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Computer Science - IGCSE > Data Transmission > Flashcards

Data Transmission Flashcards

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1
Q

What is data broken down into to be transmitted?

A

Packets.

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2
Q

Why does data need to be broken up?

A

To make transmission possible and practical across wires and radio waves.

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3
Q

What is in a packet and its functions?

A

Packet header - need to know information.
Payload - the actual data
Trailer - the indication of the end and error detection systems.

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4
Q

What is in the packet header?

A
  • Destination address
  • Originator’s address (both IP)
  • Packet number (means of identification)
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5
Q

Describe the process of packet switching. [5]

A
  • Data is broken down into packets
  • Each packet could take a different route.
  • A router controls the route a packet takes
  • Packets may arrive out of order
  • Once the last packet has arrived, packets are reordered (according to the packet number)
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6
Q

What is a network?

A

Computers and devices that are joined together using cables or wireless technology.

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7
Q

What is a router?

A

A network component that examines a data packet to obtain its destination address and the forward the packet to this address.

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8
Q

How does serial data transmission work?

A

Data is sent ne bit at a time along one wire.

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9
Q

How does parallel data transmission work?

A

Data is sent one bit at time along multiple wires so multiple bits are transferred at a time.

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10
Q

How does simplex data transmission work?

A

Transmission of data in one direction only.

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11
Q

How does half-duplex data transmission work?

A

Transmission of data in both directions but only one direction at a time.

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12
Q

How does full-duplex data transmission work?

A

Data can be transmitted in both directions at the same time.

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13
Q

Advantages and disadvantages of serial transmission. [4]

A

A.
- Data arrives in order of sequence.
- Less chance of interference because data is sent along a single wire.
- Cheaper because it needs only one wire.
- good for long distances.
D.
- it is slow
- the need of additional data called the start and stop bit to indicate transmission.

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14
Q

Advantages and disadvantages of parallel transmission. [4]

A

A.
- transmission is quicker
- it is a common use of transmission in devices
D.
- increased risk of data being skewed because data does no arrive in order and needs to be reordered.
- more chance of interference and error due to multiple wires.
- more expensive.
- only good for short distances max 5 meters.

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15
Q

Use of simplex transmission and its benefit & drawback.

A

Connecting a keyboard to a monitor.
There is no need to send data in both directions.

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16
Q

Use of half-duplex transmission and its benefit & drawback.

A

Wi-Fi. This is needed because Wi-Fi transmits and receives on the same channel.

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17
Q

Use of full-duplex transmission and its benefits & drawbacks.

A

Telephone conversations. Both people can speak and communicate at the same time.

18
Q

A company has a central file server that is located 500m away from the main office where employees work at their computers.
Employees send and retrieve files to and from the file server on a daily basis, often at the same time.
Identify the type of data transmission that would be most suitable for the given context. Explain why this would be the most suitable method of transmission. [6]

A
19
Q

What does USB stand for?

A

Universal Serial Bus

20
Q

How does a USB interface transmit data?

A

It uses serial data transmission but a special type is used that is designed to transmit data at high speeds.

21
Q

Advantages and disadvantages of USB. [6]

A

A.
- it is a simple interface and connects in only one way allowing less errors in connecting devices.
- fast transmission.
- it is a standard and included in many devices.
- it is automatically detected when connected.
- an also be used to power a device.
D.
- longest length is 5 meters
- is not the fastest method of transmission.

22
Q

Why do we need to encrypt our data?

A

Data is valuable especially when it contains personal details. People may try stealing this data during transmission fo their own gain.
Encryption protects our data.

23
Q

Describe the process of symmetric encryption.

A

Plain text is encrypted into cypher text using an encryption key
The cypher text and encryption key are sent separately to the receiving device.
The same key is used to decrypt the cipher text back into its plain form.

24
Q

What is a safety hazard of symmetric encryption?

A

The encryption key can be intercepted along with the encrypted data.

25
Q

Why do we need to check for errors after data transmission?

A

Transmission of data is not always a perfect process and errors can occur.

26
Q

How can errors occur during data transmission?

A

Due to interference e.g data loss, data gain and data change.

27
Q

What are the error detection methods?

A

Parity Check
Checksum
Echo Check

28
Q

Describe how a parity check works?

A
  • The parity checking protocol determines whether bits in a transmission have been corrupted.
  • Every byte transmitted has one of its bits allocated as a parity bit and it is usually the first or last bit of data in the byte.
  • Before transmission, the parity check is set to be an odd or even parity.
  • If odd parity is used then there must be an odd number of 1’s in the byte, including the parity bit
  • If even parity is used then there must be an even number of 1’s in the byte, including the parity bit
  • The value of the parity bit is determined by counting the number of 1’s in the byte, including the parity bit
    If the number of 1’s does not match the agreed parity then an error has occurred
29
Q

How do errors occur during parity checks?

A
  • When using parity bits, an error occurs when the number of total bits does not match the agreed parity
30
Q

How do parity blocks and bytes work as an error detecting method?

A
  • Each byte row calculates the horizontal parity as a parity bit as normal
  • Each bit column calculates the vertical parity for each row. This is (stored as) the parity byte. It is calculated before transmission and sent with the parity block
  • Each parity bit tracks if a flip error occurred in a byte while the parity byte calculates if an error occurred in a bit column
  • By cross referencing both horizontal and vertical parity values the error can be pinpointed
31
Q

Describe how checksum is used to detect errors.

A
  • Data is sent in blocks and an additional checksum value is added at the end of the block
  • Checksums are custom user-created algorithms that perform mathematical calculations on data.
  • When a block of data is to be transmitted, the checksum is first calculated and then transmitted with the rest of the data.
  • When the data is received the checksum value is calculated based on the received data and compared to the checksum value received. - If they are the same then the data does not contain any errors
  • If an error does occur then a resend request is sent and the data is retransmitted
32
Q

What errors can occur using checksum?

A

It is worth noting that the checksum value itself may become corrupted or contain errors. Checksums may be sent multiple times and tallied. The most common checksum is then assumed to be the correct one.

33
Q

Describe how an echo check works.

A
  • Echo checks involve transmitting the received data back to the sender. The sender then checks the data to see if any errors occurred during transmission
34
Q

What error can occur with echo check?

A

-This method isn’t reliable as an error could have occurred when the sender transmits the data or when the receiver transmits the data. Neither will know when the error occurred.

35
Q

What problems are there with the parity check method?

A
  • bits may have been re arranged.
  • there may be a change of an even number of bits and they add up to the parity bit.
36
Q

How do check digits work?

A

Very similarly to checksum.
- A value is calculated from the data before transmission and is sent/stored along with the data.
- When the data is received the check digit value is calculated based on the received data and compared to the check digit value received. If they are the same then the data does not contain any errors
If an error does occur then a resend request is sent and the data is retransmitted

37
Q

What is the difference between check digit and checksum?

A

Check digit is used to check for error detection during entry.

38
Q

Where are check digits used?

A

Bar codes
ISBN - international standard book numbers

39
Q

What is ARQ?

A

Automatic Repeat Request

40
Q

How does ARQ work?

A
  • it uses acknowledgments and timeout to see if data has arrived correctly after transmission.
41
Q

Define acknowledgments and timeouts

A

Acknowledgments are messages that are sent from one device to another to indicate whether data is received correctly.

Timeout is a period of time that is set and used to wait for an acknowledgment to be received.

42
Q

Explain when a positive and negative acknowledgment is used and what happens if none is sent or received.

A
  • If an error is detected the receiver sends a negative acknowledgement transmission to indicate the data is corrupted
  • If no error is detected the receiver sends a positive acknowledgement transmission meaning the data is correct

If the receiver does not send any acknowledgement transmission then the sender waits for a certain time period known as a time-out before automatically resending the data
This process is repeated until all data has been received and acknowledged

Computer Science - IGCSE (4 decks)

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