2- Data transmission

Question 1

Types and methods of data transmission

Answer 1

1- Data packets
2- Data transmission
3- Universal serial bus (USB)

Question 2

1-Data packets

Answer 2

• Data sent over long distances is usually broken up into data packets (sometimes called datagrams).
• The packets of data are usually quite small, typically 64KiB, which are much easier to control than a long continuous stream of data.

So Data packet is : – a small part of a message/data that is transmitted over a network; after transmission all the data packets are reassembled to form the original message/data

Question 3

A typical packet is split up into

Answer 3

» a packet header
» the payload
» a trailer

Question 4

packet header

Answer 4

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 5

payload

Answer 5

the actual data being carried in a data packe

Question 6

packet trailer

Answer 6

the part of a data packet that indicates the end of the data packet and cyclic redundancy check error ( CRC)

Question 7

Packet switching

Answer 7

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

Question 8

A video conference is taking place between delegates in the USA and Mauritius. Packet switching is used to send video and sound data between delegates.
Describe:
a) the potential problems with sound and video quality
b) how the problems in part a) could be overcome.

Answer 8

a) Video conferencing is in real time. Video and audio data is split up into data packets before it is sent over the internet. This means that each data packet could potentially follow a different route from delegate to delegate. Data packets could therefore take different times from sender to recipient, or some may even become ‘lost’. The arriving packets also need to be reassembled into the correct sequence, which can also cause a time delay. All of this could lead to gaps in data received, freezing of images, drop out or voice and mouth ‘out of synchronisation’ – this would be caused by audio and video data taking different times from sender to receiver.
b) Since the problems seem to be due to timing issues, the best solution would be to eliminate these time differences. A single path for all data packets would resolve this; that could be achieved using a dedicated communication link between delegates.

Question 9

Benefits of packet switching

Answer 9

• 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 10

Drawbacks of packet switching

Answer 10

• Packets can be lost and need to be re-sent.
• The method doesn’t work well
  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 11

Explain how packet switching could be used to download some software from a website

Answer 11

• The software is broken up into data packets; each numbered in sequence and each has the same MAC/IP addresses.
• Each data packet is sent to its destination via its own route; the route taken is ‘decided’ by the routers within the network which take various factors into consideration.
• When the data packets arrive at their destination, they are reassembled into the correct order according to their sequence numbers.
• A CRC check is carried out on each data packet to ensure no errors occurred during the transmission of the software.
• Note: mention of hop numbers can also be included in the description and should gain credit.

Question 12

Data being shared between two computers is split up into a number
of data packets prior to transmission. The data packets contain a header and a trailer.
a) Give three pieces of information found in the header.
b) The trailer contains a cyclic redundancy check (CRC).
Name another item found in the trailer.

c) i) Explain how it is possible for data packets to be lost during their transmission across a network.
ii) Describe how it is possible to deal with lost packets so that they don’t cause network problems, such as ‘clogging up’ the system.

Answer 12

a - IP addresses of sender and recipient.
- Packet sequence number.
- Packet size.
b - Some indication of the end of the packet.

c i) Data packets keep ‘bouncing’ around from
router to router and never actually reach their destination; this then becomes a lost data packet.
ii) Each data packet is given a hop number; every time a packet leaves a router, the hop number is decreased by 1; if the hop number reaches 0, and it hasn’t yet reached its destination, then the data packet is deleted from the system and a request is made for it to be re-sent.

Question 13

router

Answer 13

a device that enables data packets to be moved between different networks, for example to join a LAN to a WAN

Question 14

real time streaming

Answer 14

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

Question 15

hopping/hop number

Answer 15

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 16

2- Data transmission

Answer 16

• Data transmission can be either over a short distance (for example, computer to printer) or over longer distances (for example, from one computer to another in a global network).
• Essentially, three factors need to be considered when transmitting data:
  » the direction of data transmission
  » the method of transmission
  » how will data be synchronised
• These factors are usually considered by a communication protocol.

Question 17

The modes of data transmission

Answer 17

• Simplex : mode occurs when data can be sent in ONE DIRECTION ONLY.
• Half-duplex : mode occurs when data is sent in BOTH DIRECTIONS but NOT AT THE SAME TIME.
• Full-duplex : mode occurs when data can be sent in BOTH DIRECTIONS AT THE SAME TIME.

Question 18

Types of data transmission

Answer 18

• Serial data transmission : occurs when data is sent ONE BIT AT A TIME over a
  SINGLE WIRE/CHANNEL. Bits are sent one after the other as a single stream.
  Serial data transmission works well over long distance.
• Parallel data transmission : occurs when SEVERAL BITS OF DATA (usually one byte) are sent down SEVERAL CHANNELS/WIRES all at the same time. Each channel/wire transmits one bit.
  Parallel data transmission works well over short distances.

Question 19

Features of serial transmission

Answer 19

• less risk of external interference than with parallel (due to fewer wires)
• more reliable transmission over longer distances
• transmitted bits won’t have the risk of being skewed (that is, out of synchronisation)
• used to send data over long distances (for example, telephone lines)
• less expensive than parallel due to fewer hardware requirements

Question 20

Features of parallel transmission

Answer 20

• faster rate of data transmission than serial
• works well over shorter distances (for example, used in internal pathways on computer circuit boards)
• since several channels/wires used to transmit data, the bits can arrive out of synchronisation (skewed)
• preferred method when speed is important
• if data is time-sensitive, parallel is the most appropriate transmission method
• parallel ports require more hardware, making them more expensive to implement than serial ports
• easier to program input/output operations when parallel used

Question 21

3- Universal serial bus (USB)

Answer 21

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

Question 22

the benefits of using the USB system

Answer 22

• devices plugged into the computer are automatically detected and device drivers are automatically loaded up
• connections can only fit one way preventing incorrect connections being made
• it has become an industry standard, which means considerable support is available
• can support different data transmission rates (from 1.5 Mbps to 5 Gbps)
• no need for external power source since cable supplies +5V power
• USB protocol notifies the transmitter to re- transmit data if any errors are detected; this leads to error-free data transmission
• it is relatively easy to add more USB ports if necessary, by using USB hubs
• USB is backward compatible (that is, older versions are still supported)

Question 23

The drawbacks of using the USB system

Answer 23

• standard USB only supports a maximum cable length of 5 m; beyond that, USB hubs are needed to extend the cable length.
• even though USB is backward compatible, very early USB standards (V1) may not always be supported by the latest computers.
• even the latest version 3 (V3) and version 4 (V4) USB-C systems have a data transfer rate which is slow compared to, for example, Ethernet connections (Note: USB V2 has a maximum data transfer rate of 480 Mbps.)

Question 24

Errors can occur during data transmission due to:

Answer 24

• electrical interference – can corrupt data.
• packet switching – can lead to data being lost or out of synchronisation.
• skewing of data – bits arrive at their destination no longer synchronised.

Question 25

methods used to check for errors following data transmission.

Answer 25

1- parity checks 2- checksum 3- echo check

Question 26

1- Parity checks

Answer 26

- A parity check is a method used to check whether data has been changed or corrupted following data transmission. - Parity can be even (even number of 1-bits) or odd (odd number of 1-bits). The left-most bit is reserved for a parity bit. - If two bits have been changed (for example, 11110000 changed to 00111100) the parity may stay the same and wouldn’t be picked up by a parity check. In such cases, another error detection method, such as a checksum, needs to be carried out.

Question 27

Parity blocks

Answer 27

- a horizontal and vertical parity check on a block of data being transmitted. - can be used to determine exactly which bit has been corrupted or changed following data transmission.

Question 28

2- Checksum

Answer 28

- A checksum is another method used to check if data has been changed/ corrupted following data transmission. - The checksum is calculated by the sending computer, from the block of data using an agreed algorithm. The data is then sent as a block along with the checksum. The checksum is re-calculated by the receiving computer using the same algorithm used by the sending computer. Any differences in the checksum indicates an erro

Question 29

3- Echo check

Answer 29

- An echo check requires data to be sent back to the sending computer where it is compared with the data originally sent. Any errors and the data is re-sent. - This is not a very reliable method; if the two sets of data are different, it is not known whether the error occurred when sending the data originally or if the error occurred when sending the data back for checking.

Question 30

Check digits

Answer 30

- A check digit is the final digit included in a code; it is calculated from all the other digits in the code. - Check digits are used for barcodes on products, such as International Standard Book Numbers (ISBN) and Vehicle Identification Numbers (VIN). - Check digits are used to identify errors in data entry caused by mis-typing or mis-scanning a barcode.

Question 31

Types of errors can be checked by Check digits

Answer 31

- incorrect digits entered - transposition errors - omitted or extra digits in the number - phonetic errors (for example, 13 (thirteen) instead of 30 (thirty)).

Question 32

Two common methods used to generate a check digit

Answer 32

ISBN 13 Modulo-11

Question 33

Automatic Repeat Request (ARQ)

Answer 33

is a way to check data following data transmission. It is often used by mobile phone networks to guarantee data integrity.

Question 34

Explain how Automatic Repeat Request (ARQ) can be used to detect errors following data transmission.

Answer 34

- ARQs 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. - However, 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 timeout 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.

Question 35

acknowledgement

Answer 35

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

Question 36

timeout

Answer 36

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

Question 37

Explain why echo check is not a very reliable method for detecting errors

Answer 37

The problem with echo check is it is unclear where the error occurred. Did it occur during the original transmission from sender to receiver, or did it occur when the receiver sent the data back to the sender? Because of this, it is not a very reliable method of detecting where an error might have occurred

Question 38

The purpose of encryption

Answer 38

- When data is transmitted over public networks, there is always the risk of it being intercepted. - Using encryption and encryption keys helps solve this problem or certainly makes it more difficult for the hacker. - Encryption alters the data into a form that is unreadable by anybody for whom the data is not intended.

Question 39

plaintext

Answer 39

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

Question 40

ciphertext

Answer 40

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

Question 41

Symmetric encryption

Answer 41

- uses an encryption key; the same key is used to encrypt and decrypt the encoded message. - modern computers could ‘crack’ this encryption key in a matter of seconds. - The real difficulty is keeping the encryption key a secret, Therefore, the issue of security is always the main drawback of symmetrical encryption, since a single encryption key is required for both sender and recipient

Question 42

Asymmetric encryption

Answer 42

- was developed to overcome the security problems associated with symmetric encryption. It makes use of two keys called the public key and the private key. - public key (made available to everybody). - private key (only known to the computer user). - Both types of key are needed to encrypt and decrypt messages.

Question 43

quantum computer

Answer 43

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