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Computer science > Unit 2 - Data Transmission > Flashcards

Unit 2 - Data Transmission Flashcards

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1
Q

What are data packets?

A

Data that is sent over long distances is usually broken up into smaller parts known as data packets. The packets of data are usually quite small, typically 64KiB, which are much easier to control than a long continuous stream of data.

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2
Q

Why do you split the data into different packets?

A

The idea of splitting up data in this way means each packet an be sent along a different route to its destination. This would clearly be of great benefit if a particular transmission route was out of action or very busy. The only obvious drawback of splitting data into packets is the need to reassemble the data when it reaches its destination.

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3
Q

What are the 3 parts of a data packet?

A
  • the header
  • the payload
  • the trailer
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4
Q

What does the packet header consist of?

A
  • The IP address of the sending device (sender)
  • the IP address of the receiving device (destination)
  • the sequence number of the packet (allowing correct reassembly)
  • packet size (in bytes)
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5
Q

What is the payload?

A

The payload is the contents that need to be sent (the actual data in the packets - 64 KiB)

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6
Q

What is the packet trailer?

A
  • some way of identifying the end of the packet

- Has a checksum (an error checking method) for the destination to check it transmitted correctly (it is error-free)

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7
Q

What is packet switching?

A

Packet switching is a way of transmitting data in which a message is broke up into a number of packets. Each packet can then be sent independently from start point to end point.

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8
Q

What is the process of packet switching?

A
  1. Break down data into packets
  2. Send the packets through the network
  3. Routers control the route taken
  4. Reorder packets into correct order
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9
Q

How are the different packets taken over different routes to get to their destination? What might happen as a result of this?

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
  • unfortunately, packets can reach the destination in a different order to that in which they were sent
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10
Q

Describe the process of packet switching:

A

To send data, such as a file through the Internet, it is first split into fragments known as packets. Packet switching is then undertaken by each router on the network. Each packet is directed to another closer router. Each packet may take a different route through the network. Packets may arrive out of order. Once the last packet has arrived, the packets are reordered.

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11
Q

Is data transmission over a long distance or a short distance?

A

It is both:

  • short distance e.g. computer to a printer
  • long distance e.g. computer to another in a global network
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12
Q

What three factors need to be considered when transmitting data?

A
  • the direction of data transmission (for example can data transmit in one direction only, or in other directions)
  • the method of transmission (for example, how many bits can be sent at the same time)
  • how will data be synchronized (that is, how to make sure the received data is in the correct order)
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13
Q

Name 4 ways data can be transferred from one point to another:

A
  • copper cable using electrical pulses
  • wirelessly using radio frequency
  • optical fiber using light
  • infrared
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14
Q

What is bandwidth?

A

The amount of data that can be carried t one time, the bigger the bandwidth, the more data that can be carried.

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15
Q

What is serial transmission?

A
  • bits are sent one signal at a time over a single wire - one bit is sent at a time and once that bit has been received the next bit is sent.
  • very high data transfer rates can be achieved
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16
Q

What does serial transmission use?

A

usually uses USB - Universal Serial Bus cables

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17
Q

What is parallel transmission?

A
  • is when several bits of data (usually one byte) are sent down serval channels/wires all at the same time
  • each channel/wire transmits one bit
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18
Q

Why shouldn’t you use parallel data transmission for longer distances?

A
  • Parallel data transmission works well over short distances. Over longer distances (for example, over 20 metres), data can become skewed (that is, the data can arrive unsynchronised) and bits can arrive out of order. The longer the wire, the worse this can become.
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19
Q

When is parallel data transmission useful?

A

It is faster than serial transmission. The internal circuits in a computer use parallel data transmission since the distance travelled between components is very short and high-speed transmission is essential

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20
Q

What is skew?

A

Each wire in a parallel cable has slightly different properties:

  • Signals, therefore, arrive at slightly different times
  • The receiver must wait until all lines are received until the next set of bits may be sent
  • Transmission needs to be over very short distances to reduce the problem
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21
Q

What is “Crosstalk”?

A
  • Crosstalk may occur when a signal on one line creates a disturbance in another line
  • This may cause a bit to be reversed in a parallel line
  • The longer a cable is, the more chance there is of crosstalk
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22
Q

What are the advantages of serial transmission?

A
  • By using serial transmission, faster rates are possible without the issues of crosstalk or skew
  • This is why modern transmission methods such as SATA (for hard drives) and USB (for external devices) are now most common
  • This also allows for longer cables without the risks of greater crosstalk and skew
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23
Q

What is simplex transmission?

A
  • data travels in one direction only e.g. used for sending data from a keyboard to the CPU
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24
Q

What is half-duplex transmission?

A

Data can travel in both directions along a single cable, but not simultaneously:

  • The full bandwidth is available to both the sender and receiver but they cannot send at the same time as receiving
  • used in parallel printer cables: text is sent to a printer and the printer can send ‘Out of paper’ messages back, but only when the computer has finished sending text
  • walkie-talkies
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25
Q

What is duplex transmission?

A

Data can travel in both directions simultaneously using two cables:

  • Ethernet network cables make use of two twister pairs of cable – one for sending and one for receiving
  • Fibre optic cables use two fibres – one for transmitting and one for receiving
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26
Q

What are the advantages of serial over parallel?

A
  • Smaller, simpler, and cheaper connectors such as those used for smartphones
  • “Crosstalk” creates interference between parallel lines, and can result in corrupted words which then need to be retransmitted.
  • Serial links are more reliable over much greater distances than parallel links (for example telephone lines)
  • Less risk of external interference than with parallel (due to fewer wires)
  • transmitted bits won’t have the risk of being skewed (that is, out of synchronization)
  • less expensive than parallel due to fewer hardware requirements
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27
Q

What are the advantages of parallel over serial?

A
  • Faster rate of data transmission than serial
  • Works well over shorter distances (for example, used in internal pathways on computer circuit boards)
  • preferred method when speed is important
  • if data is time-sensitive, parallel is the most appropriate transmission method
  • easier to program input/output operations when parallel used
28
Q

What are the disadvantages of parallel?

A
  • Since several channels/wires used to transmit data, the bits can arrive out of synchronisation (skewed)
  • Parallel ports require more hardware, making them more expensive to implement than serial ports
29
Q

What is a USB?

A

AS the name suggests, the Universal Serial Bus (USB) is a form of serial data transmission.

USB is now the most common type of input/output port found on computers and has led to standardization method for the transfer of data between devices and a computer.

30
Q

What kind of data transmission does a USB follow.

A

Both half-duplex and full-duplex

31
Q

What does a USB consist of?

A

The USB cable consists of four wired shield cables, with two wires for power (red and black). The other two wires (white and green) are for data transmission.

32
Q

How does the computer detect that a device is present (USB)?

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 recognized, 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.

33
Q

What are the benefits of USB systems?

A
  • Devices plugged into the computer and 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 1.5Mbps to 5Gbps)
  • 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)
34
Q

What are the drawbacks of USB Systems?

A
  • Standard USB only supports a maximum cable length of 5m; 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)
35
Q

What 3 factors can cause errors to occur during data transmission?

A
  • interference (all types of cables 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 can cause data corruption if the bits arrive out of synchronization)
36
Q

What can errors in the data transmission cause?

A
  • Data to be lost
  • Data to be gained
  • Data to be changed
37
Q

What are 5 ways to check for errors?

A
  • Parity bits
  • Check digits
  • Check sums
  • Echo checks
  • Automatic Repeat reQuests (ARQ)
38
Q

why is checking for errors important?

A

There is always the chance that data has been corrupted (changed in some way) during transmission. This is true regardless of the distance of transmission. To avoid having corrupted data it is really important that computers check for errors and correct them wherever possible.

39
Q

What is parity check?

A

It is a form of error detection. An extra bit (the parity bit) is added to a string of binary code to endure the number of 1 bits are either even or odd.

40
Q

How does parity check work?

A
  • The parity bit is set according to whether the parity being used is even or odd. For an even parity bit, the parity bit is set to 1 or 0 so that the total number of 1s transmitted are an even number.
  • before data is transferred, an agreement is made between sender and receiver regarding which type of parity is being used.
41
Q

What happens when there is an error with paroty check?

A
  • When data is transmitted, the parity bit is set at the transmitting end and parity is checked at the receiving end
  • If the wrong number of bits are found, an error has occurred
  • It isn’t possible with parity checks to know which bit was incorrectly transmitted
    If a transmission error is found, the receiving device notifies the transmitting end and the data is resent
42
Q

What is a checksum?

A

Barcodes and other numbers have check digits which are used to make sure a number has been entered correctly
- A barcode scanner may misread the number and requestit to be scanned again

43
Q

What real word examples have a check digit?

A
  • Printed books have a unique barcode with an ISBN (International Standard Book Number)
  • Other products contain bar codes using formats such as UPC (Universal Product Code)
  • Both codes make use of check digits to make sure they have been scanned or entered correctly
44
Q

How does a checksum work?

A
  • both the sender and receiver agree on a calculation and a number to divide the sum of these numbers by (a divider)
  • interested in the remainder value, this is the checksum
  • add all the numbers together, then divide it by the divider
  • the remainder is the checksum value
45
Q

How does the checksum detect an error?

A

The receiving computer will do the same calculation and if the remainder is different then the information is incorrect and it needs to be resent.

46
Q

How does an echo check work?

A
  • with an echo check, the receiving computer send a copy of the data immediately back to the sending computer for comparison
  • the sending computer compares the two sets of data to check if any errors occurred during the transmission process
  • if an error has occurred, the data will be transmitted again
47
Q

What are the drawbacks of echo checks?

A
  • if the two sets of data are different you will have no way of knowing whether the error occurred when originally sent, or when it was sent back
  • echo checks require a lot of extra data to be transmitted
48
Q

What is a ARQ?

A

An Automatic Repeat ReQuest is an error-control protocol that automatically initiates a call to retransmit any data packet or frame after receiving flawed or incorrect data.

49
Q

How do ARQs work?

A
  • The sender first sends a block of data
  • The receiving computer then sends back an acknowledgement
  • When the sender receives the acknowledgement it sends the next block of data
  • If the acknowledgement is not received after a period of time, a timeout occurs and the data is resent
  • This process continues until the acknowledgement is received
50
Q

What are ARQs typically used?

A

ARQs are often used to assure reliable transmissions over an unreliable service.

51
Q

What is a negative acknowledgement?

A
  • When a data block is correctly received an acknowledgement (ACK) is sent
  • When the receiver detects an error in a data block that has been sent, a negative acknowledgment (NACK) is sent
52
Q

What are the benefits of a negative acknowledgement?

A

By sending a NACK, the sender doesn’t need to wait so long in a timeout – it can immediately send the data block again

53
Q

What is encryption?

A

Encryption is the process of encoding data or a message so that it cannot be understood by anyone other than its intended recipient.

54
Q

What is the opposite of encryption?

A

decryption

55
Q

Why do we need encryption?

A
  • Large amounts of personal data are now stored and transmitted on computers
  • Companies and other organisations also have highly confidential data that they don’t want others looking at
  • In paper systems, sensitive data was held in locked cabinets and locked rooms now, sensitive data is often transmitted via the Internet which is a public network
  • Data is also stored on computers and servers on the Internet which may be hacked
  • If a hacker gains access to the data, they won’t be able to understand it if it is encrypted
56
Q

What are some examples of things that need to be encrypted:

A

Most communication sent by the internet are encrypted in some way:

  • purchases made online are encrypted to try to prevent theft of credit card details
  • tools enable a user to encrypt a document, such as a spreadsheet, before sending it to a colleague via the internet.
  • satellite TV transmissions are encrypted to prevent users who do not subscribe from watching TV shows
57
Q

What is the journey the text takes on to become encrypted

A

The original data being sent is known as PLAINTEXT. Once it has gone through an Encryption Algorithm it produces CIPHERTEXT. It is then decrypted back into PLAINTEXT using a key.

58
Q

What are the 2 types of encryption methods?

A
  • symmetric encryption
59
Q

What is symmetric encryption?

A

In Symmetric-Key encryption the message is encrypted by using a key and the same key is used to decrypt the message which makes it easy to use but less secure.

It also requires a safe method to transfer the key from one party to another. Communicating parties must have the same key in order to achieve secure communication.

60
Q

What is asymmetric encryption?

A

IN today’s digital world, there has been a need to develop a different approach to encryption called asymmetric encryption. With this approach, a pair of linked keys is used and consists of a public key, used to encrypt the data and a private key, used to decrypt the data. Both keys are different but related.

The public key is available to everyone who wishes to send a message. On the other hand, the private key is kept at a secure place by the owner of the public key.

61
Q

What is the difference between symmetric and asymmetric key encryption?

A

Symmetric:

  • It only requires a single key for both encryption and decryption
  • The size of ciphertext is the same or smaller than the original plain text.
  • the encryption process is very fast.
  • It is used when a large amount of data is required to transfer.
  • It only provides confidentiality
  • Examples: 3DES, AES, DES and RC4
  • In symmetric key encryption, resource utilization is low compared to asymmetric key encryption

Asymmetric:

  • it requires two keys, one to encrypt and one to decrypt.
  • the size of ciphertext is the same or larger than the original plain text.
  • it is used to transfer small amount of data
  • it provides confidentiality, authenticity and validation
  • Examples: Diffie-Hellman, ECC, El Gamal, DSA and RSA
  • In asymmetric key encryption, resource utilization is high
62
Q

What is check digit?

A

A check digit is an additional digit at the end of a set of other numbers designed to check for mistakes in input or transmission

63
Q

When are check digits used?

A

Check digits are used to identify errors in data entry caused by mistyping or mis-scanning a barcode.

64
Q

What type of errors can check digit recognize?

A
  • an incorrect digit entered
  • transposition errors where two numbers changed order
  • omitted or extra digits
  • phonetic errors (13 (thirteen) instead of 30 (thirty))
65
Q

What is check digit used for?

A
  • ISBN

- VIN

66
Q

What are the 2 common methods of calculating the check digit?

A
  • ISBN 13

- Modulo-11

Computer science (8 decks)

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