Multiplexing FDM and TDM
- 1. Presented by: Ali Ahmad Farooq
- 2. INTRODUCTION Under the simplest conditions, a medium can carry only one signal at any moment in time If we try to pass multiple signals through a common medium , they will possibly interfere with each other. When two or more signals with same frequency pass at the same time through a common medium the interference phenomena occurs
- 3. INTRODUCTION This means we have to devise a way to avoid the interference of the signals Which means that multiple signals i. Should have different frequency ii. Must not travel at same time iii. Must not travel through same medium For multiple signals to share a medium , the medium must somehow be divided , so that each signal receives a portion of the total bandwidth.
- 4. Need for MULTIPLEXING Transmission services are very expensive (leased lines , packet switched networks) Multiplexing and compression techniques save the business money As the data capacity of line increases , it will become more cost effective for a company Most data services require modest data rate support
- 5. TYPES The current techniques that can accomplish multiplexing includes : • Frequency division multiplexingFDM • Time division multiplexing TDM
- 6. MULTIPLEXING Multiplexing is nothing but sharing of medium Multiplexor (MUX) De-multiplexor(DEMUX) Sometimes just called a MUX Simple block diagram of Mux-Demux pair
- 7. Frequency Division Multiplexing Frequency spectrum is divided among multiple logical channels Each user can have access to its own assigned frequencies or logical channels at all the time that the individual user is active
- 8. Frequency division multiplexing Logic diagram f1= Lowest frequency of the band f6= Highest frequency of the band fn= subcarrier frequency
- 9. Frequency division multiplexing Sends the signal in several distinct frequency ranges Each signal is modulated on the different carrier frequency which are separated by guard bands The bandwidth of the transmission medium must exceed the required bandwidth of all signals
- 10. Frequency division multiplexing Used in cable TV to carry multiple video channels on a single cable network Broadcast radio and TV and the AMPS cellular phone systems use FDM Since it involves analog signaling , it is more susceptible to noise This is the oldest technique of multiplexing
- 12. FDM System Transmitter In FDM each user is assigned non-overlapping frequency ranges so that multiple signals can be transmitted at the same time Multiple message signals {m1(t), m2(t)…mn(t)} from multiple sources are modulated over different subcarrier frequencies (f1,f2…fn) and send them to an adder Now this composite signal (Adder output) is modulated over carrier frequency (fc) and transmitted over channel
- 14. FDM System Receiver In FDM receiver we first get the transmitted which was modulated using carrier frequency (fc) and then pass this composite signal through suitable band-pass filters These band-pass filters have the same frequency as the subcarriers of FDM transmitters Now these signals s1,s2…sn are passed through respective demodulators to get the actual message signals {m1(t), m2(t)…mn(t)}
- 15. Disadvantages of FDM The problem with FDM is that it cannot utilize the full capacity of the system We need to ensure that the adjacent bands do not overlap each other , otherwise the signal in one band may interfere the signal in other band Although system has the capacity still in some cases the channel cannot pass the actual signal
- 16. TIME DIVISION MULTIPLEXING In Time division multiplexing the time is divided into multiple smaller units called slots and each user is given a slot to transmit the signal Each user has the entire bandwidth of the channel for a short interval of time
- 17. TIME DIVISION MULTIPLEXING Logic diagram
- 18. TIME DIVISION MULTIPLEXING Used for digital signals or analog signals carrying digital data Data rate of the transmission medium must exceed the required data rate of all the signals Clock synchronization is one of the major issue in TDM systems as a small mismatch in the timing may ruin the utility of overall system
- 19. Synchronous TDM
- 20. Asynchronous TDM