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Computer Science > Data transmission > Flashcards

Data transmission Flashcards

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1
Q

What are the three sections data packets are structured from

A
  • A packer header
  • The payload
  • A trailer
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2
Q

What is found in the header

A
  • IP address of the source device
  • IP address of the destination device
  • Sequence number of the packet
  • Size of the packet (in bytes)
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3
Q

What is found in the payload

A
  • The actual data in the packets
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4
Q

What is found in the trailer

A
  • Method of identifying the end of the packet
  • Some form of error checking to ensure packets arrive error-free
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5
Q

What is packet switching

A

a method of data transmission in which a message is broken up into a number of packets. Each packet can then be sent independently
from start point to end point. At the destination, the packets will need to be reassembled into their correct order.

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6
Q

Describe the process of 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
  • unfortunately, packets can reach the destination in a different order to that in
    which they were sent
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7
Q

What are 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
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8
Q

What are 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.
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9
Q

What are the three transmission modes

A
  • Simplex mode
  • Half duplex mode
  • Full duplex mode
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10
Q

What is simplex mode

A

occurs when data can be sent in ONE DIRECTION ONLY (for example, from sender to receiver). An example of this would be sending data from a computer to a printer

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11
Q

What is half duplex mode

A

occurs when data is sent in BOTH DIRECTIONS but NOT AT THE SAME TIME (for example, data can be sent from ‘A’ to ‘B’ and from ‘B’ to ‘A’
along the same transmission line, but they can’t both be done at the same time).
An example of this would be a walkie-talkie where a message can be sent in one
direction only at a time; but messages can be both received and sent.

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12
Q

What is full duplex mode

A

occurs when data can be sent in BOTH DIRECTIONS AT THE SAME TIME (for example, data can be sent from ‘A’ to ‘B’ and from ‘B’ to ‘A’
along the same transmission line simultaneously). An example of this would be a
broadband internet connection

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13
Q

What are the two types of data transmissions

A
  • Serial data transmission
  • Parallel data transmission
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14
Q

What is serial data transmission

A

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.

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15
Q

What is parallel data transmission

A

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

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16
Q

What are some advantages of serial data transmission

A
  • 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 if the amount of data being sent is relatively small since transmission rate is slower than parallel (for example, USB uses this method of data transmission)
  • used to send data over long distances (for example, telephone lines)
  • less expensive than parallel due to fewer hardware requirements
17
Q

What are the advantages and disadvantages of parallel data transmission

A
  • 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
18
Q

What is a USB

A

a form of serial data
transmission. USB is now the most common type of input/output port found on
computers and has led to a standardisation method for the transfer of data
between devices and a computer. It is important to note that USB allows both
half-duplex and full-duplex data transmission.

19
Q

What are the benefits of a USB system

A
  • 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 1.5Mbps 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)
20
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
    480Mbps.)
21
Q

What are three things that can cause errors during tansmission

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).
22
Q

What are the three main types of error checks

A
  • Parity checks
  • Checksum
  • Echo check
23
Q

How is parity checking carried out

A

The parity can be either called EVEN (that is, an even number of 1-bits in the
byte) or ODD (that is, an odd number of 1-bits in the byte). One of the bits in
the byte (usually the most significant bit or left-most bit) is reserved for a parity
bit. The parity bit is set according to whether the parity being used is even or
odd. For example, consider the byte:
1 1 0 1 1 0 0
parity bit
In this example, if the byte is using even parity, then the parity bit needs to be
set to 0, since there is already an even number of 1-bits in the byte (four 1-bits).
We thus get:
0 1 1 0 1 1 0 0
parity bit
In this example, if the byte is using odd parity, then the parity bit needs to be
set to 1, since we need to have an odd number of 1-bits in the byte. We thus get:
1 1 1 0 1 1 0 0
parity bit
Before data is transferred, an agreement is made between sender and receiver
regarding which type of parity is being used. Parity checks are therefore being
used as a type of transmission protocol.

24
Q

Solve this parity when it is odd or even:
_ 1 1 1 0 0 1 1

A

Odd parity:
(1) 1 1 0 0 1 1

Even parity:
(0) 1 1 0 0 1 1

25
Q

How is checksum carried out

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
26
Q

How is echo check carried out

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.
27
Q

Why are check digits used

A

They are used to identify errors in data entry caused by mis-typing or mis-scanning

28
Q

How to calculate the check digit from 12 digits

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.
29
Q

Find the ckeck digit of the ISBN 9 7 8 0 3 4 0 9 8 3 8 2

A
  1. 9 + 8 + 3 + 0 + 8 + 8 = 36
  2. 3 × (7 + 0 + 4 + 9 + 3 + 2) = 75
  3. (36 + 75)/10 = 111/10 = 11 remainder 1
  4. 10 – 1 = 9 the check digit
30
Q

What is the purpose of encryption

A

Encryption alters data into a form that is unreadable by anybody for whom the
data is not intended. It cannot prevent the data being intercepted, but it stops it
from making any sense to the eavesdropper. This is particularly important if the
data is sensitive or confidential (for example, credit card/bank details, medical
history or legal documents).

31
Q

What is symmetric encryption

A

Symmetric encryption uses an encryption key; the same key is used to encrypt and decrypt the encoded message. First of all, consider a simple system that uses a 10-digit denary encryption key (this gives 1 × 1010 possible codes); and a
decryption key.

32
Q

What is asymmetric encryption

A

Asymmetric encryption 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.

Computer Science (3 decks)

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