1. 1 Multiplexing Introduction  Multiplexing  The set of techniques that allow the simultaneous transmission of multiple signals across a single data link  Possible ways to link multiple pair of devices  Many to one/one to many

2. 2 Many to One/one to Many DEMUXDEMUX MUXMUX 1 path 5 channels MUX = Multiplexer DEMUX = Demultiplexer Path = physical link No multiplexing Multiplexing Channel = a portion of a path, carries transmission between a pair of devices

3. 3 Multiplexing multiplexing Frequency-division multiplexing (FDM) Time-division multiplexing (TDM) Wave-division multiplexing (WDM) Asynchronous Synchronous

4. 4 Frequency-Division Multiplexing FDM Process Channel 1 Channel 2 Channel 3 MUXMUX DEMUXDEMUX Guard band Analog technique Link bandwidth > sum (channel bandwidth) Similar to road traffic  merging into one multilane-road

5. 5 FDM Process  Process  A sending device generates signal  A multiplexer  modulates each signal into different frequencies (f1, f2, f3, etc.)  Combines them into a composite signal  Sends it over a media link  A demultilexer  Uses a series of filters to decompose the multiplexed signal  individual signals  demodulators  Demodulators separate signals from their carriers and pass them to the waiting receivers

6. 6 FDM, Time Domain

7. 7 Demultiplexing, Time Domain

8. 8 Multiplexing, Frequency Domain

9. 9 Demultiplexing, Frequency Domain

10. 10 Wave-Division Multiplexing (WDM)  Similar to the FDM  Combining different signals of different frequencies  Involve light signals transmitted through fiber optic channel  Prism is used to combine and split light sources

11. 11 Time-division Multiplexing (TDM)  A digital process  Transmission medium data rate > sum(data rate required by sending devices)  Link is subdivided and interleaved into portions  Similar to a ski lift operation  Synchronous  Asynchronous

12. 12 Synchronous TDM : Time slot MUXMUX DEMUXDEMUX 231 2 3 1 Time slot A time slot is allocated to each device Devices take turn to send its data When its allocated time slot comes up If there is nothing to be sent, the time slot is empty Synchronous : fixed/portion

13. 13 Synchronous TDM : Frame MUXMUX …. Frame 1Frame 2Frame n Number of inputs : 3 Number of slots in each frame : 3 One or more slots dedicated to each sending device

14. 14 Synchronous TDM : Interleaving AAAAA BB CCC CBA C A A A Multiplexer Compared to a fast rotating switch Sender

15. 15 Synchronous TDM : Interleaving Demultiplexer CBA C A A A AAAAA BB CCC Weakness : wasted slots

16. 16 Synchronous TDM : Framing Bits CBA1CBA0C A1 A0 A1 10101 Synchronization pattern One or more synchronization bits are added to the beginning of each frame, to help the demultiplexer separate the time slots accurately

17. 17 Synchronous TDM : Bit Stuffing  Reminding :  Frame  Fixed size  Contains slots  Each device can be allocated multiple slots, depends on its data rate  Reality :  If device A is 3 times faster than device B  A might get 3 slots : B gets one slot  BUT : if A is 3 ½ times faster than B  A gets 3 ½ slots? No. A will get 4 slot  ½ Of the 4 th slot is filled by extra bits : called bit stuffing

18. 18 Asynchronous TDM (Statistical TDM)  Synchronous TDM does not guarantee that the full capacity of a link is used  Only a portion of the time slot is in used at a given instant  Asynchronous TDM is designed to avoid the waste of a link  Asynchronous = not fixed, flexible

19. 19 Asynchronous TDM  Sum (speed of all input lines ) <= link capacity  If there are n input lines  The frame will contain no more than m slots, where m < n  Asynchronous TDM can support more devices than a synchronous TDM

20. 20 Asynchronous TDM MUXMUX …. Frame 1Frame 2 Frame n Number of inputs : 5 Number of slots in each frame : 3

21. 21 Asynchronous TDM  The number of time slots in an async TDM is based on a statistical analysis of the number of input lines that are likely to be transmitting at any given time  How it works : the multiplexer  Scans the input lines  Accepts portions of data until a frame is filled  Sends the frame across the link  If there are not enough data to fill the frame,  The frame is transmitted on partially-filled  Transmits the frame

22. 22 Example of TDM Frames a. Only three lines sending data

23. 23 Example of TDM Frames b. Only four lines sending data

24. 24 Example of TDM Frames c. All five lines sending data

25. 25 TDM : Addressing and Overhead  Data from each device might not carried in the same slot (no fixed position)  Addressing is required (e.g A1, C3)  The address is for local use only  Added by the multiplexer  Discarded by the demultiplexer