Chapter 3

Time Division Multiplexing

➢ The concept of Time Division Multiplexing
➢ TDM Examples
➢ Frame Synchronization
➢TDM Hierarchy
➢ Packet Transmission

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

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 Time Division Multiplex

➢ Channel gets the whole spectrum for a certain

amount of time
➢ Advantages: Channels ki
⚫ only one carrier in the
medium at any time c
k1 k2 k3 k4 k5 k6

⚫ throughput high even f

for many users
➢ Disadvantages:
⚫ precise
synchronization
t
necessary

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 Time and Frequency Division Multiplex

➢ A channel gets a certain frequency band for a

certain amount of time (e.g. GSM)
➢ Advantages:
Channels k i
⚫ better protection against tapping
⚫ protection against frequency k1 k2 k3 k4 k5 k6
selective interference c
f
⚫ higher data rates compared to
code multiplex
➢ Precise coordination
required
t

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 Code Division Multiplex
Channels ki

k1 k2 k3 k4 k5 k6

➢ Each channel has unique code c

➢ All channels use same spectrum at same time

➢ Advantages:
⚫ bandwidth efficient
f
⚫ no coordination and synchronization
⚫ good protection against interference
➢ Disadvantages:
⚫ lower user data rates t
⚫ more complex signal regeneration
➢ Implemented using spread spectrum technology
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 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

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

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

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 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.
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 Time Division Multiplexing

Conceptual diagram of multiplexing-demultiplexing.

PAM TDM System

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Illustrating 4-Channel PAM TDM Multiplexing

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 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 channel with

Interleaving different bit rates using two
multiplexers Eeng 360 12
Block diagram of TDM system.
PAM TDM System

A Typical Framing Structure for TDM

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 Time Division Multiplexing

Frame structure of a certain TDM signal

Composite Signal Format

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 Time Division Multiplexing

Pulse width of TDM PAM:

Ts 1
=
3 3 fs

Pulse width of TDM PCM:

Ts
3n

1
fs = fs satisfies Nyquist rate
Ts
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 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.

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 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

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 TDM Example (Multiplexing Analog and Digital)
➢Source 1: 2 kHz
bandwidth.
➢Source 2: 4 kHz
16 ksam/s
bandwidth.
➢ Source 3: 2 kHz
bandwidth.
64 kb/s
➢ Source 4-11:
Digital 7200 bits/sec.

8x7.2=57.6 kb/s
Use stuff bits to
complete 7.2 to 8
128 kb/s
kb/s.
Now 8 and 64
rates are complete
multıples

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 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

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TDM PAM for Radio Telemetry

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CCITT Digital TDM Hierarchy

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 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.

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 Summary

➢ 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)

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