Global Telecommunications

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Global Telecommunications

• Data transport networks

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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

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Local Area Networks

• LANs are typically confined to smaller distances
• Within a room or building
• Owned by a single entity
• Includes ownership of all computers, media,
  software
• LANs have topologies
• Star, Bus, Ring, etc.
• Ethernet architecture very common

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WANs

• US telephone system uses circuit switching
• Call placed, circuit established to destination
  phone through number of switches
• Digital phone system
• Voice converted to digital through PCM and then
  multiplexed
• 64kbps capacity reserved for each call
• No other traffic reduces this capacity during the
  call
• Data files do not need such a circuit established
• Data files sent through a packet switching network

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

• T-1 Carrier Services
• Developed in the 1960s to increase data speeds on
  regular copper lines
• Involves 2 pairs of Cu wire one for sending and
  one for receiving
• Digital system so it uses PCM
• 24 channels of 64kbps each (1.5Mbps)
• Can have T-2 (28 T-1 lines) T-3, T-4

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Synchronous Optical Network

• SONET uses fiber optic cables for data
  transmission
• Difference between T-1 and SONET lies in
  transmission speeds
• SONET has maximum speed of 10000 Mbps
• Much better error detection
• Most organization lease SONET lines for Internet
  access

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Virtual Private Networks (VPNs)

• Secure, encrypted connection between two points
  on the Internet
• VPNs encrypt and encapsulate information into IP
  packets
• IP packets sent across internet through process
  known as tunneling
• Used mostly to replace existing companys
  international networks

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Point-to-Point Tunneling (PPTP)

• Protocol developed by Microsoft and other
  independent vendors
• Uses Internet as the connection between remote
  users and LAN or between LANs
• PPTP wraps other protocols into an IP protocol
  (the basis for the Internet)

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PPTP contd.

• PPTP allows creation on private link on the
  internet
• No need for dedicated lines
• Is the basis for most VPNs
• Less expensive than frame relay and ATM
• Less secure encryption is weak

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How VPNs Work.
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Global Satellite Services

• GEO, MEO and LEO Satellites, IntelSat, and Global
  Positioning Systems

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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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Intelsat

• Began as an inter-governmental consortium in 1964
• Launched worlds first commercial communication
  satellite in 1965
• By 1969, had a fleet
• Broadcast live Neil Armstrongs first steps on
  the moon
• In 1974, establish the hot-line between the
  Kremlin and the White House
• In mid-1980s, developed more powerful satellites
  allowed smaller ground equipment for live
  broadcasts
• In 1997, introduced pay-as-you-go, shared access
  satellite coverage for low demand, rural areas.
• 2001 became a private company providing turnkey
  connectivity solutions

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Intelsats fleet

• 20 Geosynchronous satellites
• Cover 200 countries
• 4 nines reliability (99.997)
• 18000 earth stations, and millions of VSATs
• Offer
• Bandwidth-on-demand
• Point-to-point
• Point-to-multipoint
• C- and Ku-band capacity

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Latest Satellites launched

• Last of the Intelsat IX series launched Feb 15,
  2003 (went live in March)
• Offers more powerful C-band coverage for
• Europe
• Africa
• The Americas
• Ku spot beam coverage for Europe and Africa

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Global Positioning Satellites

• 24 MEO satellites owned by US DoD (last launched
  in 1994)
• 21 active, 3 spare
• 11,000 miles above earth
• Five control stations around the world make sure
  satellites operating correctly
• Can tell your position within 300 feet
• Possible to get as accurate as 3 feet using
  special calculations.

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How GPS works

• 24 satellites, each takes 12 hours to orbit the
  earth
• You can receive signals from 6 from any point on
  earth
• Satellites have up to 4 cesium and rubidium
  clocks (accurate to 3 billionths of a second)

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The signals

• GPS satellites transmit 2 low power radio signals
  L1 and L2 (civilians can only use L1)
• 25-50 watts (compare to 100,000 watts for an FM
  radio station)
• L1 sends 3 pieces of data
• Pseudorandom code (ID of transmitting satellite)
• Ephemeris data (tells current data and time)
• Almanac data (tells GPS receiver where every
  satellite should be at any time of day)
• The data indicates when the signal was
  transmitted from the satellite
• The receiver indicates when it was received.
• It is now possible to calculate distance from
  satellite
• Three more such signals and you can be pinpointed.

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Galileo

• The EU wants to compete
• Galileo will be under civilian control
• 30 satellites (273 spare)
• Inter-operable with GPS
• Dual frequency design allows positioning within 1
  meter.

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Then there was LEO

• Iridium project
• Initially began as a plan for 77 LEO satellites
  (atomic number 77 Iridium)
• Scaled down to 66 (should be called Dysporium)
• Cost 5billion (Mostly Motorola)
• Sold 2 years ago for 25 million