Physical Layer
in the session, we will talk about the following parts.
1.bandwidth and delay
2.physical media
3.full duplex/half duplex/simplex
4.modulation type(electrical anologue to digital: base band and band pass)
5.mutiplexing( time mutiplexing and frenquency mutiplexing code division multiplexing)
6.propogation speed(Nyquist theorem and shannon)
-
bandwidth and delay
transmission delay : T-delay
propogation delay: P-delay
latency = L = M/R(massage/rate) +p-delay
signal attenuation: reduction in amplitude(strength) of a signal -
pysical medium
2.1
wires(twisted coaxial cable)
distance: short
cost: cheap, easy to use
bandwidth: moderate
delay: middle
2.2
fiber
distance: long
cost: expensive, hard to use
bandwidth: high
delay: low
2.3
wireless
easy to deploy
mobility and broadcast
transmission interface must be managed
2.4
satellite
rapidly set up anywhere/anytime (after satellite have been launched)
broadcast to large regions
limited bandwidth and interface to manage -
link terminology’
full duplex
half duplex
simplex -
electrical anologue signal to digital
modulation type
NAZ: non-return-to-zero: base band
A sin(Ft +P): band pass
A: amplitude
F:frequency
P:phase -
multiplexing
time divison multiplexing
frequency division multiplexing
code divison multiplexing -
Nyquist theorem
max data rate = 2B log2 V bits/sec
6.1 if a signal has bandwidth B, it can be fully recontructed by sampling with 2B rate
6.2 if signal has V levels, each symbol can be represented by log2V bits -
shannon:
shannon’s theorem relates the data rate to the bandwidth(B) and signal strength(S) relative to the noise(N)
max data rate = B log2 (1+S/N) bits/sec
SNR of 20 db = S/N = 100
