Wireless Communication: Satellites

1. Wireless Communication:Satellites

2. Wireless Transmission • Directional • Focuses electromagnetic beam in direction of receiver • Terrestrial microwave • Satellite microwave • Omni directional • Spreads the electromagnetic signal in all directions • AM and FM radio • 3G networks • Smart watches

3. Terrestrial Microwave • Parabolic dish antenna sends signal to receiving dish • Line-of-sight • Typically on towers to avoid obstacles • Frequencies in the gigahertz range

4. What is a telecommunications satellite?

5. Telecommunications satellites • Space-based cluster of radio repeaters (called transponders) • Link • terrestrial radio transmitters to satellite receiver (uplink) • Satellite transmitters to terrestrial receivers (downlink)

6. Orbits • Mostly geostationary (GEO) • Circular orbit • 22,235 miles above earth • Fixed point above surface • Almost always a point on Equator • Must be separated by at least 4 degrees

7. Satellite services • Wide Area Broadcasting • Single transmitter to multiple receivers • Wide Area Report-Back • Multiple transmitters to a single receiver • Example VSATs (very small aperture terminals) • Also have microwave transmitters and receivers • Allows for spot-beam transmission (point- to-point data communications) • Can switch between beams upon request (Demand Assigned Multiple Access –DAMA) • Multi-beam satellites link widely dispersed mobile and fixed point users

8. Earth-based equipment • Original microwave transmitters and receivers were large installations • Dishes measuring 100 feet in diameter • Modern antennas about 3 feet in diameter

9. A Modern GEO satellite (IntelSat 900 series) • May have more than 72 separate microwave transponders • Each transponder handles multiple simultaneous users (protocol called Time Division Multiple Access) • Transponder consists of • Receiver tuned to frequency of uplink • Frequency shifter (to lower frequency to that of transmitter) • Power amplifier

10. IntelSat 902 (launched August 30, 2001)

11. Frequency ranges • Most transponders operate in 36MHz bandwidth • Use this bandwidth for • voice telephony (400 2-way channels/transponder) • Data communication (120Mbs) • TV and FM Radio

12. C-band, Ku-band, Ka-band • Most GEO satellites operate in the C-Band frequencies • Uplink at 6 GHz • Downlink at 4 GHz • Ku-band also used • Uplink at 14 GHz • Downlink at 11 GHz • Above bands best suited for minimal atmospheric attenuation • Few slots left… forcing companies to look at Ka band (uplink:30 GHZ , downlink: 20 GHz)

13. MEO Satellites • Exist between the first and second Van Allen Radiation belts • Peak height is ~ 9000 miles\ • Typical is about 4000 miles • Need less power than GEO satellites to reach. • Example GPS satellites

14. Global Positioning Systems • A constellation of 24 DoD satellites orbiting about 10,000 miles above earth’s surface • First launched in 1978; complete set by 1994; replaced every ten years or so.. • Solar-powered; Each circles earth about twice a day • Also have 5 ground stations (control segments) • monitor the GPS satellites, checking both their operational health and their exact position in space. • Five monitor stations: Hawaii, Ascension Island, Diego Garcia, Kwajalein, and Colorado Springs.

15. GPS Constellation

16. How they work • To determine position • GPS satellites emit 3 bits of information in its signal (L1 for civilians; L2 for military): • Pseudorandom code (ID which identifies specific satellite) • Ephemeris data (status of satellite and current data and time) • Almanac data (tells exactly where that satellite and all others are supposed to be at any given time during the day) • Finding your location • Compare time a signal is transmitted to when it is received – tells how far away satellite is… receiver knows it is on the surface of an imaginary sphere centered around the GPS satellite • With similar distance measurements from other satellites, receiver can determine location (intersection of at least three spheres) • GPS receiver must lock on to 3 satellites to give 2D location; 4 satellites to give altitude as well. • Accurate up to ~10-15 meters; DGPS and Augmented GPS can go down to a few centimeters.

17. Sources of Error for GPS • Ionosphere and troposphere delays — The satellite signal slows as it passes through the atmosphere. • Signal multipath — This occurs when the GPS signal is reflected off objects such as tall buildings or large rock surfaces before it reaches the receiver. This increases the travel time of the signal, thereby causing errors. • Receiver clock errors — A receiver's built-in clock is not as accurate as the atomic clocks onboard the GPS satellites. Therefore, it may have very slight timing errors. • Orbital errors — Also known as ephemeris errors, these are inaccuracies of the satellite's reported location. • Number of satellites visible — The more satellites a GPS receiver can "see," the better the accuracy. Buildings, terrain, electronic interference, or sometimes even dense foliage can block signal reception, causing position errors or possibly no position reading at all. GPS units typically will not work indoors, underwater or underground. • Satellite geometry/shading — This refers to the relative position of the satellites at any given time. Ideal satellite geometry exits when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping. • Intentional degradation of the satellite signal — Selective Availability (SA) is an intentional degradation of the signal once imposed by the U.S. Department of Defence. The government turned off SA in May 2000, which significantly improved the accuracy of civilian GPS receivers. Source: http://www.pocketgps.co.uk/howgpsworks.php

18. LEO Satellites • Lowest of the satellites – below the first radiation belt • Typically orbit at ~600 miles • Much less power needed than for GEO and MEO • Can be accessed using smaller devices such as phones. • Available anywhere in the world. • Geostationary?

19. Companies on the forefront: Teledesic • Offer “Internet-in-the-Sky” • Main shareholders Craig McCaw and Bill Gates • McCaw also has taken over ICO Global Communications • Wanted Iridium but has backed out

20. Teledesic • Again, series of LEO satellites • 24 pole orbiting satellite rings, 15 degrees apart • 12 satellites in each ring (total = 288 LEO satellites) • Worldwide switching.. Satellites pass on data through laser • Will map IP packets on latitudes and longitudes .. Average will be 5 satellite hops in 75 ms • Supposed to start in 2002; offer 2Mbps Internet access from terminals starting at $1000 each • Postponed to 2005

21. Optical Transmission • Cutting edge • Uses modulated monochromatic light to carry data from transmitter to receiver • Optical wavelengths are suited for high rate broadband communications • Laser-based (up to 1000 times faster than coaxial)

22. Other landline transmission paths

23. T-Carrier Lines • Dedicated telephone line • T1 carries data at about 1.544 Mbps • Each T1 is broken down into 24 channels of 64Kbps each • Each channel can carry either data or voice • T3 can go up to 44.736 Mbps (672 channels)

24. Cable Modems • Designed to work over cable lines (HFC- hybrid fiber coaxial) • Speed is about 10Mbps • Process • Coaxial cable has enough free bandwidth • IP packets modulated and sent to user’s PC • Signal hits splitter that shunts data to modem • Cable modem demodulates into Ethernet packets • Slower on the upload • Users share bandwidth • Comparison - download 857 pages of Moby Dick • Cable Modem: all 857 pages in ~ 2 seconds • 56K Modem: about 3 pages in 2 seconds

25. Digital Subscriber Lines (DSL) • Pumps data at high rates to PCs using ordinary copper lines. • Based on the 4KHz frequency cut • North American DSL market reaches 4.7 million (11/27/2001) – Telechoice survey

26. Flavors of DSL • Referred to as xDSL • ADSL (asymmetric) • Approximately 8Mbits/sec download • Maximum of 640Kbits/sec upload • SDSL (symmetric) • Equal rates for upload and download (~ 1.5Mbits/sec) • VDSL (Very high) • Up to 55 Mbits/sec • Only 1000’ from telco

27. Wireless Data Communication Networks • High frequency radio waves… mostly for mobile users • Send and receive data on a LAN or via fax, email, Internet • Services include • Cellular Digital Packet Data • Packet Radio Systems • Personal Communication Systems

28. Data Transport Networks • connect variety of computers and other devices • could be devices in same building • local area networks • could be devices in different countries • packet switching networks vs. circuit switching

29. Packet Switching Network Host DC Host node Host Berlin NY node node Cairo PADs Host node

30. X.25 Protocol (56K-64K bps) • Popular protocol for PSNs in the 1970s • Relatively slow… runs on 56K lines • Packet Switched technology • File broken down into discrete packets before being transmitted • Packets traverse different paths , at different times before being reassembled at destination • Efficient in apportioning bandwidth based on availability • Inefficient in that error control information is also saved … unnecessary if network clean

31. Frame Relay (56K-45M bps) • Dedicated, packet-switched service • Sends data in variable length packets – also called frames • Variable length makes it efficient • Works best when a few branches/subsidiaries need to share files with each other

32. International Frame Relay • High speed packet-switching protocols in WANs that span countries • Variable length packets… best suited for data and images… not for voice or video • At highest speeds, can be used for real-time data

33. International Frame Relays contd. • Cuts costs of connections to foreign countries • Set up by one telecommunications carrier • May not serve every country in an MNC’s global network • Many carriers overbook capacity of frame-relay networks.. Can cause packet discards

34. Asynchronous Transfer Mode • A type of transport service on WANs • Handles all types of data… including voice and video… on single network • Most Fortune 1000 companies have some form of ATM • Unlike TCP/IP, ATM is connection-oriented • Sender, receiver set fixed path on network before sending data • Information arrives in order it was sent

35. ATM : How does it work? • ATM network transfers data in small fixed-length packets – 53 bytes each • Packets are known as cells… all cells with same source/destination follow same network path • Real-time data takes precedence over other types.. Voice always get priority over email cells • Small, constant cell size allows more efficient network usage – less delay at ATM switch • “Cell tax” make Gigabit Ethernet more attractive

36. Local Area Networks Topologies and Collision Detection

37. What do we know so far? • Data communications involves • Exchange of digital information • Between two or more devices • Across a transmission medium • How are these devices connected?

38. Private Branch Exchanges (PBX) • Special computer that handles phone calls within a company • Carry both voice and data • Can store, transfer, hold and redial calls • Can also be used to transfer data between computers • Does not require special wiring • PCs can be plugged or unplugged anywhere in the building • Supported by commercial vendors (no internal expertise needed) • Geographic scope limited to several hundred feet • Cannot handle large amounts of data

39. Local Area Networks (LAN) • Connect several buildings in close proximity • Typically within 2000 feet • Requires own communication lines • Have higher transmission speeds • Typically used to connect PCs and shared printers

40. Typical LAN Components Network Server (with network software) LAN Another LAN Gateway

41. Network Topologies • In the case of LANs, the shape of the network defines its topology • Star • Bus • Ring

42. Star Network Topology Host Computer - Used to connect a smaller number of computers - depends on health of host computer

43. Bus Network Topology • Central line (“bus”) that may be TP, Coaxial, or fiber • All messages broadcast to entire network • Software identifies which device receives message • Bus network can only handle one message at a time • Can slow down at peak hours • Collisions may occur

44. Ring Network Topology • - Each computer part of a closed loop • messages passed from one device to another • Only passes in one direction

45. Designed for multiple devices sharing a single communication cable Devloped by Bob Metcalfe of Xerox in 1973 Tried to link a Xerox Alto computer to a printer Ethernet

46. Ethernet Terms Medium, Segment, Node, frames

47. CSMA / CD • An analogy • Imagine a group of people sitting at a table • They are having a polite conversation • Everyone can hear others speak • They wait for conversations directed at them • Wait for a pause in conversation before speaking • Two people waiting for lull speak up at same time • Must repeat themselves

48. Contention issues • All devices on a bus or ring can send messages • Devices keep listening to the network to check for messages meant for them • What happens if messages are sent at the same time? • Messages can sometimes collide and be garbled or lost • LANs must have a predetermined way to deal with these conflicts or contentions

49. CSMA/CD(Collision Sensing Multiple Access/Collision Detection) • This is used in traditional bus network topologies • Ethernet uses bus topology with CSMA/CD • Any device on the bus can send a message • If the line is idle two devices may send at same time • Device recognize collision and send message again after random period of time

50. Limitations of Ethernet Networks • Mostly relate to length of cable segments • Electrical signals attenuate as they travel longer distances • Segment must be short enough for devices to hear each other clearly • Places limit on size on network • Network diameter • Since CSMA/CD only allows one device to communicate at a time, limits number of devices without degrading performance