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UNIT 8
Network architecture refers to the layout of the network, consisting of the hardware, software,
connectivity, communication protocols and mode of transmission, such as wired or wireless. Types of
network classified according to the areas covered such as LAN, MAN and WAN. Network topologies are
categorized according to the layout of equipments and computers such as star, loop, bus, or mesh
topologies. There are many communication protocols used in the networking technology. It is important
to know about the network architecture as networks play a very important role in today's world.
Network architecture, is the logical and structural layout of the network consisting of transmission
equipment, software and communication protocols and infrastructure (wired or wireless) transmission
of
data
and
connectivity
between
components.
A logical topology is how devices appear connected to the user. A physical topology is how they are
actually interconnected with wires and cables. For example, in a shared Ethernet network that uses hubs
rather than switches, the logical topology appears as if every node is connected to a common bus that
runs from node to node. However, its physical topology is a star, in which every node on the network
connects to a central hub.
There are 4 different network topologies: star network, a bus or line network, a loop or ring network,
and a mesh network.
8.1 Star Topology
The physical star Topology uses a central controlling hub with dedicated legs pointing in all directions –
like points of a star. Each network device has a dedicated point-to-point link to the central hub. There is
no direct link between these computers and the computers can communicate via central controller only.
This strategy prevents troublesome collisions and keeps the lines of communications open and free of
traffic.
The routing function is performed by the central controller which centrally controls communication
between any two computers by establishing a logical path between them. It means that if one computer
A wants to send data to another computer B, Computer A sends the data to the controller & this
controller then sends the data to computer B.
Advantages of Star Topology
The benefits of star topology are:
1. It is easier to add new node or modify any existing node without disturbing network i.e. expansion is
easier.
2. Addition of new node does not increase communication delay.
3. If any local computer or link fails, the entire system does not collapse. Only that link or computer is
affected.
4. It is easy to find device and cable problems i.e. fault identification and isolation is easier.
5. Media faults are automatically isolated to the failed segment.
Disadvantages of Star Topology
The disadvantages are considered as follows:
1. If the central controller or hub fails, entire system collapses.
2. Cabling cost is more as each node is connected individually to the hub.
3. Requires more cable than most topologies
4. Moderately difficult to install
A clustered star is defined as topology which consists of two or more than two cluster nodes through
which other computers are connected to form a simple star topology.
8.2 Bus Topology
In bus topologies, all computers are connected to a single cable or “trunk or backbone”, by a transceiver
either directly or by using a short drop cable. All ends of the cable must be terminated, that is plugged
into a device such as a computer or terminator. Most bus topologies use coax cables.
The number of computers on a bus network will affect network performance, since only one computer
at a time can send data; the more computers you have on the network the more computers there will
be waiting send data. A line break at any point along the trunk cable will result in total network failure.
Computers on a bus only listen for data being sent they do not move data from one computer to the
next, this is called passive topology.
Advantages of Bus Topology
The advantages of physical bus topology are:

It uses established standards and it is relatively easy to install and the use for small networks.

It requires less media than other topologies.

Failure of one node does not affect the network functioning.

Cost is less as only one main cable is required and least amount of cable is required to connect
computers.

Expansion is easier. New node can be easily added by using a connector.
Disadvantages of Bus Topology
The disadvantages of bus Topology are:

If the main central line fails the entire network collapses.

The bus networks are difficult to reconfigure, especially when the acceptable number of
connections or maximum distances have been reached.

They are also difficult to troubleshoot because everything happens on a single media segment.
This can have dangerous consequences because any break in the cabling brings the network to
its knee.

Sharing a single communication channel results in slower access time.

In this topology, higher network traffic slows down the bus speed. Only one device transmits at
a time, other devices wait for their turn. As a result there is no coordination between the
devices for reservation of transmission time slots, so data collisions are frequent.
Ring Topology
The physical ring Topology is a circular loop of point-to-point links. Each device connects
directly to the ring or indirectly through and interface device or drop cable. Message travel
around the ring from node to node in a very organized manner. Each workstation checks the message
for a matching destination address. If the address doesn’t match the node simply regenerates the
message and sends it on its way. If the address matches, the node accepts the message and sends a
reply to the originating sender.
• In ring topology, the various nodes are connected in form of a ring or circle (physical ring), in which
data flows in a circle, from one station to another station.
• It has no beginning or end that needs to be terminated.
• In this topology, each device or node has a dedicated point to point line configuration with only two
devices on either side of it.
• Signal is passed along the ring in one direction from one station to another until it reaches destination.
• Each device in ring incorporates a repeater.
• When a device receives a signal intended for another device, its repeater regenerates the bits and
passes them along.
Advantages of Ring Topology
The advantages of Ring Topology are:
1. They are very easy to troubleshoot because each device incorporates a repeater.
2. A special internal feature called beaconing allows troubled workstations to identify themselves
quickly.
3. There is no master computer on controller. Every computer has equal chance to place the data
and access the token.
4. There are no collisions.
5. Data packets travel at greater speeds.
6. It is easier to locate the problems with device and cable i.e. fault isolation is simplified. If one
device does not receive a signal within a specified time, it can issue an alarm. This alarm alerts
the network operator to the problem and its location.
Disadvantages of Ring Topology
The disadvantages of ring topologies are:
A ring network requires more cable than a bus network.
A break in cable ring brings down entire network (in case of single ring).
Adding or removing the node disturbs the network activity.
In ring network, communication delay is directly proportional to the number of nodes in the
network. Hence addition of new nodes in the network also increases communication delay.
5. It is considerably difficult to install and reconfigure ring Topology
6. Media failure on unidirectional or single loop causes complete network failure.
1.
2.
3.
4.
Mesh Topology
A Mesh topology provides each device with a point-to-point connection to every other device in the
network. These are most commonly used in WAN’s, which connect networks over telecommunication
links. Mesh topologies use routers to determine the best path. Mesh networks provide redundancy, in
the event of a link failure; meshed networks enable data to be routed through any other site connected
to the network. Because each device has a point-to-point connection to every other device, mesh
topologies are the most expensive and difficult to maintain.
In mesh topology, each node is connected to every other node in the network i.e. each node has a
dedicated point to point link to every other node as shown. Dedicated means that the link carries the
traffic only between two devices it connects.
In this way there exist multiple paths between two nodes of the network. In case of failure of one path,
the other one can be used.
Advantages of Mesh Topology
1. It is robust as the failure of one node does not collapse the entire system. If one link fails, the
entire system continues to work.
2. There is no traffic congestion problem as dedicated links are being used.
3. Dedicated links ensure faster transmission without any delay.
4. Dedicated links also ensure data privacy and security.
5. Point to point links makes fault identification and isolation easier.
Disadvantages of Mesh
1. Uses the most cabling to implement.
2. Has a high administrative overhead.
8.3 Client Server Network Model
The client-server model is a distributed communication framework of network processes among service
requestors, clients and service providers. The client-server connection is established through a network
or
the
Internet.
A client-server network has a node that functions as a server which provides resources (e.g. programs,
disk, printers) for other nodes (client computers) and manages clients’ access to the network resources.
Corporate networks are typically client-server with one or more servers that store corporate
information and employees' computers as clients.
The client-server model is a core network computing concept also building functionality for email
exchange and Web/database access. Web technologies and protocols built around the client-server
model are:
Hypertext Transfer Protocol (HTTP)
Domain Name System (DNS)
Simple Mail Transfer Protocol (SMTP)
Telnet
Clients include Web browsers, chat applications, and email software, among others. Servers include
Web, database, application, chat and email, etc.
Advantages of Client/Server Architecture
The client/server model is particularly recommended for networks requiring a high degree of reliability,
the main advantages are:
1. Centralized resources: The server is the centre of the network, it can manage resources that are
common to all users, for example: a central database would be used to avoid problems caused
by redundant and inconsistent data
2. Improved security: As the number of entry points giving access to data is not so important
3. Server level administration: As clients do not play a major role in this model, they require less
administration
4. Scalable network: It is possible to remove or add clients without affecting the operation of the
network and without the need for major modification
Disadvantages of the client/server model
Client/Server architecture also has the following drawbacks:
1. Increased cost: due to the technical complexity of the server cost of peripheral rises.
2. A weak link: Since the server is the only weak link in the client/server network but fortunately,
the server is highly fault tolerant (primarily due to the RAID system)
8.4 Peer-to-peer Network architecture model
Peer-to-peer is a communications model in which each party has the same capabilities and either
party can initiate a communication session. Other models with which it might be contrasted include
the client/server model and the master/slave model. In some cases, peer-to-peer communications is
implemented by giving each communication node both server and client capabilities. In recent usage,
peer-to-peer has come to describe applications in which users can use the Internet to exchange files
with each other directly or through a mediating server.
In contrast to client-server networks there is no dedicated server in peer-to-peer architecture. Thus
each computer in such a network is part server and part client. This means that each computer on the
network is free to share its own resources. A computer which is connected to a printer may even share
the printer so that all other computers may access it over the network.
Advantages of peer-to-peer architecture
Nonetheless, peer-to-peer architecture does have several advantages:
1. reduced cost (the costs involved in such a network are hardware, cabling and maintenance )
2. well tested simplicity
Disadvantages of peer-to-peer architecture
Peer-to-peer networks have many disadvantages:
1. this system is not centralized, making administration difficult
2. lack of security
3. no link in the network is reliable Therefore, peer-to-peer networks are only useful for a small
number of computers (generally about 10), and only suitable for applications that do not require
a high level of security (it is not advisable in a business network containing sensitive data).