1. Data and Computer Communications
Lecture 3
Data Transmission
Decibels
Digital Data
Anaologue Data
Transmission Losses

2. Decibels & Signal Strength
As a signal propagates along a transmission media, there will be loss or attenuation of signal strength.
Alexander Graham Bell discovered that the human ear responded logarithmically to power difference and invented a unit called the Bel. The most common form is the decibel which is 1/10 of a Bel.
The decibel is a measure of the ratio between the power of two signal levels.
Ndb=10 log10 P2/P1
Ndb = Number of decibels
P1 = Input power
P2 = Output power level

3. dB’s
Decibel values refer to relative magnitudes or changes in relative magnitude, not to an absolute level.
A loss from 1000mW to 500mW is a loss of 3dB.
A loss of 3 dB halves the power level
A gain of 3 dB doubles the power

4. Data Rate and Bandwidth
The greater the bandwidth, the higher the information-carrying capacity
Any transmission system has a limited band of frequencies
This limits the data rate that can be carried
Conclusions
Any digital waveform will have infinite bandwidth
BUT the transmission system will limit the bandwidth that can be transmitted
AND, for any given medium, the greater the bandwidth transmitted, the greater the cost
HOWEVER, limiting the bandwidth creates distortions

5. Finding the Bandwidth
A square wave represents a a binary stream
The duration of each pulse is 1/f
Date rate is 2f bits per second (bps)
Bandwidth of signal:
Upper freq – fundamental frequency

6. Analog and Digital Data Transmission
Entities that convey meaning
Signals
Electric or electromagnetic representations of data
Transmission
Communication of data by propagation and processing of signals

7. Data Analog Digital Continuous values within some interval
e.g. sound, video
Digital
Discrete values
e.g. text, integers

8. Acoustic Spectrum (Analog)

9. Signals Means by which data are propagated Analog Digital
Continuously variable
Various media
wire, fiber optic, space
Speech bandwidth 100Hz to 7kHz
Telephone bandwidth 300Hz to 3400Hz
Video bandwidth 4MHz
Digital
Use two DC components

10. Data and Signals
Usually use digital signals for digital data and analog signals for analog data
Can use analog signal to carry digital data
Modem
Can use digital signal to carry analog data
Compact Disc audio

11. Analog Signals Carrying Analog and Digital Data

12. Digital Signals Carrying Analog and Digital Data

13. Analog Transmission
Analog signal transmitted without regard to content
May be analog or digital data
Attenuated over distance
Use amplifiers to boost signal
Amplifiers also amplifies noise (SNR)

14. Digital Transmission Concerned with content
Integrity endangered by noise, attenuation etc.
Repeaters used
Repeater receives signal
Extracts bit pattern
Retransmits
Attenuation is overcome
Noise is not amplified

15. Advantages of Digital Transmission
Digital technology
Low cost LSI/VLSI technology
Data integrity
Longer distances over lower quality lines
Uses repeaters as opposed to amplifiers
Capacity utilization
High bandwidth links now economical
High degree of multiplexing easier with digital techniques
Security & Privacy
Encryption can be readily applied
Integration
By digitising analog data, analog and digital data can be treated similarly

16. Transmission Impairments
Signal received may differ from signal transmitted. Why?
Analog - degradation of signal quality
Digital - bit errors
Caused by
Attenuation and attenuation distortion
Delay distortion
Noise

17. Attenuation Signal strength falls off with distance
Reduction tends to be logarithmic and is expressed in dB’s per unit distance
Depends on medium
Received signal strength:
must be strong enough to be detected
must be sufficiently higher than noise to be received without error
Attenuation is an increasing function of frequency

18. Attenuation Solutions
Problems 1 & 2 can be solved by ensuring the signal strength is maintained using amplifiers.
Problem 3 requires one of two solutions
Equalization of attenuation across a band of frequencies ( Coils used in telephone circuits)
Variation in amplification of signal at different frequencies

19. Delay Distortion Only in guided media
Propagation velocity varies with frequency
Light passing through a prism caused the separation of white light
Velocity will tend to be faster at center of bandwidth
Intersymbol interference is where component frequencies of one bit position overlap with the next bit position

20. Noise Additional signals inserted between transmitter and receiver
May be divided into four categories
Thermal
Intermodulation
Crosstalk
Impulse noise

21. Thermal Noise Due to thermal agitation of electrons
Uniformly distributed
White noise
No = kT (W/Hz)
No Noise power density per 1 Hz
K Boltzmann’s constant x 10-23J/oK
T Temperature in degrees Kelvin

22. Intermodulation Noise
When signals of different frequecies share the same medium, intermodulation noise may occur
Intermodulation noise are signals that are the sum and difference of original frequencies sharing the medium
Intermodulation occurs when there is some form of nonlinearality occurs in the transmitter, medium or receiver.

23. Crosstalk Crosstalk A signal from one line is picked up by another
Electrical coupling from adjacent twisted pair
Same order of magnitude as thermal noise
All noise so far is reasonabley predictable and of a constant amplitude. This allows for some form of compensation circuit / layout

24. Impulse Noise
Noncontinuos noise consisting of irregular pulses or spikes
e.g. External electromagnetic interference
Short duration
High amplitude
There are a variety of causes
Electrical switchgear
Lightning
Circuit faults

25. Channel Capacity
Channel capacity is the maximum rate at which data can be transmitted over a certain communication path.
Data rate
In bits per second (bps)
Rate at which data can be communicated
Bandwidth
In cycles per second of Hertz
As constrained by transmitter and medium
Noise
Average level of noise over the communications path
Error rate
Rate at which errors occur

26. Nyquist Bandwidth
We would like to make the most efficient use of available bandwidth.
For digital data we would like the highest data rate possible at a particular error rate for a given bandwidth.
Nyquist formulated this limation as follows:
For binary signals (two voltage levels)
C = 2B
C = capacity
B = Bandwidth
A bandwidth of 3100hz gives a capacity of 6200 bps

27. Multilevel Signalling
With multilevel signaling, each level representing more than one bit
Nyquists formula becomes
C = 2B log2 M
M = number of discrete signal or voltage levels
C = Capacity
Assuming M = 8, this gives a capacity of bps for the previous example.

28. Signal-to-Noise Ratio
Ratio of the power in a signal to the power contained in the noise that’s present at a particular point in the transmission
Typically measured at a receiver
Signal-to-noise ratio (SNR, or S/N)
A high SNR means a high-quality signal, low number of required intermediate repeaters
SNR sets upper bound on achievable data rate

29. Shannon Capacity Formula
Equation:
Represents theoretical maximum that can be achieved
In practice, only much lower rates achieved
Formula assumes white noise (thermal noise)
Impulse noise is not accounted for
Attenuation distortion or delay distortion not accounted for

30. Example of Nyquist and Shannon Formulations
Spectrum of a channel between 3 MHz and 4 MHz ; SNRdB = 24 dB
Using Shannon’s formula

31. Example of Nyquist and Shannon Formulations
How many signaling levels are required?

32. Required Reading
Stallings chapter 3