计算机网络物理层相关知识点总结(English)

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)

1. 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

2. 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

3. link terminology’
   full duplex
   half duplex
   simplex

4. 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

5. multiplexing
   time divison multiplexing
   frequency division multiplexing
   code divison multiplexing

6. 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

7. 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

最后

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