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IGCSE Computer Science > Chapter 2-Data Transmission > Flashcards

Chapter 2-Data Transmission Flashcards

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1
Q

What are data packets+ 2 other names

A

Packets, datagrams
A small part of a message/data that is sent over a network which after transmission reassemble to form the original message/data

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2
Q

Why are data packets used

A

They are usually 64KiB, †hey are much easier to control and splitting them up means that they can be sent along different routes

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3
Q

When would data packets be particularly beneficial

A

When a particular transmission route is out of action or blocked

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4
Q

What is a drawback of packets

A

The packets all need to be reassembled at the destination

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5
Q

Parts of data packets (3)

A

Packet header
Payload
Trailer

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6
Q

packet headers components (4)

A

IP address of the sender
IP adress of the reciever
Sequence number of packet
Packet size

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7
Q

What does a payload consist of

A

The actual data in the packet
Usually 64 KiB

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8
Q

What does a packet trailer contain

A

A method of identifying end of packet- allows them to separate from eachother
A method of error checking

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9
Q

What are CRC’s

A

Cyclic redundancy checks
Sending: adds all the one bits in payload and stores as hex value in the trailer
Recieve: recalculates no of bits in payload
Compares to trailer
Asks to resend

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10
Q

What does a router do

A

Receives data packet and based on info in header decides where to send it next

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11
Q

Describe the stages of packet switching

A

The message is split up into smaller data packets.
These packets are then independently sent over the network
Along the transmission route are nodes containing router which use the header/ IP address to determine where to send the packet
The shortest possible route is chosen
Packets reach destination in the wrong order

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12
Q

What are the benefits of packet switching(4)

A

No need to tie up single communication line
Possible to overcome faulty lines by rerouting
Relatively easy to expand package usage
High data transmission rate is possible

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13
Q

What are the drawbacks of packet switching (3)

A

Packets can be lost and need to be resent
Delay while packets are being reordered
More prone to errors with real-time streaming

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14
Q

What is hopping

A

Hopping is the use of a hop number that decreases by one each time a packet leaves a router

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15
Q

What 3 factors need to be considered when transmitting data
By what are they considered

A

Direction of data(1 or 2 directions)
Method of transmission(how many bits at a time)
How will data be synchronised (to end up in the right order)
Communication protocols

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16
Q

When does simplex data transmission occur

A

When data can only be sent in One direction
Eg. Computer toPrinter

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17
Q

When does Half Duplex data transmission occur

A

When data is sent in Both directions but not at the same time
Eg. Walkie talkie

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18
Q

When does Full-duplex data transmission occur

A

When data can be sent in both directions at the Same time
Eg. Broadband network connection

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19
Q

What is serial data transmission
Benefit
Drawback
Eg

A

When data is sent over a single wire one bit at a time in a single stream(can be simplex etc.)
Data arrives in order
Can be slower than parallel
Computer->USB->printer

20
Q

What is parallel data transmission

Benefit
Drawback
Eg

A

When several bits of data are sent over multiple Chanel’s/wires at the same time
Good over short, faster than serial
Can become skewed 20m+
Internal circuits in computer

21
Q

What is skewed data

A

Data that is out of order

22
Q

What is a USB

A

a type of serial data transmission which has become the industry standard for connecting computers to devices via a USB port

23
Q

What kind of data transmission is used by USB

A

Half and full duplex

24
Q

What does a USB cable consist of

A

A 4-wire shielded cable with a red and black cable for power and green and white for data transmission
Red-5V
Black-ground
White+
Green- -

25
Q

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

A

Computer detects there is a device-voltage change in data signalers
Device is recognized- appropriate software is loaded to communicate effectively
Device is not recognized-user is prompted to download appropriate software

26
Q

What are the benefits of USB systems(8)

A

Devices are automatically detected and software is loaded
Only fits in one way -prevents incorrect connection
Support available
Can support different data transmission rates
No need for external power source
Error-free data transmission(is resent)
Easy to add more USB ports with USB hubs
Backward compatible(old versions supported)

27
Q

Drawbacks of USB (3)

A

Max cable length(5)-USB hub needed
Very early versions not compatible with new computers
Slower transfer rate than Ethernet connections

28
Q

maximum transfer rate of USB

A

480 Mbps

29
Q

Node

A

Stages in a network that can recieve and transmit data packets. (Routers = nodes in communication networks)

30
Q

Why can errors during data transmission occur (3)

A

Interference (cable suffers from electrical interference leading to lost data)
Problems during packet switching
Skewing of data (parallel data transmission, can cause corruption if bits arrive out on sync)

31
Q

How to check for data transmission errors (3)

A

Parity checks
Check sum

32
Q

Parity checks

A

Form of error checking based on the number of 1 bits in a byte of data
Makes use of even parity (even number of 1 bits) or odd parity (odd number of 1 bits)
Parity is determinedvia an agreement made between the sender and reciever before data is transferred therefore it is also a type of transmission protocol

33
Q

Error of parity checking

A

If more than one bit is modified during transmission, but the bytes retained even/odd parity no error would be flagged

34
Q

parity block

A

Horizontal and vertical parity check on a block of data being transmitted

35
Q

Checksum

A

Method to check if data has been changed or corrupted during transmission. The checksum value is calculated from the block of data being sent and sent after each blocj

36
Q

Check sum process (6)

A

When a block is about to be transmitted, a checksum is calculated from the data block
Calculation done with an agreed algorhythm between sender and receiver
Checksum transmitted with block of data
Once received check sum is recalculated from the block of data using the algorhythm
The recalculated checksum is compared with the original
If theyre the same, no errors, if their not a resend request is made

37
Q

Echo check

A

When data is sent to another device and then back to the sender again who compares the 2 sets of data to check if theres been any transmission errors. Not reliable as you wont know whether it happened ongthe way there or back

38
Q

Check digits

A

Method of identifying errors in data entry, the final digit in a code calculated from all thye other digits. Used in International Standard Book Numbers (ISBN), barcodes and Vehicle Identification Numbers.

39
Q

Types or error identified by checksum (4)

A

Phonetic (thirteen/thirty)
Incorrect digit (5203/5223)
Ommitted/extra digits (5203/52030)
Transposition errors (right digit wrong order)

40
Q

Methods used to generate check digit

A

ISBN 13
Modulo 11

41
Q

Parity block

A

Block of data sent and a parity check is done both vertically and horizontally

42
Q

ARQ’s

A

Automatic repeat requests
Uses positive/negative acknowledgements and timeouts. Receiving device receives an error detection code, if no error is detected it sends back a positive acknowledgement. If it detects an error it sends a negative acknowledgement requesting for retransmission. Data is automatically resent if time-out limit is reached until a positive acknowledgement is sent back or a predetermined number of positive acknowledgements is reached.

43
Q

Acknowledgements

A

Messages sent to the receiver indicating data has/has not been received correctly

44
Q

Where are ARQs often used

A

By mobile phone companies 60 guarantee data integrity

45
Q

Use of encryption

A

To minimise the risk of an eavesdropper (hacker intercepting data being transmitted over a public network)

46
Q

Symmetric encryption

A

Uses the same encryption key to encrypt and decrypt the encoded message

47
Q

Asymmetric encryption

A

Designed to overcome security risks associated with symmetric encryption, makes use of a public and a private key

IGCSE Computer Science (11 decks)

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