1. Chapter 3. Data Transmission
Concepts and Terminology
Data and Signal
Transmission Impairments

2. Transmission Terminology
Guided and unguided media
Simplex, Half-duplex, Full-duplex tx
Time-Domain representation
Frequency-Domain representation

3. Time Domain Representation

4. Frequency Domain Representation

5. Data Rate vs. Bandwidth Case A (transmission system bandwidth = 4MHz)
(4/p)[sin(2pft) + (1/3)sin(2p(3f)t)]  … (Treat the waveform as a bit string of 1s & 0s)
Bandwidth 4MHz = 3f – f = 2f =  f = 2MHz
Data rate = 2f bps = 4 Mbps
Case B (transmission system bandwidth = 4MHz)
(4/p)[sin(2pft) + (1/3)sin(2p(3f)t)+(1/5)sin(2p(5f)t)]
Bandwidth 4MHz = 5f – f = 4f  f = 1MHz
Data rate = 2f bps = 2 Mbps

6. Data Rate vs. Bandwidth (cont)
T= 1/f B

7. Data and Signals Analog data Digital data
a function of time and occupy a limited frequency spectrum
can be represented by a electromagnetic signal occupying the same spectrum
Digital data
can be represented by digital signals, with a different voltage level for each of the two binary digits

8. Data and Signals (cont)
Analog signal
Digital signal
Two alternatives: (1) signal occupies the same spectrum as the analog data;(2) analog data are encoded to occupy a different portion of spectrum.
Analog data are encoded using a codec to produce a digital bit stream.
Analog data
Digital data are encoded using a modem to produce analog signal.
Two alternatives: (1) signal consists of two voltage levels; (2) digital data are encoded to produce a digital signal with desired properties.
Digital data

9. Treatment of Signals Analog transmission Digital transmission
Is propagated through amplifiers; same treatment whether signal is used to represent analog data or digital data.
Assumes that the analog signal represents digital data. Signal is propagated through repeaters.
Analog signal
Not used
Digital signal represents a stream of 1s and 0s, which may represent digital data or may be an encoding of analog data. Signal is propagated through repeaters.
Digital signal

10. Analog Signal

11. Digital Signal
Data

12. Transmission Impairments
Attenuation distortion (strength of a signal)
Guided media: logarithmic
Unguided media: complex function of distance and of the makeup of atmosphere.
Increasing function of frequency
Delay distortion (peculiar to guided media)
The velocity of propagation of a signal through a guided medium varies with frequency
Inter-symbol interference
Some of the signal components of one bit position will spill over into other bit positions

13. Transmission Impairments
Noise
Thermal noise (white noise)
Due to thermal agitation of electrons
Function of temperature
Uniformly distributed across the frequency spectrum
Cannot be eliminated, place an upper bound on comm.
Inter-modulation noise
Different frequencies share the same tx medium
Crosstalk
Electrical coupling
Impulse noise

14. Channel Capacity Nyquist Bandwidth Shannon Capacity Formula
Case of a channel that is noise free
Given a bandwidth of B, the highest signal rate that can be carried is 2B
If the signals to be transmitted are binary (2 voltage levels), then the data rate is 2B bps
Shannon Capacity Formula
C = B log2 (1+SNR)
Theoretical maximum that can be achieved
Consider white noise only