Effects on TCP of Routing Strategies in Satellite Constellations
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SATELLITE-BASED INTERNET TECHNOLOGY AND SERVICES Effects on TCP of Routing Strategies in Satellite Constellations Lloyd Wood, George Pavlou, and Barry Evans, University of Surrey ABSTRACT number of satellites required to cover the Earth. A lower altitude decreases free space A broadband satellite network uses a constella- loss and propagation delay, but means that the tion of a number of similar satellites to provide service each satellite can offer is limited to wireless networking services to the Earth. A users in a smaller visible area of the ground number of these constellation networks are (the satellite’s footprint). To fully cover the under development. This article introduces the globe, more satellites are needed. This increas- types of satellite constellation networks, and es frequency reuse and overall system capacity, examines how overall performance of TCP com- but will also increase overall system construc- munications carried across such a network can tion and maintenance costs. Satellites at lower be affected by the choice of routing strategies altitudes must move faster relative to the used within the network. Constellations utilizing ground to stay in their orbits, increasing the rate direct intersatellite links are capable of using of handoff and Doppler effects between termi- multiple paths between satellites simultaneously nals and satellites. as a strategy to spread network load. This allows Most proposed systems use circular orbits more general routing strategies than shortest- with constant altitudes, since this means that path routing, but we show these strategies to be satellite overhead pass times and power levels detrimental to the performance of individual needed for communication are constant, and TCP connections. each satellite can be used for traffic throughout its orbit. Other systems have been proposed NTRODUCTION using multiple elliptical orbits, where satellites I are only intended to be available for service Broadband satellite constellation networks have while moving relatively slowly around high-alti- been proposed and are now being developed. tude apogee (e.g., the commercial Ellipso and These will provide medium- and high-capacity Pentriad proposals). In those elliptical systems wireless data services, and will interconnect with the communication payloads are unused existing telco backbones and the Internet. Once throughout the rest of the orbit. launched and operational, proposed networks, Figure 1 compares the altitudes of the sys- such as Teledesic, Spaceway, and Skybridge, can be tems discussed above. expected to transport significant amounts of TCP traffic between ground endhosts on the Internet. EOMETRY OPOLOGY AND ELAY Although the use of satellite constellations G , T , D for delivering broadband IP services is new, From a networking viewpoint, there are two fun- satellite-constellation-based systems are already damentally different approaches taken by these being used for applications such as providing: broadband constellations. Each constellation • Navigation and position information: Glob- consists of a number of similar satellites, where al Positioning System (GPS) and the satellite design is based on one of the follow- GLONASS ing approaches: • Voice telephony and low-bit-rate communi- • Each satellite is a space-based retransmitter cation services (although not financially suc- of traffic received from user terminals and cessful, the Iridium and Globalstar systems local gateways below it on the ground, have made service available, and ICO Glob- returning the traffic to the ground. This al Communications is to follow) allows isolated user terminals to exchange • Low-bit-rate messaging data services for a traffic with nearby ground stations that are variety of applications (e.g., Orbcomm) gateways into the terrestrial network. The The altitude of the satellites in the constella- satellite forms a wireless last hop to an tion is a significant factor in determining the extensive ground network. Commercial sys- 172 0163-6804/01/$10.00 © 2001 IEEE IEEE Communications Magazine • March 2001 tems taking this approach include Global- star and many geostationary satellites, and the planned ICO and Skybridge. Molnya elliptical orbit • Each satellite is a network switch that is also Pentriad, Russian television able to communicate with neighboring satel- (useful at apogee) lites by using radio or laser intersatellite links (ISLs). This allows user terminals on the ground below the satellite to exchange traffic Global Positioning with gateways to the terrestrial network or System (GPS) users below distant satellites, without requir- Glonass ing a local gateway or significant terrestrial infrastructure to do so. Commercial systems Teledesic, Skybridge, utilizing ISLs include the deployed Iridium Globalstar and the proposed Teledesic, Hughes Space- way, and Astrolink networks. Proposals with circular orbits are the most Concordia Earth Ellipso common, while proposals using ISLs are the Borealis most interesting and complex from a networking Iridium, Orbcomm and systems viewpoint, so we will focus on those. Low Earth Orbit (LEO) Geostationary satellites can be interconnect- ed to form a simple ring network around the ICO, Earth’s equator, as shown in Fig. 2a for the min- Spaceway NGSO imum three geostationary satellites needed to Medium Earth Orbit (MEO) cover most of the Earth’s surface. At low- and medium-earth-orbit altitudes the ISLs between Geostationary orbital ring (GEO) Spaceway, Astrolink, DirecPC, VSAT, satellites form a more complex dynamic mesh 0 10,000 kilometres Inmarsat, Intelsat, television etc. topology. That topology is a variation of the regions of Van Allen radiation belt peaks (high-energy protons) Manhattan network, named after the grid of orbits are not shown at actual inclination; this is a guide to altitude only streets in Manhattan, which is well-known in I theoretical computer networking for use in par- Figure 1. Orbits used by satellite constellations. allel computer architectures. Depending on the specific orbital geometry chosen, this overall satellite topology will be either: ground terminals over the course of a day, as • A fully toroidal network, created from an the Earth rotates beneath the moving satellites. inclined Walker delta or Ballard rosette con- Here, we see smooth changes in delay due to stellation [1], where the ascending (moving the orbital motion of satellites, as well as abrupt northward) and descending (moving south- changes due to handoffs and path changes; ward) planes overlap and span the full 360˚ these changes are small compared to the granu- of longitude; for example, the Hughes larity of a TCP timer. Spaceway NGSO MEO proposal (Fig. 2b). Network latency results from queuing, switch- This geometry offers the best satellite visi- ing, and link-layer medium access control (MAC) bility and highest degree of satellite capaci- bandwidth allocation as well as from serialization ty over the populated mid-latitudes. and propagation delays. With the exception of the • A form of cylindrical mesh network, from a total path propagation delay shown in Fig. 3i, near-polar Walker star constellation [2], where the speed of light is a common constraint, where the interconnected ascending and these contributing delays will vary considerably descending orbital planes of satellites each according to the specific system design. As a first cover around 180˚ of longitude and are approximation, these delays can be thought of as separate. Systems using this approach proportional to the number of wireless links tra- include Iridium and the proposed Teledesic versed, indicated in Fig. 3ii. (Teledesic’s redun- system (a proposed Teledesic design is dant “geodesic” eight-link design, shown in Fig. shown in Fig. 2c). This geometry offers the 2c, minimizes delay and the number of links tra- best satellite visibility at the poles. versed, while also providing the two ISLs needed In a near-polar star constellation, the gap for frequent smooth acquisition and handoff of a between the counter-rotating ascending and single link across the seam.) descending planes, shown in the Teledesic Examining the shortest-path propagation delay design in Fig. 2c, is known as the counter-rotat- across a network provides a best-effort goal with ing seam. Cross-seam ISLs, which interconnect which real-world performance could be compared. satellites at the edges of the cylindrical mesh to However, relying on a metric of minimum propa- form a seamless spherical network, must be gation delay for cost-based routing will tend to handed off frequently as the planes of satellites load interplane links at the highest latitudes, where move past each other. These cross-seam links distances between satellites are shortest. have not yet been demonstrated in practice; Without cross-seam links, the seam increases Iridium, the only system constructed using ISLs both delay and the number of links traversed to date, has none. when it intersects the shortest path between However, cross-seam ISLs do have a consid- ground terminals. The seam alters the shortest erable effect on network performance. Figure 3 path between the terminals so that the path shows the total propagation delay experienced lengthens and lies over the highest latitudes. The by traffic taking the shortest path across the duration of this disruption is proportional to constellation networks between two fixed separation in longitude between the terminals, IEEE Communications Magazine • March 2001 173 a) Minimum three-satellite Clarke