計網小考猜題10/10

Nyquist's theorem
ex.5.4
Use Nyquist's theorem to determine the maximum rate in bits per second at which data can be sent accross a transmission system that has a bandwidth 4000 Hz and uses four values of voltage to encode information.

Shannon theorem

PSM(Phase-Shift Modulation)

• in order to encode more bits in one cycle (according to Nyquist theorem)
• 可encode 3 bits/cycle →2B變6B
  
• 0,90,180,270
  

Error Detection

• Parity check
  
• even(湊到偶數個1)
  
• odd(湊到奇數個1)
• ascii code: 7 bits + 1 bit parity bit
  
• Checksum
  

• 會考！記得4個16位元數字一組，不夠的補零
  

• CRC(Cyclic Redundancy Check Code)(這次先不考，但會出作業)
• notation: F: frame(就是data啦~), P:除數, R: 餘數
  
• sender:
  
• F mod P=R, F'=F+R(附加在後),送出F'
• receiver:
  
• F' mod P=R'
• check:
  
• R=R' (correct) R<>R' (error)
• 重點在怎麼除：
• 字串頭為零，直接往左移
• 字串頭為一，作XOR
• 如何選擇除數？
• 待續...
  

(因為老爺說他花很多時間在這幾個機制上，所以大膽預測會考!!)
Reference:
矛与盾的较量——CRC原理篇：很好懂的介紹