1. Time Division Multiplexing
Chapter 3
Time Division Multiplexing
The concept of Time Division Multiplexing
TDM Examples
Frame Synchronization
TDM Hierarchy
Packet Transmission
Huseyin Bilgekul
Eeng360 Communication Systems I
Department of Electrical and Electronic Engineering
Eastern Mediterranean University

2. Frequency Division Multiplex
Separation of spectrum into smaller frequency bands
Channel gets band of the spectrum for the whole time
Advantages:
no dynamic coordination needed
works also for analog signals
Disadvantages:
waste of bandwidth if traffic distributed unevenly
inflexible
guard spaces
Channels ki k3 k4 k5 k6 c f t

3. Time Division Multiplex
Channel gets the whole spectrum for a certain amount of time
Advantages:
only one carrier in the medium at any time
throughput high even for many users
Disadvantages:
precise synchronization necessary
Channels ki k1 k2 k3 k4 k5 k6 c f t

4. Time and Frequency Division Multiplex
A channel gets a certain frequency band for a certain amount of time (e.g. GSM)
Advantages:
better protection against tapping
protection against frequency selective interference
higher data rates compared to code multiplex
Precise coordination required
Channels ki k1 k2 k3 k4 k5 k6 c f t

5. Code Division Multiplex
Channels ki k1 k2 k3 k4 k5 k6
Each channel has unique code
All channels use same spectrum at same time
Advantages:
bandwidth efficient
no coordination and synchronization
good protection against interference
Disadvantages:
lower user data rates
more complex signal regeneration
Implemented using spread spectrum technology c f t

6. Multiplexing
Two basic forms of multiplexing. (a) Frequency-division multiplexing (FDM) (with guardbands). (b) Time-division multiplexing (TDM); no provision is made here for synchronizing pulses.
FDM
TDM

7. TDM
Composition of one frame of a multiplexed PAM signal incorporating four voice-signals and a synchronizing pulse.

8. Frequency Division Multiplexing (FDM)
Block diagram of FDM system, showing the important constituents of the transmitter and receiver.

9. Time Division Multiplexing
Definition: Time Division Multiplexing (TDM) is the time interleaving of samples from several sources so that the information from these sources can be transmitted serially over a single communication channel.
At the Transmitter
Simultaneous transmission of several signals on a time-sharing basis.
Each signal occupies its own distinct time slot, using all frequencies, for the duration of the transmission.
Slots may be permanently assigned on demand.
At the Receiver
Decommutator (sampler) has to be synchronized with the incoming waveform  Frame Synchronization
Low pass filter
ISI – poor channel filtering
Feedthrough of one channel's signal into another channel -- Crosstalk
Applications of TDM: Digital Telephony, Data communications, Satellite Access,
Cellular radio.

10. Time Division Multiplexing
Conceptual diagram of multiplexing-demultiplexing.
PAM TDM System

11. Illustrating 4-Channel PAM TDM Multiplexing

12. Digital Time Division Multiplexing
Time Division Multiplexing (TDM) can be accomplished at bit or byte (word) level.
Channhels having different data rates can also be TDM multiplexed but must be interleaved accordingly.
Digit Interleaving
Interleaving channel with different bit rates
WORD or Byte Interleaving
Interleaving channel with different bit rates using two multiplexers

13. Block diagram of TDM system.
PAM TDM System
A Typical Framing Structure for TDM

14. Time Division Multiplexing
Frame structure of a certain TDM signal
Composite Signal Format

15. Time Division Multiplexing
Pulse width of TDM PAM:
Pulse width of TDM PCM:

16. Pulse Stuffing in TDM
Stuff bits, which are dummy bits are inserted in the TDM output data when the different inputs are not completeley synchronized or the different input rates are not related by a ratinal number.

17. Pulse Stuffing in TDM
Stuff bits, which are dummy bits are inserted in the TDM output data when the different inputs are not completeley synchronized or the different input rates are not related by a ratinal number.
Multiplexing of two data streams with bit stuffing

18. TDM Example (Multiplexing Analog and Digital)
Source 1: 2 kHz bandwidth.
Source 2: 4 kHz bandwidth.
Source 3: 2 kHz bandwidth.
Source 4-11: Digital 7200 bits/sec.
16 ksam/s
64 kb/s
8x7.2=57.6 kb/s Use stuff bits to complete 7.2 to 8 kb/s. Now 8 and 64 rates are complete multıples
128 kb/s

19. Frame Synchronization
To sort and direct the received multiplexed data to the appropriate output channel
Two ways to provide frame sync to the demultiplexer circuit
- Over a separate channel
- Deriving from the TDM signal itself
Frame sync (unique k-bits) +Information words of an N-channel TDM system

20. TDM PAM for Radio Telemetry

21. CCITT Digital TDM Hierarchy

22. Packet Transmission System
TDM is Synchronous Transfer Mode (STM) technology
- Data source is assigned a specific time slot – fixed data rate
- More efficient when sources have a fixed data rate
- Inefficient to accommodate bursty data source
Solution?
Packet Transmission System
- Partitions source data into data packets (destination address, header)
- Efficiently assigns network resources when the sources have bursty data
- Examples : Internet TCP/IP technology and the Asynchronous Transfer Mode (ATM) technology.

23. Summary How to compute the spectra for line codes
How information in analog waveforms can be represented by digital signaling
How to compute the spectra for line codes
How filtering of the digital signal, due to the communication channel affects our ability to recover the digital information at the receiver [ISI]
How we can merge information from several sources into one digital signal by using time division multiplexing (TDM)