Integrity in Data Communications Packets, Frames and Error Detection.

Integrity in DataIntegrity in Data CommunicationsCommunications

Packets, Frames and Error Packets, Frames and Error

DetectionDetection

More About PacketsMore About Packets

Networks do not transfer arbitrary amounts of Networks do not transfer arbitrary amounts of data for 2 reasons:data for 2 reasons:

1.1. Errors in large blocks cause large delays.Errors in large blocks cause large delays.

--Senders and receivers have to coordinate Senders and receivers have to coordinate transmission. Errors often occur. transmission. Errors often occur.

-Dividing data into small blocks allows the -Dividing data into small blocks allows the sending and receiving computers to make fast sending and receiving computers to make fast

re- re- transmission. transmission.

These networks are called ‘packet networks’ or packet

switching networks’

More About PacketsMore About Packets

2. 2. Computers have to share underlying connections Computers have to share underlying connections in hardware.in hardware.

-Communications channels are expensive so -Communications channels are expensive so sharing allows all to be treated equally. sharing allows all to be treated equally.

Early networks allowed an application to hold a Early networks allowed an application to hold a resource until finished.resource until finished.

Packets and Time-Division Packets and Time-Division MultiplexingMultiplexing

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

1

2

3

Computer 1 using channel to send packet.

Multiplexing occurs here.



1

2

3





1

2

3





A source with a small total amount will finish A source with a small total amount will finish promptly.promptly.

Larger amounts will take longer.Larger amounts will take longer.

Data are arriving at two or more computers Data are arriving at two or more computers simultaneously.simultaneously.

Packets and Hardware FramesPackets 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 Frame = term used for specific hardware

``packet``.``packet``.




soh block of data in frame eot




Hex 01Hex 01 Hex 04Hex 04

soh block of data in frame eot

Unprintable ASCII

Characters



``packet``.``packet``. Disadvantage is overhead.Disadvantage is overhead. Advantage is reliability.Advantage is reliability.

Byte StuffingByte Stuffing

Most networks cannot afford to reserve Most networks cannot afford to reserve characters.characters.

Systems never confuse data with control Systems never confuse data with control information.information.

So extra bits or bytes are inserted to change So extra bits or bytes are inserted to change data for transmission.data for transmission.

i.e. bit or byte stuffingi.e. bit or byte stuffing

The esc character is Hex 1BThe esc character is Hex 1B

Byte StuffingByte Stuffing

Character CharactersCharacter Characters

in Data Sentin Data Sent

______________________________________________________________________

sohsoh esc x esc x

eoteot esc y esc y

escesc esc z esc z

Transmission ErrorsTransmission Errors

Parity Checking (RS 232)Parity Checking (RS 232) ChecksumsChecksums

16 bit checksums16 bit checksums

Break data into 16 bit (2byte) segmentsBreak data into 16 bit (2byte) segmentsSum the valuesSum the valuesSend the sum in with the transmissionSend the sum in with the transmissionReceiver compares answers after transmissionReceiver compares answers after transmission


ChecksumsChecksums

H e l l o w o r l d .H e l l o w o r l d .


ChecksumsChecksums


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


ChecksumsChecksums


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

4865 + 6C6C + 6F20 + 776F + 726C + 642E + carry = 71FC

EthernetEthernet

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 carrierHardware monitors bus for carrier No carrier – Sender transmitsNo carrier – Sender transmits Carrier – sender waitsCarrier – sender waits CollisionsCollisions

Senders use a random number generator to Senders use a random number generator to calculate delay timecalculate delay time

If collision occurs again the range of the If collision occurs again the range of the random number generator is increased.random number generator is increased.

ATM TechnologyATM Technology

Designed for voice, video & dataDesigned for voice, video & data

Voice and video require low delay and jitterVoice and video require low delay and jitter

Video also requires much higher data rateVideo also requires much higher data rate


Packets should maximize payloadPackets should maximize payload8 Kbytes is common in some networks8 Kbytes is common in some networksPhone systems use an 8 bit audio sample Phone systems use an 8 bit audio sample

every 125 microsecond (millionth of second)every 125 microsecond (millionth of second)Sender must delay more than a second to Sender must delay more than a second to

accumulate enough samples to fill a packetaccumulate enough samples to fill a packetTelephone systems employ echo cancellation Telephone systems employ echo cancellation

techniquestechniquesLarge packets also create an echo problem Large packets also create an echo problem

ATM divides all data into fixed ``cells``ATM divides all data into fixed ``cells``48 octets for data48 octets for data5 octets of header information5 octets of header information


Nortel has developed 6.4 TbpsNortel has developed 6.4 Tbps Commercial platform of 6.4 Tbps was available Commercial platform of 6.4 Tbps was available

in 2001.in 2001. Uses Dense-wavelength division multiplexing.Uses Dense-wavelength division multiplexing. Designed to deliver 99.9999% reliabilityDesigned to deliver 99.9999% reliability

The End.The End.

Integrity in Data Communications Packets, Frames and Error Detection.

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