Data Communication and networking, chapitre 6

1. CH6 - BANDWIDTH
UTILISATION:
MULTIPLEXING AND
SPREADING
FDM, TDM, WDM, Spread
Spectrum, FHSS, DSSS,
Multiplexing,
Demultiplexing, Framing…
By Alban

2. INTRODUCTION
Bandwidth utilization is the wise use of available bandwidth to achieve
specific goals. Efficiency can be achieved by multiplexing; privacy and
anti-jamming can be achieved by spreading.

3. MULTIPLEXING
Multiplexing is the set of techniques that allows the simultaneous
transmission of multiple signals across a single data link;
The opposite operation consisting in retrieving multiple signals from
a single data link is called Demultiplexing;
Multiplexing combines signals from several sources to achieve
bandwidth efficiency

4. MULTIPLEXING
In multiplexed system, n lines share the bandwidth of one link
The bandwidth of a link may be greater than the bandwidth needs of
the devices connected to it

5. MULTIPLEXING
There are three basic multiplexing techniques: frequency-division
multiplexing, wavelength-division multiplexing, and time-division
multiplexing.
Multiplexin
g
Frequency-
division
multiplexing
Wavelength-
division
multiplexing
Time-division
multiplexing
Analog Analog Digital

6. FREQUENCY-DIVISION
MULTIPLEXING (FDM)
FDM is an analog multiplexing technique that combines analog
signals.
It can be applied when the bandwidth of a link (in hertz) is greater
than the combined bandwidths of the signals to be transmitted.
 Signals generated by each sending device modulate different carrier frequencies.
Channel: Carrier frequencies separated by sufficient
bandwidth to accommodate the modulated signal

7. FDM : MULTIPLEXING PROCESS
Each source generates a signal of a similar frequency range
Inside the MUX, these input signals modulate different frequencies (f
1 , f 2 , f 3 )
The resulting modulated signals
are then combined into a single
composite signal that is sent out
over a media link that has
enough bandwidth to
accommodate it

8. FDM: DEMULTIPLEXING PROCESS
Demultiplexer uses a series of filters to decompose the multiplexed signal
into its constituent component signals
The individual signals are then passed to demodulators
Each demodulator separates
original analog signal from
their carriers and passes it to
the outline

9. FDM
Even though FDM is analog muxing, FDM can be used to send digital
signals
 Digital data can be converted to an analog signal before FMS’s muxing

10. WAVELENGTH-DIVISION
MULTIPLEXING (WDM)
Conceptually same than FDM. Instead involves optical signal.
WDM is an analog multiplexing technique to combine optical signals.
 Designed to use the high-data-rate capability of fiber-optic cable
 The main idea is to combine different signals of different frequencies same as FDM
Very narrow bands of light from different sources are combined to
make a wider band of light.
WDM is designed to use the high bandwidth capability of fiber-

11. WAVELENGTH-DIVISION
MULTIPLEXING (WDM)
A multiplexer combines several input beams of light, each containing
a narrow band of frequencies, into one output beam of a wider band
of frequencies
A demultiplexer reverses the process
Use prisms as MUX and DEMUX

12. WAVELENGTH-DIVISION
MULTIPLEXING (WDM)
WDM is used in SONET (ch.14)
Dense WDM : muxing a very large number of channels by spacing
channels very close to one another

13. TIME-DIVISION MULTIPLEXING
(TDM)
Instead of sharing a portion of the bandwidth as in FDM, time is shared in TDM.
Each connection occupies a portion of time in the link
TDM is a digital multiplexing technique for combining several low-rate channels
into one high-rate one.
In TDM one data is sent using the full bandwidth available in the medium
Frame
Time slot

14. TIME-DIVISION MULTIPLEXING
2 types of schemes
 Synchronous TDM
 Statistical TDM

15. SYNCHRONOUS TDM
In synchronous TDM, the data rate of the link is n times faster, and
the unit duration is n times shorter.
 In synchronous TDM, a round of data units from each input connection is collected
into a frame
 For n connections, a frame is divided into n time slots and one slot is allocated for
each unit
Time slots are grouped into
frames and a frame consists of
one complete cycle of time slots

16. SYNCHRONOUS TDM-
INTERLEAVING
The process of taking a group of bits from each input line for multiplexing
is called interleaving
TDM has two fast-rotating switches, one on the MUXside and other on the
side
 The switches are synchronized and rotate at the same speed
 On MUX, as the switch opens in front of a connection, that connection has opportunity to
send a unit onto the path -> interleaving
If a source does not
have data to send,
the corresponding
slot in the output
frame is empty.

17. TDM - DATA RATE MANAGEMENT
Set of techniques used to handle a disparity in the input data rates
20
kpbs20
kpbs40
kpbs40
kpbs40
kpbs
40
kpbs
160
kpbs
50
kpbs
25
kpbs25
kpbs25
kpbs
125
kpbs
25
kpbs
25
kpbs
50
kpbs
50
kpbs
46
kpbs
150
kpbs
50
kpbs
Pulse
stuffing
Multilevel multiplexing is a technique
used when the data rate of an input
line is a multiple of others.
Multiple-Slot Allocation - Sometimes it
is more efficient to allot more than one
slot in
a frame to a single input line.
Pulse Stuffing - used to make the
highest input data rate the dominant
data rate and then add dummy bits to
the input lines with lower rates. This
will increase their rates.

18. TDM – FRAME SYNCHRONIZATION
If the multiplexer and the demultiplexer are not synchronized, a bit
belonging to one channel may be received by the wrong channel.
To solve this problem, in synchronous TDM, one or more
synchronization bits are usually added to the beginning of each
frame.

19. STATISTICAL TIME-DIVISION
MULTIPLEXING
In statistical time-division multiplexing, slots are dynamically
allocated to improve bandwidth efficiency.
 Only when an input line has a data to send is it given a slot in the output frame
 A slot needs to carry data as well as the address of the destination

20. SPREAD SPECTRUM
Spread spectrum is designed to be used in wireless applications
Its main goal is to provide a relative protection against interferences, eavesdrop or
jamming.
To achieve these goals, spread spectrum techniques add redundancy
It combines signals from different sources to fit into a larger bandwidth.
B ss >> B (where B : required bandwidth for each station)
 B ss : expanded bandwidth for SS
The expanded bandwidth allows the source to wrap its message in protective
envelope for a more secure transmission

21. SPREAD SPECTRUM - FREQUENCY
HOPPING SPREAD
SPECTRUM(FHSS)
Uses M different carrier frequencies that are modulated by the source
signal
 at one moment, the signal modulates one carrier frequency;
 at the next moment, the signal modulates another carrier frequency;… ;
 finally M signal frequencies are modulated in the long run
FHSS Frequency selection in FHSS

22. FREQUENCY HOPPING SPREAD
SPECTRUM(FHSS)
Main advantages :
 Prevention against message interception
 Anti-jamming effect

23. FHSS & FDM
In FDM, each station uses 1/M of the bandwidth, but the allocation is
fixed
In FHSS, each station uses 1/M of the bandwidth, but the allocation
changes hop by hop

24. SPREAD SPECTRUM - DIRECT
SEQUENCE SPREAD SPECTRUM
(DSSS)
In DSSS each bit is assigned a code of n bits, called chips, where the
chip rate is n times that of the data bit.
If the intruder dose not know the chips, the spread signal can be
protected.
X
Original
signal
Spread
signal
Chips
generat
or
Modulator