Data link layer in OSI model

This article will help you in getting a good understanding of the topic called the Data link layer in the OSI model.

It is the second bottom most layer in the OSI model. It is the most complicated layer among all layers in the OSI model and it involves many complex functioning procedures and methodologies.

The data link layer works between two hosts which are having a direct connection. Here the direct connection is either point-to-point or broadcast type. The data link consists of two sub-layers which are given below:

1. Logical link control
2. Media access control

a) Logical Link Control: This sub-layer deals with various aspects like protocols, flow, and error control.

b) Media Access control: This sub-layer deals with the original control of media.

The main purpose of the data link layer is to make sure that transferred information is error-free. From the network layer, the data link layer gets information in the form of packets and divides those packets into frames. These frames are bit-by-bit transferred to the physical layer from the data link layer. Functions of the data link layer are error control, flow control, addressing, frame synchronization, access control, providing service to the network layer, and reliable delivery.

The packets which are sent by the network layer are broken down into frames based on the Network Interface CARD(NIC) and the frame size. This layer stores or preserves the MAC address of the sender and receiver in the header.

MAC (Media Access Control) address is a 12-digit address that is used to uniquely identify a network device in the network.

For example:
32-04-2C-27-G5 is having 6 octets of which the first 3 represent the OUI and the next three octets represent the NIC).

The MAC address of the receiver can be found or identified by using Address Resolution Protocol(ARP) request onto the wire for asking the question “ whose IP address is this?”. The host will be responding to this question by sending the MAC address.

Functionality of Data Link Layer

• Error control: The main functionality of  data link layer in the OSI model is error control. This layer needs to identify errors and correct them by using techniques. Due to several reasons, data can be corrupted. To make the data error-free, the data link layer uses techniques like error detection and correction. Error detection checks whether the error occurred or not, and error corrections find out the exact number of corrupted bits and their location.
• Flow control: If the speed of the sender is slower than the receiver, overflow occurs, and some of the frames will be lost in this process. The data link layer allows the sender and receiver to transfer data simultaneously such that no overflow will occur in the transfer of data. There are 2 types of flow control processes they are \
  • Stop and wait for flow control
  • Sliding window

a. Stop and wait for flow control: It pushes the sender after sending the data to stop and wait by the receiver.

b. Sliding Window: It is the process in which both the sender and receiver decide the number of frames after which the acknowledgment should be given. This process consumes less time as it uses few resources.

• Addressing: The data link layer binds the source and destination physical addresses (MAC addresses) at the header of each frame to verify delivery. A physical address is always a unique address that is given to the system at the time of manufacturing, and it will not be changed.
• Framing: The data link layer receives the packet from the network layer and divides that packet into frames, adding some bits to the header and also at the end of the frame, after which it will be transferred to the physical layer. Framing is the way of connecting two devices and also sending bit by bit meaningful data to the receiver.
• Providing service to the network layer: It provides an interface to the network layer for transferring the data packets. Acknowledged connectionless services, unacknowledged connectionless services, and acknowledged connection-oriented services are the various services which are provided by the data link layer.

This is all about the Data link layer of the OSI model. Hope you understood this topic.