Integrity in Data Communications

1
Integrity in Data Communications

• Packets, Frames and Error
• Detection

2
More About Packets

• Networks do not transfer arbitrary amounts of
  data for 2 reasons
• 1. Errors in large blocks cause large delays.
• -Senders and receivers have to
  coordinate transmission. Errors often
  occur.
• -Dividing data into small blocks
  allows the sending and receiving
  computers to make fast re-
  transmission.

These networks are called packet networks or
packet switching networks
3
More About Packets

• 2. Computers have to share underlying
  connections in hardware.
• -Communications channels are
  expensive so sharing allows all to be
  treated equally.
• Early networks allowed an application to hold a
  resource until finished.

4
Packets and Time-Division Multiplexing

• Computers take turns sending and receiving small
  packets of data.

1
Computer 1 using channel to send packet.
2
Multiplexing occurs here.
3
5
Packets and Time-Division Multiplexing

• Computers take turns sending and receiving small
  packets of data.

1
Computer 2 using channel to send packet.
2
Multiplexing occurs here.
3
6
Packets and Time-Division Multiplexing

• Computers take turns sending and receiving small
  packets of data.

1
Computer 3 using channel to send packet.
2
Multiplexing occurs here.
3
7
Packets and Time-Division Multiplexing

• Computers take turns sending and receiving small
  packets of data.
• A source with a small total amount will finish
  promptly.
• Larger amounts will take longer.
• Data are arriving at two or more computers
  simultaneously.

8
Packets and Hardware Frames

• Packet small block of data (General)
• Hardware technology defines size.
• Frame term used for specific hardware
  packet.

9
Packets and Hardware Frames

• Packet small block of data (General)
• Hardware technology defines size.
• Frame term used for specific hardware
  packet.

soh block of
data in frame eot
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Packets and Hardware Frames

• Packet small block of data (General)
• Hardware technology defines size.
• Frame term used for specific hardware
  packet.
• Hex 01 Hex 04

Unprintable ASCII Characters
soh block of
data in frame eot
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Packets and Hardware Frames

• Packet small block of data (General)
• Hardware technology defines size.
• Frame term used for specific hardware
  packet.
• Disadvantage is overhead.
• Advantage is reliability.

12
Byte Stuffing

• Most networks cannot afford to reserve
  characters.
• Systems never confuse data with control
  information.
• So extra bits or bytes are inserted to change
  data for transmission.
• i.e. bit or byte stuffing
• The esc character is Hex 1B

13
Byte Stuffing

• Character
  Characters
• in Data
  Sent
• ___________________________________
• soh esc x
• eot esc y
• esc esc z

14
Transmission Errors

• Parity Checking (RS 232)
• Checksums
• 16 bit checksums
• Break data into 16 bit (2byte) segments
• Sum the values
• Send the sum in with the transmission
• Receiver compares answers after transmission

15
Transmission Errors

• Checksums
• H e l l o w o r l
  d .

16
Transmission Errors

• Checksums
• H e l l o w o r l
  d .

48 65 6C 6C 6F 20 77
6F 72 6C 64 2E
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Transmission Errors

• Checksums
• H e l l o w o r l
  d .

48 65 6C 6C 6F 20 77
6F 72 6C 64 2E
4865 6C6C 6F20 776F 726C 642E carry
71FC
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Ethernet

• Bus topology (10/100 Megabits)
• Gigabit Ethernet (Uses fibre as well)
• Hardware monitors bus for carrier
• No carrier Sender transmits
• Carrier sender waits
• Collisions
• Senders use a random number generator to
  calculate delay time
• If collision occurs again the range of the random
  number generator is increased.

19
ATM Technology

• Designed for voice, video data
• Voice and video require low delay and jitter
• Video also requires much higher data rate

20
ATM Technology

• Packets should maximize payload
• 8 Kbytes is common in some networks
• Phone systems use an 8 bit audio sample every 125
  microsecond (millionth of second)
• Sender must delay more than a second to
  accumulate enough samples to fill a packet
• Telephone systems employ echo cancellation
  techniques
• Large packets also create an echo problem ATM
  divides all data into fixed cells
• 48 octets for data
• 5 octets of header information

21
ATM Technology

• Nortel has developed 6.4 Tbps
• Commercial platform of 6.4 Tbps was available in
  2001.
• Uses Dense-wavelength division multiplexing.
• Designed to deliver 99.9999 reliability

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The End.