ECIS560: Lecture 2

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ECIS560 Lecture 2

• Fundamentals of Telecommunications

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Data Communications

• exchange of digital information between two
  devices using an electronic transmission medium

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Types of Signals

• Analog signals
• used for voice communication
• has a continuous waveform
• Digital signals
• discrete
• not continuous
• 0s and 1s

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Digital vs. Analog
Amplitude
Analog Transmission
Time
1
1
1
1
() voltage
Amplitude
0
0
0
0
0
(-) voltage
Time
Digital Transmission
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Advantages of Digital Signals

• Can be converted to decimal number
• Used for error detection and encryption
• Language of computers
• Easier to recover after distortion
• Signals weaken due to resistance in a medium
• Waveform shape gets distorted

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Converting Analog to Digital

• Pulse Code Modulation (PCM)
• Like getting a ticker quote every 10 minutes
• Approximates the actual signal curve
• In PCM
• Measure the signal height every 1/8000th of a
  second
• 8 bits used to report the height at each
  measurement
• 8800064,000 bits per second to provide
  approximation of analog signal
• 64Kbps represents a single voice line in digital
  telecommunications

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Digital to Analog Conversion

• Needed to transmit computer signals over
  telephone lines
• Analog signal characteristics
• Amplitude
• Intensity of the wave (height)
• Wavelength
• Distance between comparable points on the wave
• Frequency
• Number of up and down cycles per second (Hz)
• Phase
• Relative state of the amplitude

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Amplitude Modulation
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Frequency Modulation
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Multiplexing

• Allows multiple signals to be sent over same
  medium at same time
• Modes of multiplexing
• Frequency Division (FDM)
• Time Division (TDM)

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Frequency Division Multiplexing
X X X X
X X X X X X
Y Y Y Y Y Y
Y Y Y Y
Z Z Z Z Z Z
Z Z Z Z

• originally designed so multiple voice
• streams could be placed on same telephone line

• Multiple analog signals superimposed but on
• different frequency spectra

• Involves pair of multiplexers

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Time Division Multiplexing
X X X X
Y Y Y Y
X
Y
Z
X
Y
Z
X
Y
Z
Z Z Z Z

• Each signal allotted a time slot
• Creates a composite stream with slots dedicated
  to data sources
• If data source is not sending, slot goes unused
  wasteful
• Instead, use statistical TDM in which slots are
  dynamically allocated
• If there is big demand, buffers are used.

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Transmission Media

• the physical path along which the data is carried
• Types
• twisted pair
• coaxial
• fiber optics and free space
• satellite
• terrestrial

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Bandwidth

• Measure of data that can flow in unit of time
• Analog involves range of frequencies
• 300-2400 Hz for telephone wires
• Digital involves bits per second
• Varies from medium to medium

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Transmission Media

• Twisted Pair
• pair of wires twisted along entire length
• usually copper with an insulating coat
• Unshielded Twisted Pair (UTP) popular with LANs
• CAT3 (voice) and CAT5 are common
• CAT5 used for both voice and data
• 100Mbs transmission speed
• Limited segment length signals needs
  regeneration every 100 meters

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Transmission Media

• Coaxial cable
• thick insulated copper wire
• Longer segment lengths
• can carry up to 200 Mb/second
• less interference due to shielding
• Uses FDM to transmit 1000s of voice channels and
  100s of TV channels
• Not popular in LANS
• More difficult to work with than UTP

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Transmission Media

• Fiber Optics cable
• thousands of little fiber optic strands
• May be glass or plastic
• Thickness of a human hair
• Inner core surrounded by glass (cladding)
• Can be single mode or multimode
• Single mode
• Expensive, bigger capacity, long segment length
• 8/125
• Multimode
• Cheaper, less capacity
• 62.5/125
• Data transmitted as pulses of light
• 500 Kb/sec to several GB/sec

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A typical optic fiber

• Core made of silica and germania
• Optic cladding is pure silica
• Mix of different refractive indices allows for
• total internal reflection

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Advantages of fiber optics

• Nearly infinite capacity
• Single fiber can carry 40000 telephone calls or
  250 channels of television
• High transmission rates at greater distances
• Immune to interference and electricity
• Does not corrode (being glass)
• Smaller and lighter than coaxial or twisted pair
• Extremely secure

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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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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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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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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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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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Ethernet

• Developed by Xerox (Metcalfe) and later submitted
  to IEEE by consortium (Digital, Intel, Xerox)
• Uses variety of media
• Bus topology
• Data throughput varies from 10Mbps to 100Mbps to
  1Gbps

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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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Tunneling

• Process of routing one protocol through another
• Like an astronaut traveling between space ships
• Astronaut (the non-IP message) needs to get from
  spacecraft A (LAN 1) to spacecraft B (LAN 2)
• Puts on a space suit (the wrap)
• Travels through vacuum (IP)

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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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Next Class

• Research Question
• Wi-Fi or Bluetooth?
• Quiz 2
• GP1 due
• Please remember to turn in Intent to Submit to
  Entrepreneurship Center before January 17th
  deadline.