Multiplexing Categories-Cont

Interleaving o    Synchronous TDM can be compared to a very fast rotating switch o    As  the  switch  opens  in  front  of  a  device,  the  device  has  the  opportunity  to  send  a specifies amount of data on to the path o    The switch moves from device to device at a constant rate and in a fixed order o    This process is called INTERLEAVING o    Interleaving can be done by BITS, BYTES or by any other DATA UNIT o    In other words MUX can take one byte from each device, then another byte from each device and so on o    In a given system interleaved units will always be of the same size

o    Fig,, shows interleaving and frame building o    In the example we interleave the various TXs by character (equal to 1 byte each) but the concept is the same for data units of any length o    Each device is sending a different message o    The MUX interleaves the different and forms them into FRAMES before putting them onto the link o    At the receiver the DEMUX decomposes each frame by extracting each character o    As  a  character  is  removed  from  a  frame,  it  is  passed  to  the  appropriate  receiving device Weakness of Synchronous TDM Figure o    Both figures point out  major weakness in Synchronous TDM o    By assigning each timeslot to a specific I/p line, we end up with empty slots whenever not all the lines are active o    In  figure  only  the  first  three  frames  are  completely  filled,  The  last  3  frames  have  a collective 6 empty slots o    Having 6 empty slots out of 24 means that a quarter of a capacity of the link is wasted o    Framing Bits o    Because  the  time  slots  order  in  a  synchronous  TDM  does  not  vary  from  frame  to frame, very little overhead information need to be included in each frame o    The   order   of   receipt   tells   the   DEMUX   where   to   direct   each   time   slot   so   no ADDRESSING is necessary Demultiplexing Process o    Demultiplexer decomposes each frame by extracting each data unit in turn o    Weakness of synchronous TDM -Waste of empty slots Framing Bits o    Various factor however can cause timing inconsistencies. o     For this reason one or more synchronization bits are added to the beginning of each frame o    These bits called Framing bits follow a pattern frame to frame that allows a DEMUX to synchronize with the incoming stream so that it can separate time slots accurately o    This synch info consist of one bit /frame alternating b/w 0 and 1.