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IGCSE O Level Computer science > Chapter 2: Data Transmission > Flashcards

Chapter 2: Data Transmission Flashcards

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1
Q

Define a data packet

A

a small part of a message/data that is transmitted over a network

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2
Q

Define packet header

A

the part of the data packet that contains the IP addresses of the sender and receiver, and includes the packet number which allows reassembly of the data packets

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3
Q

Define packet trailer

A

The part of a data packet test indicates the end of the data packet and CRC.

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4
Q

Define CRC

A

An error checking method in which all the 1-bits I’m the data packet payload are added and the total is stored in the packet trailer.

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5
Q

Define payload

A

The actual data being carried on a data packet

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6
Q

Define node

A

Stages in a network that can receive and transmit data packets

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7
Q

Define packet switching

A

A method of transmission in which a message is broken into many data packets which can be sent along pathways independently to each other

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8
Q

Define router

A

A device that enables data packets to be moved between different networks

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9
Q

Define real time streaming

A

the transmission of data over a network for live events where the data is sent as soon as it is received or generated

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10
Q

Define hop/hopping number

A

a number in a data packet header used to stop data packets that never reach their destination from ‘clogging up’ the data paths/routes

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11
Q

Define simplex

A

data that can be sent on one direction only

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12
Q

Define half-duplex

A

data that can be sent in both directions but not at the same time

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13
Q

Define full duplex

A

data that can be sent in both directions at the same time (simultaneously)

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14
Q

Define serial data transmission

A

sending data down one channel/wire one bit at a time

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15
Q

Define parallel data transmission

A

sending data down several channels/wires several bits at a time (usually 1 byte)

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16
Q

Define skewed data

A

data that arrives at the destination with the bits no longer synchronised

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17
Q

Define USB

A

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

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18
Q

Define parity check

A

a method used to check if data has been transferred correctly; it makes use of even parity (an even number of 1-bits) or odd parity (an odd number of 1-bits)

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19
Q

Define parity bit

A

a bit (either 0 or 1) added to a byte of data in the most significant bit position; this ensures that the byte follows the correct even parity or odd parity protocol

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20
Q

Define parity block

A

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

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21
Q

Define parity block

A

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

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22
Q

Define parity byte

A

an extra byte of data sent at the end of a parity block; it is composed of the
parity bits generated from a vertical parity check of the data block

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23
Q

Define checksum

A

a verification method used to check if data transferred has been altered or corrupted; calculated from the block of data of data being sent; the checksum value is sent after each data block

24
Q

Define ARQ

A

a method of checking transmitted data for errors; it makes use of acknowledgement and timeout to automatically request re-sending of data if the time interval before positive acknowledgement is too long

25
Q

Define acknowledgment

A

a message sent to the receiver indicating that data has been received correctly (used in the ARQ error detection method)

26
Q

Define timeout

A

the time interval allowed to elapse before an acknowledgement is received (in the ARQ error detection method)

27
Q

Define echo check

A

a method used to check if data has been transferred correctly; data is sent to a receiver and then immediately sent back to the sender; the sender then checks if the received data matches the sent data

28
Q

Define check digit

A

an additional digit appended to a number to check if the entered number is error-free; check digit is a data entry check and not a data transmission check

29
Q

Define eavesdropper

A

another name for a hacker who intercepts data being transmitted on a wired or wireless network

30
Q

Define encryption

A

the process of making data meaningless using encryption keys; without the correct decryption key the data cannot be decoded (unscrambled)

31
Q

Define plaintext

A

the original text/message before it is put through an encryption algorithm

32
Q

Define cipher text

A

encrypted data that is the result of putting a plaintext message through an
encryption algorithm

33
Q

Define encryption algorithm

A

a complex piece of software that takes plaintext and generates a ciphertext

34
Q

Define symmetric encryption

A

a type of encryption in which the same encryption key is used both to encrypt and decrypt a message

35
Q

Define asymmetric encryption

A

a type of encryption that uses public keys and private keys to ensure data is secure

36
Q

Define public key

A

a type of encryption key that is known to all users

37
Q

Define private key

A

a type of encryption key which is known only to the single computer/user

38
Q

Define quantum computer

A

a computer that can perform very fast calculations; it can perform calculations that are based on probability rather than simple 0 or 1 values; this gives
a quantum computer the potential to process considerably more data than existing computers

39
Q

What does a packet header consist of

A

» the IP address of the sending device

» the IP address of the receiving device

» the sequence number of the packet (this is to ensure that all the packets can
be reassembled into the correct order once they reach the destination)

» packet size (this is to ensure the receiving station can check if all of the
packets have arrived intact).

40
Q

What does packet trailer consist of

A

» some way of identifying the end of the packet; this is essential to allow each packet to be separated from each other as they travel from sending to receiving station
» an error checking method; cyclic redundancy checks

41
Q

How does CRC error checking method work

A

– this involves the sending computer adding up all the 1-bits in the payload
and storing this as a hex value in the trailer before it is sent

– once the packet arrives, the receiving computer recalculates the number of
1-bits in the payload

– the computer then checks this value against the one sent in the trailer

– if the two values match, then no transmission errors have occurred;
otherwise the packet needs to be re-sent.

42
Q

Describe packet switching

A

» each packet will follow its own path (route)

» routers will determine the route of each packet

» routing selection depends on the number of packets waiting to be processed
at each node

» the shortest possible path available is always selected – this may not always
be the shortest path that could be taken, since certain parts of the route may
be too busy or not suitable

» packets can reach the destination in a different order to that in
which they were sent.

43
Q

State the benefits of packet switching

A

» there is no need to tie up a single communication line

» it is possible to overcome failed, busy or faulty lines by simply re-routing
packets

» it is relatively easy to expand package usage

» a high data transmission rate is possible.

44
Q

State the drawbacks of packet switching

A

» packets can be lost and need to be re-sent

» the method is more prone to errors with real-time streaming (for example, a
live sporting event being transmitted over the internet)

» there is a delay at the destination whilst the packets are being re-ordered.

45
Q

How are packets lost

A

they keep ‘bouncing’ around from router to router and never actually reach their destination.

46
Q

How does hopping work

A

Each packet has a maximum op number to start with.

Once a hop number reaches zero, and the packet hasn’t reached its destination,

then the packet is deleted when it reaches the next router.

The missing packets will then be flagged by the receiving computer

a request to re-send these packets will be made.

47
Q

What are the factors needed to be considered when transmitting data

A

» the direction of data transmission (for example, can data transmit in one direction only, or in both directions)

» the method of transmission (for example, how many bits can be sent at the same time)

» how will data be synchronised (that is, how to make sure the received data is in the correct order).

48
Q

What happens When a device is plugged into a computer using one of the USB ports

A

» the computer automatically detects that a device is present (this is due to a small change in the voltage on the data signal wires in the USB cable)

» the device is automatically recognised, and the appropriate device driver software is loaded up so that the computer and device can communicate effectively

» if a new device is detected, the computer will look for the device driver that matches the device; if this is not available, the user is prompted to download the appropriate driver software (some systems do this automatically and the user will see a notice asking for permission to connect to the device website).

49
Q

State errors that can occur during data transmission

A

» interference (all types of cable can suffer from electrical interference, which can cause data to be corrupted or even lost)

» problems during packet switching (this can lead to data loss – or it is even possible to gain data!)

» skewing of data (this occurs during parallel data transmission and can cause data corruption if the bits arrive out of synchronisation).

50
Q

Describe the checksum process

A

» when a block of data is about to be transmitted, the checksum is calculated from the block of data

» the calculation is done using an agreed algorithm (this algorithm has been agreed by sender and receiver)

» the checksum is then transmitted with the block of data

» at the receiving end, the checksum is recalculated by the computer using the
block of data (the agreed algorithm is used to find the checksum)

» the re-calculated checksum is then compared to the checksum sent with the
data block

» if the two checksums are the same, then no transmission errors have occurred;
otherwise a request is made to re-send the block of data.

51
Q

Describe echo check

A

» a copy of the data is sent back to the sender

» the returned data is compared with the original data by the sender’s computer

» if there are no differences, then the data was sent without error

» if the two sets of data are different, then an error occurred at some stage
during the data transmission.

52
Q

What kind of errors can be detected by check digit

A

» an incorrect digit entered, for example 5327 entered instead of 5307

» transposition errors where two numbers have changed order, for example 5037
instead of 5307

» omitted or extra digits, for example 537 instead of 5307 or 53107 instead
of 5307

» phonetic errors, for example 13 (thirteen), instead of 30 (thirty).

53
Q

Describe ISBN-13 method

A

1) add all the odd numbered digits together

2) add all the even numbered digits together and multiply the result by 3

3) add the results from 1 and 2 together and divide by 10

4) take the remainder, if it is zero then use this value, otherwise subtract the remainder from 10 to find the check digit.

54
Q

Recalculating ISBN-13 method

A

1) add all the odd numbered digits together, including the check digit

2) add all the even number of digits together and multiply the result by 3

3) add the results from 1 and 2 together and divide by 10

4) the number is correct if the remainder is zero.

55
Q

Define modulo-11

A

1 each digit in the number is given a weighting of 8, 7, 6, 5, 4, 3 or 2 starting from the
left (weightings start from 8 since the number will become eight-digit when the
check digit is added)
2 the digit is multiplied by its weighting and then each value is added to make a total
3 the total is divided by 11
4 the remainder is then subtracted from 11 to find the check digit (note if the
remainder is 10 then the check digit ‘X’ is used).

56
Q

Recalculateing modulo-11

A

1 each digit in the number is given a weighting of 8, 7, 6, 5, 4, 3, 2 or 1 starting from
the left

2 the digit is multiplied by its weighting and then each value is added to make a total

3 the total is divided by 11

4 the number is correct if the remainder is zero

57
Q

Describe ARQ

A

» ARQ uses positive and negative acknowledgements and timeout
» the receiving device receives an error detection code as part of the data transmission (this is typically a Cyclic Redundancy Check ; this is used to detect whether the received data contains any transmission errors

» if no error is detected, a positive acknowledgement is sent back to the sending device

» if an error is detected, the receiving device now sends a negative acknowledgement to the sending device and requests re-transmission of the data

» a time-out is used by the sending device by waiting a pre-determined amount of time ….

» if no acknowledgement of any type has been received by the sending device within this time limit, it automatically re-sends the data until a positive acknowledgement is received ….

» or until a pre-determined number of re-transmissions has taken place

» ARQ is often used by mobile phone networks to guarantee data integrity.

IGCSE O Level Computer science (3 decks)

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