Transmission Media -Cont

™   WAVELENGTH o    Another characteristic of a signal traveling through the TX Medium o    This  concept  binds  the  frequency  of  the  signal  to  the  propagation  speed  of medium o    It  is  customary  to  talk  about  wavelength  when  talking  about  TX  of  light  in Optical Fiber o    The wavelength is the distance a simple signal can travel in one period o    The wavelength depends on both the frequency and the medium -Wavelength = Propagation speed * period -Wavelength = Propagation speed / frequency ™   Shannon Capacity In  1944,  Claude  Shannon  introduced  a  formula  to  determine  the  theoretical highest data rate for a channel: C = B log 2 (1 + S/N) in bps -B: bandwidth of the channel -S/N: signal to noise ratio Examples Š    Extremely noisy channel -    S/N » 0 -    C = B log2 (1+0) = 0 Š    Telephone line -    Bandwidth is 3000 Hz, S/N ratio is 3162 (35 dB) -    C = 3000 log2 (1+3162) = 34,860 bps Media Comparison When evaluating the suitability of a particular medium to a specific application, 5 factors should be kept in mind: ŠCOST: This is the cost of materials plus installation ŠSPEED: Speed is the max no. of bits per second that a medium can transmit reliably -Speed  varies  with  frequency,  with  physical  size  of  medium  and  Transmission Equipment ŠATTENUATION: Tendency of EM signal to become weak or distorted over signal ŠEM  Interference:  EMI  is  the  susceptibility  of  the  medium  to  external  EM  energy introduced into the link that interferes with the intelligibility of a signal -Familiar effects of EMI are static(audio) and snow (visual) ŠSECURITY: How easy it is for an unauthorized device to listen on the link? -Twisted pair is intercept able -Optical Fiber is more secure