X.21

• CCITT standard for direct digital connection to digital telephone network

• uses only 8 signal lines, on 15 pin connector

• data rate of at least 56 Kbps

• instead of more circuits (RS 449 approach)

  use more logic - transmit and receive line provides user and control information
  

• defines both bit and byte synchronization

• acts as physical layer standard for X.25

  X.21bis: interim standard allows X.21 to be used with analog essentially same as RS-232!
  

Previous slide

Next slide

Back to first slide

View graphic version