Transport layer

The transport layer provides the flow of data between computers, as the name suggests. The Transmission Control Protocol, TCP and the User Datagram Protocol reside in this layer. In the transport layer, the data is segmented into parts to achieve a steady flow so that the differences in connection bandwidth between the host computers does not matter in the circulation of the data through the network.

Network layer
The Network Layer helps in transporting the data between the networks. At the beginning of the segments a network header is placed, which consists of the IP addresses of the sender and the receiver terminals. This header remains constant which is just read during the process of communication. The IP or Network trailer is also present at the end of the package, which contains the checksum to validate the data that is transferred between the networks. The Network Layer is employed by the routers to handle the network addresses.



Data link Layer
The data link layer takes input bits from layer one and arranges them into data structures called frames. It uses a frame header containing the sender and receiver addresses. These values do not remain constant and change while passing through the network nodes, routers or servers.


Physical Layer
The Physical Layer consists of the electrical part of the process of communication, the binary signals or bits that are transmitted and received in the process of exchange of data during the communication. The binary signal is a combination of 1's and 0's that may be repeated.

A mesh topology is a computer network in which every node has a dedicated point-to-point connection to every other node in the network. A mesh topology has n*(n-1)/2 routes that connect 'n' devices in the network. Mesh topology work on the concept of routes. Message that is to be delivered to the destination can take any possible shortest and easiest routes that it can find.

Mesh Topology

Advantages of Mesh topology

• As it has multiple routes between the source and the destination, if one route fails, there would be a number of other routes through which data transfer can take place.
• Traffic problem is minimized in this topology.
• Troubleshooting is easier compared to other networks like star and ring.
• Heavy load of data transfer has little impact on the performance of this topology.
• This network can be easily expanded without affecting the existing users.

Disadvantages of Mesh topology

• It is expensive as it requires more cable than other topologies.
• Installation is difficult and complicated.

A tree topology consists of bus and star topologies framed in a common network. The tree like structure of this network allows the colleges, schools, universities and other institutions to maintain their own network as a branch of a bigger network through which they can maximize their area of access to the network. This type of network topology is most suitable for such networks which are widely spread and divided into many branches.
On tree topology the hubs of each smaller topologies are connected to the central hub that controls the entire network. However, some nodes can be directly connected to the central hub.

Tree topology

Advantages of tree topology

• It is possible to form a point to point connection with tree topology.
• All the computers can access other computers which are in other larger and immediate networks.
• Tree topology is best suited for branched out networks.

Disadvantages of tree topology

• In this topology, the length of the network depends on the type of cable utilized to form the network.
• The entire tree topology would fail if the central trunk fails as it forms the backbone of the network.
• The tree topology is comparatively more complex than the bus and star topologies. If it gets bigger, it would be difficult to configure and manage it.
• It is expensive as more hubs and cables are required to install the network.

A network topology refers to the interconnection of various nodes of a computer network. In other words, it defines the layout of the various devices that are part of the computer network.Topology is the virtual "shape" of the network, but it may not actually represent the actual physical design of the network that the various devices are connected to each other.
There are two types of network topologies: physical and logical.

A physical topology refers to the physical design of the computer network that includes the computers and other devices, their location and the various cables installed in the network.

A logical topology means the mode of flow of data in the network in contrast to its physical arrangement.

There are mainly six types of network topologies listed as under:

• bus
• star
• ring
• tree
• mesh
• hybrid

On the basis of the area a network covers, it can be divided into the following types:

    *  LAN      -  Local Area Network
    *  WLAN  - Wireless Local Area Network
    *  MAN     - Metropolitan Area Network
    *  WAN     - Wide Area Network
    * SAN       -  Storage Area Network, System Area Network, Server Area Network, Small Area Network

    * CAN      - Campus Area Network, Controller Area Network, or sometimes Cluster Area Network
    * PAN      - Personal Area Network
    * DAN      - Desk Area Network


The original categories of computer networks were only LAN, MAN and WAN. The other categories evolved during the advancements in the technology of computer networks.

LAN (Local Area Network)

A LAN is a network of computers and devices over a relatively short geographical area. An office building networked with computers,a school or college or a home usually contains a single LAN, though sometimes one building will contain a few small LANs (probably one per room), and occasionally a LAN will span a group of nearby buildings. In addition to operating in a limited space, LANs are also controlled by a single person or organization. A LAN also tend to use certain connectivity technologies, primarily Ethernet or Token Ring.

 WAN - Wide Area Network
A WAN  spans a large physical distance compared to a LAN. The internet is the largest and the most common example of a WAN, that spans the whole Earth. A WAN can be considered as a collection of several LANs that are dispersed throughout the whole world. The network device Router connects several LANs to form the WAN. The Router also maintains both a LAN address and a WAN address in IP networking.

WLAN  - Wireless Local Area Network
WLAN is also a type of LAN(local area network) based on wireless network technology mostly referred as Wi-Fi. In contrast to a LAN, in WLAN no wires are used; radio signals are the medium for communication. To access any wireless network around, wireless network cards must be installed in the system. The wireless cards are connected to the Wireless Routers for the purpose of communication among the computers.

A computer network is a combination of computers and other computer related devices connected by communications channels for the purpose of sharing files and resources among the users. The network may be of many types, with unique features of its own.
The primary objective of computer network is to share the files among various users. Among the various networks of computers, the internet is definitely the most important inventions of modern age. It is a huge source of information on any topic, it brought the era of emails and point to point communication through voice and video, allowing anyone to communicate with anyone else in the world; and it is also a great medium of new kind of business - the online business. Everyone in the world today depend so much on the computer networks and specially the internet, that it has become a part of our daily life.

fig: Computer Networks