data transmission

what are data packets

what are data packets

small units of data used for sending data over long distances

what is the typical size of a data packet

64KiB

why are data packets used

easier to control, can avoid busy transmission routes

What is the structure of a typical data packet?

header, payload, trailer.

What information does the packet header contain?

IP of sending device, IP of receiving device, sequence number, packet size

What is the payload in a data packet?

actual data being sent

What is the purpose of the packet trailer?

identify end of packet, includes error checker

what is a typical error checker in a packet

CRC

what does CRC stand for

cyclic redundancy check

why does a packet trailer need an error checking method

ensure data integrity

How does the CRC work in data packets?

Sending computer adds 1-bits and stores as hex value

Receiving computer recalculates 1-bits and compares values

If values match, no errors

What is packet switching?

process where data packets are sent from a sender via multiple possible routes

What is the main benefit of splitting data into packets?

each packet can take different routes to the destination which can avoid busy transmission routes

What is the main drawback of splitting data into packets?

data needs to be reassemble at destination.

How do packets find their route in packet switching?

Each packet follows the shortest available path

What is a key issue with packet switching regarding order?

can arrive in a different order from which they were sent.

List four benefits of packet switching.

can re-route packets when a line is busy

easy to expand package usage

high data transmission rate

no need to tie up a single communication line

List three drawbacks of packet switching.

packets can be lost

prone to real-time streaming errors

delay at destination when being reordered

What is the problem of lost packets in packet switching?

Packets can get lost by bouncing between routers and clogging the network.

How is the lost packet problem in packet switching overcome?

hopping

what is hopping

where a hop number in the packet header decreases with each number

What are the three key factors to consider in data transmission?

direction, method, data synching

What is simplex data transmission?

data sent one direction

What is an example of simplex data transmission?

computer to printer

What is half-duplex data transmission?

both directions, not at the same time

What is an example of half-duplex data transmission?

a walkie-talkie

What is full-duplex data transmission?

sent both ways simultaneously

What is an example of full-duplex data transmission?

broadband internet connection

What is serial data transmission?

one bit at a time over a single wire

6 characteristics of serial data transmission

one bit at a time

single channel

can be simplex, half-duplex, full duplex

best over long distance

slower data rate

arrives synched

What is an example of serial data transmission?

computer to a printer via a USB connection.

What is parallel data transmission?

Several bits of data simultaneously down several channels

4 characteristics of parallel data transmission

multiple bits simultaneously

multiple channels

When is parallel data transmission used?

High-speed network connections.

What modes can parallel data transmission operate in?

simplex, half-duplex, full-duplex

Over what distance does parallel data transmission work well?

works well over short distances.

What happens to parallel data transmission over longer distances?

data can become skewed, arriving unsynched

How does the length of the wire affect parallel data transmission?

longer the wire, the worse the data skew

How does the speed of parallel data transmission compare to serial data transmission?

parallel is faster than serial

Where is parallel data transmission commonly used within a computer?

internal circuits in a computer

What type of data transmission does USB use?

serial

What types of data transmission does USB support?

half-duplex, full-duplex

How many wires are in a USB cable

four

what are the colour wires in a USB cable

red, black, green, white

what is the red wire in a USB for

power

what is the black wire in a USB for

power

what is the green wire in a USB for

data transmission

what is the white wire in a USB for

data transmission

What happens when a device is plugged into a computer using a USB port?

computer detects device due to voltage change, driver software loaded, prompts user to download if necessary

What is USB-C

24-pin symmetrical connector

features of USB-C

fits in its port either way round

smaller and thinner than older USBs

offer 100 watt

carry 10GBps of data

support 4k video delivery

Is USB-C backward compatible, and with which versions?

it is backward compatible with USB 2.0 and 3.0

What is the significance of USB-C becoming more common?

become the new industry standard format.

Why is it important to check for errors during data transmission?

computers are unable to understand text if the words are corrupted

what does the built-in dictionary do in data transmission

tries to understand text that is corrupted with words it has stored

What are the causes of errors during data transmission?

interference

problems during packet switching

skewing of data

How does interference affect data transmission?

causes corruption or lose of data due to electrical disturbances

What issues can arise from problems during packet switching?

data loss or even the gain of incorrect data.

What is skewing of data

bits to arrive out of synch

how does skewed data affect data transmission

data corruption.

What are the methods used to check for errors in data transmission?

parity checks

checksum

echo check

What can happen if data corruption occurs during transmission?

data may become unintelligible leading to errors and miscommunication

What is parity checking?

based on the number of 1-bits in a byte of data

What are the two types of parity?

EVEN and ODD

what is even parity checking

the 1s in a byte add to an even number

what is odd parity checking

when the 1s in a byte add to an odd number

How is the parity bit set?

determining which bit is reserving the left-most bit for the parity bit.

What must be agreed upon before data is transferred using parity checks?

An agreement must be made between the sender and receiver on which parity is being used

What happens if even parity is used and the received byte has an odd number of 1-bits?

an error is flagged.

What is the limitation of parity checking?

If two bits changed value

what is the result of two bits changed in a parity

data may be impossible to locate

How does the parity block method work?

sending a block of data and totaling the number of 1-bits horizontally and vertically

What is a checksum?

sending an additional checksum value with the data block.

process of using a checksum to verify data integrity

calculate checksum from data block

transmit checksum and data block

recalculate checksum at receiving side

compare recalculation

if they match, no error

What happens if the checksums do not match?

request to resend data block is made

What is echo check?

data sent to another device is sent back again to the sender to check for errors

How does echo check work?

copy sent back to sender

sender compares with original data

if no differences, data was sent without errors

What is the limitation of echo check?

it isn’t known whether the error occurred when sending the data

What is a check digit?

final digit included in a code, calculated from all the other digits in the code

Where are check digits commonly used?

barcodes, ISBN, VIN

what does ISBN stand for

International Standard Book Numbers

what does VIN stand for

Vehicle Identification Numbers

What types of errors can check digits usually detect?

incorrect number entered, transposition errors, omitted or extra digits, phonetic errors

what error in a check digit would be 5327 when it is supposed to be 5307

incorrect digit entered

what error in a check digit would be 5037 when it is supposed to be 5307

transposition error

what error in a check digit would be 537 when it is supposed to be 5307

omitted/extra digit

what error in a check digit would be 13 when it is supposed to be 30

phonetic error

What are two common methods to generate a check digit?

ISBN 13

Modulo-11

What is the check digit in ISBN 13?

the thirteenth digit in the number.

How is the check digit generated in ISBN 13? (Step 1)

Add all the odd-numbered digits together.

How is the check digit generated in ISBN 13? (Step 2)

Add all the even-numbered digits together and multiply the result by 3.

How is the check digit generated in ISBN 13? (Step 3)

Add the results from steps 1 and 2 together and divide by 10.

How is the check digit generated in ISBN 13? (Step 4)

Take the remainder; if the value isn’t zero then subtract the remainder from 10 to find the check digit.

What are the methods to verify data has arrived unchanged?

parity

echo

ARQ

what does ARQ stand for

Automatic Repeat Request

What does ARQ use to check data?

positive, negative acknowledgements

timeout intervals

What is the purpose of a positive acknowledgement in ARQ?

data has been received correctly

What happens if an error is detected in ARQ

A negative acknowledgement is sent back

What is the role of a timeout in ARQ?

time interval allowed to elapse before an acknowledgement is received

what happens if the ARQ time interval is elapsed and nothing is received

the data is re-sent.