Flashcards
Physical Layer
Flashcards do conjunto (26)
- 1What is the physical layer concerned with?- Properties of the physical media
- Effective representation of data as signals on physical media
- Hardware to generate and receive signals - 2Define physical media propertiesCharacteristics of the physical media over which signals are transmitted
- 3Different types of media have different properties, what are they?- Bandwidth
- Signalling Rate
- Data Representation
- Distance
- Environmental Noise
- Bit Error Rate - 4Signals can be considered as a function of time g(t): what are the two basic distinctions that can be made?- g is a continuous function (analogue)
- g is a discrete function (digital) - 5During transmission signals are subject to distortion: name 2- Attenuation (decay)
- External interference (noise) - 6Define BandwidthFrequency range across which the channel can transmit data
- 7Define Bit RateNumber of bits per second a channel can transmit data
- 8Baud RateThe rate at which symbols are transmitted and received
- 9When converting digital data to a suitable form so that it can be sent across an analogue network, the carrier wave must be modulated, name 3 different techniques for this...1. Amplitude Shift Keying - ASK
2. Frequency Shift Keying - FSK
3. Phase Shift Keying - PSK - 10What is Amplitude Shift Keying?- Amplitude of waveform is varied according to binary data to be transmitted
- Can now use two amplitude levels to modulate wave in accordance to binary data
- Receiver demodulates signal and converts back to binary data - 11How does Frequency Shift Keying work?Frequency of carrier is varied according to binary data to be transmitted, e.g. higher frequencies for 0 and lower for 1
- 12How does Phase Shift Keying (PSK) work?Phase of carrier is varied according to binary data being transmitted
- 13What are the main issues linked to modulation (Amplitude Shift Keying)?- It is very sensitive to noise which can lead to errors
- Frequency modulation requires more bandwidth (2x amplitude modulation)
- Phase mod. uses same bandwidth as amplitude modulation, but detection phase less sensitive to noise that amplitude - 14What does QPSK stand for and what is it?Quadrature Phase Shift Keying
Extended phase modulation - Several bits can be transmitted per unit of time, not just 1, transmit symbols instead! - 15What is the main issue with QPSK?Transmitting more bits per time unit (symbol) increases chances of errors.
More symbols = less phase difference between symbols - 16How are digital signals usually generated?Changing the voltage level of a cable
- 17What are the 3 most common ways of sharing media?1. Sharing time among users - TDM
2. Dividing frequency among users - FDM
3. Allocating individual codes to users - CDMA - 18What does TDM stand for and what is it?Time Division Multiplexing
Each user is allocated a time slot when the channel is dedicated to them, after this slot the channel is dedicated to another user - 19What type of signal is TDM typically used for?Digital Signals
- 20What does FDM stand for and what is it?Frequency Division Multiplexing
Bandwidth of channel is divided up amongst users - 21What type of signal is FDM typically used for?Analogue Signals
- 22What does CDMA stand for and what is it?Code Division Multiple Access
Each user has unique code/chip sequence
This code is used to encode 1's and 0's of binary data
Receiver tunes to unique chips codes and extracts users data - 23What is the calculation for signal-to-noise ratio (SNR)?SNR(dBs) = 10 log10 Signal Power
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Noise Power - 24Name 2 ways of boosting the power of a signal- Amplifiers: Amplify original signal AND any corruption to it (ANALOGUE)
- Repeaters: Regenerate original signal and retransmit this - 25What type of signally is more reliable, analogue or digital and why?Digital Signalling
More reliable and resistant to corruption - 26—