UNIT V DATA LINK AND PHYSICAL LAYERS

Data Link Layer – Framing – Flow control – Error control – Data-Link Layer
Protocols – HDLC – PPP - Media Access Control – Ethernet Basics – CSMA/CD –
Virtual LAN – Wireless LAN (802.11) - Physical Layer: Data and Signals -
Performance – Transmission media- Switching – Circuit Switching.
1.DATA LINK LAYER
The Data-link layer is the second layer from the bottom in the OSI (Open System
Interconnection) network architecture model. It is responsible for the node-to-node
delivery of data. Its major role is to ensure error-free transmission of information. DLL
is also responsible to encode, decode and organize the outgoing and incoming data. This
is considered the most complex layer of the OSI model as it hides all the underlying
complexities of the hardware from the other above layers.

Sub-layers of Data Link Layer:

The data link layer is further divided into two sub-layers, which are as follows:

Logical Link Control (LLC):This sublayer of the data link layer deals with multiplexing,
the flow of data among applications and other services, and LLC is responsible for
providing error messages and acknowledgments as well.

Media Access Control (MAC):MAC sublayer manages the device’s interaction,

responsible for addressing frames, and also controls physical media access.

The data link layer receives the information in the form of packets from the Network
layer, it divides packets into frames and sends those frames bit-by-bit to the underlying
physical layer.

Functions of the Data-link Layer:

1. Framing: The packet received from the Network layer is known as a frame in the
Data link layer. At the sender’s side, DLL receives packets from the Network layer and
divides them into small frames, then, sends each frame bit-by-bit to the physical layer.
It also attaches some special bits (for error control and addressing) at the header and
end of the frame. At the receiver’s end, DLL takes bits from the Physical layer organizes
them into the frame, and sends them to the Network layer.

2. Addressing: The data link layer encapsulates the source and destination’s MAC
address/ physical address in the header of each frame to ensure node-to-node delivery.
MAC address is the unique hardware address that is assigned to the device while
manufacturing.

3. Error Control: Data can get corrupted due to various reasons like noise, attenuation,
etc. So, it is the responsibility of the data link layer, to detect the error in the transmitted
data and correct it using error detection and correction techniques respectively. DLL
adds error detection bits into the frame’s header, so that receiver can check received
data is correct or not.

4. Flow Control: If the receiver’s receiving speed is lower than the sender’s sending
speed, then this can lead to an overflow in the receiver’s buffer and some frames may
get lost. So, it’s the responsibility of DLL to synchronize the sender’s and receiver’s
speeds and establish flow control between them.

5. Access Control: When multiple devices share the same communication channel
there is a high probability of collision, so it’s the responsibility of DLL to check which
device has control over the channel and CSMA/CD and CSMA/CA can be used to avoid
collisions and loss of frames in the channel.

2.FRAMING IN DATA LINK LAYER

Frames are the units of digital transmission, particularly in computer networks

and telecommunications. Frames are comparable to the packets of energy called
photons in the case of light energy. Frame is continuously used in Time Division
Multiplexing process.
Framing is a point-to-point connection between two computers or devices
consisting of a wire in which data is transmitted as a stream of bits. However, these bits
must be framed into discernible blocks of information. Framing is a function of the data
link layer. It provides a way for a sender to transmit a set of bits that are meaningful to
the receiver. Ethernet, token ring, frame relay, and other data link layer technologies
have their own frame structures. Frames have headers that contain information such as
error-checking codes.
At the data link layer, it extracts the message from the sender and provides it to the
receiver by providing the sender’s and receiver’s addresses. The advantage of using
frames is that data is broken up into recoverable chunks that can easily be checked for
corruption.
The process of dividing the data into frames and reassembling it is transparent to the
user and is handled by the data link layer.
Framing is an important aspect of data link layer protocol design because it allows the
transmission of data to be organized and controlled, ensuring that the data is delivered
accurately and efficiently.
Problems in Framing
 Detecting start of the frame: When a frame is transmitted, every station must
be able to detect it. Station detects frames by looking out for a special
sequence of bits that marks the beginning of the frame i.e. SFD (Starting
Frame Delimiter).
 How does the station detect a frame: Every station listens to link for SFD
pattern through a sequential circuit. If SFD is detected, sequential circuit
alerts station. Station checks destination address to accept or reject frame.
 Detecting end of frame: When to stop reading the frame.
 Handling errors: Framing errors may occur due to noise or other
transmission errors, which can cause a station to misinterpret the frame.
Therefore, error detection and correction mechanisms, such as cyclic
redundancy check (CRC), are used to ensure the integrity of the frame.
 Framing overhead: Every frame has a header and a trailer that contains
control information such as source and destination address, error detection
code, and other protocol-related information. This overhead reduces the
available bandwidth for data transmission, especially for small-sized frames.
 Framing incompatibility: Different networking devices and protocols may
use different framing methods, which can lead to framing incompatibility
issues. For example, if a device using one framing method sends data to a
device using a different framing method, the receiving device may not be able
to correctly interpret the frame.
 Framing synchronization: Stations must be synchronized with each other to
avoid collisions and ensure reliable communication. Synchronization
requires that all stations agree on the frame boundaries and timing, which
can be challenging in complex networks with many devices and varying
traffic loads.
 Framing efficiency: Framing should be designed to minimize the amount of
data overhead while maximizing the available bandwidth for data
transmission. Inefficient framing methods can lead to lower network
performance and higher latency.

Types of framing
There are two types of framing:
1. Fixed-size: The frame is of fixed size and there is no need to provide boundaries to
the frame, the length of the frame itself acts as a delimiter.
 Drawback: It suffers from internal fragmentation if the data size is less than
the frame size
  Solution: Padding

2. Variable size: In this, there is a need to define the end of the frame as well as the
beginning of the next frame to distinguish. This can be done in two ways:
1. Length field – We can introduce a length field in the frame to indicate the
length of the frame. Used in Ethernet(802.3). The problem with this is that
sometimes the length field might get corrupted.
2. End Delimiter (ED) – We can introduce an ED(pattern) to indicate the end of
the frame. Used in Token Ring. The problem with this is that ED can occur in
the data. This can be solved by:
1. Character/Byte Stuffing: Used when frames consist of characters. If data
contains ED then, a byte is stuffed into data to differentiate it from ED.
Let ED = “$” –> if data contains ‘$’ anywhere, it can be escaped using ‘\O’
character.
–> if data contains ‘\O$’ then, use ‘\O\O\O$'($ is escaped using \O and \O is
escaped using \O).

Disadvantage – It is very costly and obsolete method.

2. Bit Stuffing: Let ED = 01111 and if data = 01111
–> Sender stuffs a bit to break the pattern i.e. here appends a 0 in data = 011101.
–> Receiver receives the frame.
–> If data contains 011101, receiver removes the 0 and reads the data.
Examples:
 If Data –> 011100011110 and ED –> 0111 then, find data after bit stuffing.
--> 011010001101100
 If Data –> 110001001 and ED –> 1000 then, find data after bit stuffing?
--> 11001010011
framing in the Data Link Layer also presents some challenges, which include:

Variable frame length: The length of frames can vary depending on the data being
transmitted, which can lead to inefficiencies in transmission. To address this issue,
protocols such as HDLC and PPP use a flag sequence to mark the start and end of each
frame.
Bit stuffing: Bit stuffing is a technique used to prevent data from being interpreted as
control characters by inserting extra bits into the data stream. However, bit stuffing can
lead to issues with synchronization and increase the overhead of the transmission.
Synchronization: Synchronization is critical for ensuring that data frames are
transmitted and received correctly. However, synchronization can be challenging,
particularly in high-speed networks where frames are transmitted rapidly.
Error detection: Data Link Layer protocols use various techniques to detect errors in
the transmitted data, such as checksums and CRCs. However, these techniques are not
foolproof and can miss some types of errors.

Efficiency: Efficient use of available bandwidth is critical for ensuring that data is
transmitted quickly and reliably. However, the overhead associated with framing and
error detection can reduce the overall efficiency of the transmission.

3.FLOW CONTROL IN DATA LINK LAYER

Flow control is design issue at Data Link Layer. It is a technique that generally observes
the proper flow of data from sender to receiver. It is very essential because it is possible
for sender to transmit data or information at very fast rate and hence receiver can
receive this information and process it. This can happen only if receiver has very high
load of traffic as compared to sender, or if receiver has power of processing less as
compared to sender. Flow control is basically a technique that gives permission to two
of stations that are working and processing at different speeds to just communicate
with one another. Flow control in Data Link Layer simply restricts and coordinates
number of frames or amount of data sender can send just before it waits for an
acknowledgement from receiver. Flow control is actually set of procedures that
explains sender about how much data or frames it can transfer or transmit before data
overwhelms receiver. The receiving device also contains only limited amount of speed
and memory to store data. This is why receiving device should be able to tell or inform
the sender about stopping the transmission or transferring of data on temporary basis
before it reaches limit. It also needs buffer, large block of memory for just storing data
or frames until they are processed flow control can also be understand as a speed
matching mechanism for two stations.

Approaches to Flow Control : Flow Control is classified into two categories:

 Feedback – based Flow Control : In this control technique, sender simply
transmits data or information or frame to receiver, then receiver transmits
data back to sender and also allows sender to transmit more amount of data
or tell sender about how receiver is processing or doing. This simply means
that sender transmits data or frames after it has received acknowledgements
from user.
 Rate – based Flow Control : In this control technique, usually when sender
sends or transfer data at faster speed to receiver and receiver is not being
able to receive data at the speed, then mechanism known as built-in
mechanism in protocol will just limit or restricts overall rate at which data or
information is being transferred or transmitted by sender without any
feedback or acknowledgement from receiver.
Techniques of Flow Control in Data Link Layer : There are basically two types of
techniques being developed to control the flow of data
1. Stop-and-Wait Flow Control : This method is the easiest and simplest form of flow
control. In this method, basically message or data is broken down into various multiple
frames, and then receiver indicates its readiness to receive frame of data. When
acknowledgement is received, then only sender will send or transfer the next frame.
This process is continued until sender transmits EOT (End of Transmission) frame. In
this method, only one of frames can be in transmission at a time. It leads to inefficiency
i.e. less productivity if propagation delay is very much longer than the transmission
delay and Ultimately In this method sender sent single frame and receiver take one
frame at a time and sent acknowledgement(which is next frame number only) for new
frame.
Advantages –
 This method is very easiest and simple and each of the frames is checked and
acknowledged well.
 This method is also very accurate.
Disadvantages –
 This method is fairly slow.
 In this, only one packet or frame can be sent at a time.
 It is very inefficient and makes the transmission process very slow.
2. Sliding Window Flow Control : This method is required where reliable in-order
delivery of packets or frames is very much needed like in data link layer. It is point to
point protocol that assumes that none of the other entity tries to communicate until
current data or frame transfer gets completed. In this method, sender transmits or
sends various frames or packets before receiving any acknowledgement. In this method,
both the sender and receiver agree upon total number of data frames after which
acknowledgement is needed to be transmitted. Data Link Layer requires and uses this
method that simply allows sender to have more than one unacknowledged packet “in-
flight” at a time. This increases and improves network throughput. and Ultimately In
this method sender sent multiple frame but receiver take one by one and after
completing one frame acknowledge(which is next frame number only) for new frame.
Advantages –
 It performs much better than stop-and-wait flow control.
 This method increases efficiency.
 Multiples frames can be sent one after another.
Disadvantages –
  The main issue is complexity at the sender and receiver due to the
transferring of multiple frames.
 The receiver might receive data frames or packets out the sequence.

4.ERROR CONTROL
Data-link layer uses the techniques of error control simply to ensure and confirm that
all the data frames or packets, i.e. bit streams of data, are transmitted or transferred
from sender to receiver with certain accuracy. Using or providing error control at this
data link layer is an optimization, it was never a requirement. Error control is basically
process in data link layer of detecting or identifying and re-transmitting data frames
that might be lost or corrupted during transmission. In both of these cases, receiver or
destination does not receive correct data frame and sender or source does not even
know anything about any such loss regarding data frames. Therefore, in such type of
cases, both sender and receiver are provided with some essential protocols that are
required to detect or identify such types of errors as loss of data frames. The Data-link
layer follows a technique known as re-transmission of frames to detect or identify
transit errors and also to take necessary actions that are required to reduce or remove
such errors. Each and every time an error is detected during transmission, particular
data frames are retransmitted and this process is known as ARQ (Automatic Repeat
Request).
Ways of doing Error Control : There are basically two ways of doing Error control as
given below :

Ways of Error Control

1. Error Detection : Error detection, as the name suggests, simply means

detection or identification of errors. These errors may occur due to noise or
any other impairments during transmission from transmitter to the receiver,
in communication system. It is a class of techniques for detecting garbled i.e.
unclear and distorted data or messages.
2. Error Correction : Error correction, as the name suggests, simply means
correction or solving or fixing of errors. It simply means reconstruction and
rehabilitation of original data that is error-free. But error correction method
is very costly and very hard.
Various Techniques for Error Control : There are various techniques of error control as
given below :

Techniques of Error Control

1. Stop-and-Wait ARQ : Stop-and-Wait ARQ is also known as alternating bit protocol.

It is one of the simplest flow and error control techniques or mechanisms. This
mechanism is generally required in telecommunications to transmit data or
information between two connected devices. Receiver simply indicates its readiness to
receive data for each frame. In these, sender sends information or data packets to
receiver. Sender then stops and waits for ACK (Acknowledgment) from receiver.
Further, if ACK does not arrive within given time period i.e., time-out, sender then again
resends frame and waits for ACK. But, if sender receives ACK, then it will transmit the
next data packet to receiver and then again wait for ACK from receiver. This process to
stop and wait continues until sender has no data frame or packet to send.
2. Sliding Window ARQ : This technique is generally used for continuous transmission
error control. It is further categorized into two categories as given below :
 Go-Back-N ARQ : Go-Back-N ARQ is form of ARQ protocol in which
transmission process continues to send or transmit total number of frames
that are specified by window size even without receiving an ACK
(Acknowledgement) packet from the receiver. It uses sliding window flow
control protocol. If no errors occur, then operation is identical to sliding
window.
 Selective Repeat ARQ : Selective Repeat ARQ is also form of ARQ protocol in
which only suspected or damaged or lost data frames are only retransmitted.
 This technique is similar to Go-Back-N ARQ though much more efficient than
the Go-Back-N ARQ technique due to reason that it reduces number of
retransmission. In this, the sender only retransmits frames for which NAK is
received. But this technique is used less because of more complexity between
sender and receiver and each frame must be needed to be acknowledged
individually.

The main difference between Go Back ARQ and Selective Repeat ARQ is that in Go Back
ARQ, the sender has to retransmit the whole window of frame again if any of the frame
is lost but in Selective Repeat ARQ only the data frame that is lost is retransmitted.

6.DATA-LINK LAYER PROTOCOLS

The Data Link Layer protocols are Ethernet, token ring, FDDI and PPP. An important
characteristic of a Data Link Layer is that datagram can be handled by different link layer
protocols on different links in a path. For example, the datagram is handled by Ethernet
on the irst link, PPP on the second link.

7.HIGH-LEVEL DATA LINK CONTROL (HDLC)

High-level Data Link Control (HDLC) is a group of communication protocols of the data
link layer for transmitting data between network points or nodes. Since it is a data link
protocol, data is organized into frames. A frame is transmitted via the network to the
destination that verifies its successful arrival. It is a bit - oriented protocol that is
applicable for both point - to - point and multipoint communications.
Transfer Modes
HDLC supports two types of transfer modes, normal response mode and asynchronous
balanced mode.
 Normal Response Mode (NRM) − Here, two types of stations are there, a
primary station that send commands and secondary station that can
respond to received commands. It is used for both point - to - point and
multipoint communications.
 Asynchronous Balanced Mode (ABM) − Here, the con iguration is balanced, i.e. each
station can both send commands and respond to commands. It is used for only point - to
- point communications.

HDLC Frame
HDLC is a bit - oriented protocol where each frame contains up to six fields. The structure
varies according to the type of frame. The fields of a HDLC frame are −
 Flag − It is an 8-bit sequence that marks the beginning and the end of the
frame. The bit pattern of the flag is 01111110.
 Address − It contains the address of the receiver. If the frame is sent by the
primary station, it contains the address(es) of the secondary station(s). If it
is sent by the secondary station, it contains the address of the primary
station. The address field may be from 1 byte to several bytes.
 Control − It is 1 or 2 bytes containing low and error control information.
 Payload − This carries the data from the network layer. Its length may vary
from one network to another.
 FCS − It is a 2 byte or 4 bytes frame check sequence for error detection. The
standard code used is CRC (cyclic redundancy code).

Types of HDLC Frames

There are three types of HDLC frames. The type of frame is determined by the control
field of the frame −
 I-frame − I-frames or Information frames carry user data from the network
layer. They also include flow and error control information that is
piggybacked on user data. The first bit of control field of I-frame is 0.
  S-frame − S-frames or Supervisory frames do not contain information field.
They are used for flow and error control when piggybacking is not required.
The first two bits of control field of S-frame is 10.
 U-frame − U-frames or Un-numbered frames are used for myriad
miscellaneous functions, like link management. It may contain an
information field, if required. The first two bits of control field of U-frame is
11.

8.POINT-TO-POINT PROTOCOL (PPP)

Point - to - Point Protocol (PPP) is a communication protocol of the data link layer that
is used to transmit multiprotocol data between two directly connected (point-to-point)
computers. It is a byte - oriented protocol that is widely used in broadband
communications having heavy loads and high speeds. Since it is a data link layer protocol,
data is transmitted in frames. It is also known as RFC 1661.
Services Provided by PPP
The main services provided by Point - to - Point Protocol are −
 Defining the frame format of the data to be transmitted.
 Defining the procedure of establishing link between two points and
exchange of data.
 Stating the method of encapsulation of network layer data in the frame.
 Stating authentication rules of the communicating devices.
 Providing address for network communication.
 Providing connections over multiple links.
 Supporting a variety of network layer protocols by providing a range os
services.
Components of PPP
Point - to - Point Protocol is a layered protocol having three components −
 Encapsulation Component − It encapsulates the datagram so that it can be
transmitted over the specified physical layer.
 Link Control Protocol (LCP) − It is responsible for establishing, con iguring,
testing, maintaining and terminating links for transmission. It also imparts
negotiation for set up of options and use of features by the two endpoints
of the links.
 Authentication Protocols (AP) − These protocols authenticate endpoints for
use of services. The two authentication protocols of PPP are −
o Password Authentication Protocol (PAP)
o Challenge Handshake Authentication Protocol (CHAP)
 Network Control Protocols (NCPs) − These protocols are used for
negotiating the parameters and facilities for the network layer. For every
higher-layer protocol supported by PPP, one NCP is there. Some of the NCPs
of PPP are −
o Internet Protocol Control Protocol (IPCP)
o OSI Network Layer Control Protocol (OSINLCP)
o Internetwork Packet Exchange Control Protocol (IPXCP)
o DECnet Phase IV Control Protocol (DNCP)
o NetBIOS Frames Control Protocol (NBFCP)
o IPv6 Control Protocol (IPV6CP)

PPP Frame
PPP is a byte - oriented protocol where each field of the frame is composed of one or
more bytes. The fields of a PPP frame are −
 Flag − 1 byte that marks the beginning and the end of the frame. The bit
pattern of the flag is 01111110.
  Address − 1 byte which is set to 11111111 in case of broadcast.
 Control − 1 byte set to a constant value of 11000000.
 Protocol − 1 or 2 bytes that de ine the type of data contained in the payload
field.
 Payload − This carries the data from the network layer. The maximum
length of the payload field is 1500 bytes. However, this may be negotiated
between the endpoints of communication.
 FCS − It is a 2 byte or 4 bytes frame check sequence for error detection. The
standard code used is CRC (cyclic redundancy code)

Byte Stuffing in PPP Frame − Byte stuf ing is used is PPP payload ield whenever the lag
sequence appears in the message, so that the receiver does not consider it as the end of
the frame. The escape byte, 01111101, is stuffed before every byte that contains the same
byte as the flag byte or the escape byte. The receiver on receiving the message removes
the escape byte before passing it onto the network layer.

9.MEDIA ACCESS CONTROL

A media access control is a network data transfer policy that determines how data is
transmitted between two computer terminals through a network cable. The media
access control policy involves sub-layers of the data link layer 2 in the OSI reference
model.

The essence of the MAC protocol is to ensure non-collision and eases the transfer of data
packets between two computer terminals. A collision takes place when two or more
terminals transmit data/information simultaneously. This leads to a breakdown of
communication, which can prove costly for organizations that lean heavily on data
transmission.

Media Access Control Methods

This network channel through which data is transmitted between terminal nodes to
avoid collision has three various ways of accomplishing this purpose. They include:
 Carrier sense multiple access with collision avoidance (CSMA/CA)
 Carrier sense multiple access with collision detection (CSMA/CD)
 Demand priority
 Token passing
10.CARRIER SENSE MULTIPLE ACCESS WITH COLLISION AVOIDANCE (CSMA/CA)
Carrier sense multiple access with collision avoidance (CSMA/CA) is a media
access control policy that regulates how data packets are transmitted between two
computer nodes. This method avoids collision by configuring each computer terminal
to make a signal before transmission. The signal is carried out by the transmitting
computer to avoid a collision.
Multiple access implies that many computers are attempting to transmit data.
Collision avoidance means that when a computer node transmitting data states its
intention, the other waits at a specific length of time before resending the data.
CSMA/CA is data traffic regulation is slow and adds cost in having each computer
node signal its intention before transmitting data. It used only on Apple networks.

Carrier Sense Multiple Access with Collision Detection (CSMA/CD)

Carrier sense multiple access with collision detection (CSMA/CD) is the opposite
of CSMA/CA. Instead of detecting data to transmit signal intention to prevent a collision,
it observes the cable to detect the signal before transmitting.
Collision detection means that when a collision is detected by the media access
control policy, transmitting by the network stations stops at a random length of time
before transmitting starts again.
It is faster than CSMA/CA as it functions in a network station that involves fewer
data frames being transmitted. CSMA/CD is not as efficient as CSMA/CA in preventing
network collisions. This is because it only detects huge data traffic in the network cable.
Huge data traffic increases the possibility of a collision taking place. It is used on the
Ethernet network.

Demand Priority
The demand priority is an improved version of the Carrier sense multiple access
with collision detection (CSMA/CD). This data control policy uses an ‘active hub’ in
regulating how a network is accessed. Demand priority requires that the network
terminals obtain authorization from the active hub before data can be transmitted.
Another distinct feature of this MAC control policy is that data can be transmitted
between the two network terminals at the same time without collision. In the Ethernet
media, demand priority directs that data is transmitted directly to the receiving
network terminal.

Token Passing
This media access control method uses free token passing to prevent a collision. Only a
computer that possesses a free token, which is a small data frame, is authorized to
transmit. Transmission occurs from a network terminal that has a higher priority than
one with a low priority.
Token passing flourishes in an environment where a large number of short data frames
are transmitted. This media access control policy is highly efficient in avoiding a
collision. Possession of the free token is the only key to transmitting data by a network
node. Each terminal holds this free token for a specific amount of time if the network
with the high priority does not have data to transmit, the token is passed to the
adjoining station in the network.
Media access control regulates how a network is accessed by computer terminals and
transmits from one terminal to the other without collision. This is achieved through
CSMA/CD, CSMA/CA, demand priority, or Token passing.

11. VIRTUAL LAN

Virtual Local Area Networks or Virtual LANs (VLANs) are a logical group of computers
that appear to be on the same LAN irrespective of the configuration of the underlying
physical network. Network administrators partition the networks to match the
functional requirements of the VLANs so that each VLAN comprise of a subset of ports
on a single or multiple switches or bridges. This allows computers and devices in a VLAN
to communicate in the simulated environment as if it is a separate LAN.
Features of VLANs
 A VLAN forms sub-network grouping together devices on separate physical
LANs.
 VLAN's help the network manager to segment LANs logically into different
broadcast domains.
 VLANs function at layer 2, i.e. Data Link Layer of the OSI model.
 There may be one or more network bridges or switches to form multiple,
independent VLANs.
 Using VLANs, network administrators can easily partition a single switched
network into multiple networks depending upon the functional and
security requirements of their systems.
 VLANs eliminate the requirement to run new cables or reconfiguring
physical connections in the present network infrastructure.
 VLANs help large organizations to re-partition devices aiming improved
traffic management.
 VLANs also provide better security management allowing partitioning of
devices according to their security criteria and also by ensuring a higher
degree of control connected devices.
 VLANs are more flexible than physical LANs since they are formed by logical
connections. This aids is quicker and cheaper reconfiguration of devices
when the logical partitioning needs to be changed.

Types of VLANs

 Protocol VLAN − Here, the traf ic is handled based on the protocol used. A
switch or bridge segregates, forwards or discards frames the come to it
based upon the traffics protocol.
 Port-based VLAN − This is also called static VLAN. Here, the network
administrator assigns the ports on the switch / bridge to form a virtual
network.
 Dynamic VLAN − Here, the network administrator simply de ines network
membership according to device characteristics.
12.Wireless LAN (802.11)

Wireless LANs are those Local Area Networks that use high frequency radio waves
instead of cables for connecting the devices in LAN. Users connected by WLANs can move
around within the area of network coverage. Most WLANs are based upon the standard
IEEE 802.11 or WiFi.
IEEE 802.11 Architecture
The components of an IEEE 802.11 architecture are as follows
1) Stations (STA) − Stations comprise all devices and equipments that are connected to
the wireless LAN. A station can be of two types:

 Wireless Access Pointz (WAP) − WAPs or simply access points (AP) are
generally wireless routers that form the base stations or access.
 Client. − Clients are workstations, computers, laptops, printers,
smartphones, etc.
Each station has a wireless network interface controller.
2) Basic Service Set (BSS) −A basic service set is a group of stations communicating at
physical layer level. BSS can be of two categories depending upon mode of operation:

 Infrastructure BSS − Here, the devices communicate with other devices

through access points.
 Independent BSS − Here, the devices communicate in peer-to-peer basis in
an ad hoc manner.
3) Extended Service Set (ESS) − It is a set of all connected BSS.
4) Distribution System (DS) − It connects access points in ESS.

Advantages of WLANs
 They provide clutter free homes, offices and other networked places.
 The LANs are scalable in nature, i.e. devices may be added or removed from
the network at a greater ease than wired LANs.
  The system is portable within the network coverage and access to the
network is not bounded by the length of the cables.
 Installation and setup is much easier than wired counterparts.
 The equipment and setup costs are reduced.

Disadvantages of WLANs
 Since radio waves are used for communications, the signals are noisier with
more interference from nearby systems.
 Greater care is needed for encrypting information. Also, they are more
prone to errors. So, they require greater bandwidth than the wired LANs.
 WLANs are slower than wired LANs.

13.PHYSICAL LAYER: DATA AND SIGNALS

Physical Layer
 The main functionality of the physical layer is to transmit the individual bits from one
node to another node.
 It is the lowest layer of the OSI model.
 It establishes, maintains and deactivates the physical connection.
 It speci ies the mechanical, electrical and procedural network interface speci ications.
Functions of a Physical layer:
 Line Con iguration: It de ines the way how two or more devices can be connected
physically.
 Data Transmission: It de ines the transmission mode whether it is simplex, half-
duplex or full-duplex mode between the two devices on thenetwork.
 Topology: It de ines the way how network devices are arranged.
 Signals: It determines the type of the signal used for transmitting the information.

Data-Link Layer
 This layer is responsible for the error-free transfer of data frames.
 It de ines the format of the data on the network.
 It provides a reliable and ef icient communication between two or more devices.
 It is mainly responsible for the unique identi ication of each device that
resides on a local network.
*List out the functions of the Data Link Layer(Apr/May 2021)(2)
Functions of the Data-link layer
 Physical Addressing: The Data link layer adds a header to the frame that contains a
destination address. The frame is transmitted to the destination address mentioned in the
header.
 Flow Control: Flow control is the main functionality of the Data-link layer. It is the
technique through which the constant data rate is maintained on both the sides so that
no data get corrupted. It ensures that the transmitting station such as a server with higher
processing speed does not exceed the receiving station, with lower processing speed.
Error Control: Error control is achieved by adding a calculated value CRC (Cyclic
Redundancy Check) that is placed to the Data link layer's trailer which is added to the
message frame before it is sent to the physical layer. If any error seems to occur, then the
receiver sends the acknowledgment for the retransmission of the corrupted frames.
 Access Control: When two or more devices are connected to the same communication
channel, then the data link layer protocols are used to determine which device has
control over the link at a given time.

Network Layer
 It is a layer 3 that manages device addressing, tracks the location of devices on the
network.
 It determines the best path to move data from source to the destination based on the
network conditions, the priority of service, and other factors.
 The Data link layer is responsible for routing and forwarding the packets.
 Routers are the layer 3 devices, they are speci ied in this layer and used to provide the
routing services within an internetwork.
 The protocols used to route the network traf ic are known as Network layer protocols.
Examples of protocols are IP and Ipv6.

Functions of Network Layer:

 Addressing: A Network layer adds the source and destination address to the header of
the frame. Addressing is used to identify the device on the internet.
 Routing: Routing is the major component of the network layer, and it determines the
best optimal path out of the multiple paths from source to the destination.

Transport Layer
 The Transport layer is a Layer 4 ensures that messages are transmitted in the order in
which they are sent and there is no duplication of data.
 The main responsibility of the transport layer is to transfer the data completely.
 It receives the data from the upper layer and converts them into smaller units known
assignments.
 This layer can be termed as an end-to-end layer as it provides a point-to-point
connection between source and destination to deliver the data reliably.
Functions of Transport Layer:
 Service-point addressing: Computers run several programs simultaneously due to this
reason, the transmission of data from source to the destination not only from one computer
to another computer but also from one process to another process. The transport layer adds
the header that contains the address known as a service-point address or port address. The
responsibility of the network layer is to transmit the data from one computer to another
computer and the responsibility of the transport layer is to transmit the message to the
correct process.
 Segmentation and reassembly: When the transport layer receives the message
from the upper layer, it divides the message into multiple segments, and each
segment is assigned with a sequence number that uniquely identi ies each segment.
When the message has arrived at the destination, then the transport layer
reassembles the message based on their sequence numbers.
 Connection control: Transport layer provides two services Connection-oriented
service and connectionless service. A connectionless service treats each segment as an
individual packet, and they all travel in different routes to reach the destination. A
connection-oriented service makes a connection with the transport layer at the
destination machine before delivering the packets. In connection-oriented service, all the
packets travel in the single route.
 Flow control: The transport layer also responsible for low control but it is performed
end-to- end rather than across a single link.
 Error control: The transport layer is also responsible for Error control. Error control
is performed end-to-end rather than across the single link. The sender transport layer
ensures that message reach at the destination without any error.

Session Layer
 It is a layer 3 in the OSI model.
 The Session layer is used to establish, maintain and synchronizes the interaction
between communicating devices.
Functions of Session layer:
 Dialog control: Session layer acts as a dialog controller that creates a dialog between
two processes or we can say that it allows the communication between two processes
which can be either half-duplex or full-duplex.
 Synchronization: Session layer adds some checkpoints when transmitting the data in
a sequence. If some error occurs in the middle of the transmission of data, then the
transmission will take place again from the checkpoint. This process is known as
Synchronization and recovery.
Presentation Layer
 A Presentation layer is mainly concerned with the syntax and semantics of the
information exchanged between the two systems.
 It acts as a data translator for a network.
 This layer is a part of the operating system that converts the data from one
presentation format to another format.
 The Presentation layer is also known as the syntax layer.

Functions of Presentation layer:

 Translation: The processes in two systems exchange the information in the form of
character strings, numbers and so on. Different computers use different encoding
methods, the presentation layer handles the interoperability between the different
encoding methods. It converts the data from sender-dependent format into a common
format and changes the common format into receiver-dependent format at the receiving
end.
 Encryption: Encryption is needed to maintain privacy. Encryption is a process of
converting the sender-transmitted information into another form and sends the
resulting message over the network.
 Compression: Data compression is a process of compressing the data, i.e., it reduces
the number of bits to be transmitted. Data compression is very important in multimedia
such as text, audio, video.

Application Layer
 An application layer serves as a window for users and application processes to access
network service.
 It handles issues such as network transparency, resource allocation, etc.
 An application layer is not an application, but it performs the application layer
functions.
 This layer provides the network services to the end-users.
Functions of Application layer:
 File transfer, access, and management (FTAM): An application layer allows a user to
access the iles in a remote computer, to retrieve the iles from a computer and to manage
the iles in a remote computer.
 Mail services: An application layer provides the facility for email forwarding and
storage.
 Directory services: An application provides the distributed database sources and is
used to provide that global information about various objects.
15.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.
 The main functionality of the transmission media is to carry the information in the form
of bits through LAN(Local Area Network).
 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 iber based network, the bits in the form of light pulses.

1. Guided Media:
It is also referred to as Wired or Bounded transmission media. Signals being transmitted
are directed and con ined in a narrow pathway by using physical links.
Features:
 High Speed
 Secure
 Used for comparatively shorter distances
There are 3 major types of Guided Media

(i) Twisted Pair Cable –

It consists of 2 separately insulated conductor wires wound about each other. Generally,
several such pairs are bundled together in a protective sheath. They are the most widely
used Transmission Media. Twisted Pair is of two types
1. Unshielded Twisted Pair(UTP):
This type of cable has the ability to block interference and does not depend on a
physical shield for this purpose. It is used for telephonic applications.
Advantages:
 Least expensive
 Easy to install
 High speed
 capacity
Disadvantages:
 Susceptible to external interference
 Lower capacity and performance in comparison to STP
 Short distance transmission due to attenuation
2. Shielded Twisted Pair (STP):
This type of cable consists of a special jacket to block external interference. It is used in
fast-data-rate Ethernet and in voice and data channels of telephone lines.
Advantages:
 Better performance at a higher data rate in comparison to UTP
 Eliminates crosstalk
 Comparatively faster Disadvantages:
 Comparatively dif icult to install and manufacture
 More expensive
 Bulky
(ii) Coaxial Cable –
It has an outer plastic covering containing 2 parallel conductors each having a
separate insulated protection cover. Coaxial cable transmits information in two modes:
 Baseband mode( dedicated cable bandwidth)
 Broadband mode(cable bandwidth is split into separate ranges).
 Cable TVs and analog television networks widely use Coaxial cables.
Advantages
 High Bandwidth
 Better noise Immunity
 Easy to install and expand
 Inexpensive
Disadvantages:
 Single cable failure can disrupt the entire network
(iii) Optical Fiber Cable –
It uses the concept of re lection of light through a core made up of glass or plastic.
The core is surrounded by a less dense glass or plastic covering called the cladding. It is
used for transmission of large volumes of data.
Advantages:
 Increased capacity and bandwidth
 Light weight
 Less signal attenuation
 Resistance to corrosive materials
Disadvantages:
 Dif icult to install and maintain
 High cost
 Fragile
 unidirectional, i.e., will need another iber, if we need bidirectional communication
2. Unguided Media:
It is also referred to as Wireless or Unbounded transmission media. No physical
medium is required for the transmission of electromagnetic signals.
Features:
 Signal is broadcasted through air
 Less Secure
 Used for larger distances
There are 3 major types of Unguided Media:
(i) Radiowaves –
These are easy to generate and can penetrate through buildings. The sending and
receiving antennas need not be aligned. Frequency Range:3KHz – 1GHz. AM and FM
radios
andcordless phones use Radio waves for transmission.
Further Categorized as (i) Terrestrial and (ii) Satellite.

(ii) Microwaves –
It is a line of sight transmission i.e. the sending and receiving antennas need to be
properly aligned with each other. The distance covered by the signal is directly proportional
to the height of the antenna. Frequency Range:1GHz – 300GHz. These are majorly used for
mobile phone communication and television distribution.

(iii) Infrared –
Infrared waves are used for very short distance communication. They cannot penetrate
through obstacles. This prevents interference between systems. Frequency
Range:300GHz – 400THz. It isused in TV remotes, wireless mouse, keyboard, printer, etc.

16.SWITCHING -CIRCUIT SWITCHING.

*distinguish between packet switching and circuit switching. (apr/may 2017)(2)
*describe circuit switching and packet switching with examples.(nov/dec 2019)(13)
*differentiate circuit switching and packet switching with suitable application example
(nov/dec 2021)(2)
*differentiate circuit switching networks and packet switching networks (nov/dec2020) (2)
A circuit-switched network consists of a set of switches connected by physical links. A
connection between two stations is a dedicated path made of one or more links.
However, each connection uses only one dedicated channel on each link. Each link is
normally divided into n channels by using FDM or TDM. Figure shows a trivial circuit-
switched network with four switches and four links. Each link is divided into n (n is 3 in
the figure) channels by using FDM or TDM.

Circuit switching takes place at the physical layer.

 Before starting communication, the stations must make a reservation for the resources
to be used during the communication. These resources, such as channels (bandwidth in
FDM and time slots in TDM), switch buffers, switch processing time, and switch
input/output ports, must remain dedicated during the entire duration of data transfer
until the teardown phase.
 Data transferred between the two stations are not packetized (physical layer transfer
of the signal). The data are a continuous flow sent by the source station and received by
the destination station, although there may be periods of silence.
 There is no addressing involved during data transfer. The switches route the data
based on their occupied band (FDM) or time slot (TDM). Of course, there is end-to end
addressing used during the setup phase, as we will see shortly
Three Phases:
The actual communication in a circuit-switched network requires three phases:
connection setup, data transfer, and connection teardown.
Setup Phase :
Before the two parties (or multiple parties in a conference call) can communicate, a
dedicated circuit (combination of channels in links) needs to be established. The end
systems are normally connected through dedicated lines to the switches, so connection
setup means creating dedicated channels between the switches.
Data-Transfer Phase :l
After the establishment of the dedicated circuit (channels), the two parties can transfer
data.
Teardown Phase:
When one of the parties needs to disconnect, a signal is sent to each switch to release the
resources.
Delay
Although a circuit-switched network normally has low efficiency, the delay in this type
of network is minimal. During data transfer the data are not delayed at each switch; the
resources are allocated for the duration of the connection. Figure shows the idea of delay
in a circuit-switched network when only two switches are involved.

Figure shows, there is no waiting time at each switch. The total delay is due to the time
needed to create the connection, transfer data, and disconnect the circuit. The delay
caused by the setup is the sum of four parts: the propagation time of the source computer
request (slope of the first gray box), the request signal transfer time (height of the first
gray box), the propagation time of the acknowledgment from the destination computer
(slope of the second gray box), and the signal transfer time of the acknowledgment
(height of the second gray box). The delay due to data transfer is the sum of two parts:
the propagation time (slope of the colored box) and data transfer time (height of the
colored box), which can be very long.
The third box shows the time needed to tear down the circuit. We have shown the case
in which the receiver requests disconnection, which creates the maximum delay.