Part II: Packet Transmission Packets on a Network

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Part II Packet TransmissionPackets on a Network
Fall 2005

• Packets, Frames, LAN, WAN, Hardware Addresses,
  Bridges, Switches, Routing and Protocols

Qutaibah Malluhi Computer Science and
Engineering Qatar University
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Packets, Frames and Error Detection
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Packet and Circuit Switching

• Circuit-switched networks (connection oriented)
• Form a dedicated connection (circuit) between two
  points
• Guaranteed capacity but high-cost (cost is fixed
  and is independent of the traffic)
• E.g. telephone system
• Computer networks are often called packet
  switched networks (Connectionless)
• Data divided into small pieces (packets)
• Each packet is sent individually

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Motivation for Packet Switching

• Coordination - helps transmitter and receiver
  determine which data have been received correctly
  and which have not
• Better more efficient control on transmission
  errors
• fair Access - each computer can only send one
  packet at a time
• Resource sharing - allows multiple computers to
  have more effective sharing of the expensive
  network infrastructure
• Most network use shared media which interconnect
  all computers
• However - only one source can transmit data at a
  time

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Dedicated (non packet-switched) Access

• 5MB file transferred across network with 56Kbps
  capacity will require 12 minutes
• 5x106 bytes 8 bits/byte
• ----------------------------------------------
  11.9 minutes
• 60 secs/minute 56x103 bits/second
• All other computers will be forced to wait 12
  minutes before initiating other transfers

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Packet Switched Access

• If a 5 MB file is broken into packets, other
  computers must only wait until packet (not entire
  file) has been sent
• From previous example, suppose file is broken
  into 1000 byte packets
• Each packet takes less than 0.15 seconds to
  transmit
• 1000 bytes 8 bits/byte
• ----------------------------------- 0.143
  seconds
• 56 103 bits/second
• Other computer must only wait 0.143 seconds
  before beginning to transmit
• Note
• If both files are both 5MB long, each now takes
  24 minutes to transmit
• BUT if second file is only 10KB long, it will be
  transmitted in only 2.8 seconds, while 5MB file
  still takes roughly 12 minutes

Packet Switching provides fair and prompt access
to shared network resources
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Packets Allow Time Division Multiplexing

• Dividing data into small packets conceptually
  permits providing a form of time-division
  multiplexing

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Packets and frames

• Packet is generic'' term that refers to a small
  block of data
• Each hardware technology uses different packet
  format
• Frame or hardware frame denotes a packet of a
  specific format on a specific hardware technology
  
• Need to define a standard format for data to
  indicate the beginning and end of the frame
• Header and trailer used to frame'' the data
• Can choose two unused data values for framing.
  E.g., if data is limited to printable ASCII, can
  use
• start of header'' (soh)
• end of text'' (eot)

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Frame Delimiters

• Incurs extra overhead
• soh and eot take time to transmit, but carry no
  data
• Accommodates transmission problems
• Missing eot indicates sending computer crashed
• Missing soh indicates receiving computer missed
  beginning of message
• Bad frame is discarded

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Data Stuffing

• May not afford to reserve two special characters
  for framing
• E.g., transmitting arbitrary binary data
• soh and eot can be part of data will be
  misinterpreted as framing data
• Solution Bit stuffing and byte stuffing
• Inserting extra data to encode reserved bytes
• Sender and receiver must agree to encode special
  characters for unambiguous transmission
• Example
• Byte stuffing translates each reserved byte into
  two unreserved bytes
• For example, can use esc as prefix, followed by
  x for soh, y for eot and z for esc

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Byte Stuffing

• Sender scans data and translates each reserved
  byte into the appropriate encoding pair of bytes
• Receiver interprets pairs of bytes and retrieves
  encoded byte Data still framed by soh and eot

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Bit-Oriented Frames/ Bit Stuffing

• Delineate frame with a special bit pattern
  01111110
• If bit pattern occurs at the data, use
  bit-stuffing
• sender insert bit 0 after five consecutive 1s
• receiver if receive five consecutive 1s, check
  next bit
• if it is 0, remove it
• if they are 10, mark the end of the frame
• if they are 11, error

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Transmission Errors

• Transmission errors can occur due to interference
  from external electromagnetic signals
• Data can be changed
• Data can be lost
• Unwanted data can be generated

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Error detection and correction

• Error detection
• send additional information so incorrect data can
  be detected and rejected
• retransmission used to recover from errors
• Use for low error rate in data communication
• Error correction
• send additional information so incorrect data can
  be detected, corrected and accepted
• higher overhead, used only if retransmission is
  impractical, such as simplex transmission, long
  delay, high error rate
• No error control
• real time traffic (e.g. real time voice and
  video)
• does not require a 100 error rate

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Error Detection Techniques

• Will See three techniques
• Parity
• Checksum
• Cyclic redundancy check (CRC)
• Checksum and CRC are two widely used techniques

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Parity Error Detection

• A parity bit is an extra bit transmitted with a
  data item
• Parity refers to the number of bits set to 1 in
  the data item
• Even parity - an even number of bits are 1
• data 10011101, parity bit 1
• use XOR to compute
• Odd parity - an odd number of bits are 1
• data 10011101, parity bit 0
• use XNOR to compute
• Procedure
• Transmitter and receiver agree on which parity to
  use
• Sender computes parity bit and sends it with data
• If a transmission error changes one of the bits
  in the data from a 1 to a 0 or from a 0 to a 1,
  parity of resulting bits will be wrong
• Receiver detects error in data because of
  incorrect parity
• Parity can only detect odd or practically
  single-bit errors (not even bit errors)

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Checksum

• Treat data as a sequence of integers
• Can be 8-, 16- or 32-bit integers
• Compute and send arithmetic sum Easy to compute
• Checksum computed over data
• Checksum appended to frame
• Typically use 1s-complement arithmetic Add
  carry to result
• Example - 16-bit checksum with 1s complement
  arithmetic
• Handles multiple bit errors
• Cannot handle all errors

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Checksum May Fail to Detect Errors

• Second bit reversed in each item
• Checksum is the same

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Cyclic Redundancy Check (CRC)

• Idea given a k-bit frame, the transmitter
  generates an n-bit sequence known as the Cyclic
  Redundancy Check (CRC), so that the resulting
  (kn)-bit frame is exactly divisible (modulo-2)
  by some predetermined number called a generator
  (specified by a generator polynomial).
• Consider data in message as coefficients of a
  polynomial
• Divide that coefficient set by a known polynomial
• Transmit remainder as CRC
• Good error detection properties
• In practice can detect 99.98 of errors
• Easy to implement in hardware

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Cyclic Redundancy Check (CRC)

• Commonly used polynomial generators
• CRC-12
• CRC-16
• CRC-CCITT
• Assume the degree of the generator is n
• n zero bits appended to the end of the frame
• Modulo-2 operations used to determine the
  remainder CRC
• Sender sends the data plus CRC (notice that data
  plus CRC should be divisible by the generator
  polynomial)
• Receiver performs the modulo-2 operations on
  (kn)-bit frame (original data plus CRC) using
  the same generator
• If the remainder is zeros, assume NO error.
  Otherwise, discard the frame.

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Insight on CRC Computation

• Data bits are 10011
• Generator polynomial (x3 1) has degree 3
• I.e., 3 bit CRC
• Therefore, append three check bits to the data
  bits, producing the code word 10011000 (x7 x4
  x3).
• If we divide the data codeword by the generator
  polynomial, the result has a remainder of 1.
• The remainder constitute the CRC
• Subtracting (same as adding/XORing) to the
  original data codeword word produces 10011001
  (x7 x4 x3 1), which, is what get sent by the
  sender.
• Notice that this should be divisible by the
  generator
• If the receiver divides what he receives by the
  generator polynomial, the result should be zero
  if there are no errors.

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CRC Computation Example (1)
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Building Blocks For CRC

• Exclusive or
• Shift register
• a shows status before shift
• b shows status after shift
• output same as top bit

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Example Of CRC Hardware

• Computes 16-bit CRC
• Registers initialized to zero
• Bits of message shifted in
• CRC found in registers

Uses the generator polynomial
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CRC Computation Example (1)
input
ShiftReg. _____
InputBit _____
000001010100000001010100 001
10011000
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CRC Computation Example (2)



Input 11100101000
ShiftReg. _____
InputBit _____
0000 0001 0011 0111 1110 0111 1111 0101 1011 1101
0001 0010 0100
1 1 1 0 0 1 0 1 0 0 0 0
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Illustration Of Frame Using CRC

• Error detection typically done for each frame
• CRC cover data only
• Error in frame typically causes receiver to
  discard frame

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Summary

• Packet technology
• Invented to provide fair access in shared network
• Sender divides data into small packets that
  travel independently
• Packets specific to a particular hardware network
  technology are called frames
• Each frame has a specific format that identifies
  the beginning and end of the frame
• Data (bit or byte)-stuffing are needed when
  special characters appear in data
• Error detection and correction is used to
  identify and isolate transmission errors
• To detect transmission errors
• Sender adds information to packet
• Receiver checks