What Does the Data Link Layer Do in Networks?

TL;DR

The data link layer provides hop-to-hop delivery, controlling communication between consecutive nodes rather than managing the complete source-to-destination journey. It also performs node-level flow control, detects transmission errors efficiently, and controls access when multiple devices share one channel, using methods such as Stop-and-Wait, Go-Back-N, Selective Repeat, CRC, CSMA/CD, ALOHA, and token-based approaches.

Transcript

Hello friends, Welcome to Gate Smashers the topic is data link layer and data link layer in OSI model is 2nd layer from bottom. You can also call it as a layer in between network layer and physical layer. It takes data from network layer and provides data to physical layer and then the physical layer sends that data further in the network so that i... Read More

Key Insights

• The data link layer is the second layer from the bottom of the OSI model, located between the network layer and the physical layer. It receives data from the network layer and supplies that data to the physical layer for further transmission.
• Hop-to-hop delivery is the primary responsibility of the data link layer. It moves a message from one node or router to the next node, while the network layer handles the broader task of moving data from the original source to the final destination.
• Communication within one network can be provided by the data link layer using a MAC address or physical address. When data must cross different networks, routers connect those networks, and the message advances through consecutive hops before reaching the receiver.
• Flow control is the process of regulating how quickly a source sends messages to the next node. It prevents a router or other node from becoming filled with messages when the sender produces data faster than the receiving node can accommodate.
• Data-link flow control operates from node to node, while transport-layer flow control operates from source to destination. The data link layer considers the capacity of each intermediate node, whereas the transport layer considers the capacity and message acceptance of the final destination machine.
• Stop-and-Wait, Go-Back-N, and Selective Repeat are three algorithms used for flow control at the data link layer. These methods regulate transmission across individual hops so that messages can move through intermediate nodes without exceeding their receiving capacity.
• Error control at the data link layer detects errors on individual hops, including single-bit changes and burst errors involving multiple bits. Detecting an error at an intermediate node can reduce the distance traveled before the damaged message must be sent again.
• Access control determines how multiple devices use a shared communication channel within a network. Methods named for this responsibility include CSMA/CD, Ethernet, ALOHA, Slotted ALOHA, Token Ring, and Token Bus, particularly when several devices connect through one high-bandwidth wire.

Questions & Answers

Q: What is the data link layer in the OSI model?

The data link layer is the second layer from the bottom of the OSI model. It sits between the network layer and the physical layer, accepting data from the network layer and providing it to the physical layer. The physical layer then sends the data farther through the network. Its responsibilities include hop-to-hop delivery, flow control, error control, and access control.

Q: What is hop-to-hop delivery in computer networks?

Hop-to-hop delivery means moving data from one node or router to the next node along its path. The data link layer does not independently manage the complete journey from the original sender to the final receiver. Instead, it handles each consecutive link, repeating this node-to-node process until the message reaches a node in the destination network.

Q: How does the data link layer support communication within a LAN?

The data link layer can support communication between devices located within the same network or LAN by using MAC addresses, also called physical addresses. For example, one device can communicate with another device in that network without requiring the network layer to fulfill the local delivery responsibility. The data link layer manages communication over the network's connected topology and shared media.

Q: How does data-link flow control work?

Data-link flow control regulates the speed at which messages are sent to the next node. If a sender produces messages too quickly, the receiving router or node can become filled because its buffer has limited capacity. The data link layer therefore controls message flow separately at each hop, considering how many messages and what message size the next node can accept.

Q: What is the difference between data-link and transport-layer flow control?

Data-link flow control operates from one node to the next and considers the capacity of each intermediate router or node. Transport-layer flow control operates from the source to the final destination and considers the destination machine's capacity, window size, accepted message count, and message size. The distinction is therefore hop-to-hop control versus complete source-to-destination control.

Q: Which flow-control algorithms are used by the data link layer?

The three flow-control algorithms identified for the data link layer are Stop-and-Wait, Go-Back-N, and Selective Repeat. They are used to regulate how messages move from one node to another. Their shared purpose is to prevent the sender from overwhelming the next node and to keep transmission aligned with that node's capacity to accept messages.

Q: Why does the data link layer perform error control?

The data link layer performs error control so errors can be detected at an intermediate hop instead of only after data reaches the final receiver. Errors can include a single bit changing from zero to one or multiple bits changing in a burst error. Earlier detection allows the message to be resent after traveling a shorter distance, which takes less time than destination-only detection.

Q: What methods are associated with data-link access control?

Access control manages how multiple devices use one shared communication channel within a network. The methods identified include CSMA/CD, which is associated with collision detection and Ethernet, as well as ALOHA, Slotted ALOHA, Token Ring, and Token Bus. These approaches address situations where several devices are connected through the same high-bandwidth wire and need channel access.

Summary & Key Takeaways

• The data link layer is the second layer from the bottom of the OSI model, positioned between the network and physical layers. It accepts data from the network layer and passes it to the physical layer. Its central delivery responsibility is moving data between consecutive nodes or hops along a communication path.

• Flow control at the data link layer prevents a node from receiving messages faster than its capacity allows. It operates separately on each hop, unlike transport-layer flow control, which considers the final destination. Stop-and-Wait, Go-Back-N, and Selective Repeat are identified as important algorithms for controlling this node-to-node message flow.

• Error control allows intermediate nodes to detect damaged data before it travels all the way to the destination. CRC is associated with data-link error checking, while checksum is associated with transport-layer checking. Access control addresses shared-channel communication through methods including CSMA/CD, ALOHA, Slotted ALOHA, Token Ring, and Token Bus.