1. City University
Course Title: Computer Networks
Course Code: CSE317
Topic Name: Topology
&
OSI Model
&
CSMA
Submitted By
Nishat Tasnim Ali
ID:1834902591
Batch: 49th
Submitted To
Pranab Bandhu Nath
Senior Lecturer
Department of CSE
City University

2. Topology
The way in which network links its nodes (computers or other communication
devices) together is called network topology. (P-364, computer fundamental)
There are two types of topology:
1. Physical Topology: It defines layout of the topology.
2. Logical Topology: It defines the way of data transfer. It is also known as
signal topology.
Some common topologies are:
1) Star Topology:
It has multiple nodes connected to a host node. The host node performs
routing function and centrally (Hub) controls communication between other
nodes creating a logical path.
Example: High speed LAN.
Advantages:
a) It has minimal line cost because only n-1 lines are required for
connecting n nodes.
b) Transmission delays do not increase by adding new nodes to a
network.
c) Easy install and easy fault detection.
d) If any node other than host node fails, the remaining nodes will be
unaffected.
Disadvantages:
a) The system depends on host node; so if host node fails, entire network
fails.
b) Hub requires more resources and regular maintenance.
2) Ring Topology:
Each node has two communicating subordinates but there is no master node
controlling other nodes. The only decision a node has to take is whether the
data is for its own use.

3. Example: token ring.
Advantages:
a) It works well where there is no central node for making routing
decision.
b) It is more reliable then star network because if one node fails,
alternate routing is possible.
c) Easy install.
Disadvantages:
a) Addition of new nodes increases communication delay.
b) It requires more complicated control software than star.
c) Data traffic issues as data is circulating in a ring.
d) A link failure can fail the entire network.
3) Bus Topology:
There is only a single transmission line for all nodes. That cable is also known
as backbone cable. The bus is connected to various taps and droplines. Taps
are the connectors while droplines are the cable that connects the backbone
cable with the computers.
Example: Ethernet cable.
Advantages:
a) Less cable is required.
b) Cost efficient.
c) Simple to install.
d) If a node fails, the other nodes will be unaffected.
Disadvantages:
a) If the bus fails, the entire network will fail.
b) A limited number of nodes can be connected due to bus length.
c) Less security and risks cause data are sent to all the nodes.
d) If more nodes added the strength of signal decreases.
e) Congestion and traffic.

4. 4) Tree Topology:
All the nodes are directly or indirectly connected to the main hub. The sub-
hubs are connected to each other. It is a combination of star and bus network.
Advantages:
a) It can cover large distance network.
b) Less or no data loss.
c) A large numbers of node can be connected.
d) If one of the node fails, others aren’t affected.
e) Fault detection is easy.
Disadvantages:
a) Cabling and hardware cost a lot.
b) Complex implementation.
c) Hard to manage a large tree network.
d) If main bus fails, the entire network will fail.
5) Mesh Topology:
Direct connection takes place between the nodes. If there are ‘n’ devices in
network then each device has ‘(n-1)’ input-output ports and links.
Example: the Internet (WAN).
Advantages:
a) No traffic issues.
b) Very fast communication.
c) Maintain privacy and security.
d) Facilitate direct communication.
e) If a node fails, others won’t be affected.
Disadvantages:
a) Costly.
b) Complex implementation.
c) Maintenance are very difficult.
d) High cabling requires.

5. 6) Hybrid Topology:
It is a combination of two or more topologies. Practically, they are the widely
used. All topologies are interconnected according to the need to form hybrid.
Advantages:
a) It can handles lot of nodes.
b) It has flexibility to modify the network according to our need.
c) Very reliable.
Disadvantages:
a) Expensive
b) Complex design.
c) Multi station access unit is required.
OSI Model: (Open System Interconnection Model)
The OSI was published in 1984 by International Organization for Standardization
(ISO). The OSI model is a layered framework for the design of network systems
that allows communication between all types of computer systems. It consists of
seven separate but related layers, each of which defines a part of the process of
moving information across a network.
1. Physical Layer:
It is the lowest layer of the OSI model. It electrically or optically
transmitting raw unstructured data bits across the network from physical
layer sending device to receiving device. At physical we might find
resources like hub, cabling or modems.
2. Data Link Layer:
Here, nodes are used to perform node to node data transfer where data is
packaged into frames. It also corrects error from the physical layer.
It has two sub-layers: media access control (MAC) and logical link control
(LLC).
3. Network Layer:

6. It is responsible for receiving frames from the data link layer and deliver
them to the destined addresses inside the frame. It uses IP address to find
destination. Routers are used at this layer.
4. Transport Layer:
It manages delivery and check errors of data packet. It also regulate size,
sequencing and transfer of data between systems and hosts. TCP is the
common example of this layer.
5. Session Layer:
Connection between machines is set up, managed and terminated at this
layer. It also provides authentication and reconnections.
6. Presentation Layer:
This layer formats or translate data for the application layer. It is also called
syntax layer. It can handle the encryption and decryption required by
application layer.
7. Application Layer:
This layer identifies resource availability, synchronizes communication and
communication partners. At this layer both end-user and application layer
interact directly with software application.
CSMA
CSMA is a mechanism that senses the state of the shared channel to prevent or
recover data packets from a collision. It is divided into two parts:
1. CSMA/CD (Collision Detection): It is used to detect a collision in MAC
layer.
Advantages:
a) It is used for collision detection on a shared channel in a short time.
b) It is used to avoid waste of transmission.
c) It share same amount of bandwidth at each station.
Disadvantages:
a) It is not suitable for long distance networks.
b) Beyond the range it cannot detect collisions.
c) If multiple device are added detection performance is reduced.

7. 2. CSMA/CA (Collision Avoidance): It is a network protocol that uses to
avoid a collision rather allowing it to occur and doesn’t deal with the
recovery of packets after collision.
Advantages:
a) If data packets is large the chances of collision is less.
b) It controls data packet and sends when receiver wants to send.
c) It is used to prevent collision than detection.
d) Avoids wasted transmission of data over channel.
e) It is suited for wireless transmission in a network.
f) Avoids unnecessary data traffic.
Disadvantages:
a) It consumes more bandwidth by each station.
b) It is less efficient than CSMA/CD.
c) It takes much waiting time to transmit data packets.
Difference between CSMA/CD & CSMA/CA
CSMA/CD CSMA/CA
1. It is the collision detection protocol. 1. It is the collision avoidance
protocol.
2. It is used in 802.3 Ethernet network
cable.
2. It is used in 802.11 Ethernet
network cable.
3. It works in wired network. 3. It works in wireless network.
4. It minimizes the recovery time. 4. It minimizes the risk of collision.
5. When data packets conflicts it
resends the data frame.
5. It waits until the channel is busy and
doesn’t recover after collision.
6. The efficiency is high. 6. The efficiency is less than
CSMA/CD.
References:
[1] https://www.forcepoint.com/cyber-edu/osi-
model#:~:text=The%20OSI%20Model%20(Open%20Systems,between%20diffe
rent%20products%20and%20software.
[2] https://www.javatpoint.com/osi-model

8. [3] Data Communications and Networking Information Center:
(mheducation.com)
[4] [PDF] Computer Fundamentals By P. K. Sinha Free Download –
Learnengineering.in
[5] https://www.geeksforgeeks.org/layers-of-osi-model/
[6] https://www.imperva.com/learn/application-security/osi-model/
[7] https://www.javatpoint.com/csma-ca-vs-csma-
cd#:~:text=CSMA%20is%20a%20mechanism%20that,and%20data%20integrity
%20is%20maintained