geostationary satellites can drift from theirdesignated location due to gravitational perturbations-- these effectsare not modelled in \ns.   Two kinds of geostationary satellites can be modelled.  Traditional``bent-pipe'' geostationary satellites are merely repeaters in orbit--all packets received by such satellites on an uplink channel are pipedthrough at RF frequencies to a corresponding downlink, and the satellite nodeis not visible to routing protocols.   Newer satellites willincreasingly use baseband processing, both to regenerate the digital signal andto perform fast packet switching on-boardthe spacecraft.  In the simulations, these satellites can be modelled more like traditional \ns~nodes with classifiers and routing agents.    Previously, users could simulate geostationary satellite links by simplysimulating a long delay link using traditional \ns~links and nodes.  Thekey enhancement of these satellite extensions with respect to geostationarysatellites is the capability to simulate MAC protocols.  Users can nowdefine many terminals at different locations on the Earth's surface andconnect them to the same satellite uplink and downlink channels, and thepropagation delays in the system (which are slightly different for eachuser) are accurately modelled.  In addition, the uplink and downlink channelscan be defined differently (perhaps with different bandwidths or error models).%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%\subsection{Low-earth-orbiting satellites}\label{sec:satellite/overview/leo}\begin{figure}    \centerline{\includegraphics{sat-constellation}}    \caption{Example of a polar-orbiting LEO constellation.  This figurewas generated using the SaVi software package from the geometry center at theUniversity of Minnesota.}    \label{fig:constellation}\end{figure}Polar orbiting satellite systems, such as Iridium and the proposed Teledesic system, canbe modelled in \ns.   In particular, the simulator supports the specificationof satellites that orbit in purely circular planes, for which the neighboring planes are co-rotating.There are other non-geostationary constellation configurations  possible (e.g., Walker constellations)-- the interested user may develop newconstellation classes to simulate these other constellation types.  Inparticular, this would mainly require defining new intersatellite link handoff procedures.The following are the parameters of satellite constellations that can currentlybe simulated:\begin{itemize}        \item {\bf Basic constellation definition} Includes satellite altitude,number of satellites, number of planes, number of satellites per plane.        \item {\bf Orbits} Orbit inclination can range continuouslyfrom 0 to 180 degrees (inclination greater than 90 degrees corresponds toretrograde orbits).  Orbit eccentricity is not modeled.  Nodal precession is not modeled.  Intersatellite spacing within a given plane is fixed.  Relativephasing between planes is fixed (although some systems may not control phasingbetween planes).        \item {\bf Intersatellite (ISL) links} For polar orbiting constellations,intraplane, interplane, and crossseam ISLs can be defined.  Intraplane ISLsexist between satellites in the same plane and are never deactivated or handed off.  Interplane ISLs exist between satellites of neighboring co-rotating planes.  These links are deactivated near the poles (abovethe ``ISL latitude threshold'' in the table) because the antenna pointing mechanism cannot track these links in the polar regions.  Like intraplane ISLs,interplane ISLs are never handed off.  Crossseam ISLs may exist in a constellation between satellites in counter-rotating planes (where the planes form a so-called ``seam'' in the topology).   GEO ISLs can also bedefined for constellations of geostationary satellites.        \item {\bf Ground to satellite (GSL) links}  Multiple terminalscan be connected to a single GSL satellite channel.  GSL links for GEO satellites are static, while GSL links for LEO channels are periodically handed off as described below.          \item {\bf Elevation mask} The elevation angle above which a GSL link can be operational.  Currently, if the (LEO) satellite serving a terminaldrops below the elevation mask, the terminal searches for a new satelliteabove the elevation mask.  Satellite terminals check for handoff opportunitiesaccording to a timeout interval specified by the user.  Each terminalinitiates handoffs asynchronously; it would be possible also to definea system in which each handoff occurs synchronously in the system.\end{itemize}The following table lists parameters used for example simulation scriptsof the Iridium\footnote{Asidefrom the link bandwidths (Iridium is a narrowband system only), theseparameters are very close to what a broadband version of the Iridium systemmight look like.}  and Teledesic\footnote{These Teledesic constellation parameters are subject to change; thanks to Marie-Jose Montpetit of Teledesic for providingtentative parameters as of January 1999.  The link bandwidths are notnecessarily accurate.} systems.\begin{table}[h]\begin{center}{\tt\begin{tabular}{|c||c|c|}\hline& {\bf Iridium} & {\bf Teledesic}\\\hline\hline{\bf Altitude} & \rm 780 km& \rm 1375 km\\\hline{\bf Planes} & \rm 6& \rm 12\\\hline{\bf Satellites per plane} & \rm 11 & \rm 24\\\hline{\bf Inclination (deg)} & \rm 86.4 & \rm 84.7\\\hline{\bf Interplane separation (deg)} & \rm 31.6 & \rm 15\\\hline{\bf Seam separation (deg)} & \rm 22 & \rm 15\\\hline{\bf Elevation mask (deg)} & \rm 8.2 & \rm 40\\\hline{\bf Intraplane phasing} & \rm yes & \rm yes\\\hline{\bf Interplane phasing} & \rm yes & \rm no\\\hline{\bf ISLs per satellite} & \rm 4  & \rm 8\\\hline{\bf ISL bandwidth} & \rm 25 Mb/s  & \rm 155 Mb/s\\\hline{\bf Up/downlink bandwidth} & \rm 1.5 Mb/s  & \rm 1.5 Mb/s\\\hline{\bf Cross-seam ISLs} & \rm no & \rm yes\\\hline{\bf ISL latitude threshold (deg)} & \rm 60 & \rm 60\\\hline\end{tabular}}\end{center}\caption{Simulation parameters used for modeling a broadband version ofthe Iridium system and the proposed 288-satellite Teledesic system.Both systems are examples of polar orbiting constellations.}\end{table}\clearpage%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%\section{Using the satellite extensions}\label{sec:satellite/usage}\begin{figure}    \centerline{\includegraphics{sat-spherical}}    \caption{Spherical coordinate system used by satellite nodes}    \label{fig:spherical}\end{figure}%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%%\subsection{Nodes and node positions}\label{sec:satellite/usage/nodes}There are two basic kinds of satellite nodes:  {\em geostationary}  and {\em non-geostationary} satellite nodes.  In addition, {\em terminal} nodescan be placed on the Earth's surface.  As is explained later in Section \ref{sec:satellite/implementation},each of these three different types of nodes is actually implemented with the same \code{class SatNode} object, but with different position,handoff manager,  and link objects attached.  The position object keeps track of the satellite node's location in the coordinate system as a function of the elapsed simulation time.This position information is used to determine link propagation delays andappropriate times for link handoffs. Figure \ref{fig:spherical} illustrates the spherical coordinate system,and the corresponding Cartesian coordinate system.The coordinate system is centered at the Earth's center, and the $z$ axis coincides with the Earth's axis of rotation.  $(R,\theta,\phi) = (6378 km, 90^o, 0^o)$ corresponds to $0^o$ longitude (prime meridian) on the equator.Specifically, there is one class of satellite node \code{Class Node/SatNode},to which one of three types of \code{Position} objects may be attached.  Each \code{SatNode} and \code{Position} object is a split OTcl/C++ object,but most of the code resides in C++.  The following types of position objects exist: \begin{itemize}\item \code{Position/Sat/Term} A terminal is specified by its latitude andlongitude.  Latitude ranges from $[-90, 90]$ and longitude ranges from$[-180, 180]$, with negative values corresponding to south and west, respectively.  As simulation time evolves, the terminals move alongwith the Earth's surface.  The  Simulator instproc \code{satnode} can be used to create a terminal with an attached position object as follows:\begin{program}$ns satnode terminal $lat $lon\end{program}\item \code{Position/Sat/Geo} A geostationary satellite is specified by its longitude above the equator.  As simulation time evolves, the geostationarysatellite moves through the coordinate system with the same orbital periodas that of the Earth's rotation.  The longitude ranges from $[-180,180]$degrees.  The Simulator instproc \code{satnode} can be used to create a geostationary satellite with an attached position object as follows:\begin{program}$ns satnode geo $lon\end{program}\item \code{Position/Sat/Polar} A polar orbiting satellite has a purelycircular orbit along a fixed plane in the coordinate system; the Earthrotates underneath this orbital plane, so there is both an east-west anda north-south component to the track of a polar satellite's footprint onthe Earth's surface.  Strictly speaking, the polar position object canbe used to model the movement of any circular orbit in a fixed plane;  we use the term ``polar'' here because we later use such satellites to model polar-orbiting constellations.Satellite orbits are usually specified by six parameters:  {\em altitude},