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Introduction

Module

Physical Layer

-​ Analog and Digital Transmission -​ Signal and composite signals, time and frequency domain Digital signal transmission (baseband and broadband transmission) Transmission impediments (attenuation and noise)

  • Digital Signals

  • Analog Signals

  • Peak Amplitude

  • Period and Frequency

Two types of graphs - Time-domain plot - Frequency-domain plot

Digital Signals

Bit Rate
  • Bit rate is the number of bits sent in 1s , expressed in bits per second. Pasted image 20250914122826.png

Example 2.3

100 pages per minute, 10/6 pages per second
A page is an average of 24 lines with 80 characters in each line, each character requires 8 bits

24 * 80 * 8 * 100 / 60 -> 1.536/60 Mbps
Bit Length

The distance occupied by 1 bit $Bit length$ = $1 / (bit rate)$

Transmission of Digital Signals

1) Baseband - No conversion to analog signals 2) Broadband - Converts to analog signals

Signal Impairment

1) Attenuation - Loss of energy, requires amplification $10\log_{10}{(P2/P1)}$

2) Distortion - Signal changes its form or shape. 3) Noise - Thermal Noise - Random motion of electrons - Induced Noise - External Sources - Crosstalk - Effect of one wire on other - Impulse Noise - spike in signal

Signal-to-Noise- Ratio (SNR)

$SNR = (Average signal power) / (Average noise power)$

  • Described in decibels $SNRdb = 10\log{10}{SNR}$

Data Rate Limits

  • Bandwidth available
  • Level of the signals we use
  • The quality of the channel (level of noise)

Nyquist Bit Rate

BitRate = $2 * B * \log{2}{L}$

Example

We need to send 265 kbps over a noiseless (ideal) channel with a bandwidth of 20 kHz. How many signal levels do we need? We can use the Nyquist formula as shown: $265000 = 2 * 20,000 * log{2}{L} \implies 98.7 levels$ Sound Level should be in powers of 2, so either accept $L = 64 (265 kbps)$ or $L = 128 (280 kbps)$

Shannon Capacity (Noisy Channel)

Channels are always noisy.

$C = B * log{2}{(1 + SNR})$

Topic

SNR, Data rate limits (Nyquist bit rate for

noiseless channel and Shannon Capacity for noisy channel) Digital-to-digital transmission (line coding – NRZ, NRZ-L and NRZ-I, RZ, Manchester Analog signal to digital data:

Digital Transmission

Digital-to-Digital Conversion
  • Line coding is the process of converting digital data to digital signals

Line Coding

NRZ

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  • Cons - baseline wander (average of the signal to distinguish high and low signals)
  • Non-return to zero inverted
NRZI

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Manchester Encoding and Differential Manchester
  • Dr Thomas and IEEE standards <- (Often used)
Analog-to-Digital Conversion
  • Pulse Code Modulation (PCM)
    • Sampling, Encoding and Quantizing
    • Sampling is the process of measuring the amplitude at regular intervals
    • $f_s>=2*f_{max}$
    • $BitRate = f_s * n * c$
    • $c \implies number of channels$
    • $n \implies number of bits per sample$
    • $B_{min} = n_b * B_{analog}$ Sampling generates pulses whose amplitudes match the sampled signal. These pulses are sent in a sequence for transmission.

$Quantization$ - Converting each sampled amplitude into a finite set of discrete levels. $$ Δ = (Vmax − Vmin)/L, L = 2^n$$

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