Integrity in Data Communications Packets, Frames and Error Detection

More About Packets Networks do not transfer arbitrary amounts of data for 2 reasons: 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. - 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. -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’

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

Packets and Time-Division Multiplexing Computers take turns sending and receiving small packets of data. Computers take turns sending and receiving small packets of data Computer 1 using channel to send packet. Multiplexing occurs here.

Packets and Time-Division Multiplexing Computers take turns sending and receiving small packets of data. Computers take turns sending and receiving small packets of data Computer 2 using channel to send packet. Multiplexing occurs here.

Packets and Time-Division Multiplexing Computers take turns sending and receiving small packets of data. Computers take turns sending and receiving small packets of data Computer 3 using channel to send packet. Multiplexing occurs here.

Packets and Time-Division Multiplexing Computers take turns sending and receiving small packets of data. 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. Data are arriving at two or more computers simultaneously.

Packets and Hardware Frames Packet = small block of data (General) Packet = small block of data (General) Hardware technology defines size. Hardware technology defines size. Frame = term used for specific hardware ``packet``. Frame = term used for specific hardware ``packet``.

Packets and Hardware Frames Packet = small block of data (General) Packet = small block of data (General) Hardware technology defines size. Hardware technology defines size. Frame = term used for specific hardware ``packet``. Frame = term used for specific hardware ``packet``. soh block of data in frame eot

Packets and Hardware Frames Packet = small block of data (General) Packet = small block of data (General) Hardware technology defines size. Hardware technology defines size. Frame = term used for specific hardware ``packet``. Frame = term used for specific hardware ``packet``. Hex 01 Hex 04 Hex 01 Hex 04 soh block of data in frame eot Unprintable ASCII Characters

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

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

Byte Stuffing Character Characters Character Characters in Data Sent in Data Sent___________________________________ soh esc x eot esc y esc esc z

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

Transmission Errors Checksums Checksums H e l l o w o r l d. H e l l o w o r l d.

Transmission Errors Checksums Checksums H e l l o w o r l d. H e l l o w o r l d C 6C 6F F 72 6C 64 2E

Transmission Errors Checksums Checksums H e l l o w o r l d. H e l l o w o r l d C 6C 6F F 72 6C 64 2E C6C + 6F F + 726C + 642E + carry = 71FC

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

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

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

ATM Technology Nortel has developed 6.4 Tbps Nortel has developed 6.4 Tbps Commercial platform of 6.4 Tbps was available in Commercial platform of 6.4 Tbps was available in Uses Dense-wavelength division multiplexing. Uses Dense-wavelength division multiplexing. Designed to deliver % reliability Designed to deliver % reliability

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