3. Multiplexing is the set of techniques that allows the
simultaneous transmission of multiple signals across a single data
link.
To make efficient use of high-speed telecommunication lines,
some form of multiplexing is used.
Multiplexing allows several transmission sources to share a larger
transmission capacity.
4. The figure below depicts the multiplexing function in its simplest
form. There are n inputs to a multiplexer. The multiplexer is
connected by a single data link to a demultiplexer.
The link is able to carry n separate channels of data. The multiplexer
combines (multiplexes) data from the n input lines and transmits over a
higher-capacity data link.
The demultiplexer accepts the multiplexed data stream, separates
(demultiplexes) the data according to channel, and delivers data to the
appropriate output lines.
6. Frequency-division multiplexing (FDM) is a technique by which
the total bandwidth available in a communication medium is
divided into a series of non-overlapping frequency bands, each of
which is used to carry a separate signal.
This allows a single transmission medium such as a cable or
optical fiber to be shared by multiple independent signals.
Another use is to carry separate serial bits or segments of a
higher rate signal in parallel.
7. The multiple separate information (modulation) signals that are
sent over an FDM system, such as the video signals of the
television channels that are sent over a cable TV system, are
called baseband signals.
At the source end, for each frequency channel, an electronic
oscillator generates a carrier signal, a steady oscillating
waveform at a single frequency that serves to carry information.
The carrier is much higher in frequency than the baseband signal.
The carrier signal and the baseband signal are combined in a
modulator circuit.
The modulator alters some aspect of the carrier signal, such as
its amplitude, frequency, or phase, with the baseband signal,
"piggybacking" the data onto the carrier.
Using the modulation technique, the input signals are
transmitted into frequency bands and then combined to form a
composite signal.
8. A generic depiction of an FDM system is shown in the figure.
A number of analog or digital signals are to be multiplexed onto
the same transmission medium.
Each signal is modulated onto a carrier ; because multiple
carriers are to be used, each is referred to as a subcarrier.
Any type of modulation may be used.
9. The resulting analog, modulated signals are then summed to
produce a composite baseband signal mb(t).
The figure below shows the result.
The spectrum of signal mi(t) is shifted to be centered on fi.
fi must be chosen so that the bandwidths of the various signals
do not significantly overlap. Otherwise, it will be impossible to
recover the original signals.
The composite signal may then be shifted as a whole to another
carrier frequency by an additional modulation step.
10. This second modulation step need not use the same modulation
technique as the first.
The FDM signal s(t) has a total bandwidth B. This analog signal
may be transmitted over a suitable medium.
At the receiving end, the FDM signal is demodulated to retrieve
mb(t) which is then passed through n bandpass filters, each filter
centered on fi and having a bandwidth Bi, n<= i <= n.
In this way, the signal is again split into its component parts.
Each component is then demodulated to recover the original
signal.
11. The following diagram conceptually represents multiplexing using
FDM.
It has 4 frequency bands, each of which can carry signal from 1
sender to 1 receiver. Each of the 4 senders is allocated a frequency
band.
The four frequency bands are multiplexed and sent via the
communication channel. At the receiving end, a demultiplexer
regenerates the original four signals as outputs.
Here, if the frequency bands are of 150 KHz bandwidth separated by
10KHz guard bands, then the capacity of the communication channel
should be at least 630 KHz (channels : 150 × 4 + guard bands : 10 ×
3).
12. It allows sharing of a single transmission medium like a copper
cable or a fiber optic cable, among multiple independent signals
generated by multiple users.
FDM has been popularly used to multiplex calls in telephone
networks. It can also be used in cellular networks, wireless
networks and for satellite communications.
13. Pros:
FDM process is very simple and easy modulation.
A Large number of signals can be sent through an FDM simultaneously.
It does not require any synchronization between sender and receiver.
Cons:
FDM technique is used only when low-speed channels are required.
It suffers the problem of crosstalk.
A Large number of modulators are required.
It requires a high bandwidth channel.
14. In general, carrier systems are mechanisms that provide a means
to send signals from more than one source over a single physical
channel. Analog carrier system is the electronic mechanism that
implemented FDM.
In frequency division multiplexing the composite signal
transmitted across the medium is analog. The input signals may
be either digital or analog.
In the case of digital input, the input signals must be passed
through modems to be converted to analog. In either case, each
input analog signal must then be modulated to move it to the
appropriate frequency band.
15. The true potential of optical fiber is fully exploited when
multiple beams of light at different frequencies are transmitted
on the same fiber. This is a form of frequency division
multiplexing (FDM) but is commonly called wavelength division
multiplexing (WDM).
With WDM, the light streaming through the fiber consists of many
colors, or wavelengths, each carrying a separate channel of data.
A typical WDM system has the same general architecture as other
FDM systems.
A number of sources generate a laser beam at different
wavelengths. These are sent to a multiplexer, which consolidates
the sources for transmission over a single fiber line.
The term dense wavelength division multiplexing (DWDM)
connotes the use of more channels, more closely spaced, than
ordinary WDM. In general, a channel spacing of 200 GHz or less
could be considered dense.
16. A typical WDM system has the same general architecture as other
FDM systems. A number of sources generate a laser beam at
different wavelengths.
These are sent to a multiplexer, which consolidates the sources
for transmission over a single fiber line.
Optical amplifiers, typically spaced tens of kilometers apart,
amplify all of the wavelengths simultaneously.
Finally, the composite signal arrives at a demultiplexer, where
the component channels are separated and sent to receivers at
the destination point.
17. Time division multiplexing (TDM) is a digital multiplexing method
of transmitting and receiving independent signals over a common
signal path by means of synchronized switches at each end of
transmission line, so that each signal appears on the line only a
fraction of time.
TYPES OF TDM:
Synchronous Time Division Multiplexing
Asynchronous or Statistical Time Division Multiplexing.
18. Synchronous time division multiplexing is the technique that
allow multiple signals to be sent over a single faster connection
by sectioning a link into time slots.
In synchronous TDM, each device is given same time slot to
transmit the data over the link, irrespective of the fact that the
device has any data to transmit or not. Hence the name
Synchronous TDM.
Each device places its data onto the link when its time
slot arrives. Each device is given the possession of line turn by
turn.
If any device does not have data to send then its time slot
remains empty.
The various time slots are organized into frames and each frame
consists of one or more time slots dedicated to each sending
device.
If there are n sending devices, there will be n slots in frame, one
slot for each device.
19. A generic depiction of a synchronous TDM system is provided
here:
A number of signals [mi(t), i = 1, n] are to be multiplexed onto
the same transmission medium. The signals carry digital data and
are generally digital signals.
The incoming data from each source are briefly buffered. The
buffers are scanned sequentially to form a composite digital data
stream mc(t).
The scan operation is sufficiently rapid so that each buffer is
emptied before more data can arrive. Thus, the data rate of
mc(t) must at least equal the sum of the data rates of the mi(t).
20. The digital signal mc(t) may be transmitted directly, or passed
through a modem so that an analog signal is transmitted. In
either case, transmission is typically synchronous.
The transmitted data may have a format something like the
figure b. The data are organized into frames. Each frame
contains a cycle of time slots. In each frame, one or more slots
are dedicated to each data source.
The sequence of slots dedicated to one source, from frame to
frame, is called a channel. The slot length equals the transmitter
buffer length, typically a bit or a byte (character).
21. At the receiver, the interleaved data are demultiplexed and
routed to the appropriate destination buffer.
For each input source mi(t), there is an identical output
destination that will receive the output data at the same rate at
which it was generated.
Synchronous TDM is called synchronous not because synchronous
transmission is used, but because the time slots are pre-assigned
to sources and fixed. The time slots for each source are
transmitted whether or not the source has data to send.
22. It is used in digital audio mixing system, Pulse Code
Modulation(PCM) transmission system, the optical data
transmission system or optical fiber communication etc.
It is used in SONET(Synchronous Optical Networking).
The main application of TDM(Time Division Multiplexing) is to
transmit or receive different data and signal using a common
single transmission line.
23. Pros:
An order is maintained
No addressing information is required, channel capacity should
be large
Cons:
The channel capacity cannot be fully utilized. Some of the
slots go empty in certain frames. This is wastage of bandwidth.
The capacity of single communication line that is used to carry
the various transmission should be greater than the total speed
of input lines.
24. No headers and trailers
Data link control protocols not needed
Flow control :
Data rate of multiplexed line is fixed
If one channel receiver can not receive data, the others must carry on
The corresponding source must be quenched
This leaves empty slots
Error control :
Errors are detected and handled by individual channel systems
Framing: There is a basic requirement for framing because of
not providing flag or SYNC characters to bracket TDM frames,
some means is needed to assure frame synchronization. It is
clearly important to maintain framing synchronization because,
if the source and destination are out of step, data on all
channels are lost.
25. Pulse Stuffing: With pulse stuffing, the outgoing data rate of the
multiplexer, excluding framing bits, is higher than the sum of the
maximum instantaneous incoming rates. The stuffed pulses are
inserted at fixed locations in the multiplexer frame format so
that they may be identified and removed at the demultiplexer.
26. SONET (Synchronous Optical Network) is an optical transmission
interface originally proposed by BellCore and standardized by
ANSI.
A compatible version, referred to as Synchronous Digital
Hierarchy (SDH), has been published by ITU-T in Recommendation
G.707.2
SONET is intended to provide a specification for taking advantage
of the high-speed digital transmission capability of optical fiber.
The SONET specification defines a hierarchy of standardized
digital data rates
The basic SONET building block is the STS-1 frame, which consists
of 810 octets and is transmitted once every 125 ms, for an
overall data rate of 51.84 Mbps
27. Synchronous time-division multiplexing is possible when the
attainable data rate (called bandwidth) of the medium exceeds
the data rate of digital signals to be transmitted.
If the data signal is analog, it has to be converted to digital.
Multiple digital signals or analog signals carrying digital data can
be carried on a single transmission path by interleaving portions
of each signal in time through digital carrier systems.
28. Statistical Time Division multiplexing is a technique that allows
the numbers of data of users to be multiplexed over the channel
more than the channel can afford.
In statistical time division multiplexing, slots are dynamically
allocated to improve bandwidth efficiency, each slot contains an
address part that identifies the source of the data.
Statistical TDM is a technique in which time slots are not fixed as
in the case of synchronous TDM. In statistical multiplexing, the
number of slots in each frame is less than the number of input
lines.
It allocates a slot for an input line if the line has data to send;
otherwise ,it skips the line and checks the next line. The
bandwidth of the output link is normally less than the total
bandwidth of the each channel.
29. It’s input lines with a buffer, associated to each of them. During
the input, the multiplexer scans the input buffers, collecting
data until the frame is filled and send the frame. At the receiving
end, the demultiplexer receives the frame and distributes the
data to the appropriate buffers.
It allocates the time slots on demand to separate input channels
while saving the channel capacity.
The frames in statistical TDM need not be synchronized ,so we do
not need synchronization bits.
30. In statistical TDM, there are five lines or five channels, and
considered the first slot channel C and channel E doesn't
transferring any data. And In the second time slot channel A
and channel C doesn't transferring any data.
But in this figure, we use some header there. There a, b, d, e
are the header of A,B,D,E slots.
31. Actually in statistical time division multiplexing we use a
header or address part that identifies the source of the data
for each slots. For this reason, every slot can identify the
upcoming data and fill all the slots with that data.
It allocates a slot for an input line if the line has data to
send; otherwise, it skips the line and checks the next line and
for that no slot is wasted.
32. The MPEG transport stream for digital TV transmission. Statistical
multiplexing is used to allow several video, audio and data
streams of different data rates to be transmitted over a
bandwidth-limited channel.
The UDP and TCP protocols, where data streams from several
application processes are multiplexed together.
The X.25 and Frame relay packet-switching protocols, where the
packets have varying lengths, and the channel number is denoted
virtual connection identifier (VCI).
The Asynchronous Transfer Mode packet-switched protocol,
where the packets have fixed length.
33. Serial
no.
Parameter FDM Synchronous
TDM
Statistical
TDM
01. Line utilization
efficiency
Poor Good Very good
02. Flexibility Poor Good Very good
03. Channel capacity Poor Good Excellent
04. Error control Not possible Not possible Possible
05. Multidrop capacity Very good Difficult to
achieve
Possible
06. Transmission delay Does not
exist
Low Random
07. Cost High Low Moderate
34. There are four multiplexing techniques used for sharing channel
capacity among multiple transmitter/receiver stations. These
techniques differ from the FDM and TDM techniques because no
physical multiplexer is involved.
Individual stations are assigned a frequency band or a sequence
of time slots and transmit directly on the channel and not
through a multiplexer.
The four techniques are:
Frequency-Division Duplex (FDD)
Time-Division Duplex (TDD)
Frequency-Division Multiple Access (FDMA)
Time-Division Multiple Access (TDMA)
35. Frequency-division duplexing (FDD) is a method for establishing a
full-duplex communications link that uses two different radio
frequencies for transmitter and receiver operation.
Examples:
Asymmetric digital subscriber line (ADSL) and very-high-bitrate digital
subscriber line (VDSL)
Cellular systems, including the UMTS/WCDMA Frequency Division
Duplexing mode and the CDMA2000 system
IEEE 802.16 WiMax Frequency Division Duplexing mode
Because the FDD technique uses different frequency bands for
send and receive operations, the sending and receiving data
signals don't interfere with each other. This makes FDD a better
choice than Time Division Duplex (TDD) for symmetric traffic such
as voice applications in broadband wireless networks.
36. FDD operation normally assigns the transmitter and receiver to
different communication channels. One frequency is used to
communicate in one direction, and the other frequency is
required to communicate in the opposite direction.
The transmit direction and receive direction frequencies are
separated by a defined frequency offset.
37. In time-division duplex (TDD),time rather than frequency is used
to separate the transmission and reception of the signals, and
thus a single frequency is assigned to a user for bot h directions.
TDD provides quasi-simultaneous bi-directional flow of
information.
Time-division duplex is method where data are transmitted in
one direction at a time, with transmission alternating between
the two direction.
38. Time division duplex (TDD) communication system, an
unconnected base station that is not directly connected to a core
network transmits uplink backhaul signals to a connected base
station.
The unconnected base station is transmitting downlink signals to
one or more user equipment (UE) devices and receives downlink
backhaul signals from the connected base station while receiving
uplink signals from one or more UE devices.
After determining the transmission schedule of the connected
base station, the unconnected base station selects a transmission
schedule that is orthogonal to the connected base station
transmission schedule.
Depending on the particular implementation, the unconnected
base station may be a repeater base station, a relay base station,
or a self-backhauled base station.
39. FDMA is one of the most common analogue multiple access
methods. The frequency band is divided into channels of equal
bandwidth so that each conversation is carried on a different
frequency.
Advantages of FDMA:
Reduces the bit rate information and the use of efficient numerical
codes increases the capacity
It reduces the cost and lowers the inter symbol interference (ISI)
Equalization is not necessary.
Disadvantages of FDMA:
The maximum flow rate per channel is fixed and small.
Guard bands lead to a waste of capacity.
Hardware implies narrowband filters, which cannot be realized in VLSI
and therefore increases the cost.
40. In FDMA method, guard bands are used between the adjacent
signal spectra to minimize crosstalk between the channels.
A specific frequency band is given to one person, and it will
received by identifying each of the frequency on the receiving
end.
It is often used in the first generation of analog mobile phone.
41. Time Division Multiple Access (TDMA) is a digital cellular
telephone communication technology.
It facilitates many users to share the same frequency without
interference. Its technology divides a signal into different
timeslots, and increases the data carrying capacity.
Advantages of TDMA:
Permits flexible rates
Can withstand gusty or variable bit rate traffic.
No guard band required for the wideband system.
No narrowband filter required for the wideband system
Disadvantages of TDMA:
High data rates of broadband systems require complex equalization.
Electronics operating at high bit rates increase energy consumption.
Complex signal processing is required to synchronize within short slots.
42. The frequency of the system is divided into sub-bands, and TDMA
is used for the multiple access in each sub-band. Sub-bands are
known as carrier frequencies.
In the following example, the frequency band has been shared by
three users. Each user is assigned definite timeslots to send and
receive data.
The user ‘B’ sends after user ‘A,’ and user ‘C’ sends thereafter.
In this way, the peak power becomes a problem and larger by the
burst communication.
