Categories of Data Link Protocols

Categories of Data Link Protocols Data link protocols can be divided into two sub-groups:

Asynchronous Protocols: Treat each character in a Bit stream independently Synchronous Protocols: Take the whole bit stream and chop it into characters of equal Size ¾   Asynchronous Protocols Employed mainly in Modems Inherent Slowness is a disadvantage: Requires addition of start and stop bits and extended spaces b/w frames, so these are mainly replaced with High-speed synchronous mechanisms Not Complex and Inexpensive to Implement Transmission does not require timing coordination; Timing is done by using extra bits •     Different Asynchronous Protocols A variety of Asynchronous protocols have been developed We will discuss some of the important ones 9   XMODEM In  1979  Ward  Christiansen  designed  a  File  transfer  protocol  for  Telephone-line communication b/w PCs called XMODEM Half Duplex and Stop-and-Wait ARQ protocol XMODEM o    The first field is a One Byte start of header (SOH) field o    The second field is a two-byte Header. -The first header byte , the Sequence number carries the Frame number -The second header byte is used to check the validity of the sequence number o    The fixed data field holds 128 bytes of data o    The last field CRC checks for errors in the data field only Transmission in XMODEM o    TX begins with sending of a NAK frame from the receiver to the sender o    Each  time,  the  sender  sends  a  frame,  it  must  wait  for  an  ACK  before  sending  next frame o    If NAK is received instead of ACK, the last frame is sent again o    A frame is also resent if no response arrives from a receiver after a fixed time period o    A sender can also receive cancel (CAN) to abort TX Control Frames in XMODEM Control frames from the receiver -ACK: Acknowledgement -NAK: Error or start of transmission -CAN: Aborts the transmission o    1024-Byte data unit o    Two CANs to abort Transmission o    ITU-T CRC-16 for Error Checking o    Multiple files can be sent simultaneously