Role of Physical Layer Physical Layer Signals – bit Bite Rate- no. of bits sent in one seconds Bit length- distance one bit occupies on the transmission medium = propagation speed x bit duration Attenuation- Loss of energy as it travels on the medium. To compensate for loss, we may use different components such as amplifiers, Decible- It is the unit that measure the relative strength (power or voltage) of two signals one signal at two different points. db = 10 log10(P2/P1)       or    db = 20 log10 (V2/V1) It should be noted that db will be +ve if signal is amplified and –ve if signal is lost/attenuated Nyquist bit rate :For noiseless channel, the nyquist bit rate define theoretical maximum bit rate as Nyquist bit