LAYER MATTERS

1.Data-Link layer:~

The Internet is made up of end-hosts, links and routers. Data is delivered hop-by-hop over each link in turn. Data is delivered in packets. A packet consists of the data we want to be delivered, along with a header that tells the network where the packet is to be delivered, where it came from and so on.

The data link layer is the protocal layer in a program that handles the moving of data into and out of a physical link in a network. The data link layer is Layer 2 in the Open Systems Interconnection architecture model for a set of telecommunication protocols.

The data link layer has two sublayers: the logical link control (LLC) sublayer and the media access control (MAC)sublayer.

Functions of the data link layer

The data link layer has three main functions:

• It handles problems that occur as a result of bit transmission errors.
• It ensures data flows at a pace that doesn’t overwhelm sending and receiving devices.
• It permits the transmission of data to Layer 3, the network layer, where it is addressed and routed.

2.Network layer:~

The most important layer is the Network layer. It delivers packets end-to-end across the Internet from the source to the destination. A packet is an important basic building block in networks.

A packet is the name we give to a self-contained collection of data, plus a header that describes what the data is, where it is going and where it came from.Network layer packets are called datagrams. They consist of some data and a head containing the “To” and “From” addresses — just like we put the “To:” and “From” addresses on a letter.

Functions of the Network layer

The network layer provides the means of transferring variable-length network packets from a source to a destination host via one or more networks.Functions of the network layer include:

*Connectionless communication:-IP is connectionless, in that a data packet can travel from a sender to a recipient without the recipient having to send an acknowledgement. Connection-oriented protocols exist at other, higher layers of the OSI model.

*Host addressing:-Every host in the network must have a unique address that determines where it is. This address is normally assigned from a hierarchical system.

• Message forwarding:-Since many networks are partitioned into subnetworks and connect to other networks for wide-area communications, networks use specialized hosts, called gateways or routers, to forward packets between networks.

3.Transport Layer:~

The most common Transport Layer is TCP(Transmission Control Protocol).TCP makes sure that data sent by an application at one end of the Internet is correctly delivered –in the right order -to the application at the other end of the Internet. If the Network Layers drops some datagrams, TCP will retransmit them, multiple times if need-be. If the Network Layer delivers them out of order –perhaps because two packets follow a different path to their destination — TCP will put the data back into the right order again.

Functions of the Transport layer:~

The transport layer is responsible for error-free, end-to-end delivery of data from the source host to the destination host. It corresponds to the transport layer of the OSI model. It facilitates the communicating hosts to carry on a conversation. It provides an interface for the users to the underlying network.

4.Application Layer:~

An application layer is an abstraction layer that specifies the shared communications protocols and interface methods used by hosts in a communications network. The application layer abstraction is used in both of the standard models of computer networking: the Internet Protocol Suite (TCP/IP) and the OSI model.There are of course many thousands of applications that use the Internet. While each application is different, it can reuse the Transport Layer by using the well-defined API from the Application Layer to the TCP or UDP service beneath.

Functions of the Application layer:~

The application layer only standardizes communication and depends upon the underlying transport layer protocols to establish host-to-host data transfer channels and manage the data exchange in a client-server or peer-to-peer networking model.