What is the data link layer in the OSI model?

The data link layer is the second layer of the OSI model and responsible for bits being sent correctly in the form of data packets. It not only looks after portioning and monitoring during the transfer, it’s also actively involved in troubleshooting.

The data link layer is the second layer in the OSI model. This conceptual model structures the processes needed by computers and network technology to allow communication across hardware and software. Within this model, the data link layer works as a protocol layer and ensures frames are transferred correctly within a physical connection.

The OSI reference model is split into seven different layers that are built one on top of the other. Each layer has its own specific tasks. The individual layers are as follows:

1. Physical layer
2. Data link layer
3. Network layer
4. Transport layer
5. Session layer
6. Presentation layer
7. Application layer

To understand what functions the data link layer has, it’s necessary to first take a look at the layer below it. In the physical layer, physical connections between two systems are activated, monitored and deactivated. This means that data can be sent from a sender to a receiver. This layer does not, however, evaluate the data. It simply transfers it without checking its content or structure.

This is where the data link layer comes in. It codifies, decodifies and organizes the individual bits and then prepares them as data frames. This means that it creates data packets, turning larger data packets into smaller units.

The data is then forwarded from the data link layer to the network layer, the layer situated directly above it. In the network layer, it is then given an address and forwarded on. The data link layer ensures that the frames are transferred without any errors and, therefore, makes unsecured system connections secure.

The communication which takes place on the data link layer can be carried out with or without a connection. If a connection is not present, all data which is to be transferred will contain a source and target address. However, if a connection is being used, logical connections between the sender and the receiver will be established first.

In addition to the tasks mentioned above, the data link layer is also responsible for checking frames during and after data transfer as well as identifying and correcting any errors. To do this, it first analyses the bit pattern within the frame so it can detect problems and errors early on.

If there is an irregularity, the data link layer informs the layers above it. The receiver can then re-sort the frames if they were delivered in the wrong order. Meanwhile, the data link layer checks to see whether the individual packets have remained intact.

To do this, the OSI data link layer examines the data flow to see if a physical connection is overloaded and then intervenes accordingly. If there are any limitations, this information is sent to devices nearby and, if possible, rerouted.

The data link layer is broken down into two sublayers: The logical MAC (Medium Access Control) is adjacent to the physical layer (1) and the LLC (Logical Link Control) connects to the network layer (Layer 3). The services provided by the data link layer can be separated into three categories:

If frames are sent using this method, the recipient will provide no confirmation, even if frames have been received as intended. If data packets are damaged or get lost, there is no restoration. This type of transfer is, therefore, only suitable for exceptionally secure connections or if a higher level can troubleshoot. This method can be used when instant data transfer is needed and is more important than a complete transfer.

In most cases, this type of transfer is preferred since each transfer is confirmed. If a transfer does not reach its specified location or some parts of it get lost, there will be no confirmation, and the data transfer should be started again. This ensures that all frames reach the receiver.

The most secure method is using connection-oriented services. This gives each data packet a specific number, which is logged at both the sender and receiver’s end. These then connect before every transfer. Both sides are thus able to ensure that every packet is transferred just once and reaches its destination without errors.

There are many data link layer protocols. These include among others:

• 1-Wire
• ARCnet
• ARP
• ATM
• Ethernet
• HDLC
• LLC/MAC
• MIL-STD-1553
• PPP
• SpaceWire
• Token ring
• UNI/O
• V.120
• X.75