dB =10log P1 P2

! Synchronous bit pipe
! Sending side of DLC supplies the sending side modem bits at a fixed rate
(one bit per T seconds)
! Idle fill (dummy bits) when no data
! Intermittent Synchronous bit pipe
! Supplies synchronously when there is data to be sent
! Sends nothing when no data
! Receiver complications:
! need to distinguish between 0, 1 and idle
! Re-synchronize with sender at end of idle period
! Asynchronous character pipe
! Bits within a character sent at a fixed rate
! Characters separated by variable delays
! We need to decide where a frame starts and
ends
! Character based framing
! Uses special characters
! SYN for idle and fill
! STX Start transmission
! ETX End Transmission
! Bit Oriented Framing
! Special string of bits 01111110
! Start, end and fill
! Length count
! Gives the frame length in a header field
33
35
• In asynch serial communication, the electrical interface is held in the mark position between
characters. The start of transmission of a character is signaled by a drop in signal level to the space
level. At this point, the receiver starts its clock. After one bit time (the start bit) come 7 or 8 bits of
true data followed by one or more stop bits at the mark level. The receiver tries to
sample the signal in the middle of each bit time. The byte will be read correctly if the line is still in
the intended state when the last stop bit is read.
! Frames consist of integer number of bytes
! Asynchronous transmission systems using ASCII to transmit printable
characters
! Octets with HEX value