What Is a Data Link Layer?

The data link layer is the second layer of the open systems interconnection (OSI) model, which is a model that breaks down computer systems into seven distinct layers. The data link layer creates and disbands communication points between network devices and is responsible for how systems communicate over a network. The seven layers of the OSI model all work together and encompass functionality ranging from the physical setup to how apps run on the user interface. The seven layers are:

• Layer 1: Physical layer

• Layer 2: Data link layer

• Layer 3: Network layer

• Layer 4: Transport layer

• Layer 5: Session layer

• Layer 6: Presentation layer

• Layer 7: Application layer

To understand computer systems in depth, learning about each layer individually can help you see how the layers fit together and facilitate more advanced functionality. Explore more about the data link layer and its function, including how it works, the benefits of this layer, and how to start working with OSI systems. 

What is a data link layer, and how does it work?

A data link layer (layer 2) sits between the physical and network layers in the OSI model. This layer can receive and transmit information to and from both layers, with each responsible for a different subset of information. From the network layer, it gets information such as packet data and logical addresses (e.g., MAC addresses). After receiving information from the network layer, it breaks it into data frames before decoding and recoding it into a format the physical layer can understand (e.g., a bitstream). The physical layer can later pass a bitstream back to the data link layer, which gets converted back to packets for the network layer. The data link layer aims to facilitate an error-free transfer of information between layers so that data can be transmitted properly between devices. 

Read more: Information Technology (IT) Terms: A to Z Glossary

Sublayers of the data link layer

The data link layer consists of two sublayers: the media access control (MAC) sublayer and the logical link control (LLC) sublayer. The MAC sublayer manages data flow between devices so that information doesn’t collide on transmission pathways. It does this by connecting MAC addresses, ensuring a clear flow of information between source and destination addresses. The LLC sublayer provides logic for data linkages, including checking for errors, identifying protocols, and synchronizing frames. 

Why is the OSI model important?

The OSI model is excellent for troubleshooting network problems, and the seven layers serve as a framework for engineering professionals. Some important functions include:

• Understanding of complex network systems: IT professionals and engineers use the layers to organize complex network systems, and each layer has a function.

• Research and development: Breaking a complex system into smaller parts lets engineers know exactly which part needs work.

• Flexibility: The OSI model breaks down at each level by tasks, which means standardization is flexible when building complex systems.

• Security: The OSI model is used to identify data security risks as it can address where the majority of data is stored and whether it is safe. 

Read more: 6 Network Certifications for Your IT Career

What are the benefits of a data link layer?

The data link layer is vital to the OSI process. It performs various services, such as:

• Framing data

• Controlling data transmission flow

• Ensuring reliable data delivery

• Detecting errors

• Multiplexing data streams

Every link in the OSI model works together to perform a function. Tasks are assigned between layers, with each layer holding an important role in the overall system function. The data link layer interfaces between the network and physical layers. Without it, the information transmitted between the layers would be unchecked, and the system would become confused. The data link layer plays an important role in organizing, reframing, translating, and directing information in an efficient and sensible way. 

5 data link layer functions 

As discussed previously, the data link layer provides many functions that contribute to a smooth flow of information between devices in a network. By understanding each step in-depth, you can explore more granular details of what the data link layer does and how different aspects of computer systems relate to the function of this layer. 

1. LLC and MAC

You’ll know from above that the data link layer breaks down into two sublayers: the logical link control (LLC) and the media access control (MAC). As its name suggests, the LLC provides the logic for the data link. It is responsible for sequencing, cyclic redundancy checks, and destination information. The MAC conserves network life by allocating access efficiently to nodes.

2. Data framing 

Data framing is a way for a host to transmit information to a receiver. It is the point-to-point contact, and it frames information. Typically, the data link layer breaks down information into a series of a few hundred bytes. Once this information is ready to transmit to the physical layer, the MAC sublayer uses addresses to identify the source and where it is going. 

3. Flow control

Flow control is a useful function of the LLC as it provides an equal flow speed of data exchange. This ensures data transmission stays within the network capacity, and that devices on the network send and receive information sequentially to avoid data collisions. 

4. Error detection

The data link layer has an excellent capacity for error detection (cyclical redundancy checking) with a function to detect data corruption and take appropriate action. 

5. Threat reduction 

The data link layer is a prime target for attackers to infiltrate a frame, such as through MAC flooding. As each frame has a header, body, and trailer, if an attacker can view any of these, it compromises your data. For this reason, security at layer 2 needs to be tight, so access and control must be limited. You generally achieve this through encryption, using MAC address filtering, and disabling ports.

How to start working with OSI systems

IT and networking professionals generally work with OSI models in professions involving computer networks and systems, such as network engineering. If you’re looking for work in these fields, you typically need a bachelor’s degree in a relevant major as a minimum requirement. Gaining professional certifications is also useful to prove you have certain relevant technical knowledge and experience and shows employers you're committed to your career.

A great way to build experience is through an internship, an entry-level job, such as a help desk technician, or by starting out in a programming role. Experience through personal projects is also highly valuable. 

Read more: What Are Computer Information Systems? Definition, Degree, and Careers

Getting started in networking with Coursera

The data link layer is the second layer of the OSI model, which is a network systems model describing how computers and other devices transmit and process information. The data link layer plays an important role in information processing and transmission, and understanding this layer can help you understand more intricate details of how computer systems operate.

If you’re looking to start your career in computer networking or learn more about OSI and its seven layers, a great starting point is the Networking Fundamentals beginner course, delivered by Akamai on Coursera. This beginner’s course introduces you to essential aspects of networking.