Chapter 2: Data Transmission

Question 1

Define a data packet

Answer 1

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

Question 2

Define packet header

Answer 2

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

Question 3

Define packet trailer

Answer 3

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

Question 4

Define CRC

Answer 4

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.

Question 5

Define payload

Answer 5

The actual data being carried on a data packet

Question 6

Define node

Answer 6

Stages in a network that can receive and transmit data packets

Question 7

Define packet switching

Answer 7

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

Question 8

Define router

Answer 8

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

Question 9

Define real time streaming

Answer 9

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

Question 10

Define hop/hopping number

Answer 10

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

Question 11

Define simplex

Answer 11

data that can be sent on one direction only

Question 12

Define half-duplex

Answer 12

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

Question 13

Define full duplex

Answer 13

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

Question 14

Define serial data transmission

Answer 14

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

Question 15

Define parallel data transmission

Answer 15

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

Question 16

Define skewed data

Answer 16

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

Question 17

Define USB

Answer 17

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

Question 18

Define parity check

Answer 18

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)

Question 19

Define parity bit

Answer 19

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

Question 20

Define parity block

Answer 20

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

Question 21

Define parity block

Answer 21

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

Question 22

Define parity byte

Answer 22

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

Question 23

Define checksum

Answer 23

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

Question 24

Define ARQ

Answer 24

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

Question 25

Define acknowledgment

Answer 25

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

Question 26

Define timeout

Answer 26

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

Question 27

Define echo check

Answer 27

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

Question 28

Define check digit

Answer 28

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

Question 29

Define eavesdropper

Answer 29

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

Question 30

Define encryption

Answer 30

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

Question 31

Define plaintext

Answer 31

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

Question 32

Define cipher text

Answer 32

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

Question 33

Define encryption algorithm

Answer 33

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

Question 34

Define symmetric encryption

Answer 34

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

Question 35

Define asymmetric encryption

Answer 35

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

Question 36

Define public key

Answer 36

a type of encryption key that is known to all users

Question 37

Define private key

Answer 37

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

Question 38

Define quantum computer

Answer 38

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

Question 39

What does a packet header consist of

Answer 39

» 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).

Question 40

What does packet trailer consist of

Answer 40

» 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

Question 41

How does CRC error checking method work

Answer 41

– 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.

Question 42

Describe packet switching

Answer 42

» 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.

Question 43

State the benefits of packet switching

Answer 43

» 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.

Question 44

State the drawbacks of packet switching

Answer 44

» 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.

Question 45

How are packets lost

Answer 45

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

Question 46

How does hopping work

Answer 46

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.

Question 47

What are the factors needed to be considered when transmitting data

Answer 47

» 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).

Question 48

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

Answer 48

» 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).

Question 49

State errors that can occur during data transmission

Answer 49

» 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).

Question 50

Describe the checksum process

Answer 50

» 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.

Question 51

Describe echo check

Answer 51

» 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.

Question 52

What kind of errors can be detected by check digit

Answer 52

» 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).

Question 53

Describe ISBN-13 method

Answer 53

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.

Question 54

Recalculating ISBN-13 method

Answer 54

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.

Question 55

Define modulo-11

Answer 55

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).

Question 56

Recalculateing modulo-11

Answer 56

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

Question 57

Describe ARQ

Answer 57

» 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.