1. Transmission Problems
Computer Networks
Transmission Problems

2. Transmission Impairments
Signal received at receiver may differ from the signal transmitted by transmitted
Analog: Degradation of Signal Quality
Digital: Bit Errors
Reason:
Attenuation
Delay Distortion
Dispersion
Noise

3. Attenuation Signal strength falls off with distance
Signals must be sufficiently strong so that the receiver will be able to detect and interpret them
Should maintain a sufficient high level to make them distinguishable from noise
Too strong signals can overload the circuitry of the transmitter and result in distortion
They should take into account that attenuation increases with the frequency
Greatly depends upon medium
Amplifiers (Repeaters) are used to deal with attenuation problem
deciBel
Unit to show loss or gain of strength

4. Attenuation

5. Delay Distortion
Difference frequencies have different propagation velocities on a guided transmission medium like cable
Highest velocity near center of frequency
Low velocity near edges
Velocity difference create phase difference at receiver
Check frequencies of different cable channels when the reception is not very good

6. Delay Distortion

7. Delay Distortion Problem Limit Max bit rate
Edge (Higher) frequencies are in more trouble
As bit rate increases, some of the frequency components associated with each bit transmission are delayed and start to interfere with frequency components associated with a later bit

8. Dispersion Signals tends to spread as they travel
Amount of spreading depends upon frequency

9. Noise
Additional signals that insert into the signal transmitted by transmitter while it was on its way to receiver through transmission system
Several Types:
Thermal
Inter-modulation
Crosstalk
Impulse

10. Noise
Noise
Signal
Signal + Noise

11. Thermal Thermal noise causes due to thermal agitation of electrons
Signals may be disturbed, additional may be created
White noise
Uniformly distributed/affect all frequencies
Cannot be eliminated
Places upper bound on communication system performance

12. Inter-modulation
Occurs when multiple signals/frequencies are sharing a medium
Effect is to produce signal at a frequency which is sum/difference of two original frequencies
Example
Signal f1 and f2 inter-modulate:
f1+f2
The resultant may also be an original signal traveling on the same medium
That signal will be affected by the inter-modulated noise

13. Inter-modulation Causes
Transmitter, receiver or transmitting medium’s malfunctioning or overload by excessive signal strength

14. Cross Talk Signal from one line is picked up be another
Foreign signal enters in path of transmitted signal
Example
Telephone Systems
You could be able to listen to someone else conversation
Microwaves
Sometimes the Sound systems catches the radio signals!

15. Impulse Noise
All the noise types discussed so far are reasonably predictable and have usually constant magnitude
Procedures may be developed to cope with those types of noises
Impulse noise is, however, non-continuous noise consisting of irregular pulses/noise spikes which are usually of short duration and high amplitude
Causes
External Electromagnetic Interference
Lightening
Faults in Communication Systems