Love-Electronics: Applications of multiplexer in communication engineering

TABLE OF CONTENTS

· 1.INTRODUCTION

· 2.MULTIPLEXERS

Ø a.Digital multiplexers

Ø b.Chaining multiplexers

· 3.TYPES OF MULTIPLEXING

Ø a.Space-division multiplexing

Ø b.Frequency-division multiplexing

Ø c.Time-division multiplexing

Ø d.Code-division multiplexing

· 4.APPLICATIONS OF MULTIPLEXING

· 5.REFERENCES

INTRODUCTION

In telecommunications and computer networks, multiplexing (also known as muxing) is a process where multiple analog message signals or digital data streams are combined into one signal over a shared medium. The aim is to share an expensive resource. For example, in telecommunications, several phone calls may be transferred using one wire. It originated in telegraphy, and is now widely applied in communications.

The multiplexed signal is transmitted over a communication channel, which may be a physical transmission medium. The multiplexing divides the capacity of the low-level communication channel into several higher-level logical channels, one for each message signal or data stream to be transferred. A reverse process, known as demultiplexing, can extract the original channels on the receiver side.

A device that performs the multiplexing is called a multiplexer (MUX), and a device that performs the reverse process is called a demultiplexer (DEMUX).

Inverse multiplexing (IMUX) has the opposite aim as multiplexing, namely to break one data stream into several streams, transfer them simultaneously over several communication channels, and recreate the original data stream.

MULTIPLEXERS

In electronics, a multiplexer or mux (occasionally the terms muldex or muldem are also found for a combination multiplexer-demultiplexer) is a device that performs multiplexing; it selects one of many analog or digital input signals and forwards the selected input into a single line. A multiplexer of 2ⁿ inputs has n select lines, which are used to select which input line to send to the output.

An electronic multiplexer makes it possible for several signals to share one device or resource, for example one A/D converter or one communication line, instead of having one device per input signal.

On the other end, a demultiplexer (or demux) is a device taking a single input signal and selecting one of many data-output-lines, which is connected to the single input. A multiplexer is often used with a complementary demultiplexer on the receiving end.

An electronic multiplexer can be considered as a multiple-input, single-output switch, and a demultiplexer as a single-input, multiple-output switch. The schematic symbol for a multiplexer is an isosceles trapezoid with the longer parallel side containing the input pins and the short parallel side containing the output pin. The schematic on the right shows a 2-to-1 multiplexer on the left and an equivalent switch on the right. The sel wire connects the desired input to the output.

Digital multiplexers:-

In digital circuit design, the selector wires are of digital value. In the case of a 2-to-1 multiplexer, a logic value of 0 would connect $\scriptstyle I_0$ to the output while a logic value of 1 would connect $\scriptstyle I_1$ to the output. In larger multiplexers, the number of selector pins is equal to $\scriptstyle \left \lceil \log_2(n) \right \rceil$ where $\scriptstyle n$ is the number of inputs.

For example, 9 to 16 inputs would require no fewer than 4 selector pins and 17 to 32 inputs would require no fewer than 5 selector pins. The binary value expressed on these selector pins determines the selected input pin.

A 2-to-1 multiplexer has a boolean equation where $\scriptstyle A$ and $\scriptstyle B$ are the two inputs, $\scriptstyle S$ is the selector input, and $\scriptstyle Z$ is the output:

$Z = ( A \cdot \overline{S}) + (B \cdot S)$

A 2-to-1 mux

Which can be expressed as a truth table:

$\scriptstyle S$	$\scriptstyle A$	$\scriptstyle B$	$\scriptstyle Z$
0	1	1	1
	1	0	1
	0	1	0
	0	0	0
1	1	1	1
	1	0	0
	0	1	1
	0	0	0

This truth table shows that when $\scriptstyle S=0$ then $\scriptstyle Z=A$ but when $\scriptstyle S=1$ then $\scriptstyle Z=B$ . A straightforward realization of this 2-to-1 multiplexer would need 2 AND gates, an OR gate, and a NOT gate.

Larger multiplexers are also common and, as stated above, requires $\scriptstyle \left \lceil \log_2(n) \right \rceil$ selector pins for n inputs. Other common sizes are 4-to-1, 8-to-1, and 16-to-1. Since digital logic uses binary values, powers of 2 are used (4, 8, 16) to maximally control a number of inputs for the given number of selector inputs.

4-to-1 mux

8-to-1 mux

16-to-1 mux

The boolean equation for a 4-to-1 multiplexer is:

$F = (A \cdot \overline{S_0} \cdot \overline{S_1}) + (B \cdot \overline {S_0} \cdot S_1) + (C \cdot S_0 \cdot \overline{S_1}) + (D \cdot S_0 \cdot S_1)$

Two realizations for creating a 4-to-1 multiplexer are shown below:

These are two realizations of a 4-to-1 multiplexer:

one realized from a decoder, AND gates, and an OR gate
one realized from 3-state buffers and AND gates (the AND gates are acting as the decoder)

Note that the subscripts on the $\scriptstyle I_n$ inputs indicate the decimal value of the binary control inputs at which that input is let through.

Chaining multiplexers:-

Larger multiplexers can be constructed by using smaller multiplexers by chaining them together. For example, an 8-to-1 multiplexer can be made with two 4-to-1 and one 2-to-1 multiplexers. The two 4-to-1 multiplexer outputs are fed into the 2-to-1 with the selector pins on the 4-to-1's put in parallel giving a total number of selector inputs to 3, which is equivalent to an 8-to-1.

List of ICs which provide multiplexing:-

The 7400 series has several ICs that contain multiplexer(s):

S.No.	IC No.	Function	Output State
1	74157	Quad 2:1 mux.	Output same as input given
2	74158	Quad 2:1 mux.	Output is inverted input
3	74153	Dual 4:1 mux.	Output same as input
4	74352	Dual 4:1 mux.	Output is inverted input
5	74151A	8:1 mux.	Both outputs available (i.e., complementary outputs)
6	74151	8:1 mux.	Output is inverted input
7	74150	16:1 mux.	Output is inverted input

Digital demultiplexers:-

Demultiplexers take one data input and a number of selection inputs, and they have several outputs. They forward the data input to one of the outputs depending on the values of the selection inputs. Demultiplexers are sometimes convenient for designing general purpose logic, because if the demultiplexer's input is always true, the demultiplexer acts as a decoder. This means that any function of the selection bits can be constructed by logically OR-ing the correct set of outputs.

Example: A Single Bit 1-to-4 Line Demultiplexer

List of ICs which provide demultiplexing:-

The 7400 series has several ICs that contain demultiplexer(s):

S.No.	IC No.	Function	Output State
1	74139	Dual 1:4 demux.	Output is inverted input
3	74156	Dual 1:4 demux.	Output is open collector
4	74138	3:8 demux.	Output is inverted input
5	74154	1:16 demux.	Output is inverted input
6	74159	1:16 demux.	Output is open collector and same as input

TYPES OF MULTIPLEXING

Multiplexing technologies may be divided into several types, all of which have significant variations: space-division multiplexing (SDM), frequency-division multiplexing (FDM), time-division multiplexing (TDM), and code division multiplexing (CDM). Variable bit rate digital bit streams may be transferred efficiently over a fixed bandwidth channel by means of statistical multiplexing, for example packet mode communication. Packet mode communication is an asynchronous mode time-domain multiplexing which resembles time-division multiplexing.

Digital bit streams can be transferred over an analog channel by means of code-division multiplexing (CDM) techniques such as frequency-hopping spread spectrum (FHSS) and direct-sequence spread spectrum (DSSS).

In wireless communications, multiplexing can also be accomplished through alternating polarization (horizontal/vertical or clockwise/counterclockwise) on each adjacent channel and satellite, or through phased multi-antenna array combined with a Multiple-input multiple-output communications (MIMO) scheme.

Space-division multiplexing:-

In wired communication, space-division multiplexing simply implies different point-to-point wires for different channels. Examples include an analogue stereo audio cable, with one pair of wires for the left channel and another for the right channel, and a multipair telephone cable. Another example is a switched star network such as the analog telephone access network (although inside the telephone exchange or between the exchanges, other multiplexing techniques are typically employed) or a switched Ethernet network. A third example is a mesh network. Wired space-division multiplexing is typically not considered as multiplexing.

In wireless communication, space-division multiplexing is achieved by multiple antenna elements forming a phased array antenna. Examples are multiple-input and multiple-output (MIMO), single-input and multiple-output (SIMO) and multiple-input and single-output (MISO) multiplexing. For example, a IEEE 802.11n wireless router with N antennas makes it possible to communicate with N multiplexed channels, each with a peak bit rate of 54 Mbit/s, thus increasing the total peak bit rate with a factor N. Different antennas would give different multi-path propagation (echo) signatures, making it possible for digital signal processing techniques to separate different signals from each other. These techniques may also be utilized for space diversity (improved robustness to fading) or beamforming (improved selectivity) rather than multiplexing.

Frequency-division multiplexing:-

Frequency-division multiplexing (FDM): The spectrums of each input signal are swifted in several distinct frequency ranges.

Frequency-division multiplexing (FDM) is inherently an analog technology. FDM achieves the combining of several digital signals into one medium by sending signals in several distinct frequency ranges over that medium.

One of FDM's most common applications is cable television. Only one cable reaches a customer's home but the service provider can send multiple television channels or signals simultaneously over that cable to all subscribers. Receivers must tune to the appropriate frequency (channel) to access the desired signal.

A variant technology, called wavelength-division multiplexing (WDM) is used in optical communications.

Time-division multiplexing:-

Time-division multiplexing (TDM).

Time-division multiplexing (TDM) is a digital technology. TDM involves sequencing groups of a few bits or bytes from each individual input stream, one after the other, and in such a way that they can be associated with the appropriate receiver. If done sufficiently and quickly, the receiving devices will not detect that some of the circuit time was used to serve another logical communication path.

Consider an application requiring four terminals at an airport to reach a central computer. Each terminal communicated at 2400 bps, so rather than acquire four individual circuits to carry such a low-speed transmission, the airline has installed a pair of multiplexers. A pair of 9600 bps modems and one dedicated analog communications circuit from the airport ticket desk back to the airline data center are also installed.

Further information: Time-division multiplexing

Code-division multiplexing:-

Code division multiplexing (CDM) is a technique in which each channel transmits its bits as a coded channel-specific sequence of pulses. This coded transmission typically is accomplished by transmitting a unique time-dependent series of short pulses, which are placed within chip times within the larger bit time. All channels, each with a different code, can be transmitted on the same fiber and asynchronously demultiplexed. Other widely used multiple access techniques are Time Division Multiple Access (TDMA) and Frequency Division Multiple Access (FDMA).

Code Division Multiplex techniques are used as an access technology, namely Code Division Multiple Access (CDMA), in Universal Mobile Telecommunications System (UMTS) standard for the third generation (3G) mobile communication identified by the ITU. Another important application of the CDMA is the Global Positioning System (GPS).

However, the term Code Division Multiple access (CDMA) is also widely used to refer to a group of specific implementations of CDMA defined by Qualcomm for use in digital cellular telephony, which include IS-95 and IS-2000. The two different uses of this term can be confusing. Actually, CDMA (the Qualcomm standard) and UMTS have been competing for adoption in many markets.

APPLICATIONS OF MULTIPLEXER

1.Telegraphy:-

The earliest communication technology using electrical wires, and therefore sharing an interest in the economies afforded by multiplexing, was the electric telegraph. Early experiments allowed two separate messages to travel in opposite directions simultaneously, first using an electric battery at both ends, then at only one end.

Émile Baudot developed a time-multiplexing system of multiple Hughes machines in the 1870s.
In 1874, the quadruplex telegraph developed by Thomas Edison transmitted two messages in each direction simultaneously, for a total of four messages transiting the same wire at the same time.
Several workers were investigating acoustic telegraphy, a frequency-division multiplexing technique, which led to the invention of the telephone.

2.Telephony:-

In telephony, a customer's telephone line now typically ends at the remote concentrator box down the street, where it is multiplexed along with other telephone lines for that neighborhood or other similar area. The multiplexed signal is then carried to the central switching office on significantly fewer wires and for much further distances than a customer's line can practically go. This is likewise also true for digital subscriber lines (DSL).

Fiber in the loop (FITL) is a common method of multiplexing, which uses optical fiber as the backbone. It not only connects POTS phone lines with the rest of the PSTN, but also replaces DSL by connecting directly to Ethernet wired into the home. Asynchronous Transfer Mode is often the communications protocol used.

Because all of the phone (and data) lines have been clumped together, none of them can be accessed except through a demultiplexer. This provides for more-secure communications, though they are not typically encrypted.

The concept is also now used in cable TV, which is increasingly offering the same services as telephone companies. IPTV also depends on multiplexing.

3.Video processing:-

In video editing and processing systems, multiplexing refers to the process of interleaving audio and video into one coherent MPEG transport stream (time-division multiplexing).

In digital video, such a transport stream is normally a feature of a container format which may include metadata and other information, such as subtitles. The audio and video streams may have variable bit rate. Software that produces such a transport stream and/or container is commonly called a statistical multiplexor or muxer. A demuxer is software that extracts or otherwise makes available for separate processing the components of such a stream or container.

4.Digital broadcasting:-

In digital television and digital radio systems, several variable bit-rate data streams are multiplexed together to a fixed bitrate transport stream by means of statistical multiplexing. This makes it possible to transfer several video and audio channels simultaneously over the same frequency channel, together with various services.

In the digital television systems, this may involve several standard definition television (SDTV) programmes (particularly on DVB-T, DVB-S2, ISDB and ATSC-C), or one HDTV, possibly with a single SDTV companion channel over one 6 to 8 MHz-wide TV channel. The device that accomplishes this is called a statistical multiplexer. In several of these systems, the multiplexing results in an MPEG transport stream. The newer DVB standards DVB-S2 and DVB-T2 has the capacity to carry several HDTV channels in one multiplex. Even the original DVB standards can carry more HDTV channels in a multiplex if the most advanced MPEG-4 compressions hardware is used.

On communications satellites which carry broadcast television networks and radio networks, this is known as multiple channel per carrier or MCPC. Where multiplexing is not practical (such as where there are different sources using a single transponder), single channel per carrier mode is used.

Signal multiplexing of satellite TV and radio channels is typically carried out in a central signal playout and uplink centre, such as ASTRA Platform Services in Germany, which provides playout, digital archiving, encryption, and satellite uplinks, as well as multiplexing, for hundreds of digital TV and radio channels.

In digital radio, both the Eureka 147 system of digital audio broadcasting and the in-band on-channel HD Radio, FMeXtra, and Digital Radio Mondiale systems can multiplex channels. This is essentially required with DAB-type transmissions (where a multiplex is called an ensemble), but is entirely optional with IBOC systems.

5.Analog broadcasting:-

In FM broadcasting and other analog radio media, multiplexing is a term commonly given to the process of adding subcarriers to the audio signal before it enters the transmitter, where modulation occurs. Multiplexing in this sense is sometimes known as MPX, which in turn is also an old term for stereophonic FM, seen on stereo systems since the 1960s.