The role of communications satellites in computer networking is simple in concept (just another layer 1 & 2 protocol) and exotic in execution (...3, 2, 1 blastoff!). In this module we will discuss some satellite concepts in general and then get down to the physical and link layer protocols in particular.
First lets define a few terms:
An orbit is characterized by two attributes:
There are three altitudes of interest:
There are three orbital inclinations of interest:
These attributes are illustrated in the diagram below:
Each of the three orbital altitudes has communications applications as tabulated below. There is a particularly good web site produced by NASA that provides graphic annimations of the orbital paths of all unclassified satellites. Click here to take a look (NASA, 2000) .
|Altitude||100-300 miles||6000-12000 miles||22,282 miles|
|Rotation Period||90 minutes||5-12 hours||24 hours|
|Time in Sight||15 minutes||2-4 hours||Always|
In this course we will focus on geostationary satelltes; i.e., those that appear to be fixed in space overhead. They are particularly convenient in that they do not require tracking antennas.
In order to maintain that "bent pipe" in the sky, communications satellites need six major on-board subsystems:
Each subsystem is described below.
Geostationary satellites are so far from the earth that they need directional antennas in order to communicate. A directional antenna uses a parabolic reflector (commonly referred to as a dish) to focus the radiated energy from the transmitter (analogous to a flashlight) and to focus the incoming energy to the receiver (analouous to a telescope mirror). This ability to focus energy is referred to as "antenna gain."
The illustration below uses the flashlight analogy to illustrate the concept of antenna gain. Consider a flashlight bulb illuminating an otherwise dark room. It will illuminate the room uniformly in every direction, but nowhere in the room will there be enough light to read by. This is illustrated under "Without Antenna Gain" below where we have three "light beams" hitting the wall in the location where we would like to be reading.. Now let us place a flashlight style reflector behind the bulb to focus most of the energy radiated by the bulb in the same direction. Now we have nine light beams hitting the wall (enough to read by). This is illustrated under "With Antenna Gain" below. We can calculate the "gain" provided by the reflector (antenna) as 9/3 = a gain of 3.
Without Antenna Gain
With Antenna Gain
The communications subsystem is the reason communications satellites exist. Consisting of an array of transponders sharing uplink (Rx) and downlink (Tx) antennas, the communications subsystem is the "bent pipe" referred to in the overview. Each transponder consists of three components:
All these transponders need electricity to operate. Power is supplied by on-board batteries that are recharged by solar radiation.
In order to be sure the communications subsystem is operating correctly it is necessary for ground based personnel to be able to monitor its operational status. This is the function of the telemetry subsystem. As illustrated below, an on-board computer with links to all satellite systems provides this status data (called telemetry data in space-talk) to a special telemetry downlink. Based on this information it may be necessary to take corrective action from the ground; e.g., if a transponder fails it is important to be able to switch in a spare ASAP (each transponder generates between $100,000 and $200,000 per month in lease revenue). This is done from the ground via a command uplink that provides a communication path to the on-board computer.
In order for a geostationary communications satellite to continue to function, it must remain stationary with respect to all the earth station antennas that are pointed at it. To correct for the orbital perturbations that all satellites are subject to, each satellite carries a thrust subsystem to give it an occasional nudge to keep it "On Station."
Remaining on-station is only half the battle. Additionally, the satellite's antennas must always be aimed at the same spot on the surface. This requires gyroscopic stabilization of the satellite body. This is accomplished with gyroscopes in one of two configurations:
Whenever a customer is spending significant money to lease a communications transponder, it is reasonable to expect some quantitative assurance of system availability. There are four real and/or potential causes of satellite communications outages and they are summarized in the table below.
|Eclipses||Sun transit outages||Satellite MTBF||Rain|
We will describe the most common satellite communications protocols as before:
We will assume the system model illustrated below. The application will be a gasoline station at which the customer can pay via credit card at the pump. One of many company gas stations is illustrated on the left side of the diagram below with a LAN connecting the gas pumps to an earth station on the roof of the station. The corporate credit approval server is on the right side of the space link.
The space link occupies the physical and link layers as shown below. the link layer is split into Logical Link Control (LLC - the same as we discussed in the LAN module) and one of two media access control protocols:
|Transport||UDP (User Datagram Protocol)||TCP (Transmission Control Protocol)|
|Network||IP (Internet Protocol)|
For purposes of discussion lets assume three gas stations bearing MAC addresses 1 - 3, each having a VSAT antenna on the roof. The corporate credit approval server is located at a facility called a Hub. Communication from the hub to the population of gas stations (for credit card approvals) is via a TDM frame uplinked from the hub to the satellite, and downlinked from the satellite to the entire country to be received by all of the gas stations in the population. The TDM frame consists of three slots numbered 1 - 3 that correspond to each of the three gas stations. This model is illustrated in the diagram below.
It is also necessary to establish communication from each of the gas stations to the hub (for customers' credit card numbers and sales data). This is accomplished by assigning each gas station a recuring time slot in which it can turn on its transmitter and uplink data to the satellite for subsequent downlinking to the hub. All stations transmit on the same frequency, but they don't collide with each other because the time slots are non-overlaping.
Both of these protocols execute simultaneously thereby providing full duplex communication between the credit card server and the gas pumps.
The network is illustrated in the form of OSI reference model stacks. The credit approval server is on the right, and one representative gas pump is on the left. The next two diagrams illustrate the credit solicitation path and the credit approval path respectively.
The Hughes product known as DirecPC provides an excellent case study in the real world application of a satellite link in an internet. It's functionality is described step-by step below.
Conventional ISP Service
Hughes Communications, Inc. (1996) Web Station. Available HTTP http://www.hcisat.com/index.html
Hughes Network Systems. (1997). Available HTTP http://www.hns.com/
Helius, Inc. (1997). Expanding Network Horizons. Available HTTP http://www.helius.com/
Lyngemark, C. (1998) SATCO DX Satellite Chart. Available HTTP http://www.satcodx.com/index.html
Mustafa Mir, Rizwan. (1997). Satellite Data Networks. Available HTTP http://www.cis.ohio-state.edu/~jain/cis788-97/satellite_data/index.htm Mirrored here
Irridium LLC. (1999). Calling Planet Earth. Available HTTP http://www.iridium.com/
NASA. (2000). Liftoff to Space Exploration. Available HTTP http://liftoff.msfc.nasa.gov/realtime/jtrack/3d/JTrack3d.html
Copyright © 2002. All rights reserved. Author: Thomas Naugler