Physical Layer

 Physical Layer

 The physical layer is responsible for transmitting raw bits over a communication channel.

 Ø  Design issues deal with mechanical, electrical, and timing interfaces, and the physical transmission medium

 Ø  There are a wide variety of physical media over which data may be transmitted.

   E.g. wire, cable, fibre, radio, satellite.

   The media are rated according to their transmission speed (bit rate) and likelihood of errors (bit error rate)

  For each medium there are fundamental limit on transmission speed

 Ø  No medium gives error free transmission (although some are better than others)

 

Additional limitations of transmission media

Signal Attenuationis the phenomenon whereby the Amplitudeof a signal decreases as it propagates along a transmission line.

Attenuation is a function of distance and frequency of signal

Repeaters are used to increase the power of the signal at appropriate intervals

Signal Distortioninvolves the Shape of the signal becoming altered as it propagates along the line.

One cause of distortion is the different attenuation rates for different frequency components of the signal.

This can be addressed using an equalizer

Delay distortionis caused by different frequency components of the signal propagating at slightly different speeds.

If the frequency components of one bit are delayed sufficiently they will overlapwith the component of the next bit resulting in Inter Symbol Interference (ISI).