Computer Communication & Computer Networking Module 1 , 2 Notes.pdf

Digital Transmission
⮚ Data can be represented either in analog or digital form.
⮚ The computers used the digital form to store the
information.
⮚ Therefore, the data needs to be converted in digital form
so that it can be used by a computer.

DIGITAL-TO-DIGITAL CONVERSION
⮚ Digital-to-digital encoding is the representation of digital
information by a digital signal. When binary 1s and 0s
generated by the computer are translated into a sequence of
voltage pulses that can be propagated over a wire, this
process is known as digital-to-digital encoding.

Digital-to-digital encoding is divided into three categories:
⮚ Unipolar Encoding
⮚ Polar Encoding
⮚ Bipolar Encoding

Unipolar
⮚Digital transmission system sends the voltage pulses over the medium link
such as wire or cable.
⮚In most types of encoding, one voltage level represents 0, and another
voltage level represents 1.
⮚The polarity of each pulse determines whether it is positive or negative.

Polar
⮚Polar encoding is an encoding scheme that uses two voltage levels: one is
positive, and another is negative.
⮚By using two voltage levels, an average voltage level is reduced, and the
DC component problem of unipolar encoding scheme is alleviated.

⮚NRZ-L: If a bit is 0 or 1, then their voltages will be positive and negative
respectively.
⮚NRZ-I: In this scheme, 0 bit represents no change and 1 bit represents a
change in voltage level.

RZ
⮚ RZ stands for Return to zero.
⮚ There must be a signal change for each bit to achieve synchronization.
However, to change with every bit, we need to have three values: positive,
negative and zero.

Biphase
⮚ Biphase is an encoding scheme in which signal changes at the middle of
the bit interval but does not return to zero. Biphase encoding is
implemented in two different ways:
Manchester
⮚ In Manchester encoding, a negative-to-positive transition represents
binary 1, and positive-to-negative transition represents 0.
Differential Manchester
⮚ A transition means binary 0 and no transition means binary 1.

Bipolar
⮚ Bipolar encoding scheme represents three voltage levels:
positive, negative, and zero.
⮚ In Bipolar encoding scheme, zero level represents binary 0, and
binary 1 is represented by alternating positive and negative
voltages.
⮚ If the first 1 bit is represented by positive amplitude, then the
second 1 bit is represented by negative voltage, third 1 bit is
represented by the positive amplitude and so on. This
alternation can also occur even when the 1bits are not
consecutive.

ANALOG-TO-DIGITAL CONVERSION
⮚ When an analog signal is digitalized, this is called an analog-
to-digital conversion.
⮚ Suppose human sends a voice in the form of an analog
signal, we need to digitalize the analog signal which is less
prone to noise. It requires a reduction in the number of values
in an analog message so that they can be represented in the
digital stream.
⮚ In analog-to-digital conversion, the information contained in
a continuous wave form is converted in digital pulses.

TECHNIQUES FOR ANALOG-TO-DIGITAL CONVERSION
PAM
⮚ PAM stands for pulse amplitude modulation. PAM is a technique used in
analog-to-digital conversion.
⮚ PAM technique takes an analog signal, samples it, and generates a series
of digital pulses based on the result of sampling where sampling means
measuring the amplitude of a signal at equal intervals.

PCM
⮚PCM stands for Pulse Code Modulation. PCM technique is used to modify
the pulses created by PAM to form a digital signal. To achieve this, PCM
quantizes PAM pulses. Quantization is a process of assigning integral
values in a specific range to sampled instances.

Analog Transmission
DIGITAL-TO-ANALOG CONVERSION
⮚When data from one computer is sent to another via some analog carrier, it
is first converted into analog signals. Analog signals are modified to reflect
digital data. An analog signal is characterized by its amplitude, frequency,
and phase.
There are three kinds of digital-to-analog conversions:
⮚ Amplitude Shift Keying
⮚ Frequency Shift Keying
⮚ Phase Shift Keying

Analog Transmission
Amplitude Shift Keying
⮚ The amplitude of analog carrier signal is modified to reflect
binary data. When binary data represents digit 1, the
amplitude is held; otherwise it is set to 0. Both frequency and
phase remain same as in the original carrier signal.

Analog Transmission
Frequency Shift Keying
⮚ In this conversion technique, the frequency of the analog carrier signal is
modified to reflect binary data.
⮚ This technique uses two frequencies, f1 and f2. One of them, for example
f1, is chosen to represent binary digit 1 and the other one is used to
represent binary digit 0. Both amplitude and phase of the carrier wave are
kept intact.

Analog Transmission
Phase Shift Keying
In this conversion scheme, the phase of the original carrier signal is altered to
reflect the binary data. When a new binary symbol is encountered, the phase
of the signal is altered. Amplitude and frequency of the original carrier signal
is kept intact.

Analog Transmission
ANALOG-TO-ANALOG CONVERSION
Analog signals are modified to represent analog data. This conversion
is also known as Analog Modulation. Analog modulation is required when
bandpass is used. Analog to analog conversion can be done in three ways:
⮚ Amplitude Modulation
⮚ Frequency Modulation
⮚ Phase Modulation

Analog Transmission
Amplitude Modulation
⮚ The amplitude of the carrier signal is
modified to reflect the analog data.
Amplitude modulation is implemented by
means of a multiplier.
⮚ The amplitude of modulating signal
(analog data) is multiplied by the
amplitude of carrier frequency, which
then reflects analog data.
⮚The frequency and phase of carrier
signal remain unchanged.

Analog Transmission
Frequency Modulation
⮚ The frequency of the carrier signal is
modified to reflect the change in the
voltage levels of the modulating signal
(analog data).
⮚ The amplitude and phase of the carrier
signal are not altered.

Analog Transmission
Phase Modulation
⮚ The phase of carrier signal is modulated
in order to reflect the change in voltage
(amplitude) of analog data signal.
⮚ Phase modulation is practically similar to
Frequency Modulation, but in Phase
modulation frequency of the carrier signal
is not increased. Frequency of carrier is
signal is changed (made dense and sparse)
to reflect voltage change in the amplitude
of modulating signal.

Bandwidth utilization
⮚ Bandwidth utilization is the wise use of available bandwidth to
achieve specific goals.
⮚ Efficiency can be achieved by multiplexing; i.e., sharing of the
bandwidth between multiple users.

Bandwidth utilization: Multiplexing
Whenever the bandwidth of a medium linking two devices is greater than the
bandwidth needs of the devices, the link can be shared. Multiplexing is the
set of techniques that allows the (simultaneous) transmission of multiple
signals across a single data link. As data and telecommunications use
increases, so does traffic.
Types of multiplexing
⮚Frequency-Division Multiplexing
⮚ Wavelength-Division Multiplexing
⮚ Time-Division Multiplexing

Frequency-Division Multiplexing
⮚FDM is an analog multiplexing technique that combines analog signals.
⮚ It uses the concept of modulation.

Multiplexing Demultiplexing
Frequency-Division Multiplexing

Wavelength-Division Multiplexing
⮚ WDM is an analog multiplexing technique to combine optical signals.

Time Division Multiplexing
⮚ TDM is a digital multiplexing technique for combining several low-rate
digital channels into one high-rate one.

Bandwidth utilization: Spreading
⮚ A spreading code is used to map each data bit in the original signal to
multiple bits in the transmitted signal.
⮚ A collective class of signaling techniques are employed before transmitting
a signal to provide a secure communication, known as the Spread Spectrum
Modulation. The main advantage of spread spectrum communication
technique is to prevent “interference” whether it is intentional or
unintentional.

Spread Spectrum Signals
Following are some of its features −
⮚ Band of signals occupy a wide range
of frequencies.
⮚ Power density is very low.
⮚ Energy is wide spread.

FHSS and DSSS / CDMA
Spread spectrum multiple access techniques uses signals which
have a transmission bandwidth of a magnitude greater than the
minimum required RF bandwidth.
These are of two types.
⮚ Frequency Hopped Spread Spectrum (FHSS)
⮚ Direct Sequence Spread Spectrum (DSSS)

FHSS DSSS / CDMA
Multiple frequencies are used Single frequency is used
Hard to find the user’s frequency at any
instant of time
User frequency, once allotted is always the
same
Frequency reuse is allowed Frequency reuse is not allowed
Sender need not wait Sender has to wait if the spectrum is busy
Power strength of the signal is high Power strength of the signal is low
Stronger and penetrates through the
obstacles
It is weaker compared to FHSS
It is never affected by interference It can be affected by interference
It is cheaper It is expensive
This is the commonly used technique This technique is not frequently used
Difference in FHSS and DSSS / CDMA

Switching Techniques
In large networks there might be multiple paths linking sender and
receiver. Information may be switched as it travels through various
communication channels. There are three typical switching techniques
available for digital traffic.
⮚ Circuit Switching
⮚ Message Switching
⮚ Packet Switching

Circuit Switching
⮚ Circuit switching is a technique that directly connects the sender and the
receiver in an unbroken path.
⮚ Telephone switching equipment, for example, establishes a path that
connects the caller's telephone to the receiver's telephone by making a
physical connection.
⮚ With this type of switching technique, once a connection is established, a
dedicated path exists between both ends until the connection is terminated.
⮚ Routing decisions must be made when the circuit is first established, but
there are no decisions made after that time.

Circuit Switching
⮚ Circuit switching in a network operates almost the same way as the
telephone system works.
⮚ A complete end-to-end path must exist before communication can take
place.
⮚ The computer initiating the data transfer must ask for a connection to
the destination.
⮚ Once the connection has been initiated and completed to the
destination device, the destination device must acknowledge that it is
ready and willing to carry on a transfer.

Circuit switching
Advantages:
⮚ The communication channel (once established) is dedicated.
Disadvantages:
⮚ Possible long wait to establish a connection, (10 seconds, more on
long- distance or international calls.) during which no data can be
transmitted.
⮚ More expensive than any other switching techniques, because a
dedicated path is required for each connection.
⮚ Inefficient use of the communication channel, because the channel
is not used when the connected systems are not using it.

Message Switching
⮚ With message switching there is no need to establish a dedicated path
between two stations.
⮚ When a station sends a message, the destination address is appended to
the message.
⮚ The message is then transmitted through the network, in its entirety, from
node to node.
⮚ Each node receives the entire message, stores it in its entirety on disk,
and then transmits the message to the next node.
⮚ This type of network is called a store-and-forward network.

Message Switching
A message-switching node is typically a general-purpose computer. The
device needs sufficient secondary-storage capacity to store the incoming
messages, which could be long. A time delay is introduced using this type
of scheme due to store- and-forward time, plus the time required to find
the next node in the transmission path.

Message Switching
Advantages:
⮚ Channel efficiency can be greater compared to circuit- switched
systems, because more devices are sharing the channel.
⮚ Traffic congestion can be reduced, because messages may be
temporarily stored in route.
⮚ Message priorities can be established due to store-and-forward
technique.
⮚ Message broadcasting can be achieved with the use of broadcast
address appended in the message.
Disadvantages
⮚ Message switching is not compatible with interactive applications.
⮚ Store-and-forward devices are expensive, because they must have
large disks to hold potentially long messages.

Packet Switching
⮚ Packet switching can be seen as a solution that tries to combine the advantages
of message and circuit switching and to minimize the disadvantages of both.
⮚ There are two methods of packet switching:
-Datagram
- virtual circuit.

Packet Switching
⮚ In both packet switching methods, a message is broken into small
parts, called packets.
⮚ Each packet is tagged with appropriate source and destination
addresses.
⮚ Since packets have a strictly defined maximum length, they can be
stored in main memory instead of disk, therefore access delay and
cost are minimized.
⮚ Also the transmission speeds, between nodes, are optimized.
⮚ With current technology, packets are generally accepted onto the
network on a first-come, first-served basis. If the network becomes
overloaded, packets are delayed or discarded (``dropped'').

Packet Switching: Datagram
⮚ Datagram packet switching is similar to message switching in that each
packet is a self-contained unit with complete addressing information
attached.
⮚ This fact allows packets to take a variety of possible paths through the
network.
⮚ So the packets, each with the same destination address, do not follow the
same route, and they may arrive out of sequence at the exit point node (or the
destination).
⮚ Reordering is done at the destination point based on the sequence number of
the packets.
⮚ It is possible for a packet to be destroyed if one of the nodes on its way is
crashed momentarily. Thus all its queued packets may be lost.

Packet Switching:Virtual Circuit
⮚ In the virtual circuit approach, a preplanned route is established before
any data packets are sent.
⮚ A logical connection is established when
▪ a sender send a "call request packet" to the receiver and
▪ the receiver send back an acknowledge packet "call accepted packet" to
the sender if the receiver agrees on conversational parameters.
⮚ The conversational parameters can be maximum packet sizes, path to be
taken, and other variables necessary to establish and maintain the
conversation.
⮚ Virtual circuits imply acknowledgements, flow control, and error control,
so virtual circuits are reliable.
⮚ That is, they have the capability to inform upper-protocol layers if a
transmission problem occurs.

Packet Switching: Virtual Circuit
⮚ In virtual circuit, the route between stations does not mean that this is a
dedicated path, as in circuit switching.
⮚ A packet is still buffered at each node and queued for output over a line.
The difference between virtual circuit and datagram approaches:
⮚ With virtual circuit, the node does not need to make a routing decision for
each packet.
⮚ It is made only once for all packets using that virtual circuit.

Packet Switching: Virtual Circuit
VC's offer guarantees that
⮚ The packets sent arrive in the order sent
⮚ With no duplicates or omissions
⮚ With no errors (with high probability) regardless of how they are
implemented internally.

Advantages of packet switching
Advantages:
⮚ Packet switching is cost effective, because switchingdevices do not
need massive amount of secondary storage.
⮚ Packet switching offers improved delay characteristics, because there
are no long messages in the queue (maximum packet size is fixed).
⮚ Packet can be rerouted if there is any problem, such as, busy or
disabled links.
⮚ The advantage of packet switching is that many network users can
share the same channel at the same time. Packet switching can
maximize link efficiency by making optimal use of link bandwidth.

Disadvantages of packet switching
Disadvantages:
⮚ Protocols for packet switching are typically more complex.
⮚ It can add some initial costs in implementation.
⮚ If packet is lost, sender needs to retransmit the data.
⮚ Another disadvantage is that packet-switched systems still can’t
deliver the same quality as dedicated circuits in applications
requiring very little delay - like voice conversations or moving
images.

Transmission Media
⮚Transmission media is a communication channel that carries the information
from the sender to the receiver. Data is transmitted through the
electromagnetic signals.
⮚It is a physical path between transmitter and receiver in data communication.
⮚In a copper-based network, the bits in the form of electrical signals. In a fibre
based network, the bits in the form of light pulses.
⮚The electrical signals can be sent
through the copper wire, fibre optics,
atmosphere, water, and vacuum.
⮚The characteristics and quality of data
transmission are determined by the
characteristics of medium and signal.

⮚ It is defined as the physical medium through which the
signals are transmitted. It is also known as Bounded media.
Types Of Guided media
⮚ Twisted-pair cable
⮚ Coaxial cable
⮚ Fiber-optic cable
Transmission Media: Guided Media

Twisted pair:
⮚Twisted pair is a physical media made up of a pair of cables twisted with
each other. A twisted pair cable is cheap as compared to other
transmission media.
⮚ The frequency range for twisted pair cable is from 0 to 3.5KHz.
⮚ A twisted pair consists of two insulated copper wires arranged in a
regular spiral pattern

Unshielded Twisted Pair
⮚An unshielded twisted pair is
widely used in telecommunication.
⮚ categories of the unshielded
twisted pair cable:
• Category 1: used for telephone
lines that have low-speed data.
• Category 2: supports up to 4Mbps.
• Category 3: support up to 16Mbps.
• Category 4: supports up to
20Mbps.
• Category 5: supports up to
200Mbps.
Shielded Twisted Pair
⮚ A shielded twisted pair is a cable
that contains the mesh
surrounding the wire that allows
the higher transmission rate.
⮚ It has higher capacity as
compared to unshielded twisted
pair cable.
⮚ It has a higher attenuation.
⮚ It is shielded that provides the
higher data transmission rate.

Coaxial Cable
⮚Coaxial cable is very commonly used transmission media, for example, TV wire is
usually a coaxial cable. It has a higher frequency as compared to Twisted pair cable.
⮚The inner conductor of the coaxial cable is made up of copper, and the outer
conductor is made up of copper mesh. The middle core is made up of non-conductive
cover that separates the inner conductor from the outer conductor.
⮚The middle core is responsible for the data transferring whereas the copper mesh
prevents from the EMI(Electromagnetic interference).
Coaxial cable is of two types:
⮚ Baseband transmission: It is defined as the
process of transmitting a single signal at high
speed.
⮚ Broadband transmission: It is defined as the
process of transmitting multiple signals
simultaneously.

Fiber Optic
⮚ Fiber optic cable is a cable that uses electrical signals for communication.
⮚ Fiber optic is a cable that holds the optical fibers coated in plastic that are used to send
the data by pulses of light.
⮚ Fiber optics provide faster data transmission than copper wires.
⮚ Core: The optical fibre consists of a narrow strand of glass or plastic known as a core. A
core is a light transmission area of the fibre. The more the area of the core, the more light
will be transmitted into the fibre.
⮚Cladding: The concentric layer of glass is known as cladding.
⮚Jacket: The protective coating consisting of plastic is known as a jacket.

• An unguided transmission transmits the electromagnetic
waves without using any physical medium. Therefore it is
also known as wireless transmission.
• In unguided media, air is the media through which the
electromagnetic energy can flow easily.
Transmission Media: UnGuided Media

Radio waves
⮚Radio waves are the electromagnetic waves that are transmitted in all
the directions of free space.
⮚Radio waves are omnidirectional, i.e., the signals are propagated in all the
directions.
⮚The range in frequencies of radio waves is from 3Khz to 1 khz.
⮚In the case of radio waves, the sending and receiving antenna are not
aligned, i.e., the wave sent by the sending antenna can be received by any
receiving antenna.
⮚An example of the radio wave is FM radio.
Transmission Media: Unguided Media

Microwave
⮚Terrestrial Microwave Transmission
⮚ Satellite Microwave Communication
Terrestrial Microwave: transmission is a technology that transmits the
focused beam of a radio signal from one ground-based microwave
transmission antenna to another. Microwaves are the electromagnetic
waves having the frequency in the range from 1GHz to 1000 GHz.
Microwaves are unidirectional as the sending and receiving antenna is to be
aligned, i.e., the waves sent by the sending antenna are narrowly focused.
Satellite Microwave Communication: A satellite is a physical object that
revolves around the earth at a known height. Satellite communication is
more reliable nowadays as it offers more flexibility than cable and fibre
optic systems. We can communicate with any point on the globe by using
satellite communication.

Infrared
⮚ An infrared transmission is a wireless technology used for
communication over short ranges.
⮚The frequency of the infrared in the range from 300 GHz to 400 THz.
⮚It is used for short-range communication such as data transfer between
two cell phones, TV remote operation, data transfer between a computer
and cell phone resides in the same closed area.
⮚ It supports high bandwidth, and hence the data rate will be very high.

Data Link Layer
Services Provided to the Network Layer
⮚Framing
⮚Error Control
⮚Flow Control

Functions of the Data Link Layer
⮚ Provide service interface to the network layer
⮚ Dealing with transmission errors
⮚ Regulating data flow
•Slow receivers not swamped by fast senders

Functions of the Data Link Layer (2)
Relationship between packets and frames.

Data Link Layer: Framing
A character stream. (a) Without errors. (b) With one error.

Data Link Layer: Framing
Bit stuffing
(a) The original data.
(b) The data as they appear on the line.
(c) The data as they are stored in receiver’s memory after destuffing.

Data Link Layer: Error Detection and Correction
⮚ When data is transmitted from one device to another device, the system does not
guarantee whether the data received by the device is identical to the data
transmitted by another device. An Error is a situation when the message received
at the receiver end is not identical to the message transmitted.
Types Of Errors
⮚ Single-Bit Error: The only one bit of a given data unit is changed from 1 to 0 or
from 0 to 1.
⮚ Burst Error: The two or more bits are changed from 0 to 1 or from 1 to 0 is known
as Burst Error. The Burst Error is determined from the first corrupted bit to the last
corrupted bit.
Single-bit error Burst error

Error Detecting Techniques:
• Checksum
• Cyclic redundancy check
Checksum Checker
• A Checksum is verified at the receiving side. The receiver subdivides the incoming
data into equal segments of n bits each, and all these segments are added
together, and then this sum is complemented. If the complement of the sum is
zero, then the data is accepted otherwise data is rejected
The Receiver follows the given steps:
1. The block unit is divided into k sections and each of n bits.
2. All the k sections are added together by using one's complement algorithm to get
the sum.
3. The sum is complemented.
4. If the result of the sum is zero, then the data is accepted otherwise the data is dis
carded.

Cyclic Redundancy Check (CRC)
CRC is a redundancy error technique used to determine the error.
Following are the steps used in CRC for error detection:
• In CRC technique, a string of n 0s is appended to the data unit, and this n
number is less than the number of bits in a predetermined number, known
as division which is n+1 bits.
• Secondly, the newly extended data is divided by a divisor using a process is
known as binary division. The remainder generated from this division is
known as CRC remainder.
• Thirdly, the CRC remainder replaces the appended 0s at the end of the
original data. This newly generated unit is sent to the receiver.
• The receiver receives the data followed by the CRC remainder. The receiver
will treat this whole unit as a single unit, and it is divided by the same
divisor that was used to find the CRC remainder.
• If the resultant of this division is zero which means that it has no error, and
the data is accepted.

• Example: When the string 11100111 is
received at the receiving end, then
CRC checker performs the modulo-2
division. A string is divided by the
same divisor, i.e., 1001.
Remainder = 0 (No error)

Error Correction
Hamming Code
⮚Parity bits: The bit which is appended to the original data of binary bits
so that the total number of 1s is even or odd.
⮚Even parity: To check for even parity, if the total number of 1s is even,
then the value of the parity bit is 0. If the total number of 1s occurrences
is odd, then the value of the parity bit is 1.
⮚Odd Parity: To check for odd parity, if the total number of 1s is even, then
the value of parity bit is 1. If the total number of 1s is odd, then the value
of parity bit is 0.
Algorithm of Hamming code:
• An information of 'd' bits are added to the redundant bits 'r' to form d+r.
• The location of each of the (d+r) digits is assigned a decimal value.
• The 'r' bits are placed in the positions 1,2,.....2k-1.
• At the receiving end, the parity bits are recalculated. The decimal value of
the parity bits determines the position of an error.

Data Link Control: Flow Control
⮚ Ensuring the sending entity does not overwhelm the receiving entity
• Preventing buffer overflow
⮚ Transmission time
• Time taken to emit all bits into medium
⮚ Propagation time
• Time for a bit to traverse the link

Data Link Control: Stop and Wait
⮚ Source transmits frame
⮚ Destination receives frame and replies with acknowledgement
⮚ Source waits for ACK before sending next frame
⮚ Destination can stop flow by not send ACK
⮚ Works well for a few large frames

Data Link Control: Stop and Wait
⮚ Source transmits single frame
⮚ Wait for ACK
⮚ If received frame damaged, discard it
• Transmitter has timeout
• If no ACK within timeout,
retransmit
⮚ If ACK damaged, transmitter will not
recognize it
• Transmitter will retransmit
• Receive gets two copies of frame
• Use ACK0 and ACK1

Data Link Control: Fragmentation
⮚ Large block of data may be split into small frames
• Limited buffer size
• Errors detected sooner (when whole frame received)
• On error, retransmission of smaller frames is needed
• Prevents one station occupying medium for long periods
⮚ Stop and wait becomes inadequate

Data Link Control: Sliding Windows Flow Control
⮚ Allow multiple frames to be in transit
⮚ Receiver has buffer W long
⮚ Transmitter can send up to W frames without ACK
⮚ Each frame is numbered
⮚ ACK includes number of next frame expected
⮚ Sequence number bounded by size of field (k)
• Frames are numbered modulo 2k

Data Link Control: Sliding Window Diagram

Wired LANs
Ethernet
⮚ We learned that a local area network (LAN) is a computer network
that is designed for a limited geographic area such as a building or a
campus.
⮚ Although a LAN can be used as an isolated network to connect
computers in an organization for the sole purpose of sharing
resources, most LANs today are also linked to a wide area network
(WAN) or the Internet.
⮚ The LAN market has several technologies such as Ethernet, Token
Ring, Token Bus, FDDI, and ATM LAN. Some of these technologies
survived for a while, but Ethernet is by far the dominant technology.

Wireless LANs
⮚ Wireless LANs (WLANs) are wireless computer networks that use high-frequency radio waves
instead of cables for connecting the devices within a limited area forming LAN (Local Area Network).
Users connected by wireless LANs can move around within this limited area such as home, school,
campus, office building, railway platform, etc.
⮚Most WLANs are based upon the standard IEEE 802.11 standard or WiFi.
Components of WLANs
⮚The components of WLAN architecture as laid down in IEEE 802.11 are −
⮚Stations (STA) − Stations comprises of all devices and equipment that are connected to the
wireless LAN. Each station has a wireless network interface controller. A station can be of two types
• Wireless Access Point (WAP or AP)
• Client
⮚Basic Service Set (BSS) − A basic service set is a group of stations communicating at the physical
layer level. BSS can be of two categories −
• Infrastructure BSS
• Independent BSS
⮚Extended Service Set (ESS) − It is a set of all connected BSS.
⮚Distribution System (DS) − It connects access points in ESS.

Virtual Circuit Network
⮚ Virtual – circuit network is a category of
packet switching network, where a
virtual path is established between the
source and the destination systems for
data communication to occur.
⮚ This path appears to the user as if it is a
dedicated physical path, but actually is a
logical circuit allocated from a managed
pool of circuit resources as per traffic
requirements. The network resources
forming parts of this path can be shared
by other communications, however, is not
visible to this user.

Virtual Circuit Network
Phases of Virtual - Circuit Transmission
⮚ There are three phases of transmission by virtual circuits, set up, data
transfer and teardown.
1. Set up Phase − In this phase, a virtual circuit or a route is established
from the source to the destination through number of switches. The
source and destination use global addresses using which the switches
make routing table entries.
2. Data Transfer − Once the virtual circuit is set up, all packets follow the
route established during the set up phase adhering to the routing tables.
3. Teardown Phase − When data transfer is complete, the source sends a
teardown request. The destination responds using a teardown
confirmation. The switches flush their routing table entries, thus
relinquishing the circuit.

Computer Communication & Computer Networking Module 1 , 2 Notes.pdf

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