Introduction to networking

What is a network?

A computer network can be express as a system of interconnected device that can be communicate with some standards protocols. These devices communicate to exchange resources (e.g. files and printers) and services.

Below is an example of network comprise of two computers connected together:-

In the above example, the two computers are directly connected with the help of a cable. This network is used for exchanging data between just these two computers.

What if we want to stretch our network? so we use network device, either a switch or a hub, to link more than two computers together:

Now above devices on the network can commune with each other.

We will talk more about hubs and switches in next topics. For the present, just remember that these devices serve as a mid point to which all of computers connect to.

Local area network (LAN)

The term local area network (LAN) is widely used to express a network of devices in a limited area like, home, office, or a small building. This type of network is usually competent for achieving high data transfer rate at low cost. 

A typical SOHO (small office/home office) LAN include PCs, printers, switches, routers, and cabling that link all these devices with each other. Below is an example of typical LAN:-

In the above example, we have two computers which are connected with a switch. Then switch is connected with the router that provides the LAN with access to the Internet.

Some of the LAN technologies are Ethernet, Token Ring and FDDI. Most LAN networks use TCP/IP to commune. Twisted-pair cabling is usually used in a LAN network.

Ethernet is the most popular wired LAN technology that describe wiring, signaling, connectors, frame formatting, protocol, rules, etc. LANs also support the wireless LAN (WLAN) technology, defined by the IEEE 802.11 standards. 

NOTE
The term metropolitan area network describe a network in a single metropolitan area, This type of network is usually bigger than a LAN network and smaller than a WAN network. For an example, this type of network would be a network that link two companies offices inside the same city.

OSI & TCP/IP models

OSI model

OSI (Open Systems Interconnection) model was originated by the International Organization for Standardization (ISO), It was create to be a reference model for expressing the functions of  communication system. The OSI model gives a framework for creating and implementing, devices and describes how network applications on different computers can commune with some network media.

The OSI model has seven layers and each layer describing different function of data traveling between a network. below are seven layer of OSI model:

The layers are generally numbered from the last one that the Physical layer is consider to be the first layer. It is helpful to remember these layers, There will certainly be a few questions on the CCNA exam regarding them. 

So, what is the motive of these layers?

The motive of the OSI model was to help vendors and communications software developers to create interoperable network systems/devices.

Below are the brief description of each layers of the OSI model.

• Physical Layer – defines that how bits move from one device to another. and also defines that  how cables, connectors and network interface cards are supposed to work and how to send and receive bits.
• Data Link Layer – Is responsible for encapsulating a packet in a frame. A frame carry a header and a trailer that enable devices to communicate. A header carry a source and destination MAC address. A trailer carry the Frame Check Sequence field, which is used for detecting transmission errors. The data link layer has two sublayers:

1. Logical Link Control (LLC) – used for flow control and error detection.
2. Media Access Control (MAC) – used for hardware addressing and for controlling the access method.

• Network Layer – Is responsible for transferring data packets between different devices or network with the best path.

• Transport Layer – Is responsible for delivering data to the relevant application process on the host computers.

• Session Layer– defines that how to establish and terminate a session between the two host.

• Presentation Layer– defines that data formats. Compression and encryption are defined at this layer.

• Application Layer– This layer is responsible for displaying the information and images to the end user in a way that a human can recognize it..

The below table shows which protocols reside on which layer of OSI model:

TCP/IP model

The TCP/IP model was designed in the 1970s by the Defense Advance Research Project Agency (DARPA) as open, vendor neutral, public network model. like the OSI model, it illustrate general guideline for designing and implementing computer protocols. It has four layers which are given below:

The following picture show the comparison between the TCP/IP model and OSI model:

As you can see from the above layers, the TCP/IP has few layers than the OSI model. The Application, Presentation, and Session layers of the OSI model are combine into a single layer in the TCP/IP model. And also Physical and Data Link layers are called Network Access layer in the TCP/IP model. below is a brief description of four layers:

• Network Access Layer – Is a lowest layer of TCP/IP model and is responsible for delivering of data to the other devices on a directly attached networks.

• Internet Layer – Is responsible for logical transmission of data packets across the internet.

• Transport Layer –This Layer provides a end-to-end solution for reliable communications. TCP/IP depend on the transport layer to effectively control communications between two hosts.

• Application Layer – This layer provides the interface and protocols required by the users. It merge the functionalities of session layer, presentation layer and the application layer of the OSI model.

Differences between OSI and TCP/IP model

OSI stands for Open Systems Interconnection whereas TCP/IP stands to Transmission Control Protocol. OSI model follows vertical approach although TCP/IP model follows  horizontal approach. OSI model is connection oriented whereas TCP/IP model is both connection oriented or connection less.

TCP/IP Protocols Suite

The TCP/IP protocol suite is a set of protocols used on computer networks today . It provides  end-to-end connectivity by describing how data should packetized, addressed, transmitted, routed and receive on a TCP/IP network. This process is organized into four abstraction layers and each protocol in the suite belong in a particular layer.

Below are the some protocol which are included in TCP/IP protocol suite:

• ARP (Address Resolution Protocol) – used to map an IP address with a MAC address.

• IP (Internet Protocol) – used for delivering  data packets from the source to the destination host based on the IP addresses.

• ICMP (Internet Control Message Protocol) – used for detecting and reporting network error conditions. Used in ping.

• TCP (Transmission Control Protocol) – Is a connection-oriented protocol that enables reliable data transmission between two hosts.

• UDP (User Datagram Protocol) – Is a connection-less protocol for data transferring. Since a session is not created before the data transfer, there is no guarantee for data delivering.

• FTP (File Transfer Protocol) – Is used for file transferring from one host to another over TCP/IP based network.

• Telnet (Telecommunications Network) – Is responsible for taking remote of remote machine and this is a connection oriented protocol.

• DNS (Domain Name System) – Is used for translation host names to the IP address.

• HTTP (Hypertext Transfer Protocol) – Is used for transferring files on the World Wide Web(WWW).

The Below table shows which protocols belong on which layer of the TCP/IP model:-

                                                                                                             OSI & TCP/IP models