Unit– III Physical layer CND 22634 14 Marks.

Notes By Aziz sir.

3.1 Multiplexing: Basic concept, Frequency-Division Multiplexing, Wavelength-Division

Multiplexing, Synchronous Time-Division Multiplexing, Statistical Time-Division Multiplexing
3.2 Transmission medium: classification based on electromagnetic wave spectrum
3.3 Guided Media -Twisted pair(UTP,STP) cable -connector, Coaxial cable - connector,
Fiber-optic cable -connector, performance and applications
3.4 Unguided Media-Radio waves,microwaves, Infrared and their applications
3.5 Modems: classifications : Broadbandmodem, DSL –ADSL,HDSL,VDSL
3.6 Switching: Circuit-switched networks, Packet switched networks –Datagram approach,
virtual circuit approach.
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Multiplexing :- Multiplexers, often called muxes, are extremely important to telecommunications.

Their main reason for being is to reduce network costs by minimizing the number of
communications links needed between two points. As with all other computing systems,
multiplexers have evolved. Each new generation has additional intelligence, and additional
intelligence brings more benefits. The types of benefits that have accrued, for example, include
the following:
• The capability to do data compression so that you can encode certain characters with
fewer bits than normally required and free up that additional capacity for the movement of
other information.
• The capability to do error detection and correction between the two points that are being
connected to ensure that data integrity and accuracy are being maintained.
• The capability to manage transmission resources on a dynamic basis, with such things as
priority levels. If you have only one 64Kbps channel left, who gets it? Or what happens
when the link between San Francisco and Hong Kong goes down? How else can you reroute
traffic to get the high-priority information where it needs to go? Multiplexers help solve
such problems.
Frequency division Multiplexing :
In telecommunications, 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.
The most natural example of frequency-division multiplexing is radio and
television broadcasting, in which multiple radio signals at different frequencies pass through
the air at the same time. Another example is cable television, in which many television
channels are carried simultaneously on a single cable. FDM is also used by telephone systems
to transmit multiple telephone calls through high capacity trunklines, communications
satellites to transmit multiple channels of data on uplink and downlink radio beams, and
broadband DSL modems to transmit large amounts of computer data through twisted
pair telephone lines, among many other uses.

FDM transmitter Channel FDM Receiver

FDM working :- lets m1(t) , m2(t), and m3(t) are the independent message signals and they
have to transmitted on a single channel using FDM , to separate them from each other we
have to select different carrier frequencies on which we have send our message signal in the
above diagram Cos(Ω1t) = fc1 and Cos(Ω2t) = fc2, and Cos(Ω3t) = fc3 like that , we have
selected frequency fc1 , fc2, fc3 are the carrier frequency to separate the signal from one to
another. The modulated signals sent using single channel , The Composite signal having

Composite signal = m1(t) *fc1 + m2(t) *fc2 + m3(t) * fc3 + ............ and so on ......
Will be sent on single channel , at the receiver the m1(t), m2(t) and m3(t) will be separated
by using band pass filter whose center frequency is around fc1, fc2, fc3 etc, the message
signal is separated using demodulator from carrier.
Hence de –mux is done at the receiver.
Guard Band
Frequency overlap is a serious issue when it comes to frequency division multiplexing and it must be completely
avoided. Two frequency ranges can be separated by using some narrow frequency ranges called guard bands.
The guard bands avoid signal interference and enhance the quality of communication.
Time Division Multiplexing :-
1> Master clock is used at Transmitter and receiver.
2> Even if a Host does not have anything to send data a time slot is allotted to him as his
turn comes.
Statistical Time division Multiplexing :-

If a user does not have a data to be send during time allotted to him then his time is given to
the user who is ready to transmit.
3.2
Transmission medium: classification based on electromagnetic wave spectrum .
3.6 Switching: Circuit-switched networks, Packet switched networks –
Datagram approach, virtual circuit approach.

Switching is an important mechanism that provides communication between

different networks or different computer(s) and manages the data flow between the
two end points. There are three types of switching techniques –
• Circuit switching
• Packet switching
• Message switching.
Here we will discuss Circuit switching.
Circuit switching is a switching method in which a dedicated physical path is
formed between two points in a network i.e. between the sending and the receiving
devices. These dedicated paths are created by a set of switches connected by
physical links. Circuit switching is the simplest method of data communication that
has a fixed data rate and both the subscribers need to operate at this fixed rate.

Phases Of Circuit switching Communication –

It has basically three phases :

Establishment or Setup Phase – A dedicated circuit or path is established

between the sender and receiver before the actual data transfer. End-to-End
addressing i.e. source address and destination address, is required for creating a
connection between two physical devices.
Data Transfer Phase – Data transfer only starts after the setup phase is completed
and a physical, dedicated path is established.The data flow is continuous and there
may be periods of silence in data transmitting. Generally all internal connections are
made in duplex form. The switches use time slot (TDM) or the occupied band (FDM)
to route the data from the sender to the receiver and no addressing method is
involved.
 Disconnect or Teardown Phase – When one of the subscribers (either the sender
or the receiver) needs to disconnect, a disconnect signal is sent to each switch to
release the resource and break/disconnect the connection.

Advantages of Circuit Switching –

1. The data rate is fixed and dedicated as the connection is established using
dedicated physical path.
2. Once the circuit is established, there is no waiting time and the data
transmission delay is negligible.
3. Since a dedicated path is established, it is a good choice for continuous
transmission over a long duration.
Disadvantages of Circuit Switching –
1. Since the connection is dedicated it cannot be used for any other data
transmission even if the channel is free.
2. It is inefficient in terms of utilization of the system resource. As it is allocated
for the entire conversation, we can’t use the resource for other connection.
3. More bandwidth is required for the dedicated channels .
4. Establishment of physical links between senders and receivers takes huge
time prior to the actual data transfer.

Packet Switching:-

Packet Switching is the process by which a networking or

telecommunications device accepts a packet and switches it to
a telecommunications device that will take it closer to its
destination. Packet switching allows data to be sent over the
telecommunications network in short bursts or “packets” that
contain sequence numbers so that they can be reassembled at
the destination.
Wide area network (WAN) devices called switches route packets from one point on a packet-
switched network to another. Data within the same communication session might be routed
over several different paths, depending on factors such as traffic congestion and switch
availability.

The Internet is the prime example of a packet-switched network based on the TCP/IP
protocol suite. A series of routers located at various points on the Internet’s backbone
forward each packet received on the basis of destination address until the packet reaches its
ultimate destination. TCP/IP is considered a connectionless packet-switching service
because Transmission Control Protocol (TCP) connections are not kept open after data
transmission is complete.
Virtual Circuit. :-
Unlike datagram switching, virtual circuit switching sets out a data
packet path in its own way, dynamically and on a case-by-case basis.
Experts identify advantages of using a virtual circuit design, including
fewer allocated resources, packets delivered in proper order, and
reliable networking outputs. Think of the virtual circuit as a "smart"
routing system that does not adhere to strict path rules. In that sense,
the "virtual circuit" is flexible in a way that a traditional circuit printed on a
circuit board is not.
In a practical sense, telecom companies may use virtual circuits to order
packet sending. Generally, the virtual circuit allows each packet to travel
the same path, which can help with effectiveness and also with billing.
So the virtual circuit is a "dedicated pathway" for data packets. It is just
not as restrictive as datagram switching.