ECE 101 An Introduction to Information Technology Information Transmission

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ECE 101 An Introduction to Information
TechnologyInformation Transmission
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Information Path
Source of Information
Information Display
Digital Sensor
Information Receiver and Processor
Information Processor Transmitter
Transmission Medium
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Information Transmission

• Procedures for transmitting digital information
  over a communication channel
• Data sent over a channel with a limited channel
  capacity but gt data rate
• Data rate amount of data that a source
  produces in one second
• One and two-way data transmission
• Networks permit data transmission between
  remotely located computers
• networks transmit data in data packets

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Data Rate

• Source produces data that the transmitter
  converts into signal or waveforms to be sent over
  communications channel
• Twisted-pair (telephone), coaxial (TV), air
  (acoustical) or EM wave through space
• Binary transmission two distinguishable signals
  (by amplitude, frequency, phase)
• M-ary transmission more than two signals to
  represent data resulting in faster data
  transmission

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Data Rate Measurment

• Let R signal transmission rate (signals
  produced every second)
• 1/R is the time duration of each signal
• Data Rate D R log2 M

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Channel Noise

• Noise commonly from thermal energy
• Atomic (charged) particles vibrating randomly
• Disturbs the data signal
• Higher temperatures cause greater thermal motion
• ? Sensitive receivers are placed in low-temp
  environments
• Noise power level ?n2
• Maximum signal power level produced by
  transmitter ?s2

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Channel Transmission

• To transmit more data per second over a channel,
  the transmitter could increase M, the number of
  distinct signals
• Noise limits the value of M
• ?Noise level present in the transmission channel
  dictates the maximum data rate

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Decoding M-ary Signals(figure 8.2, Kuc)
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Decoding M-ary Signalsin the presence of
Noise(figure 8.3, Kuc)
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Channel Capacity

• Measures the amount of data that can be reliably
  transmitted over a channel
• Signal passing through a channel is always
  contaminated by noise
• Channel capacity C with bandwidth B is
• C B log2 (1 ?s2/ ?n2) bps
• ?s2/ ?n2 is the signal to noise ratio

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Channel Capacity

• C B log2 (1 ?s2/ ?n2) bps
• ?s2/ ?n2 is the signal to noise ratio
• Special cases
• ?n2 ? 0 C ? ?
• ( ?s2/ ?n2 ) 1 C ? B log2 (?s2/ ?n2) bps
• ?n2 ?s2 ? 0 C ? B log2 (1) 0
• Long distances attenuation occurs so ?s2 is
  decreasing, but ?n2 is increasing

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Asynchronous Data Transmission

• Sends data over a transmission one bit at a time
  or serially
• channel and receiver are idle much of the time
  waiting for data
• data are packaged in a format
• start bit
• data - one code word at a time (byte sized are
  common)
• parity bit - error detection (even or odd)
• stop bit(s) - to terminate data
• all BUT data represent over head to transmit
  serially

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Asynchronous Data Transmission and Character
Format (figures 8.4 and 8.5, Kuc)
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One-Way Data Transmission

• Typically used to control remotely a device such
  as a TV, projector, VCR, garage door
• Infrared Remote (IR) Control
• Encodes the pressed button into a sequence of IR
  light pulses
• The remote control generates a binary signal that
  consists of a sequence of light pulses modulated
  at 40 kHz for time periods of TB

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Infrared Remote Control Signal(figure 8.6, Kuc)
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Infrared Remote Control

• Binary communication, M2
• Transmits a single bit of information every TB
  seconds, or R 1/TB signals per second
• Data Rate D R log2 M 1/TB log2 2 1/TB
• Number of data bits in a code word depends upon
  the number of buttons on the remote
• n bits will take up to 2n buttons
• multiple transmission provides error correction
  by repetition the receiver counts votes

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Digital Television

• Standard TV as grid of small squares or picture
  elements (pixels) arranged in 700 columns and 400
  rows per frame
• assume each pixel is encoded with 8 bits
• TV transmits 30 frames per second
• Data rate D 67.2 ? 106 bits/second
• or D 67.2 Mbps

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MPEG

• MPEG - Motion Picture Experts Group - reduce the
  number of bits required to transmit video since
  many scenes have static parts. So may only have 2
  to 6 Mbps
• Freeze Frame video if the data rate is greater
  than the channel capacity, then each frame waits
  till all data received and the result appears as
  a series of still pictures

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Two-way Data Transmission With Modems

• Dialog between two systems
• Communication over the same channel require
  separation between the signals to distinguish
  transmitted and received signals
• Modems - transmit and receive data over telephone
  channels - data to audible tones data rates gone
  from 300 bps to over 50kbps

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Modem Data Transmission Techniques

• Use sinusoidal signals that have features that
  can be modified to represent data
• Amplitude-modulation changes amplitude only of a
  single frequency sinusoid,
• Frequency-shift keying use different frequencies
• Phase-shift keying methods change phase of a
  single frequency sinusoid
• Baud expresses number of signal intervals that
  can be reliably transmitted over a channel per
  second (same as R used earlier).

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Frequency Shift Keying (figure 8.8, Kuc)
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Frequency Shift Keying

• Frequency-shift keying uses different frequencies
• 300 to 3300 Hz bandwidth of the telephone network
• example, two different frequencies might
  represent 1s 0s
• Or, more practically, four frequencies, each one
  assigned to a two-bit value Baud rate the same,
  but the data rate doubles with the two bits per
  sample period.

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Modem Two Way Communication (figure 8.9, Kuc)
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Phase-Shift Keying

• Changes the phase at a constant frequency and
  amplitude
• Can make M-ary transmission by having each value
  have a different phase shift relative to the
  immediately preceding sinusoidal signal
• M4 dibits with dibit varying by 360/4 90o
• M8 tribits, with tribits varying by 360/845o
• Phase shift occurs every Tbaud seconds

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Phase-Shift Keying (figure 8.11, Kuc)
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Phase-Shift Keying

• Phase shift occurs every Tbaud seconds and if
  M4, every shift encodes 2 bits, so the data rate
  is twice the baud rate.
• Modem factor 1 bit/cycle 1 bps/Hz
• If M8, we transmit 3 bits every 2 cycles of the
  waveform for a modem factor of 1.5 bps/Hz

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Phase-Shift Keying with Amplitude Modulation

• Can go to quadbits, shifting the amplitude to two
  different levels and using phase shift of 45o
• Now transmit 4 bits per 2 cycles of the waveform
  for a modem factor equal to 2 bps/Hz

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AM and Phase-Shift Keying (figure 8.14, Kuc)
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Establishing Modem Communication

• No energy for 48 Tbaud
• after answering the ring, both modems listen to
  channel to determine the noise level and if
  little noise use higher data rate
• Alternation between 2 known signals for 128 Tbaud
  to synchronize the two modems
• Pseudo-random alternations between known signals
  for 384 Tbaud
• compensate for distortions in the telephone line
• Transmission of known data sequence for 48 Tbaud
  to verify all circuits are ok

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Digital Cellular Telephone

• Uses wide frequency band width radio channel to
  transmit electromagnetic signals
• Frequency band divided into channels with each
  having a transmit receive frequency
• Each user uses the first sub-baud pair as a
  control channel to communicate to all users (a
  code determines who can actually receive the
  message)
• Voice channel is assigned to a user when a call
  is made or received

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Cellular Telephone Frequency Channels (figure
8.16, Kuc)
f
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Communications(IEEE Web site)
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Satellites

• Must always be visible to the antenna with which
  it communicates
• Uses a geosynchronous orbit as the satellite
  remains stationary at 36,000 km (22,300 miles)
  above a point on the earth
• Signal delay Tt (dt dr)/c, c3?108 m/s
• Delays can be large fraction of a second hence
  one-way communications better than two

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Data Packets

• Transmission of multiple-byte units over networks
  of interconnected computers
• Five parts or fields
• address with routing information about the
  desired destination and address of the source
• data length indicating the number of bytes in the
  data field (46 to 1500 bytes)
• tag - a number that indexes the data packet
  (often single byte with numbers 0 to 255)

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Data Packets

• data field contains the information to be
  transmitted - for internet applications the data
  segment is approximately 500 bytes - compromise,
  smaller needs more packets, larger would cause
  delays for access to communication links
• cyclic redundancy clock (CRC) - error detection -
  often a one byte number simply adding up all the
  1s that are in the data and retaining the
  smallest 8 bits of the sum. This is modulo-256 of
  the sum. Alternative is parity bit

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Data Networks

• Local Area Network (LAN)
• connects computers and peripheral devices
• can use various means or protocols to transfer
  data
• Wide Area Networks (WAN)
• Connects devices wherever long-distance
  communications exist
• Most common is international network known as the
  Internet

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Star Architecture for LAN(figure 8.18, Kuc)
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Star Architecture

• All nodes connect to hub computer called a server
• fast since message only goes to server then its
  destination
• server can store message if it is not delivered
• all communication stops if the server is down
• limited number of connections to server

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Ring Architecture for LAN(figure 8.18, Kuc)
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Ring Architecture

• Each node connects to two neighboring nodes and
  the data packets flow around the loop in one
  direction.
• If the packet address corresponds to the node
  address the message is read if not it is just
  passed on
• Does not require a separate server but it
  performs properly only when all the nodes are
  operational

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Bus Architecture

• Most common LAN
• all nodes (users) connect to the same bus
• Each node can transmit and each much recognize
  its address to receive
• Doesnt require a separate server
• Additional nodes easily added
• Highly reliable since it remains operational when
  a node fails or is turned off

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Bus Architecture for LAN(figure 8.18, Kuc)
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A Wide-AreaNetwork(figure8.19, Kuc)
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Data Packets

• Recall earlier we looked at the transmission of
  data in data packets
• tag - a number that indexes the data packet
  (often single byte with numbers 0 to 255)
• data field contains the information to be
  transmitted - for internet applications the data
  segment is approximately 500 bytes - compromise,
  smaller needs more packets, larger would cause
  delays for access to communication links

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Wide-Area Network

• Consists of many switching computers or routers
  between the source and destination
• Moving packets around the wide-area network is
  packet switching
• The exact path of a particular packet is random
  otherwise bottlenecks
• More sophisticated networks offer the fastest
  paths
• Recall that each packet has the destination and a
  tag to help it arrange the packets in order

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Ethernet

• Most common communication channel for
  transmitting data packets
• Standard has capacity of 10 million bps
• Fast ethernet 100 Mbps, Gigabit ethernet 1
  billion bps
• Special data signal using two wires to transmit
  data and two wires to receive data

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Ethernet

• Hence etherner uses dedicated cables to
  interconnect computers directly
• Computer connects to network through a special
  network interface card (NIC)
• packages the data bytes from the computer into
  data packets
• at the receiving end another NIC receives the
  data packets, checks for errors, and delivers the
  data bytes (typically 46 to 1500 bytes)

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Data Packets on Ethernet

• Preamble 7 repetitions of 10101010 to
  synchronize the receiver (7 bytes)
• Start byte with a value of 10101011 to indicate
  the start of the information fields (1 byte)
• Destination Address (6 bytes)
• Source address (6 bytes)

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Data Packets on Ethernet

• Tag/Length field that indicates the packet number
  and length of data (2 bytes)
• Data varies in length (46 to 1,500 byte)
• A cyclic redundancy check (CRC) for error
  detection (4 bytes)
• Total overhead of 26 additional bytes

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Asynchronous Transfer Mode (ATM)

• Ethernet packets have variable length fields.
• To simplify server design, ATM is used
• ATM packets are always 53 bytes long (5 for
  routing and 48 for data
• All ATM packets use the same path to the
  destination, so path designate by just 5 bytes to
  reduce the routing information
• Error checking done only at the destination

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Transmission Protocols on the Internet

• Data on the internet are transmitted as data
  packets
• Methods of data transfer are protocols such as
• TCP/IP guarantees that the received data is
  correct hence reliable
• UDP/IP transmits data quickly but does not
  retransmit erroneous packets hence speed

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TCP/IP

• Transmission control protocol/Internet protocol
  (TCP/IP)
• Uses parity bits and check character to ensure
  the integrity of the data.
• When the data packet is received correctly it
  sends an acknowledgement (ACK) to the transmitter
• If ACK is not received it sends the message again
  hence the transmission rate is reduced

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UDP/IP

• Universal datagram protocol/Internet protocol or
  UDP/IP
• Transmits data with minimum delay
• it finds the quickest available route to send the
  data and does not acknowledge receipt or
  retransmit erroneous packets
• Music uses this protocol

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Internet (Introduction to Internet - S. James)

• Who runs it?
• Backbone funded by NSF
• Internal Advisory Board - helps to set standards
• Growing exponentially
• 1980s - 213 hosts on internet
• 1986 - 2,300 hosts
• now millions
• 1991 - business use gt academic use

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Internet (Introduction to Internet - S. James)

• Computers available in late 1950s
• Immediate need to communicate with one another
• ARPA Net formed (Advanced Research Projects
  Agency) in 1969
• developed Transmission Control Protocol/Internet
  Protocol (TCP/IP)

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Internet (Introduction to Internet - S. James)

• Etiquette - prescribed forms and practices of
  correct behavior
• Netiquette - rules for the internet
• avoid flame wars
• update address
• dont use all caps
• reply to questions

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Internet (Introduction to Internet - S. James)

• Advantages
• Access information anytime
• Blind to race, religion, sex, creed
• Direct cost minimal, generally your time
• Communicating by writing - tends to be more
  organized
• Send many messages of nearly any length
  relatively quickly to many people

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Internet (Introduction to Internet - S. James)

• Disadvantages
• Credibility of information
• Internet gets crowded - connection time slow
• Addictive
• People may write what they wouldnt say
  face-to-face
• Mistakes get amplified
• Junk mail

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Internet (Introduction to Internet - S. James)

• Hosts
• computers on internet that provide some service
  (such as e-mail, file transfer, web site, etc.)
• Hostname
• all computers that are registered on the internet
  have a unique host name and domain name
• teal.gmu.edu
• teal - computer name
• gmu.edu - domain name
• edu - extension

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Internet (Introduction to Internet - S. James)

• IP Address
• all computers on internet must have an Internet
  Protocol IP address
• handed out by Internet Network Information Center
• Unix
• popular operating system for computers
• runs on PCs and mainframes
• original TCP/IP computers ran Unix

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Internet (Introduction to Internet - S. James)

• Internet 2
• universities research organizations joining
  together to create another internet exclusivley
  for their use
• Internet Service Providers (ISP)
• computer companies that have the necessary
  hardware/software to allow your computer to dial
  into the ISP and in turn connect you to the
  internet
• some use cable for higher speeds rather than
  phone lines or use satellites

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Web (Introduction to Internet - S. James)

• Origin goes back to need to communicate
• Hypertext Markup Language (HTML)
• text stored in electronic form with
  cross-reference links between pages (example -
  our syllabus)
• In 1993 almost 100 computers were equipped to
  serve up HTML pages - those linked pages were
  called the World Wide Web (WWW).
• Means for referencing text on the Internet
• Web Browsers
• view graphic images were developed like Netscape
  Navigator

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Data Networking Laboratory (Room 228, ST 2)