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| Question | Answer |
| What 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 |
| Define physical media properties | Characteristics of the physical media over which signals are transmitted |
| Different types of media have different properties, what are they? | - Bandwidth - Signalling Rate - Data Representation - Distance - Environmental Noise - Bit Error Rate |
| Signals 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) |
| During transmission signals are subject to distortion: name 2 | - Attenuation (decay) - External interference (noise) |
| Define Bandwidth | Frequency range across which the channel can transmit data |
| Define Bit Rate | Number of bits per second a channel can transmit data |
| Baud Rate | The rate at which symbols are transmitted and received |
| When 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 |
| What 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 |
| How 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 |
| How does Phase Shift Keying (PSK) work? | Phase of carrier is varied according to binary data being transmitted |
| What 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 |
| What 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! |
| What is the main issue with QPSK? | Transmitting more bits per time unit (symbol) increases chances of errors. More symbols = less phase difference between symbols |
| How are digital signals usually generated? | Changing the voltage level of a cable |
| What 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 |
| What 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 |
| What type of signal is TDM typically used for? | Digital Signals |
| What does FDM stand for and what is it? | Frequency Division Multiplexing Bandwidth of channel is divided up amongst users |
| What type of signal is FDM typically used for? | Analogue Signals |
| What 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 |
| What is the calculation for signal-to-noise ratio (SNR)? | SNR(dBs) = 10 log10 Signal Power ---------------- Noise Power |
| Name 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 |
| What type of signally is more reliable, analogue or digital and why? | Digital Signalling More reliable and resistant to corruption |
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