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Integrity in Data Communications

Explore the fundamentals of data communication, packet networks, multiplexing, frames, checksums, and Ethernet and ATM technologies. Learn about transmission errors and reliability techniques in modern data networks.

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Integrity in Data Communications

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  1. Integrity in DataCommunications 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``. sohblock of data inframeeot

  10. Packets and Hardware Frames • Packet = small block of data (General) • Hardware technology defines size. • Frame = term used for specific hardware ``packet``. Hex 01Hex 04 Unprintable ASCII Characters sohblock of data inframeeot

  11. 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

  17. 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

  18. 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

  22. The End.

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