Multiplexing allows multiple transmission sources to share a larger transmission capacity through techniques like frequency division multiplexing (FDM) and time division multiplexing (TDM). FDM allocates different carrier frequencies to different signals so they do not overlap. TDM interleaves multiple digital signals in time by assigning fixed time slots. Statistical TDM dynamically allocates time slots based on demand to make more efficient use of bandwidth compared to synchronous TDM which allocates slots even if they are empty.

1. Data Communications - Multiplexing

2. Multiplexing Multiplexing allows several transmission sources to share a larger transmission capacity. 2 common forms of multiplexing Frequency division multiplexing (FDM) Time division multiplexing (TDM) Synchronous TDM Statistical / Asynchronous TDM

3. Frequency Division Multiplexing FDM Possible if useful bandwidth of medium exceeds( melebihi ) required bandwidth of channel [ bandwidth/frequency range : medium>signal ] Each signal is modulated to a different carrier frequency Carrier frequencies separated so signals do not overlap (guard bands) e.g. broadcast radio Channel allocated even if no data

4. Frequency Division Multiplexing Diagram FTM

6. Synchronous Time Division Multiplexing (TDM) Possible if data rate of medium exceeds data rate of digital signal to be transmitted [ data rate : medium>signal ] Multiple digital signals interleaved in time Interleaving can be at bit level of blocks Time slots pre-assigned to sources and fixed Time slots allocated even if no data to send

7. Time Division Multiplexing

9. TDM Link Control No headers and trailers Data link control protocols not needed Flow control Data rate of multiplexed line is fixed If one channel receiver can not receive data, the others must carry on The corresponding source must be quenched This leaves empty slots Error control Errors are detected and handled by individual channel systems

10. Synchronous TDM

11. TDM, Multiplexing

12. TDM, Demultiplexing

13. Statistical TDM In Synchronous TDM many slots are wasted Statistical TDM allocates time slots dynamically based on demand Multiplexer scans input lines (buffer) and collects data until frame full, and then send the frame Data rate on line lower than aggregate(sum of) rates of input lines Thus, statistical TDM can use a lower data rate to support as many devices as asynchronous multiplexer

14. Synchronous TDM compared with Statistical TDM

15. Asynchronous TDM

16. Frames and Addresses a. Only three lines sending data

17. Frames and Addresses c. All five lines sending data

18. Statistical TDM Frame Formats

19. Performance Output data rate less than aggregate (the sum of the data rates of the inputs) input rates May cause problems during peak periods Buffer inputs Keep buffer size to minimum to reduce delay

20. Cable Modem Outline Cable TV provider provides dedicated 2 channels to support data transfer to/from cable modem One for transmission in each direction Each channel shared by number of subscribers Scheme needed to allocate capacity (fraction of down/upstream capacity) Statistical TDM

21. Cable Modem Operation Downstream Cable scheduler delivers data in small packets If more than one subscriber active, each gets fraction of downstream capacity May get 500kbps to 1.5Mbps Also used to allocate upstream time slots to subscribers Upstream User requests timeslots on shared upstream channel Dedicated slots for this Headend scheduler sends back assignment of future time slots to subscriber

22. Cable Modem Scheme

23. Asymmetrical Digital Subscriber Line ADSL Link between subscriber and network Local loop Uses currently installed twisted pair cable Can carry broader spectrum 1 MHz or more Most widely publicized of a family of new modem technologies designed to provide high-speed digital data transmission over ordinary telephone wire

24. ADSL Design Asymmetric Greater capacity downstream than upstream ADSL uses Frequency division multiplexing (FDM) Reserve lowest 25kHz for voice Plain old telephone service (POTS) Use echo cancellation or FDM to give two bands Use FDM within bands Range 5.5km

25. ADSL Channel Configuration

26. xDSL High data rate DSL Single line DSL Very high data rate DSL Various types of xDSL Asynchronous Digital Subscriber Line (ADSL) High Bit-rate Digital Subscriber Line (HDSL) Symmetric Digital Subscriber Line (SDSL) Very High Bit-rate Digital Subscriber Line (VDSL)

27. Comparison of XDSL Alternatives