Computer Networks

Izaz A Khan

BooksCourse Book: Data Communication and Networking 4th edition by Behrouz A Forouzan.

Reference books: •James F. Kurose and Keith W. Ross, “Computer Networking – A Top-Down Approach Featuring the Internet”, Addison Wesley.•William Stallings, "Data and Computer Communications", Prentice-Hall — Sixth Edition (for those who want to review basics of networking). •Andrew S Tanenbaum “ Computer Networks”.

Marks Distribution

Mid term = 20%

Quizzes, Presentations and Assignments = 15%

Attendance and class participation = 5%

Final exam = 60%

Introduction to Computer Networks

Chapter 1

COMPUTER Networks

• Two or more computers connected together through a communication media form a computer network.

• The computers are connected in a network to exchange information and data.

• The computers connected in a network can also use resources of other computers.

Computer Network COMPONENTS1. Server: Powerful computers that provides services to the

other computers on the network.

2. Client: Computer that uses the services that a server provides. The client is less powerful than server.

3. Media: A physical connection between the devices on a network.

4. Network Adopter: Network adopter or network interface card (NIC) is a circuit board with the components necessary for sending and receiving data. It is plugged into one of the available slots on the Pc and transmission cable is attached to the connector on the NIC.

Continued5. Resources: Any thing available to a client on the network is considered a resource .Printers, data, fax devices and other network devices and information are resources.

6. User: Any person that uses a client to access resources on the network.

7. Protocols: These are written rules used for communications. They are the languages that computers use to talk to each other on a network.

Advantages of Computer Network• Networks allow data transmission among far areas also within

local areas.

• Network allows users to share common set of data files and software stored in a main system.

• Network allows users to share common hardware resources such as printers, fax machines, modem etc.

• The cost of computing is reduced to each user as compared to the development and maintain of each single computer system.

Network Criteria

1. Performance2. Reliability3. Security

Classification of Computer NetworksA. Classification by distances.B. Classification by topologies.C. Classification by switching.D. Classification by computing models.

A- Classification By distances

• LAN• MAN• WAN

B- CLASSIFICATION BY TOPOLOGIES

1- MESH TOPOLOGY

Mesh Topology To find the number of the number of duplex-mode links in a mesh topology we use

n(n-1)/2 where “n” is the number of nodes (PCs, printers, scanners etc).

Mesh Topology

Advantages:1.No high traffic problem, as each connection can carry its own data load.

2.Mesh topology is robust (means if one link is down it doesn’t effect the entire system).

3.Mesh topology has the advantage of privacy and security.

Disadvantages:1.Large amount of cabling and ports.

2.Difficult installation and reconnection.

3.Expensive.

2- STAR TOPOLOGY

Star Topology

The number of links in star topology is = n where “n” is the number of nodes.

Star Topology

Advantages:1.Less expensive than mesh topology.

2.Easy to install and configure.

3.Star topology is robust (in the case of a link failure).

Disadvantages:1.Dependency of the whole topology on one single point, the hub.

2.Although star topology requires less cabling than mesh topology, still it requires more cabling than some other topologies like bus or ring.

3- BUS TOPOLOGY

Bus Topology

The number of links in bus topology is = n where “n” is the number of nodes.

Bus Topology

Advantages:1.Ease of installation.

2.Less cabling.

3.Cheaper.

Disadvantages:1.Difficult reconnection and fault isolation.

2.A fault or break in the bus cable stops all transmission beyond the point where the fault occurs.

4- RING TOPOLOGY

Ring Topology

The number of links in star topology is = n-1 where “n” is the number of nodes.

Ring TopologyAdvantages:1.Relatively less expensive topology.

2.Easy to install and configure.

3.Simple fault isolation (Generally in a ring, a signal is circulating all the time, if one device does not receive the signal within a specific time it can issue an alarm).

Disadvantages:1.Unidirectional traffic can be a disadvantage.

2.In a simple ring, a break in the ring can disable the entire network.

c- CLASSIFICATION BY Switching

1. Circuit Switching.2. Packet Switching.

1- Packet Switching• In packet-based networks, the message gets broken

into small data packets. • These packets are sent out from the computer and

they travel around the network seeking out the most efficient route to travel as circuits become available.

• Each packet is sent with a ‘header address’ which tells it where its final destination is, so it knows where to go.

Example of packet switching• You switch on your PC and connect to your favorite

site that offers a number of applications you can download from, so you begin downloading one application at a time. Each packet has to find its own route to the destination, i.e., your computer. Each packet finds its way using the information it carries, such as the source and destination IP addresses. If network congestion occurs, the routers routers responsible for routing packets between networks will automatically select different paths to ensure data is transferred as required. This is an example of a packet-switched network.

Packet SwitchingAdvantages:

• Security• Bandwidth used up to some extent but not

the full bandwidth. • Devices of different speeds can

communicate• Not affected by line failure (redirects signal)• Availability – no waiting for a direct

connection to become available• During a crisis or disaster, when the public

telephone network might stop working, e-mails and texts can still be sent via packet switching

Packet Switching

Disadvantages:• Under heavy use there can be a delay• Data packets can get lost or become

corrupted• Protocols are needed for a reliable

transfer• Not so good for some types data streams

(e.g. real-time video streams can lose frames due to the way packets arrive out of sequence)

2- Circuit Switching• Circuit switching was designed in 1878 in order

to send telephone calls down a dedicated channel.

• This channel remains open and in use throughout the whole call and cannot be used by any other data or phone calls.

Circuit Switching• In modern circuit-switched networks, electronic

signals pass through several switches before a connection is established.

• During a call no other network traffic can use those switches.

• The resources remain dedicated to the circuit during the entire data transfer and the entire message follows the same path.

• Circuit switching can be analog or digital.

Example of Circuit switching• You pick up your land phone and dial your friend. At

that point, the telco provider creates a dedicated circuit for that session and connects you to your friend's telephone. No matter how long you keep the line open with your friend, the circuit will remain, and packets flowing between both telephones will always follow the same path.

Circuit Switching

Advantages• Circuit is dedicated to the call – no

interference, no sharing• Guaranteed the full bandwidth for the

duration of the call• Guaranteed quality of service

Circuit SwitchingDisadvantages

• It takes a relatively long time to set up the circuit.

• During a crisis or disaster, the network may become unstable or unavailable.

• It was primarily developed for voice traffic rather than data traffic.

Packet Vs Circuit SwitchingThe battle between circuit and packet technologies has been around a long time, and it is starting to be like the old story of the tortoise and the hare.

• In this case, the hare is circuit switching—fast, reliable and smart. The hare starts out fast and keeps a steady pace, while the tortoise starts slow but manages to double his speed every 100 meters.

• If the race is longer than 2 km (1.2 miles), the power of compounding favours the tortoise (packet switching).

D- Classification by computing models1. Centralized Network Computing Model.2. Distributed Network Computing Model.

1- Centralized Network Computing Models• A centralized network is a type of network where all the users

connect to a central server that is the acting agent for all communications. The server stores both the user account and communication information.

• In the centralized network computing model, the clients use the resources of high-capacity servers to process information. In this model, the clients are also referred to as dumb terminals with very low or no processing capability. The clients only connect to the server and not to each other.

• The figure in the slide shows the centralized network computing model:

Centralized Network Computing Models

Centralized Network Computing Models

Advantages:•Centralized data management: In a centralized network computing model, data is stored on the server. This increases the reliability of data because all data modifications are stored at a central location.

•High level of security: The centralized network computing model is a highly secure network model. This is because network security can be implemented and monitored centrally from the server.

•Cost effectiveness: High-end investment is required for establishing a high-capacity and secure server. On the other hand, clients require very low investment. This reduces the overall cost of setting up a centralized network.

Centralized Network Computing Models

Disadvantages:• Low performance and network speed: The centralized

network computing model consists of a server that manages numerous requests, simultaneously. This increases network traffic, consequently reducing the speed and performance of the network.

• Central point of failure: The server is the central place for storing data and processing all client requests. If the server fails, the functioning of the entire network is disrupted.

2- Distributed Network Computing Models• Distributed computer networks consist of clients and servers

connected in such a way that any system can potentially communicate with any other system.

• The platform for distributed systems has been the network linking departments, branches, and divisions of an organization.

• Data is not located in one server, but in many servers. We can have servers for different tasks like printing etc.

• The figure in the slide shows the distributed network computing model:

Distributed Network Computing Models

Centralized vs Distributed Networks

Distributed Network Computing ModelsAdvantages:•Faster data access: The distributed network model allows a node to store the information locally. As a result, data can be accessed faster than in the centralized network model.

•High reliability: In the distributed network model, no single point of failure exists because the network does not entirely depend on a single node. This ensures lower network downtime.

Distributed Network Computing ModelsDisadvantages:•Synchronization: When services run on a single server (centralized server), there is no worry about data synchronization: all the data is simply present on that machine. However, this becomes an issue in distributed network models.

•Expensive: Distributed network models are expensive compare to centralized network models in the case when we are having many different servers for different tasks in distributed network models.