1. NETWORK TOPOLOGIES
Network topology: Topology refers to the way in which the network of computers is connected.
Each topology is suited to specific tasks and has its own advantages and disadvantages.
OR
Topology is the term used to describe how devices are connected and how messages flow from
device
to device.
There are two types of network topologies they are:
- Physical network topologies: The physical topology describes the physical way the network is
wired.
- Logical network topologies: the logical topology describes the way in which messages are
sent.
In fact, system where computers are connected to each other so that we can transfer data and
information between various users is called computer networking system. A networks topology
affects its capabilities. Choosing one topology over another can impact the:
-Type of equipment needed
-Capabilities of the equipment
-Network’s growth
-Way a network is managed.
In addition to topology, these arrangements may be referred to as:
-Physical layout
-Design
-Diagram
-Map
A networks topology implies a number of conditions. For example, a particular topology can
determine not only the type of cable used but how the cabling is run through ceilings, and walls
Topology can also determine how computers communicate on the network. Different topologies
require different communication methods, and these methods have a great influence on the
network
Standard Topologies
All network designs stem from three basic topologies:
-Bus
-Star
-Ring
While these three basic topologies are themselves simple, their real-world versions often
combine features from more than one topology and can be complex.

2. TYPES OFNETWORKTOPOLOGIES
The study of network topology recognizes eight basic topologies.
 Point-to-point
 Bus
 Star
 Ring or circular
 Mesh
 Tree
 Hybrid
 Daisy chain
1. BUS TOPOLOGY
 all workstations connect to the same cable segment
 commonly used for implementing Ethernet at 10mbps
 the cable is terminated at each end
 wiring is normally done point to point
 a faulty cable or workstation will take the entire LAN down
 two wire, generally implemented using coaxial cable during the 1980's
 two wire, generally implemented using coaxial cable during the 1980's
The bus cable carries the transmitted message along the cable. As the message arrives at each
workstation, the workstation computer checks the destination address contained in the message
to see if it matches its own. If the address does not match, the workstation does nothing more.
If the workstation address matches that contained in the message, the workstation processes
the message. The message is transmitted along the cable and is visible to all computers
connected to that cable.
Fig: Bus Topology
The common link can be thought of as the backbone to the network. All devices typically
connect to the backbone with a T-connector and coax cable.

3. Fig: Bus Topologies
There are THREE common wiring implementations for bus networks
 10Base2 (thin-net, Cheaper Net) 50-ohm cable using BNC T connectors, cards provide
transceiver
 10Base5 (Thick Net) 50-ohm cable using 15-pin AUI D-type connectors and external
transceivers
 10BaseT (UTP) UTP cable using RJ45 connectors and a wiring centre
Advantages:
 Easy to implement and extend.
 Easy to install.
 Well-suited for temporary or small networks not requiring high speeds (quick setup),
resulting in faster networks.
 Cheaper than other topologies (But in recent years has became less important due
devices like a switch)
 Cost effective; only a single cable is used.

4.  Easy identification of cable faults.
 Reduced weight due to fewer wires.
Disadvantages:
 Limited cable length and number of stations.
 If there is a problem with the cable, the entire network breaks down.
 Maintenance costs may be higher in the long run.
 Performance degrades as additional computers are added or on heavy traffic (shared
bandwidth).
 Proper termination is required (loop must be in closed path).
 Significant Capacitive Load (each bus transaction must be able to stretch to most distant
link).
 It works best with limited number of nodes.
 Commonly has a slower data transfer rate than other topologies.
 Only one packet can remain on the bus during one clock pulse.
2. STAR TOPOLOGY
All the cables run from the computers to a central location, where they are all connected by a
device called a hub. Each computer on a star network communicates with a central hub that
resends the message either to all the computers or only to the destination computers. Hub can
be active or passive in the star network Active hub regenerates the electrical signal and sends it
to all the computers connected to it. Passive hub does not amplify or regenerate signal and
does not require electrical power to run. We can expand a star network by placing another star
hub.
Fig: Star Topology
Devices typically connect to the hub with Unshielded Twisted Pair (UTP) Ethernet.
Failure in the connection between the top level node and any
Subordinate node or failure in a subordinate node will not disrupt the entire network. Because
Star Network Topologies are commonly used in LANs spanning a larger geometric area than

5. Bus or Ring Network Topologies.
Fig: Star Topology
Advantages:
 Easy to add new workstations
 Centralized control
 Centralized network/hub monitoring
 Easy to detect faults and to remove parts.
 No disruptions to the network then connecting or removing devices.
Disadvantages:
 Hub failure cripples all workstations
connected to that hub.
 Hubs are slightly more expensive than thin-Ethernet
 Requires more cable length than a linear topology.
 More expensive than linear bus topologies because of the cost of the concentrators.
 Any failure in this device will halt any communication on the network. One additional
Limitation of the Star Network Topology concerns the limited number of top level
node connection points.
3. RING TOPOLOGY
Each computer is connected to the next computer ,with the last one connected to the first. Every
computer is connected to the next computer in the ring, and each retransmits what it receives
from the previous computer. The message flow around the ring in one direction. Some ring
networks do token passing. It passes around the ring until a computer wishes to send
information to another computer. The computer adds an electronic address and data and sends
it around the ring. Each computer in sequence receives the token and the information and
passes them to the next until either the electronic address matches the address of the computer
or the token returns to the origin. The receiving computer returns a message to the originator
indicating that the message has been received. The sending computer then creates another
token and place it on the network, allowing another station to capture the token and being
transmitted.

6. Fig: Ring Topologies
Use: This topology is useful in LANs. It does not rely on central host. This network can still
function if one of the computers malfunctions.
Problems: The connecting wire, cable, or optical fiber forms a closed loop. Data are passed
along the ring from one computer to another.
Advantages:
• The structure of this network is very simple and it is very easy to insert and delete any
number of nodes.
• For this reason, ring network is easily scalable and highly modular.
• Fault isolation and recovery are much simpler than tree.
• The most important advantage of this network is its point-to-point connections.
• The electronics and maintenance of point-to-point links are much simpler than multi cast
links.
Disadvantages:
• The disadvantage is it offers only two alternate routes and thereby it can manage only a
single fault.
• Another important drawback of ring network is its large diameter.
• If diameter of a network is high, the number of edges involved in each communication
will also be very high resulting in high signal attenuation and ntwork blocking probability.
• Much slower than an Ethernet network under normal load
• Network adapter cards and MAU's are much more expensive than Ethernet cards and
hubs

7. 4. FDDI TOPOLOGY
Fiber Distributed Data Interface
FDDI is based on two counter rotating 100-Mbit/sec token-passing rings. The rings
consist of point to point wiring between nodes which repeat the data as it is received.
The primary ring is used for data transmission; the secondary is used for data
transmission or to back up the primary ring in the event of a link or station failure. FDDI
supports a sustained transfer rate of about 80Mbps, a maximum of 1000 connections
(500 nodes) and a total distance of 200 kilometers end to end. There is a maximum
distance of 2 kilometers between active nodes.
FDDI Station Types
There are two main types of stations, class A which attach directly to dual rings; or class
B which attach to a station acting as a concentrator.
A concentrator is a specialized workstation that attaches to the ring and has multiple
ports that allow attachment of other devices in a physical star configuration. These may
be cascaded.
Fig: FDDI Topology
Advantages:
 FDDI supports real-time allocation of network bandwidth.
 This allows you to use a wide array of different types of traffic.

8.  FDDI has a dual ring that is fault-tolerant. The benefit here is that if a station on the ring
fails or if the cable becomes damaged, the dual ring is automatically doubled back onto
itself into a single ring.
 The FDDI compensates for wiring failures. The stations wrap within themselves when
the wiring fails.
 Optical bypass switches are used that can help prevent ring segmentation. The faild
stations are eliminated from the ring.
Disadvantages:
 There's a potential for multiple ring failures.
 As the network grows, this possibility grows larger and larger.
 The uses of fiber optic cables are expensive.
 This has kept many companies from deploying FDDI in a widespread manner. Instead,
they have been using copper wire and the similar method of CDDI.
5. HYBRID TOPOLOGY
Hybrid networks use a combination of any two or more topologies, in such a way that the
resulting network does not exhibit one of the standard topologies (e.g., bus, star, ring, etc.). For
example a tree network connected to a tree network is still a tree network topology. A hybrid
topology is always produced when two different basic network topologies are connected. Two
common examples for Hybrid network are: star ring network and star bus network
 A Star ring network consists of two or more star topologies connected using a
multistation access unit (MAU) as a centralized hub.
 A Star Bus network consists of two or more star topologies connected using a bus trunk
(the bus trunk serves as the network's backbone).
While grid and torus networks have found popularity in high-performance computing
applications, some systems have used genetic algorithms to design custom networks that have
the fewest possible hops in between different nodes. Some of the resulting layouts are nearly
incomprehensible, although they function quite well
A Snowflake topology is really a "Star of Stars" network, so it exhibits characteristics of a hybrid
network topology but is not composed of two different basic network topologies being
connected.

9. Fig: Hybrid Topology
Advantages:
 Reliable: Unlike other networks, fault detection and troubleshooting is easy in this type
of topology. The part in which fault is detected can be isolated from the rest of network
and required corrective measures can be taken, WITHOUT affecting the functioning of
rest of the network.
 Scalable: It’s easy to increase the size of network by adding new components, without
disturbing existing architecture.
 Flexible: Hybrid Network can be designed according to the requirements of the
organization and by optimizing the available resources. Special care can be given to
nodes where traffic is high as well as where chances of fault are high.
 Effective: Hybrid topology is the combination of two or more topologies, so we can
design it in such a way that strengths of constituent topologies are maximized while there
weaknesses are neutralized. For example we saw Ring Topology has good data reliability
(achieved by use of tokens) and Star topology has high tolerance capability (as each node
is not directly connected to other but through central device), so these two can be used
effectively in hybrid star-ring topology.
Disadvantages:
 Complexity of Design: One of the biggest drawbacks of hybrid topology is its design.
It’s not easy to design this type of architecture and it’s a tough job for designers.
Configuration and installation process needs to be very efficient.
 Costly Hub: The hubs used to connect two distinct networks, are very expensive. These
hubs are different from usual hubs as they need to be intelligent enough to work with
different architectures and should be function even if a part of network is down.
 Costly Infrastructure: As hybrid architectures are usually larger in scale, they require a
lot of cables, cooling systems, sophisticate network devices, etc.