Modem Lab
THE MODEM LAB BBS node 1

Call setup

1Your modem
2Who you are dialing
3The phone line

Speaker volume

The phone line

You
Them

Call timeline (click anywhere to jump)

0:00.0 / 0:00.0

Spectrum analyzer and scope (what is on the line this instant)

What is happening

Ready

Press Dial

Simulations: what could go wrong

Terminal


      

      

        
AT decoder (what the modem was told and said)

        

      

    

  


  





  

    

      
Good to know

      

        
Two lanes or one?
Up through 2400 bps, each direction got its own slice of the band. From V.32 on, both sides talk on the same frequencies at once and echo cancellers subtract each modem's own voice.

        
Why 56K needed an ISP.
56K works because the ISP's end is digital. Only one analog to digital conversion can sit between you and them, so two home modems never got past 33.6k.

        
AT means Attention.
Hayes put every command behind AT in 1981, and nearly every modem after copied it. That is why the commands in the decoder all start the same way.

        
How this works.
Standard handshakes are generated from the published specs: Bell 103 and 212A, V.21, V.22bis, V.25, V.32bis, V.8, V.34 and V.90. HST, PEP, CSP, V.FC and V.fast were proprietary and are marked as approximations.

      

    

    

      
Photo credits

      
    
    
    
  
    
  
    Modem Lab: free and open source. Sound is generated in your browser from the published standards.Made by Jason O'Connell