Notes on Data Link Layer

Communication protocols are defined in layers, the first of which is the physical layer or how nodes in a network are interconnected to one another. Each layer has its own set of rules. Data Link Layer is a part of the Open System Interconnection model (OSI).

 The data link layer checks for errors in the information and prepares the data for going on to the receiver end. It fixes any bugs present in the communication data that needs to be transmitted.

Access to get the Data

The Data Link Layer responds to service requests from the Network Layer and issues services requests to the Physical Layer. The Data Link Layer is the protocol layer that transfers data between adjacent network entities.

This layer is also a hardware layer that specifies the channel access control method and ensures the reliable transfer of data through the transmission medium. It detects and corrects mistakes that occur in the Physical Layer. There are two sublayers for the data link layer, which include Logical link control and Media Access control.

There are 3 types of access to data-

1. Random Access- Each station of the unit can access data randomly whenever it desires by following a predefined procedure.
2.  Control Access- In the control access protocol, the stations can confront one another to check which station has the authority to send and access data. The station cannot send the data without the authorization of other stations.
3. Channelised Access– This is a method used for access in which the bandwidth of the link is shared at a particular time, frequency and code to access channelled data simultaneously. 

Breaks the input data into frame

The data link layer is the second layer of the Open System Interconnection model (OSI) to facilitate communication of heterogeneous hardware or software platforms with each other. The data sent from one device needs to be received by other destination devices. For this, there is a need for a physical channel called the Physical layer. The data link layers define the network architecture because this layer is of interest to study by Local Area Networks (LAN).

While transmitting data, the data link layer breaks down the input data into small chunks that are called frames or cells. These are transmitted to the destination device depending on the technology.  As per Ethernet, the traditional technology that enables the devices to communicate with each other using network language, these input data are called frames. But in ATM (Asynchronous Transfer Mode), these are called cells.

The data link layer frames are structured according to the predefined frame layout that is oriented to the network architecture of the Network Interface Card (NIC).

Structure of frames-

1. Header- It contains a delimiter that is the text string character (,) (;) (\ /) and information of the frame followed by the frame itself.
2. Frame Check Sequence- It consists of error-detecting code that is added to a frame in a communication protocol.
3. Stop delimiter- It indicates the stop or end of the frame.

Controls The Flow Of Data

The data link layer is designed and issued to control the flow of data. It is generally developed to analyse the flow of data from sender to receiver device. Controlling the flow of data is related to the technique that gives the right to two stations to send and receive the data for communicating with each other. The sender must send the information or data at a fast rate so that the receiver can receive the information for further working and processing of it. Sometimes it is possible that the receiver has a very high load of traffic at the server or may have less power of processing. The two stations may be working at different speeds.

The flow control is a set of protocols that guides the sender about the information that needs to be transmitted to the receiver. The data link layer controls the data in 2 ways. They are-

1. Sliding window flow control- In the flow control, the data linker ensures that no other third entity enters the network of communication between sender and receiver until the complete frame is transmitted. In this control, multiple frames can be transmitted and are very efficient.
2. Stop and Wait for flow control– In this control, the sender needs to wait until the receiver sends the acknowledgement of the frame already received. The information is firstly broken down into multiple frames, and only one frame is sent at a time.

Conclusion-

The data link layer plays an important role in data transmission by being a part of the Open System Interconnection model (OSI). The data link layer helps in detecting the error present in the information shared and provided error-free communication between and receiver. It makes the communication on physical links reliable and efficient for the users.

It is a medium for functional and procedural transfer of data through different portals. But these data link frames cannot cross the local area networks, which is a concern for the designers to work on in the near future.