Transmission Problems Computer Networks Transmission Problems

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

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

Attenuation

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

Delay Distortion

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

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

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

Noise Noise Signal Signal + Noise

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

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

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

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!

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