The Data Link Layer is the sixth layer from the upper layer of the OSI model. Its responsibility is to transmit data within a physical network. Such as the transmission of data between two computer systems.
It is considered to be the most reliable system for transporting data from one node to another. It is also the most complicated and complex of all the layers.
It hides the hardware system inside and makes all the layers above look like a medium of communication only.
The data link layer works between two hosts that are directly connected in some way or the other. As we mentioned, transmission between two computer systems. This direct connection can be either point to point or broadcast.
The work of this layer becomes more difficult when it deals with multiple hosts in the collision domain because all systems in a broadcast network are linked in the same way. It converts the bit-by-bit data stream into a signal and sends it to its internal hardware.
After receiving that data packet in the form of an electric signal in the data layer hardware, it converts it into a framed formal and works to transmit it to the upper layers.
Characteristics of data link layer
Some of the following characteristics of the data layer are as follows:-
- Physical addressing
- Network topology
- Error notification
- Frame sequencing
- Flow control
Physical addressing and network addressing are two different things.
Where network addressing understands the difference between nodes and devices in any network, due to which it is easy to switch or route traffic in the network.
Physical addressing identifies devices at the link-layer level and can differentiate between all devices in the same physical medium.
Its primary form is Media Access Control, which we know as MAC.
The specifications of a network topology determine how a network is to be designed and how all the devices within it will be linked.
Some media allow devices to be connected by a bus topology while others by a ring topology. Ethernet technology uses bus topologies that operate on devices in a juniper network.
If an error occurs inside the physical link, it informs the upper layers in advance. This is a great feature of the Data Link Layer.
For example loss of network signal, loss of clocking signal between serial connections, and loss of the terminal end of T1 or T3 link.
The data layer has the ability to store the data in a single frame sequencing, which means that if any data packets are transmitted out of sequence or are not received in the correct sequence, they can be recorded at the end of the transmission.
After this, the integrity of the transmitted package with the data payload is checked (by means of bit) inside the layer two headers.
The function of flow control is so that when data is transmitted through a device it helps to capture the congestion inside the device.
These network devices then pass on this congestion information to the upper layer network protocols so that the transmission of data packets can be restored.
Sub-layers of data link layer
The data link layer has two sub-layers:-
- Logic Link Control (LLC)
- Media Access Control (MAC)
Logic Link Control (LLC):-
The job of an LLC sub-layer single link is to manage the communication between all the devices within the network. These links support fields within layer frames that enable multiple higher-level protocols to share a single physical link.
Media Access Control (MAC):-
The MAC sub-layer takes care of the access of the network protocols given to the medium of the physical network. MAC addresses are assigned to all network devices so that all devices on the same physical link can identify each other through the MAC at the data link layer.
Functions of data link layer
Some of the following functions of the data layer are as follows:-
- Error control
- Multi access
The data link layer transmits data packages from the network layer and splits them into frames. Then it sends all these frames to the hardware, bit by bit the data. Then there the data link layer takes the signals from the hardware and converts them into frames.
The data link layer works on the “layer-2 hardware addressing mechanism”. The address is unique inside the hardware. This address is given only at the time of making this hardware. The data link sends the data only by identifying the address.
When data packets and frames are sent inside the link, then both the machines start synchronizing among themselves, if this does not happen then the transfer of data is not possible.
Sometimes there is a problem in the transition of the network signal because the data bit may be faulty. In this situation, an attempt is made to recover the original data bits to rectify this mistake. Along with this, the sender sending the data is also informed about this error.
When the host tries to transfer the data inside the sub-shared link, there is a high possibility of it colliding. The link-layer then provides the mechanism of CSMA/CD which enables access to shared data across multiple systems and also prevents collisions.