Networking written by Santosh Chauhan

         Unit 1 - Physical layer

         Computer Network:-

A computer network is a group of two or more interconnected computer systems. You can establish a network connection using either cable or wireless media.

Every network involves hardware and software that connects computers and tools.

Goals and Applications of networks:- 

Network goals:- The main goals of network as follows.

Resource sharing:- 

This is the main aim of a computer network. It means to make all programs peripherals and data available to any one computer on the network to all other computers in the network without regard to the physical locations of them. Thus user at large distances can share the resources or can see data of a computer in the same way that a local user uses them. Another aspect of resource sharing is load sharing. That is if required, a job can be 

performed using various computers in network by portioning it which reduces time consumption and load both for a particular computer. 

High reliability:- 

A second goal is to provide high reliability by having alternative sources of supply. For example, all files could be replicated on two or three machines, so if one of them is Unavailable, the other copies could be available.

Cost Reduction:-

Another goal of networking is reduction of cost. Resource sharing 

automatically reduces cost and hence money can be saved. 

Improve Performance:- 

Another closely related goal is to increase the systems 

performance. The performance of a computer can be improved by adding one or more processors 

to it as the work load on it grows. For example if the system is full instead of replacing it buy a 

larger one at large expansive it is better to add more processors to it on less cost and less disruption to the user.

 

Communication Medium:-

Computer networks provide a powerful communication 

medium. A file that was updated/modified on a network can be seen by the other users on the network immediately. 

Network application:- 

Some of the network applications in different fields are the following. 

1. Marketing and sales

2. Access to remote programs.

3. Access to remote databases.

4. Value-added communication facilities

Types of Computer Networks:-

A computer network is a cluster of computers over a shared communication path that works for the purpose of sharing resources from one computer to another, provided by or located on the network nodes.

Some of the uses of computer networks are the following:

Communicating using email, video, instant messaging, etc.

Sharing devices such as printers, scanners, etc.

Sharing files

Sharing software and operating programs on remote systems

Allowing network users to easily access and maintain information

Types of Computer Networks

Personal Area Network (PAN)

Local Area Network (LAN)

Wide Area Network (WAN)

Wireless Local Area Network (WLAN)

Campus Area Network (CAN)

Metropolitan Area Network (MAN)

Storage Area Network (SAN)

System-Area Network (SAN)

Passive Optical Local Area Network (POLAN)

Enterprise Private Network (EPN)

Virtual Private Network (VPN)

Home Area Network (HAN)

These are explained as following below.

1. Personal Area Network (PAN) :

 Pan is the most basic type of computer network. This network is restrained to a single person, that is, communication between the computer devices is centred only to an individual’s work space. PAN offers a network range of 10 meters from a person to the device providing communication.

Examples of PAN are USB, computer, phone, tablet, printer, PDA, etc.

2. Local Area Network (LAN) : -

LAN is the most frequently used network. A LAN is a computer network that connects computers together through a common communication path, contained within a limited area, that is, locally. A LAN encompasses two or more computers connected over a server. The two important technologies involved in this network are Ethernet and Wi-fi. 

Examples of LAN are networking in a home, school, library, laboratory, college, office, etc.

3. Wide Area Network (WAN) :-

WAN is a type of computer network that connects computers over a large geographical distance through a shared communication path. It is not restrained to a single location but extends over many locations. WAN can also be defined as a group of local area networks that communicate with each other. 

4. Wireless Local Area Network (WLAN) : -

WLAN is a type of computer network that acts as a local area network but makes use of wireless network technology like Wi-Fi. This network doesn’t allow devices to communicate over physical cables like in LAN but allows devices to communicate wirelessly. 

The most common example of WLAN is Wi-Fi.

5. Campus Area Network (CAN) :-

CAN is bigger than a LAN but smaller than a MAN. This is a type of computer network which is usually used in places like a school or college. This network covers a limited geographical area that is, it spreads across several buildings within the campus.

Examples of CAN are networks that cover schools, colleges, buildings, etc.

6. Metropolitan Area Network (MAN) : 

A MAN is larger than a LAN but smaller than a WAN. This is the type of computer network that connects computers over a geographical distance through a shared communication path over a city, town or metropolitan area. 

               Internet.                  

Internet is a global network that connects billions of computers across the world with each other and to the World Wide Web. It uses standard internet protocol suite (TCP/IP) to connect billions of computer users worldwide. It is set up by using cables such as optical fibers and other wireless and networking technologies. At present, internet is the fastest mean of sending or exchanging information and data between computers across the worlIt is believed that the internet was developed by "Defense Advanced Projects Agency" (DARPA) department of the United States. And, it was first connected in 1969.

Why is the Internet Called a Network?

Internet is called a network as it creates a network by connecting computers and servers across the world using routers, switches and telephone lines, and other communication devices and channels. So, it can be considered a global network of physical cables such as copper telephone wires, fiber optic cables, tv cables, etc. Furthermore, even wireless connections like 3G, 4G, or Wi-Fi make use of these cables to access the Internet.

Internet is different from the World Wide Web as the World Wide Web is a network of computers and servers created by connecting them through the internet. So, the internet is the backbone of the web as it provides the technic . The internet is not owned by a single person or organization entirely. It is a concept based on physical infrastructure that connects networks with other networks to create a global network of billions of computers. As of 12 August 2016, there were more than 300 crores of internet users across the world.

Advantages of the Internet:

Instant Messaging: You can send messages or communicate to anyone using internet, such as email, voice chat, video conferencing, etc.

Get directions: Using GPS technology, you can get directions to almost every place in a city, country, etc. You can find restaurants, malls, or any other service near your location.

Online Shopping: It allows you to shop online such as you can be clothes, shoes, book movie tickets, railway tickets, flight tickets, and more.

Pay Bills: You can pay your bills online, such as electricity bills, gas bills, college fees, etc.

Online Banking: It allows you to use internet banking in which you can check your balance, receive or transfer money, get a statement, request cheque-book, etc.

Online Selling: You can sell your products or services online. It helps you reach more customers and thus increases your sales and profit.

Work from Home: In case you need to work from home, you can do it using a system with internet access. Today, many companies allow their employees to work from home.

Entertainment: You can listen to online music, watch videos or movies, play online games.

Cloud computing: It enables you to connect your computers and internet-enabled devices to cloud services such as cloud storage, cloud computing, etc.

Career building: You can search for jobs online on different job portals and send you CV through email if required  

ISP - Internet Service                   Provider.

An Internet service provider (ISP) is a company that provides customers with Internet access. It is often referred to as just “the provider.” Data may be transmitted using several technologies, including dial-up, DSL, cable modem, wireless or dedicated high-speed interconnects.

Typically, ISPs also provide their customers with the ability to communicate with one another by providing Internet email accounts, usually with numerous email addresses at the customer’s discretion.

Other services, such as telephone and television services, or personal websites or home pages may be provided as well. The services and service combinations may be unique to each ISP.

Today, ISPs are usually cable companies or mobile phone companies that offer Internet subscriptions in addition to TV or mobile communications services.

An Internet service provider is also known as an Internet access provider (IAP).

             TCP/IP Model.           

Prerequisite – Layers of OSI Model

The OSI Model we just looked at is just a reference/logical model. It was designed to describe the functions of the communication system by dividing the communication procedure into smaller and simpler components. But when we talk about the TCP/IP model, it was designed and developed by Department of Defense (DoD) in 1960s and is based on standard protocols. It stands for Transmission Control Protocol/Internet Protocol. The TCP/IP model is a concise version of the OSI model. It contains four layers, unlike seven layers in the OSI model. The layers are:

Process/Application Layer

Host-to-Host/Transport Layer

Internet Layer

Network Access/Link Layer

The diagrammatic comparison of the TCP/IP and OSI model is as follows :

Difference between TCP/IP and OSI Model:TCP/IP OSI:-

TCP refers to Transmission Control Protocol. OSI refers to Open Systems Interconnection.

TCP/IP has 4 layers. OSI has 7 layers.

TCP/IP is more OSI is less reliable

TCP/IP does not have very strict boundaries. OSI has strict boundaries

TCP/IP follow a horizontal approach. OSI follows a vertical approach.

TCP/IP uses both session and presentation layer in the application layer itself. OSI uses different session and presentation layers.

TCP/IP developed protocols then model. OSI developed model then protocol.

Transport layer in TCP/IP does not provide assurance delivery of packets. In OSI model, transport layer provides assurance delivery of packets.

TCP/IP model network layer only provides connection less services. Connection less and connection oriented both services are provided by network layer in OSI model.

Protocols cannot be replaced easily in TCP/IP model. While in OSI model, Protocols are better covered and is easy to replace with the change in technology.

The first layer is the Process layer on the behalf of the sender and Network Access layer on the behalf of the receiver. During this article, we will be talking on the behalf of the receiver.

1. Network Access Layer –

This layer corresponds to the combination of Data Link Layer and Physical Layer of the OSI model. It looks out for hardware addressing and the protocols present in this layer allows for the physical transmission of data.We just talked about ARP being a protocol of Internet layer, but there is a conflict about declaring it as a protocol of Internet Layer or Network access layer. It is described as residing in layer 3, being encapsulated by layer 2 protocols. 

2. Internet Layer –

This layer parallels the functions of OSI’s Network layer. It defines the protocols which are responsible for logical transmission of data over the entire network. The main protocols residing at this layer are :

IP – stands for Internet Protocol and it is responsible for delivering packets from the source host to the destination host by looking at the IP addresses in the packet headers. IP has 2 versions:

IPv4 and IPv6 - IPv4 is the one that most of the websites are using currently. But IPv6 is growing as the number of IPv4 addresses are limited in number when compared to the number of users.

ICMP – stands for Internet Control Message Protocol. It is encapsulated within IP datagrams and is responsible for providing hosts with information about network problems.

ARP – stands for Address Resolution Protocol. Its job is to find the hardware address of a host from a known IP address. ARP has several types: Reverse ARP, Proxy ARP, Gratuitous ARP and Inverse ARP.

Host-to-Host Layer –

This layer is analogous to the transport layer of the OSI model. It is responsible for end-to-end communication and error-free delivery of data. It shields the upper-layer applications from the complexities of data. The two main protocols present in this layer are : 

Transmission Control Protocol (TCP) – It is known to provide reliable and error-free communication between end systems. It performs sequencing and segmentation of data. It also has acknowledgment feature and controls the flow of the data through flow control mechanism. It is a very effective protocol but has a lot of overhead due to such features. Increased overhead leads to increased cost.

User Datagram Protocol (UDP) – On the other hand does not provide any such features. It is the go-to protocol if your application does not require reliable transport as it is very cost-effective. Unlike TCP, which is connection-oriented protocol, UDP is connectionless.

4. Application Layer –

This layer performs the functions of top three layers of the OSI model: Application, Presentation and Session Layer. It is responsible for node-to-node communication and controls user-interface specifications. Some of the protocols present in this layer are: HTTP, HTTPS, FTP, TFTP, Telnet, SSH, SMTP, SNMP, NTP, DNS, DHCP, NFS, X Window, LPD. Have a look at Protocols in Application Layer for some information about these protocols. Protocols other than those present in the linked article are :

HTTP and HTTPS – HTTP stands for Hypertext transfer protocol. It is used by the World Wide Web to manage communications between web browsers and servers. HTTPS stands for HTTP-Secure. It is a combination of HTTP with SSL(Secure Socket Layer). It is efficient in cases where the browser need to fill out forms, sign in, authenticate and carry out bank transactions.

SSH – SSH stands for Secure Shell. It is a terminal emulations software similar to Telnet. The reason SSH is more preferred is because of its ability to maintain the encrypted connection. It sets up a secure session over a TCP/IP connection.

NTP – NTP stands for Network Time Protocol. It is used to synchronize the clocks on our computer to one standard time source. It is very useful in situations like bank transactions. Assume the following situation without the presence of NTP. Suppose you carry out a transaction, where your computer reads the time at 2:30 PM while the server records it at 2:28 PM. The server can crash very badly if it’s out of sync.