OSI Model – Data Link Layer (Layer 2)

After Network Layer has added Source IP Address and Destination IP Address to data segments and now known as data packets, now the data packets are handed to Data Link Layer, to proceed further with the data communication process.

We need to know here that IP addresses are logical address of network location, our data packets now know the network location, but still something is missing, they need to know the exact machine where the data has to be transferred i.e. the Physical Address or MAC Address, think of it as house number, where a parcel has to be delivered.

This work of assigning MAC Address to our data packets are done by this layer, i.e. the Data Link Layer. This layer assigns the Source MAC Address and Destination MAC Address, with a Frame Check Sequence to each of our data packets and now these packets are known as Frames.

So now we have Source MAC Address and also the Destination MAC Address, by which our Frames know where exactly the Frames has to be delivered, Frame Check Sequence are added to our data packets, which runs a Cyclic Redundancy Check on each packets and places the value on the Frame Check Sequence field, which will be used for verification purpose and error notification by the same layer on the receiving side.

Switches and Bridges make decision based on this Layer 2 Address or the MAC Address.

The Data Link Layer provides the physical transmission of the data and handles error notification, network topology and flow control.

Data link Layer has two sub-layers:

           1.       MAC Sub-Layer

           2.       Logical Link Layer

MAC Sub-Layer:

MAC defines how packets are placed on the media. Physical Addressing is done here. Line discipline, error notification (not correction) ordered delivery of frames and optional flow control can be used here.

Logical Link Layer:

It acts as an interface between the MAC sub-layer and the Network Layer. It is responsible for identifying the network layer protocol and then encapsulating them. It also provides flow control and sequencing of control BITS.

User	Service
Application Layer	Web Browser, Email Clients
Presentation Layer	Formats data, compression, encryption, etc
Session Layer	Maintains dialog control, creating session and controlling session and ending session.
Transport Layer	End-to-end communication
Network Layer	Layer 3 logical addressing and routing
Data Link Layer	Layer 2 Physical Addressing and Framing with error notification.

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