Data Communications Lecture 1: Netwoking

7/27/2019 Data Communications Lecture 1: Netwoking

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Data CommunicationsMITC 213 Data Communication and Networking


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Introduction

Moving data among computers involvemany different components andmethodologies

Goal of Data Communication:

To allow different hardware and OS tocommunicate and understand eachother

Transmission media have to meet certainhardware specifications

Software used to access the transmissionmedia must also conform to standards


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concerns the transmission of digitalmessages to devices external to themessage source

It is the aim of any communicationssystem to provide the highest possibletransmission rate at the lowest possiblepower and with the least possible noise.

Data Communications


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Communications Channels

Information sent through acommunications channel has a sourcefrom which the information originates, anda destination to which the information is

delivered, has no obvious physicalpresence.

In a digital communications channel, theinformation is represented by individualdata bits, which may be encapsulated intomultibit message units.


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Any communications channel has adirection associated with it

The message source is the transmitter,and the destination is the receiver.



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Transmitter Receiver

Simplex Channel

CHANNEL TYPES

Transmitter

Receiver

Receiver

TransmitterHalf-Duplex Channel

Transmitter

Receiver

Receiver

Transmitter

Full-Duplex Channel


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Simplex

A channel whose direction of transmissionis unchanging

For example, a radio station is a simplexchannel because it always transmits the

signal to its listeners and never allowsthem to transmit back.


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Half-Duplex

single physical channel in which thedirection may be reversed.

Messages may flow in two directions, butnever at the same time, in a half-duplex

system. In a telephone call, one party speaks while

the other listens. After a pause, the otherparty speaks and the first party listens.Speaking simultaneously results in garbledsound that cannot be understood.


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Full Duplex

allows simultaneous message exchange inboth directions.

It really consists of two simplex channels,a forward channel and a reverse channel,

linking the same points. The transmission rate of the reverse

channel may be slower if it is used only forflow control of the forward channel.


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Data Transmission

the physical transfer of data over a point-to-point or point-to-multipointcommunication channel

Examples of such channels are copper

wires, optical fibers, wirelesscommunication channels, and storagemedia.

The data is often represented as anelectro-magnetic signal, such an electricalvoltage signal, a radiowave or microwavesignal or an infra-red signal.


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Data Transmission

Serialized data is not generally sent at auniform rate through a channel

there is usually a burst of regularly spacedbinary data bits followed by a pause, afterwhich the data flow resumes.

timing information should be known by thereceiver to be synchronized with thetransmitter

Failure to remain synchronized throughouta transmission will cause data to becorrupted or lost


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Types of Transmission

Synchronous Transmission

Asynchronous Transmission


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separate channels are used to transmitdata and timing information.

The timing channel transmits clock pulsesto the receiver.

Upon receipt of a clock pulse, the receiverreads the data channel and latches the bitvalue found on the channel at thatmoment.


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The data channel is not read again untilthe next clock pulse arrives.

Because the transmitter originates boththe data and the timing pulses, the

receiver will read the data channel onlywhen told to do so by the transmitter (viathe clock pulse), and synchronization isguaranteed.



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uses start and stop bits to signify thebeginning bit ASCII character wouldactually be transmitted using 10 bits

e.g.: A "0100 0001" would become "1

0100 0001 0 The extra one (or zero depending on

parity bit) at the start and end of thetransmission tells the receiver first that acharacter is coming and secondly that thecharacter has ended.


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a separate timing channel is not used.

The transmitter and receiver must bepreset in advance to an agreed-upon baudrate.

A very accurate local oscillator within thereceiver will then generate an internalclock signal that is equal to thetransmitter's within a fraction of a percent.


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uses no start and stop bits but insteadsynchronizes transmission speeds at boththe receiving and sending end of thetransmission using clock signals built into

each component. A continual stream of data is then sent

between the two nodes.

Due to there being no start and stop bitsthe data transfer rate is quicker althoughmore errors will occur



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Error Detection and Correction

Error detection is the ability to detect thepresence of errors caused by noise orother impairments during transmissionfrom the transmitter to the receiver.

Error correction is the additional ability toreconstruct the original, error-free data.


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Error Checking Mechanisms

Even Parity

Checksum

Cyclic Redundancy Check

Hamming Code


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Even Parity

A parity bit is added to a data packet for

the purpose of error detection.

In the even-parity convention, the value ofthe parity bit is chosen so that the total

number of '1' digits in the combined dataplus parity packet is an even number.

Upon receipt of the packet, the parityneeded for the data is recomputed by local

hardware and compared to the parity bitreceived with the data. If any bit haschanged state, the parity will not match,and an error will have been detected.


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Even Parity Computation

Even Parity Computation

Data Parity Bit

10110001 1

10000110 0


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Example

A wants to transmit: 1001

A computes parity bit value: 1^0^0^1 = 0

A adds parity bit and sends: 10010

B receives: 10010

B computes parity: 1^0^0^1^0 = 0

B reports correct transmission afterobserving expected even result.


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Parity Bit

This mechanism enables the detection ofsingle bit errors, because if one bit getsflipped due to line noise, there will be anincorrect number of ones in the received

data. parity is used in many hardware

applications where an operation can berepeated in case of difficulty, or where

simply detecting the error is helpful.


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Checksum

the packets that constitute a message areadded arithmetically.

A checksum number is appended to thepacket sequence so that the sum of data

plus checksum is zero. If the sum is nonzero, an error has

occurred.

As long as the sum is zero, it is highlyunlikely (but not impossible) that any datahas been corrupted during transmission.


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Checksum Computation

Checksum Computation

10110001100001100100110011111111+10100000

001100100010001000101101111000000000Sum + Checksum = Zero

Data

Arithmetic Sum

Sum Truncated to 8 bitsChecksum

mod (256)


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Cyclic Redundancy Check (CRC)

an error-detecting code

a type of function that takes as input adata stream of any length, and producesas output a value of a certain space,

commonly a 32-bit integer A CRC can be used as a checksum to

detect alteration of data duringtransmission or storage.


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Take a binary message and convert it to apolynomial then divide it by anotherpredefined polynomial called the key.

The remainder from this division is the

CRC. Now transmit both the message andthe CRC.



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The recipient of the transmission does thesame operation (divides the message bythe same key) and compares his CRC withyours.

If they differ, the message must havebeen mangled. If, on the other hand, theyare equal, the odds are pretty good thatthe message went through uncorrupted.


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Hamming Code

a linear error-correcting code named afterits inventor, Richard Hamming.

Hamming codes can detect up to twosimultaneous bit errors, and correct

single-bit errors; thus, reliablecommunication is possible when theHamming distance between thetransmitted and received bit patterns is

less than or equal to one.


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Correcting Errors

Error detection

Positive acknowledgment

Packet passed error checking routine

Negative acknowledgment

Packet failed the error-checkingroutine

Retransmission

If receiver has not received thepacket within a certain period of time

Negative acknowledgment


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Flow Control

Managing the rate of data communication

Devices on a network do not talk and

listen at the same rates

Stop-and-Go or the Stop-and-Wait

Sliding Window


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Stop-and-Go or the Stop-and-Wait

The sender does not send any moreinformation until the receiver hasindicated that it is ready to accept thenext packet.

Very inefficient reduces the capacity ofthe network


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Sliding Window

Defines a window size the max amountof data the sender can transmit beforeacknowledgment must be sent

Size is agreed upon by both parties

Both the sender and receiver maintain afinite size buffer to hold outgoing andincoming packets from the other side.


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Every packet sent by the sender, must beacknowledged by the receiver. The sendermaintains a timer for every packet sent,and any packet unacknowledged in a

certain time, is resent. The sender may send a whole window of

packets before receiving anacknowledgement for the first packet in

the window.

Sliding Window


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Sliding Window

This results in higher transfer rates, as thesender may send multiple packets withoutwaiting for each packet'sacknowledgement.

The Receiver advertises a window sizethat tells the sender how much data it canreceive, in order for the sender not to fillup the receivers buffers.


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Any questions?

Data Communications Lecture 1: Netwoking

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