Bit and Framing Errors for Asynchronous Signal Transmissions Department of Computer Science Southern Illinois University Edwardsville Fall, 2013 Dr. Hiroshi Fujinoki CS447 - Computer and Data Communication Bit_Frame_error/000

The Five Procedures Used in Asynchronous Transmissions  Detect the first up-going signal transition (look for next 0  1 transition) (Otherwise look for the next down-going signal – skip anything until next “0” - transition and use it to detect the end of the frame)  Adjust the timer at the center of the start-bit  Sample as many as N payload bits  Take the (N+1)-th bit as the parity bit  If the parity bit is low, do nothing CS447 - Computer and Data Communication

The Five Procedures Used in Synchronous Transmissions  Detect the frame marker  Receive as many bits as specified by the frame header  Re-synchronize at every bit received  Receive CRC error detection/correction code  If no error, detect the end-of-frame marker CS447 - Computer and Data Communication

START BIT     P STOP BIT Frame n Frame n+1 0 Time SENDER RECEIVER 1. Asynchronous Transmission: No error Even Parity 8 User Data 1 Stop Bit Assumptions: CS447 - Computer and Data Communication Bit_Frame_error/001

START BIT     P STOP BIT Frame n Frame n+1 0 Time SENDER RECEIVER 2. Asynchronous Transmission: Bit Error Some bits in a frame are not correctly recognized (errors within a frame) CS447 - Computer and Data Communication Bit_Frame_error/002

START BIT Frame n Frame n+1 0 Time SENDER RECEIVER 2. Asynchronous Transmission: Frame Error Frame n Frame n+1                 P P STOP BIT STOP BIT The start bit of the next frame is not correctly recognized (More serious than simple bit errors) CS447 - Computer and Data Communication Bit_Frame_error/003

Why clock-cycle rate needs to be doubled in synchronous transmission? Asynchronous Transmission cycles needed to transmit 8 bits 4 cycles Example: Synchronous Transmission 8 cycles needed to transmit 8 bits Twice as many clocks rqrd for same # of bits 8 cycles CS447 - Computer and Data Communication Example: (8 bits to be transmitted) 1 cycle 10MHz = 20Mbits 1 cycle 20MHz = 20Mbits Bit_Frame_error/004

Summary 1. Synchronous transmission is better than asynchronous transmission : 2. However, synchronous transmission requires twice as fast hardware as asynchronous transmission to do the same amount of work  - More reliable than asynchronous transmission - High link-bandwidth utilization - Faster hardware required (expensive hardware) Because clock-drift does not accumulate Because frame size is much larger This is the Trade-off CS447 - Computer and Data Communication Bit_Frame_error/005