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Integrity in Data Communications

Integrity in Data Communications

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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.