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
Eeng 360 1
Frequency Division Multiplex

Separation of spectrum into smaller frequency bands
 Channel gets band of the spectrum for the whole time
Channels
 Advantages:



no dynamic coordination needed
works also for analog signals


k3
k4
k5
k6
c
Disadvantages:

ki
f
waste of bandwidth
if traffic distributed unevenly
inflexible
guard spaces
t
Eeng 360 2
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
Channels ki
c
k1
k2
k3
k4
k5
k6
f
Disadvantages:

precise
synchronization
t
necessary
Eeng 360 3
Time and Frequency Division Multiplex

A channel gets a certain frequency band for a
certain amount of time (e.g. GSM)
 Advantages:
Channels k




better protection against tapping
protection against frequency
c
selective interference
higher data rates compared to
code multiplex
k1
k2
k3
k4
i
k5
k6
f
Precise coordination
required
t
Eeng 360 4
Code Division Multiplex
Channels ki
k1
k2
k3

k4
Each channel has unique code
 All channels use same spectrum at same time
 Advantages:




k6
c
bandwidth efficient
no coordination and synchronization
good protection against interference
f
Disadvantages:



k5
lower user data rates
more complex signal regeneration
t
Implemented using spread spectrum technology
Eeng 360 5
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
Eeng 360 6
TDM
 Composition of one frame of a multiplexed PAM signal incorporating four voicesignals and a synchronizing pulse.
Eeng 360 7
Frequency Division Multiplexing (FDM)
 Block diagram of FDM system, showing the important constituents of the transmitter
and receiver.
Eeng 360 8
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.
Eeng 360 9
Time Division Multiplexing
Conceptual diagram of multiplexing-demultiplexing.
PAM TDM System
Eeng 360 10
Illustrating 4-Channel PAM TDM Multiplexing
Eeng 360 11
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
WORD or Byte
Interleaving
Interleaving channel with
different bit rates
Interleaving channel with
different bit rates using two
multiplexers Eeng 360
12
Block diagram of TDM system.
PAM TDM System
A Typical Framing Structure for TDM
Eeng 360 13
Time Division Multiplexing
Frame structure of a certain TDM signal
Composite Signal Format
Eeng 360 14
Time Division Multiplexing
Pulse width of TDM PAM:
Ts
1

3 3 fs
Pulse width of TDM PCM:
Ts
3n
fs 
1
Ts
fs satisfies Nyquist rate
Eeng 360 15
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.
Eeng 360 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.
Multiplexing of two data streams with bit stuffing
Eeng 360 17
TDM Example (Multiplexing Analog and Digital)
Source 1: 2 kHz
bandwidth.
Source 2: 4 kHz
bandwidth.
16 ksam/s
 Source 3: 2 kHz
bandwidth.
 Source 4-11:
Digital 7200 bits/sec.
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
64 kb/s
128 kb/s
Eeng 360 18
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
Eeng 360 19
TDM PAM for Radio Telemetry
Eeng 360 20
CCITT Digital TDM Hierarchy
Eeng 360 21
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.
Eeng 360 22
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)
Eeng 360 23