Digital Transmission: Advantages

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Digital Transmission Advantages

• Produces fewer errors
• Easier to detect and correct errors, since
  transmitted data is binary (1s and 0s, only two
  distinct values))
• Permits higher maximum transmission rates
• e.g., Optical fiber designed for digital
  transmission
• More efficient
• Possible to send more digital data through a
  given circuit
• More secure
• Easier to encrypt
• Simpler to integrate voice, video and data
• Easier to combine them on the same circuit, since
  signals made up of digital data

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Data Flow (Transmission)
data flows move in one direction only, (radio or
cable television broadcasts)
data flows both ways, but only one direction at a
time (e.g., CB radio) (requires control info)
data flows in both directions at the same time
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Amplitude Modulation (AM)

• Changing the height of the wave to encode data

• One bit is encoded for each carrier wave change

• A high amplitude means a bit value of 1
• Low amplitude means a bit value of 0

• More susceptible noise than the other
  modulation methods

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Frequency Modulation (FM)

• Changing the frequency of carrier wave to
  encode data

• One bit is encoded for each carrier wave change

• Changing carrier wave to a higher frequency
  encodes a bit value of 1
• No change in carrier wave frequency means a bit
  value of 0

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Phase Modulation (PM)

• Changing the phase of the carrier wave to
  encode data

• One bit is encoded for each carrier wave change

• Changing carrier waves phase by 180o corresponds
  to a bit value of 1
• No change in carrier waves phase means a bit
  value of 0

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Bit Rate vs. Baud Rate

• bit a unit of information
• baud a unit of signaling speed
• Bit rate (or data rate) b
• Number of bits transmitted per second
• Baud rate (or symbol rate) s
• number of symbols transmitted per second
• General formula
• b s x n
• where
• b Data Rate (bits/second)
• s Symbol Rate (symbols/sec.)
• n Number of bits per symbol

Example AM n 1 ? b s Example
16-QAM n 4 ? b 4 x s
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Multiplexing

• Breaking up a higher speed circuit into several
  slower (logical) circuits
• Several devices can use it at the same time
• Requires two multiplexer one to combine one to
  separate
• Main advantage cost
• Fewer network circuits needed
• Categories of multiplexing
• Frequency division multiplexing (FDM)
• Time division multiplexing (TDM)
• Statistical time division multiplexing (STDM)

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Frequency Division Multiplexing
Makes a number of smaller channels from a larger
frequency band
3000 Hz available bandwidth
Used mostly by CATV
FDM
FDM
Host computer

• Guardbands needed to separate
  channels
• To prevent interference between channels
• Unused frequency bands ?,wasted capacity

circuit
Four terminals
Dividing the circuit horizontally
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Time Division Multiplexing
Dividing the circuit vertically

• Allows multiple channels to be used by allowing
  the channels to send data by taking turns

4 terminals sharing a circuit, with each terminal
sending one character at a time
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Statistical TDM (STDM)

• Designed to make use of the idle time slots
• (In TDM, when terminals are not using the
  multiplexed circuit, timeslots for those
  terminals are idle.)
• Uses non-dedicated time slots
• Time slots used as needed by the different
  terminals
• Complexities of STDM
• Additional addressing information needed
• Since source of a data sample is not identified
  by the time slot it occupies
• Potential response time delays (when all
  terminals try to use the multiplexed circuit
  intensively)
• Requires memory to store data (in case more data
  come in than its outgoing circuit capacity can
  handle)

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Sources of Errors and Prevention
More important
mostly on analog
 
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Error Detection Techniques

• Parity checks
• Longitudinal Redundancy Checking (LRC)
• Polynomial checking
• Checksum
• Cyclic Redundancy Check (CRC)

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Examples of Using Parity
To be sent Letter V in 7-bit ASCII 0110101
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Using LRC for Error Detection
Example Send the message DATA using ODD
parity and LRC
Letter D A T A
Parity bit 1 1 0 1
ASCII 1 0 0 0 1 0 0 1 0 0 0 0 0 1 1 0
1 0 1 0 0 1 0 0 0 0 0 1
BCC 1 1 0 1 1 1 1 1
Note that the BCCs parity bit is also determined
by parity
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Polynomial Checking

• Adds 1 or more characters to the end of message
  (based on a mathematical algorithm)
• Two types Checksum and CRC
• Checksum
• Calculated by adding decimal values of each
  character in the message,
• Dividing the total by 255. and
• Saving the remainder (1 byte value) and using it
  as the checksum
• 95 effective
• Cyclic Redundancy Check (CRC)
• Computed by calculating the remainder to a
  division problem

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Discrete ARQ
Sender
Receiver
Sends the packet, then waits to hear from
receiver.
Sends acknowledgement
Sends the next packet
Sends negative acknowledgement
Resends the packet again
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Continuous ARQ
Sender sends packets continuously without waiting
for receiver to acknowledge
Notice that acknowledgments now identify the
packet being acknowledged.
Receiver sends back a NAK for a specific packet
to be resent.
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Asynchronous Transmission
Sometimes called start-stop transmission
Used by the receiver for separating characters
and for synch.
Each character is sent independently
Sent between transmissions (a series of stop bits)
Used on point-to-point full duplex circuits
(used by Telnet when you connect to Unix/Linux
computers)
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SDLC Synchronous Data Link Control

• Bit-oriented protocol developed by IBM
• Uses a controlled media access protocol

Beginning (01111110)
Ending (01111110)
data
CRC-32
Destination Address (8 or 16 bits)

• Identifies frame type
• Information (for transferring of user data)
• Supervisory (for error and flow control)

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Transmission Control Protocol

• Links the application layer to the network layer
• Performs packetization and reassembly
• Breaking up a large message into smaller packets
• Numbering the packets and
• Reassembling them at the destination end
• Ensures reliable delivery of packets

TCP Header 192 bits (24 bytes)
used in message reassembly
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Internet Protocol (IP)

• Responsible for addressing and routing of packets
• Two versions in current in use
• IPv4 a 192 bit (24 byte) header, uses 32 bit
  addresses.
• IPv6 Mainly developed to increase IP address
  space due to the huge growth in Internet usage
  (128 bit addresses)
• Both versions have a variable length data field
• Max size depends on the data link layer protocol.
• e.g., Ethernets max message size is 1,492 bytes,
  so max size of TCP message field
• 1492 24 24 1444 bytes

IPv4 header
TCP header
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IP Packet Formats
IPv4 Header 192 bits (24 bytes)
IPv6 Header 320 bits (40 bytes)