Transmission Channel Considerations Good Bad Every medium supports transmission in a certain equency range. >)Outside this range effects such as attenuation, . degrade the signal too much Transmission and receive hardware will try to maximize the useful bandwidth in this frequency band Frequency >)Tradeoffs between cost, distance, bit rate As technology improves, these parameters change, even for the same wire ) Thanks to our ee friends Ignal 11
11 Transmission Channel Considerations Every medium supports transmission in a certain frequency range. » Outside this range, effects such as attenuation, .. degrade the signal too much Transmission and receive hardware will try to maximize the useful bandwidth in this frequency band. » Tradeoffs between cost, distance, bit rate As technology improves, these parameters change, even for the same wire. » Thanks to our EE friends Frequency Good Bad Signal
The nyquist Limit a noiseless channel of width h can at most transmit a binary signal at a rate 2 x H >)E.g. a 3000 Hz channel can transmit data at a rate of at most 6000 bits/second >)Assumes binary amplitude encoding 12
12 The Nyquist Limit A noiseless channel of width H can at most transmit a binary signal at a rate 2 x H. » E.g. a 3000 Hz channel can transmit data at a rate of at most 6000 bits/second » Assumes binary amplitude encoding
Past the Nyquist Limit More aggressive encoding can increase the channel bandwidth 》 Example: modems Same frequency -number of symbols per second Symbols have more possible values °psk Psk AM Every transmission medium supports transmission in a certain frequency range >) The channel bandwidth is determined by the transmission medium and the quality of the transmitter and receivers Channel capacity increases over time 13
13 Past the Nyquist Limit More aggressive encoding can increase the channel bandwidth. » Example: modems – Same frequency - number of symbols per second – Symbols have more possible values Every transmission medium supports transmission in a certain frequency range. » The channel bandwidth is determined by the transmission medium and the quality of the transmitter and receivers » Channel capacity increases over time psk Psk + AM
Capacity of a Noisy Channel Can't add infinite sym bols- you have to be able to tell them apart this is where noise comes in Shannon' s theorem: C=Bx log2(1+S/N) ))C: maximum capacity(bps) >)B: channel bandwidth(Hz) >)S/N: signal to noise ratio of the channel Often expressed in decibels(db=10 log(s/N) Exam ple: >) Local loop bandwidth: 3200 Hz >Typical S/N: 1000(30db) >) What is the upper limit on capacity? Modems: Teleco internally converts to 56kbit/s digital signal, which sets a limit on B and the S/N. 14
14 Capacity of a Noisy Channel Can’t add infinite symbols - you have to be able to tell them apart. This is where noise comes in. Shannon’s theorem: C = B x log2 (1 + S/N) » C: maximum capacity (bps) » B: channel bandwidth (Hz) » S/N: signal to noise ratio of the channel – Often expressed in decibels (db) = 10 log(S/N) Example: » Local loop bandwidth: 3200 Hz » Typical S/N: 1000 (30db) » What is the upper limit on capacity? – Modems: Teleco internally converts to 56kbit/s digital signal, which sets a limit on B and the S/N