4.9.1.1 Communication methods and basics Quiz: AQA Computer Science, Unit 9

The 20 questions

1. What is serial transmission?

   • Sending the bits of data one after another along a single channel
   • Sending several bits at the same time along multiple parallel wires
   • Sending the same bit several times over the channel to avoid errors
   • Sending data in one direction only along a single wire

2. Which is an advantage of serial over parallel transmission over longer distances?

   • Fewer wires are needed and skew between bits is avoided, so higher clock rates are possible over longer cables
   • Every bit arrives at the same instant over the wires, so no synchronisation of the bits is needed at the receiver
   • It requires more wires, which reduces the interference between the signals and keeps them clear over the distance
   • It is always faster, because all of the bits are sent at the same moment along the separate wires of the link

3. What distinguishes synchronous from asynchronous transmission?

   • Synchronous transmission never needs a timing signal, whereas asynchronous always requires one
   • Synchronous transmission sends one bit at a time, whereas asynchronous sends many bits at once
   • Asynchronous transmission uses a shared clock, while synchronous transmission uses start and stop bits
   • Synchronous transmission shares a timing clock between sender and receiver, while asynchronous uses start and stop bits

4. What is the purpose of start and stop bits in asynchronous transmission?

   • They store the address of the receiving device on the network so that the frame can be routed to the correct host
   • They indicate the baud rate that the sender has chosen for the transmission, so the receiver can match its timing
   • They hold the error-correction code that checks each byte being sent for errors before it is accepted by the receiver
   • They mark the beginning and end of each byte so that the receiver can synchronise with the sender

5. What is the baud rate of a communication channel?

   • The number of bits transmitted per second over the channel
   • The maximum distance a signal can travel without a repeater
   • The amount of data that can be held in a buffer at one time
   • The number of signal changes, or symbols, transmitted per second

6. What is the bit rate of a communication channel?

   • The number of bytes held in a cache at the receiver each second
   • The number of signal changes per second on the physical medium
   • The time taken for a signal to travel from the sender to the receiver
   • The number of bits transmitted per second

7. Why can the bit rate be higher than the baud rate?

   • If more than one bit is encoded in each signal change, each change carries several bits
   • Because the baud rate is always double the bit rate on a copper cable
   • Because the baud rate is measured in bytes, whereas the bit rate is measured in kilobits
   • Because the receiver adds extra bits to each signal to speed up the delivery of data

8. A channel's signal changes 2000 times per second and each change encodes 3 bits. What is the bit rate?

   • 600 bits per second
   • 2000 bits per second
   • 6000 bits per second
   • 3000 bits per second

9. Asynchronous transmission uses 1 start bit, 8 data bits and 1 stop bit for each byte. What fraction of the transmitted bits are data?

   • 80% (8 of 10 bits)
   • 8% (8 of 100 bits)
   • 90% (9 of 10 bits)
   • 100% (8 of 8 bits)

10. A link has a bit rate of 10 Mbit/s. How long does it take to send a 5 MB file, ignoring overheads?

    • 50 seconds
    • 0.5 seconds
    • 4 seconds
    • 40 seconds

11. What is bandwidth in a communication channel?

    • The maximum rate at which data can be transmitted over the channel, usually measured in bits per second
    • The delay between sending a packet and receiving its acknowledgement from the other end
    • The number of devices that a network can support at any one time
    • The amount of data stored in a buffer at the receiver at any moment

12. What is latency in a communication system?

    • The delay between data being sent and that data being received
    • The number of start bits used in each asynchronous frame of transmitted data
    • The maximum amount of data that can be transmitted per second on a link
    • The number of bits lost in each transmission because of interference on the line

13. What is a protocol in communications?

    • A physical cable that connects two devices together for data transfer
    • A program that compresses data before it is sent over a network link
    • A unit used to measure the speed of a communication channel in bits
    • An agreed set of rules that govern how devices communicate with one another

14. What is the relationship between bit rate and bandwidth?

    • Bit rate and bandwidth are unrelated, because bandwidth measures the distance between devices
    • Bit rate is inversely proportional to bandwidth, so more bandwidth gives a lower bit rate
    • Bit rate is directly proportional to bandwidth, so more bandwidth allows a higher bit rate
    • Bit rate is always equal to bandwidth, and both are measured in hertz

15. Which example uses asynchronous serial transmission?

    • A network switch passing packets between devices in a star topology
    • A parallel printer receiving the data for a page over eight wires at the same time
    • A computer streaming video over a dedicated link that shares a clock signal
    • Sending keystrokes from a keyboard to a computer one byte at a time, with no shared clock

16. Why does a parallel link suffer from skew over longer distances?

    • Bits on different wires travel at slightly different speeds, so they arrive at different times
    • Parallel links always use start bits, which add a delay to each wire in turn
    • Receivers cannot read bits when the wires are longer than one metre in length
    • The wires heat up, so each bit is sent at a different baud rate from the others

17. A protocol frame has a start bit, 7 data bits, a parity bit and a stop bit. How many bits are in each character?

    • 8
    • 9
    • 10
    • 11

18. Why are baud rate and bit rate distinct measures?

    • Baud rate counts signal changes, while bit rate counts bits, so one signal change can carry several bits
    • Baud rate counts bytes while bit rate counts packets, so the two measures are never equal in any communication link
    • Baud rate is always lower than bit rate because it excludes the parity bits that are added to every character sent
    • They are different names for the same measure, used by different makers of communication equipment for sales purposes

19. A link operates at 1200 baud and each signal change encodes 4 bits. What is the bit rate?

    • 1200 bits per second
    • 300 bits per second
    • 1600 bits per second
    • 4800 bits per second

20. In asynchronous serial transmission, why must sender and receiver agree on the baud rate in advance?

    • So the parity bit can be removed from the channel without causing any delay
    • So the receiver samples each bit at the correct interval for the signal changes
    • So the start and stop bits can be removed from each frame before it arrives
    • So the cable length can be calculated from the speed of the transmitted data

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