Multiple Access Lab

Data Communication, Lecture 13. Many stations, one medium. Watch ALOHA frames collide, draw a CSMA/CD collision in space and time, roll the dice of exponential backoff, and share one channel with CDMA codes. Works offline.

Throughput S against offered load G
pure ALOHA S = G·e−2Gslotted ALOHA S = G·e−Gdot = your G
One random run: 40 frame times
successcollision

How much gets through?

Theory SThis runVulnerable time

Forouzan Ex. 12.3 / 12.4

TfrGSurvive per second
Space (along the cable) →Time ↓
A's signalC's signal✕ collision · ● detection

What happened


  

  

    
Detect it before you finish

    

    
    
    
    
    

    
TpMin. frameMax. cable

  






  

    
Two stations keep colliding: binary exponential backoff
    
    
    Waiting ranges (Ethernet slot = 512 bit times)
    
  
    
  
    
    
    Roll the dice
    
    
    After the K-th collision each station picks R at random from 0 to 2K − 1 and waits R slots. If both pick the same R, they collide again.
    
    
    
    
    p-persistent
    
    
    
    Send in the 1st idle slotStill waiting after 3 slotsAverage slots to send
    
    
    Assumes the channel stays idle while the station waits. p = 1 is the 1-persistent method.
    
  
    

    


    
  
    
    
    Walsh codes and what each station puts on the channel
    
    
    Decoding: inner product with the sender's code, divided by N
    
  
    
  
    
    
    Share one channel with codes
    
    
    
    
    
    
    
    Bit 1 → +1, bit 0 → −1, silent → 0. The channel carries the sum of every station's data × code.