Eeng 360 1

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• 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
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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
• 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
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Time Division Multiplex

• Channel gets the whole spectrum for a certain
  amount of time
• Advantages
• only one carrier in themedium at any time
• throughput high even for many users
• Disadvantages
• precise synchronization necessary

Channels ki
k2
k3
k4
k5
k6
k1
c
f
t
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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
• better protection against tapping
• protection against frequency selective
  interference
• higher data rates compared tocode multiplex
• Precise coordinationrequired

Channels ki
k2
k3
k4
k5
k6
k1
c
f
t
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Code Division Multiplex
Channels ki
k2
k3
k4
k5
k6
k1

• 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
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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.
Multiplexing
FDM
TDM
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Composition of one frame of a multiplexed PAM
signal incorporating four voice-signals and a
synchronizing pulse.
TDM
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Block diagram of FDM system, showing the
important constituents of the transmitter and
receiver.
Frequency Division Multiplexing (FDM)
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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
Interleaving channel with different bit rates
using two multiplexers
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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
Pulse width of TDM PCM
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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 bandwidth.
• Source 3 2 kHz bandwidth.
• Source 4-11 Digital 7200 bits/sec.

16 ksam/s
64 kb/s
8x7.257.6 kb/sUse stuff bits to complete 7.2 to
8 kb/s.Now 8 and 64 rates are complete multiples
128 kb/s
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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)