LEO NETWORK COMMUNCATIONS Trina Dobson :: Paul Woolaver :: Bob Whynot

INTRODUCTION Need for LEOs. Brief history. Characteristics. Design issues. Types of LEOs. Major players. Teledesic example.

WHY LEO? Because most of the Earth’s surface has no access to terrestrial communication lines. Because GEOs cannot facilitate real- time applications. Because of an increasingly mobile workforce.

BRIEF HISTORY 1984: U of Surrey launches UoSAT : LEO network concept conceived by Motorola. 1988: Teledesic system conceived. 1990: Motorola plans to build Iridium. 1992: WARC : FCC starts granting licenses to LEO companies 1997: Iridium launches 41 satellites. 1998: Motorola invests 750 M in Teledesic.

CHARACTERISTICS 1/3 the delay of GEOs (and dropping). More in common with Terrestrial based cellular service. Autonomous network. Dynamic topology. Cheap compared to GEOs. The great packet switching network in the sky.

DESIGN ISSUES Delay needs to be minimized. Tail charges need to be minimized. Processing needs to be minimized. Battery charge is limited. No upgrades after launch. Routing algorithms. Placement of ground terminals.

TYPES OF LEOS Little LEO and Big LEO. Little LEO: –Slow data communications. –Paging/store-and-forward, messaging. –Communicate directly with ground stations. –Weigh 50 to 100 kg. –Frequencies: Downlink: 137 to 138 and to 401 MHz Uplink: 148 to MHz.

TYPES OF LEOS Big LEO (Broadband LEO) –High speed communication. –Voice, Data, Video, etc. –Generally Utilize ISLs. –Smart nodes. –Weigh 350 – 500 kg. –Frequencies: 1610 – MHz.

MAJOR PLAYERS Teledesic Iridium (???) Skybridge Globalstar Motorola Lockheed Boeing

TELEDESIC EXAMPLE 288 satellite constellation. Utilize ISLs (Intersatellite Links) via lasers. Each satellite makes a complete orbit in 100 minutes. Use of unique ATM- like protocol.

TELEDESIC Plans to interface with IP, ISDN and ATM. Communication within the network is treated as streams of short, fixed-length packets: –HEADER: destination address and sequence info –ERROR CONTROL: verify the integrity of the header –PAYLOAD: digitally encoded user data (voice, video, data, etc.) Conversion to and from packet form takes place on the edge of the network. Frequency license: –DOWNLINK: 18.8 GHz to 19.3 GHz –UPLINK: 28.6 GHz to 29.1 GHz

TELEDESIC Plans to offer two different connections: –PACKAGE 1: 64 Mbps on the downlink and 2 Mbps on the uplink. –PACKAGE 2: 64 Mbps full duplex. Teledesic estimate that an antenna will have to switch satellites every 4 minutes! Routing algorithms originally used to manage congestion in ATM networks will be used frequently in the Teledesic system. Using ISLs Teledesic hopes to reduce delay from 100 to 40 milliseconds.