What is Multiplexing?

Multiplexing is a technique used to combine and send the multiple data streams over a single
medium. The process of combining the data streams is known as multiplexing and hardware used for
multiplexing is known as a multiplexer.

Multiplexing is achieved by using a device called Multiplexer (MUX) that combines n input lines to
generate a single output line. Multiplexing follows many-to-one, i.e., n input lines and one output line.

Demultiplexing is achieved by using a device called Demultiplexer (DEMUX) available at the receiving
end. DEMUX separates a signal into its component signals (one input and n outputs). Therefore, we
can say that demultiplexing follows the one-to-many approach.

Why Multiplexing?
o The transmission medium is used to send the signal from sender to receiver. The medium can
only have one signal at a time.
o If there are multiple signals to share one medium, then the medium must be divided in such a
way that each signal is given some portion of the available bandwidth. For example: If there are
10 signals and bandwidth of medium is100 units, then the 10 unit is shared by each signal.
o When multiple signals share the common medium, there is a possibility of collision.
Multiplexing concept is used to avoid such collision.
o Transmission services are very expensive.

History of Multiplexing
o Multiplexing technique is widely used in telecommunications in which several telephone calls
are carried through a single wire.
o Multiplexing originated in telegraphy in the early 1870s and is now widely used in
communication.
o George Owen Squier developed the telephone carrier multiplexing in 1910.

1
Concept of Multiplexing

o The 'n' input lines are transmitted through a multiplexer and multiplexer combines the signals
to form a composite signal.
o The composite signal is passed through a Demultiplexer and demultiplexer separates a signal
to component signals and transfers them to their respective destinations.

Advantages of Multiplexing:
o More than one signal can be sent over a single medium.
o The bandwidth of a medium can be utilized effectively.

Multiplexing Techniques
Multiplexing techniques can be classified as:

Skip Ad

2
Frequency-division Multiplexing (FDM)
o It is an analog technique.
o Frequency Division Multiplexing is a technique in which the available bandwidth of a single
transmission medium is subdivided into several channels.

o In the above diagram, a single transmission medium is subdivided into several frequency
channels, and each frequency channel is given to different devices. Device 1 has a frequency
channel of range from 1 to 5.
o The input signals are translated into frequency bands by using modulation techniques, and
they are combined by a multiplexer to form a composite signal.
o The main aim of the FDM is to subdivide the available bandwidth into different frequency
channels and allocate them to different devices.
o Using the modulation technique, the input signals are transmitted into frequency bands and
then combined to form a composite signal.
o The carriers which are used for modulating the signals are known as sub-carriers. They are
represented as f1,f2..fn.
o FDM is mainly used in radio broadcasts and TV networks.

3
Advantages Of FDM:

o FDM is used for analog signals.

o FDM process is very simple and easy modulation.
o A Large number of signals can be sent through an FDM simultaneously.
o It does not require any synchronization between sender and receiver.

Disadvantages Of FDM:

o FDM technique is used only when low-speed channels are required.

o It suffers the problem of crosstalk.
o A Large number of modulators are required.
o It requires a high bandwidth channel.

Applications Of FDM:

o FDM is commonly used in TV networks.

o It is used in FM and AM broadcasting. Each FM radio station has different frequencies, and they
are multiplexed to form a composite signal. The multiplexed signal is transmitted in the air.

Wavelength Division Multiplexing (WDM)

o Wavelength Division Multiplexing is same as FDM except that the optical signals are
transmitted through the fibre optic cable.
o WDM is used on fibre optics to increase the capacity of a single fibre.

4
 o It is used to utilize the high data rate capability of fibre optic cable.
o It is an analog multiplexing technique.
o Optical signals from different source are combined to form a wider band of light with the help
of multiplexer.
o At the receiving end, demultiplexer separates the signals to transmit them to their respective
destinations.
o Multiplexing and Demultiplexing can be achieved by using a prism.
o Prism can perform a role of multiplexer by combining the various optical signals to form a
composite signal, and the composite signal is transmitted through a fibre optical cable.
o Prism also performs a reverse operation, i.e., demultiplexing the signal.

Time Division Multiplexing

o It is a digital technique.
o In Frequency Division Multiplexing Technique, all signals operate at the same time with
different frequency, but in case of Time Division Multiplexing technique, all signals operate at
the same frequency with different time.
o In Time Division Multiplexing technique, the total time available in the channel is distributed
among different users. Therefore, each user is allocated with different time interval known as a
Time slot at which data is to be transmitted by the sender.
o A user takes control of the channel for a fixed amount of time.
o In Time Division Multiplexing technique, data is not transmitted simultaneously rather the data
is transmitted one-by-one.
o In TDM, the signal is transmitted in the form of frames. Frames contain a cycle of time slots in
which each frame contains one or more slots dedicated to each user.
o It can be used to multiplex both digital and analog signals but mainly used to multiplex digital
signals.
5
There are two types of TDM:

o Synchronous TDM
o Asynchronous TDM

6
Synchronous TDM

o A Synchronous TDM is a technique in which time slot is preassigned to every device.

o In Synchronous TDM, each device is given some time slot irrespective of the fact that the
device contains the data or not.
o If the device does not have any data, then the slot will remain empty.
o In Synchronous TDM, signals are sent in the form of frames. Time slots are organized in the
form of frames. If a device does not have data for a particular time slot, then the empty slot will
be transmitted.
o The most popular Synchronous TDM are T-1 multiplexing, ISDN multiplexing, and SONET
multiplexing.
o If there are n devices, then there are n slots.

Concept Of Synchronous TDM

7
In the above figure, the Synchronous TDM technique is implemented. Each device is allocated with
some time slot. The time slots are transmitted irrespective of whether the sender has data to send or
not.

Disadvantages Of Synchronous TDM:

o The capacity of the channel is not fully utilized as the empty slots are also transmitted which is
having no data. In the above figure, the first frame is completely filled, but in the last two
frames, some slots are empty. Therefore, we can say that the capacity of the channel is not
utilized efficiently.
o The speed of the transmission medium should be greater than the total speed of the input
lines. An alternative approach to the Synchronous TDM is Asynchronous Time Division
Multiplexing.

Asynchronous TDM

o An asynchronous TDM is also known as Statistical TDM.

o An asynchronous TDM is a technique in which time slots are not fixed as in the case of
Synchronous TDM. Time slots are allocated to only those devices which have the data to send.
Therefore, we can say that Asynchronous Time Division multiplexor transmits only the data
from active workstations.
o An asynchronous TDM technique dynamically allocates the time slots to the devices.
o In Asynchronous TDM, total speed of the input lines can be greater than the capacity of the
channel.
o Asynchronous Time Division multiplexor accepts the incoming data streams and creates a
frame that contains only data with no empty slots.
o In Asynchronous TDM, each slot contains an address part that identifies the source of the data.
8
 o The difference between Asynchronous TDM and Synchronous TDM is that many slots in
Synchronous TDM are unutilized, but in Asynchronous TDM, slots are fully utilized. This leads
to the smaller transmission time and efficient utilization of the capacity of the channel.
o In Synchronous TDM, if there are n sending devices, then there are n time slots. In
Asynchronous TDM, if there are n sending devices, then there are m time slots where m is less
than n (m<n).
o The number of slots in a frame depends on the statistical analysis of the number of input lines.

Concept Of Asynchronous TDM

In the above diagram, there are 4 devices, but only two devices are sending the data, i.e., A and C.
Therefore, the data of A and C are only transmitted through the transmission line.

9
Switching
o When a user accesses the internet or another computer network outside their immediate
location, messages are sent through the network of transmission media. This technique of
transferring the information from one computer network to another network is known
as switching.
o Switching in a computer network is achieved by using switches. A switch is a small hardware
device which is used to join multiple computers together with one local area network (LAN).
o Network switches operate at layer 2 (Data link layer) in the OSI model.
o Switching is transparent to the user and does not require any configuration in the home
network.
o Switches are used to forward the packets based on MAC addresses.
o A Switch is used to transfer the data only to the device that has been addressed. It verifies the
destination address to route the packet appropriately.
o It is operated in full duplex mode.
o Packet collision is minimum as it directly communicates between source and destination.
o It does not broadcast the message as it works with limited bandwidth.

Why is Switching Concept required?

Switching concept is developed because of the following reasons:

o Bandwidth: It is defined as the maximum transfer rate of a cable. It is a very critical and
expensive resource. Therefore, switching techniques are used for the effective utilization of the
bandwidth of a network.
o Collision: Collision is the effect that occurs when more than one device transmits the message
over the same physical media, and they collide with each other. To overcome this problem,
switching technology is implemented so that packets do not collide with each other.

10
Advantages of Switching:

o Switch increases the bandwidth of the network.

o It reduces the workload on individual PCs as it sends the information to only that device which
has been addressed.
o It increases the overall performance of the network by reducing the traffic on the network.
o There will be less frame collision as switch creates the collision domain for each connection.

Disadvantages of Switching:

o A Switch is more expensive than network bridges.

o A Switch cannot determine the network connectivity issues easily.
o Proper designing and configuration of the switch are required to handle multicast packets.

Switching Modes
o The layer 2 switches are used for transmitting the data on the data link layer, and it also
performs error checking on transmitted and received frames.
o The layer 2 switches forward the packets with the help of MAC address.
o Different modes are used for forwarding the packets known as Switching modes.
o In switching mode, Different parts of a frame are recognized. The frame consists of several
parts such as preamble, destination MAC address, source MAC address, user's data, FCS.

There are three types of switching modes:

o Store-and-forward
o Cut-through

11
 o Fragment-free

Store-and-forward

o Store-and-forward is a technique in which the intermediate nodes store the received frame and
then check for errors before forwarding the packets to the next node.
o The layer 2 switch waits until the entire frame has received. On receiving the entire frame,
switch store the frame into the switch buffer memory. This process is known as storing the
frame.
o When the frame is stored, then the frame is checked for the errors. If any error found, the
message is discarded otherwise the message is forwarded to the next node. This process is
known as forwarding the frame.
o CRC (Cyclic Redundancy Check) technique is implemented that uses a number of bits to check
for the errors on the received frame.
o The store-and-forward technique ensures a high level of security as the destination network
will not be affected by the corrupted frames.
o Store-and-forward switches are highly reliable as it does not forward the collided frames.

Cut-through Switching
12
 o Cut-through switching is a technique in which the switch forwards the packets after the
destination address has been identified without waiting for the entire frame to be received.
o Once the frame is received, it checks the first six bytes of the frame following the preamble, the
switch checks the destination in the switching table to determine the outgoing interface port,
and forwards the frame to the destination.
o It has low latency rate as the switch does not wait for the entire frame to be received before
sending the packets to the destination.
o It has no error checking technique. Therefore, the errors can be sent with or without errors to
the receiver.
o A Cut-through switching technique has low wait time as it forwards the packets as soon as it
identifies the destination MAC address.
o In this technique, collision is not detected, if frames have collided will also be forwarded.

Fragment-free Switching

o A Fragment-free switching is an advanced technique of the Cut-through Switching.

o A Fragment-free switching is a technique that reads atleast 64 bytes of a frame before
forwarding to the next node to provide the error-free transmission.
o It combines the speed of Cut-through Switching with the error checking functionality.
o This technique checks the 64 bytes of the ethernet frame where addressing information is
available.

13
o A collision is detected within 64 bytes of the frame, the frames which are collided will not be
forwarded further.

14
Differences b/w Store-and-forward and Cut-through
Switching.

Store-and-forward Switching Cut-through Switching

Store-and-forward Switching is a technique that waits until the Cut-through Switching is a technique t
entire frame is received. bytes following the preamble to ide
address.

It performs error checking functionality. If any error is found in the It does not perform any error checkin
frame, the frame will be discarded otherwise forwarded to the next without errors will be forwarded.
node.

It has high latency rate as it waits for the entire frame to be It has low latency rate as it checks o
received before forwarding to the next node. frame to determine the destination add

It is highly reliable as it forwards only error-free packets. It is less reliable as compared to

technique as it forwards error prone pa

It has a high wait time as it waits for the entire frame to be It has low wait time as cut-through s
received before taking any forwarding decisions. the whole frame or packets.

Switching techniques
In large networks, there can be multiple paths from sender to receiver. The switching technique will
decide the best route for data transmission.

Switching technique is used to connect the systems for making one-to-one communication.

15
Classification Of Switching Techniques

Circuit Switching
o Circuit switching is a switching technique that establishes a dedicated path between sender
and receiver.
o In the Circuit Switching Technique, once the connection is established then the dedicated path
will remain to exist until the connection is terminated.
o Circuit switching in a network operates in a similar way as the telephone works.
o A complete end-to-end path must exist before the communication takes place.
o In case of circuit switching technique, when any user wants to send the data, voice, video, a
request signal is sent to the receiver then the receiver sends back the acknowledgment to
ensure the availability of the dedicated path. After receiving the acknowledgment, dedicated
path transfers the data.
o Circuit switching is used in public telephone network. It is used for voice transmission.
o Fixed data can be transferred at a time in circuit switching technology.

16
Communication through circuit switching has 3 phases:

o Circuit establishment
o Data transfer
o Circuit Disconnect

Circuit Switching can use either of the two technologies:

Space Division Switches:

o Space Division Switching is a circuit switching technology in which a single transmission path is
accomplished in a switch by using a physically separate set of crosspoints.
o Space Division Switching can be achieved by using crossbar switch. A crossbar switch is a
metallic crosspoint or semiconductor gate that can be enabled or disabled by a control unit.
o The Crossbar switch is made by using the semiconductor. For example, Xilinx crossbar switch
using FPGAs.
o Space Division Switching has high speed, high capacity, and nonblocking switches.

Space Division Switches can be categorized in two ways:

o Crossbar Switch
o Multistage Switch

Crossbar Switch

The Crossbar switch is a switch that has n input lines and n output lines. The crossbar switch has
n2 intersection points known as crosspoints.

Disadvantage of Crossbar switch:

17
The number of crosspoints increases as the number of stations is increased. Therefore, it becomes
very expensive for a large switch. The solution to this is to use a multistage switch.

Multistage Switch

o Multistage Switch is made by splitting the crossbar switch into the smaller units and then
interconnecting them.
o It reduces the number of crosspoints.
o If one path fails, then there will be an availability of another path.

Advantages Of Circuit Switching:

o In the case of Circuit Switching technique, the communication channel is dedicated.

o It has fixed bandwidth.

Disadvantages Of Circuit Switching:

o Once the dedicated path is established, the only delay occurs in the speed of data
transmission.
o It takes a long time to establish a connection approx 10 seconds during which no data can be
transmitted.
o It is more expensive than other switching techniques as a dedicated path is required for each
connection.
o It is inefficient to use because once the path is established and no data is transferred, then the
capacity of the path is wasted.
o In this case, the connection is dedicated therefore no other data can be transferred even if the
channel is free.

Message Switching
o Message Switching is a switching technique in which a message is transferred as a complete
unit and routed through intermediate nodes at which it is stored and forwarded.
o In Message Switching technique, there is no establishment of a dedicated path between the
sender and receiver.

18
 o The destination address is appended to the message. Message Switching provides a dynamic
routing as the message is routed through the intermediate nodes based on the information
available in the message.
o Message switches are programmed in such a way so that they can provide the most efficient
routes.
o Each and every node stores the entire message and then forward it to the next node. This type
of network is known as store and forward network.
o Message switching treats each message as an independent entity.

Advantages Of Message Switching

o Data channels are shared among the communicating devices that improve the efficiency of
using available bandwidth.
o Traffic congestion can be reduced because the message is temporarily stored in the nodes.
o Message priority can be used to manage the network.
o The size of the message which is sent over the network can be varied. Therefore, it supports
the data of unlimited size.

Disadvantages Of Message Switching

o The message switches must be equipped with sufficient storage to enable them to store the
messages until the message is forwarded.
o The Long delay can occur due to the storing and forwarding facility provided by the message
switching technique.

19
Packet Switching
o The packet switching is a switching technique in which the message is sent in one go, but it is
divided into smaller pieces, and they are sent individually.
o The message splits into smaller pieces known as packets and packets are given a unique
number to identify their order at the receiving end.
o Every packet contains some information in its headers such as source address, destination
address and sequence number.
o Packets will travel across the network, taking the shortest path as possible.
o All the packets are reassembled at the receiving end in correct order.
o If any packet is missing or corrupted, then the message will be sent to resend the message.
o If the correct order of the packets is reached, then the acknowledgment message will be sent.

Approaches Of Packet Switching:

There are two approaches to Packet Switching:

Datagram Packet switching:

o It is a packet switching technology in which packet is known as a datagram, is considered as an

independent entity. Each packet contains the information about the destination and switch
uses this information to forward the packet to the correct destination.
o The packets are reassembled at the receiving end in correct order.
o In Datagram Packet Switching technique, the path is not fixed.
20
 o Intermediate nodes take the routing decisions to forward the packets.
o Datagram Packet Switching is also known as connectionless switching.

Virtual Circuit Switching

o Virtual Circuit Switching is also known as connection-oriented switching.

o In the case of Virtual circuit switching, a preplanned route is established before the messages
are sent.
o Call request and call accept packets are used to establish the connection between sender and
receiver.
o In this case, the path is fixed for the duration of a logical connection.

Let's understand the concept of virtual circuit switching through a diagram:

o In the above diagram, A and B are the sender and receiver respectively. 1 and 2 are the nodes.
o Call request and call accept packets are used to establish a connection between the sender and
receiver.
o When a route is established, data will be transferred.
o After transmission of data, an acknowledgment signal is sent by the receiver that the message
has been received.
o If the user wants to terminate the connection, a clear signal is sent for the termination.

21
Differences b/w Datagram approach and Virtual Circuit
approach

Datagram approach Virtual Circuit approach

Node takes routing decisions to forward the packets. Node does not take any routing decision.

Congestion cannot occur as all the packets travel in Congestion can occur when the node is busy, an
different directions. other packets to pass through.

It is more flexible as all the packets are treated as an It is not very flexible.
independent entity.

Advantages Of Packet Switching:

o Cost-effective: In packet switching technique, switching devices do not require massive

secondary storage to store the packets, so cost is minimized to some extent. Therefore, we can
say that the packet switching technique is a cost-effective technique.
o Reliable: If any node is busy, then the packets can be rerouted. This ensures that the Packet
Switching technique provides reliable communication.
o Efficient: Packet Switching is an efficient technique. It does not require any established path
prior to the transmission, and many users can use the same communication channel
simultaneously, hence makes use of available bandwidth very efficiently.

Disadvantages Of Packet Switching:

o Packet Switching technique cannot be implemented in those applications that require low
delay and high-quality services.
o The protocols used in a packet switching technique are very complex and requires high
implementation cost.
o If the network is overloaded or corrupted, then it requires retransmission of lost packets. It can
also lead to the loss of critical information if errors are nor recovered.

22