Wireless Communication: Satellites

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Wireless CommunicationSatellites
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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

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Terrestrial Microwave

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

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What is a telecommunications satellite?
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Telecommunications satellites

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

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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

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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

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Earth-based equipment

• Original microwave transmitters and receivers
  were large installations
• Dishes measuring 100 feet in diameter
• Modern antennas about 3 feet in diameter

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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

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IntelSat 902 (launched August 30, 2001)
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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

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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 (uplink30 GHZ , downlink 20 GHz)

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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

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Global Positioning Systems

• A constellation of 24 DoD satellites orbiting
  about 10,000 miles above earths 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.

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GPS Constellation
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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.

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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
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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?

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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

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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

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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)

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Other landline transmission paths
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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)

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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 users 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

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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

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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

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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

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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

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

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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

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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

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International Frame Relays contd.

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

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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

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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

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Local Area Networks

• Topologies and Collision Detection

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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?

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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

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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

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

• In the case of LANs, the shape of the network
  defines its topology
• Star
• Bus
• Ring

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

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Ring Network Topology

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

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Ethernet

• 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

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Ethernet Terms
Medium, Segment, Node, frames
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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

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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

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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

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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

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Repeaters

• Repeaters connect multiple Ethernet segments
• Any signals heard on one segment will be heard
  and repeated on all other segments connected to
  repeater
• Allows for expanding network diameter

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Bridges

• What happens if there are a large number of
  people at the table?
• Multiple simultaneous conversations
• In large networks, devices would constantly be
  interfering and sending colliding signals
• Bridges are like repeaters that echo signals, but
  can also regulate traffic

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• -The bridge aims at reducing unnecessary traffic
  on Ethernet segments
• -If signal from A is meant for B, there is no
  point echoing it on Segment 2
• If it is meant for C or D, the frame is forwarded
  to Segment 2.
• A can simultaneously transmit to B since it only
  uses Segment 1

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Token Ring

• Used in ring topology networks
• Eg IBM token rings
• A token (series of 0,1) floats along line
• A device wishing to send message on line must
  first grab the token and keep it only then can
  it send
• Once the message has been sent, device releases
  token back into the ring
• Collisions can never occur
• Token-ring networks typically transmit data at
  either 4 or 16 Mbps.

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IBM Token Ring
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Problems with bridges

• In the Ethernet design, messages are broadcast to
  every device (or node) on the network segments.
• The bridge forwards these broadcasts to all
  connected segments
• In very large networks, this can cause congestion
• Many stations on different segments broadcast at
  the same time
• Can be as bad as if all nodes were on one segment

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Routers

• Routers are advanced network components
• They divide the network into two virtual (or
  logically independent) networks
• Broadcasts cross bridges in search of their
  desired node
• They do not cross routers
• The router forms a logical boundary of the
  network

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Fujitsu GeoStream R900 Router
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Research Question for Next Class

• What is Abilene?