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Data LinkEdit

Data Link is the layer of network architecture that sits between the physical medium and the network layer, coordinating node-to-node data transfer across a single link. It defines how devices on the same physical segment access the medium, how frames are formed and addressed, and how errors are detected and contained within a single hop. In practical networks, the data link layer is implemented in network interface hardware and in switches and bridges, enabling efficient local communication while keeping the complexities of higher-layer routing separate. See OSI model for the broader context of where this layer fits in, and Frame for the concept of a discrete unit of data moving across the link.

Overview

  • Facilitates direct communication between adjacent devices on a shared medium or between devices connected by a switch.
  • Defines the frame structure, addressing, and error-detection mechanisms that ensure frames are delivered to the correct recipient on the same link.
  • Divides responsibilities into sublayers that separate reliability and multiplexing concerns from media access control.
  • Underpins the operation of widespread technologies such as Ethernet and IEEE 802.11 wireless networks, and interfaces with higher layers for end-to-end communication.

Architecture and sublayers

Sublayers: LLC and MAC

The data link layer is commonly described as consisting of two sublayers:

  • The Logical Link Control (LLC) sublayer, which provides multiplexing of network-layer protocols (such as IP and others) and offers a common interface to the higher layers.
  • The Media Access Control (MAC) sublayer, which handles addressing, framing, and the rules for accessing the physical medium.

These sublayers work together to ensure that a device on one side of a link can send a frame that another device on the same link can recognize and verify. See MAC address for how devices are uniquely identified at this layer, and IEEE 802 for the standards that define many MAC-layer behaviors.

Frame structure and addressing

A data-link frame typically includes: - A header containing addressing information (most commonly, a destination MAC address and a source MAC address). - A payload carrying the network-layer data. - A trailer containing error-detection information (often a Cyclic Redundancy Check, or Cyclic redundancy check).

In Ethernet, the frame also carries a field that indicates the protocol of the payload (such as IPv4 or IPv6) or the length of the payload in older versions. The MAC addressing scheme uses 48-bit addresses that are globally unique to network interfaces. See MAC address and Ethernet frame for detailed specifications.

Media access control

Access to the physical medium is governed by the MAC layer, which varies by technology:

  • In wired Ethernet environments, devices often operate in a switched network where each link is a separate collision domain, and the MAC protocol has evolved from shared-medium methods like CSMA/CD to deterministic switching. See CSMA/CD and Switch for more.
  • In wireless networks, access is typically managed by CSMA/CA (carrier-sense multiple access with collision avoidance) to cope with the shared wireless medium and the hidden-node problem. See CSMA/CA and IEEE 802.11.

Data link in practice: Ethernet and wireless

  • Ethernet frames are standardized under IEEE 802.3 and form the backbone of most wired LANs, with rapid evolution toward multi-gigabit and beyond.
  • IEEE 802.11 defines the data link layer for wireless local area networks, integrating security and mobility features suitable for portable devices.
  • Bridging and switching at the data link layer enable networks to grow while maintaining fast, local frame forwarding based on MAC addresses. See Switch and Bridge (networking) for related concepts.

Security and reliability considerations

  • The data link layer provides error detection to catch corrupted frames, but it does not by itself guarantee end-to-end reliability; retransmission and delivery guarantees are typically handled at higher layers (e.g., Transport layer protocols).
  • Wireless data links introduce specific security considerations, including the need for robust encryption and authentication. Standards such as WPA2 and WPA3 address this at the link layer for wireless networks, while wired links depend on physical security and VLANs (e.g., IEEE 802.1Q) and port-based access control (e.g., IEEE 802.1X) to manage risk.
  • Network designers often use VLANs, access control lists, and secure switching practices to reduce the exposure of the data link layer to unauthorized access. See VLAN and IEEE 802.1X for related mechanisms.

Technologies and standards

  • Ethernet remains the dominant data-link technology for wired LANs, continually expanding in speed and utility while preserving backward compatibility through the MAC layer and frame formats.
  • IEEE 802.11 defines wireless link-layer behavior, including contention methods, frame structure, and security features that adapt to mobility and variable radio conditions.
  • Classic framing and addressing concepts extend to other link-layer technologies, including various forms of hop-by-hop delivery and bridging. See Frame for a broader sense of how frames can be structured across different media.

Controversies and debates (neutral presentation)

  • Open versus closed standards: In the data link arena, debates arise over open, decentralized standardization versus proprietary extensions that a single vendor can optimize. Proponents of open standards emphasize interoperability and competition, while proponents of vendor-specific enhancements argue for faster innovation and optimized performance on particular hardware.
  • Security vs. simplicity: There is ongoing uncertainty about the best balance between robust security features at the link layer and the desire for simple, low-latency operation. Some environments prioritize aggressive encryption and access controls, while others favor lighter-weight configurations to maximize throughput and ease of deployment.
  • Regulation and spectrum management: For wireless link-layer technologies, regulatory environments and spectrum allocation can shape technology choices and deployment strategies, affecting the pace of innovation and the cost of devices that must comply with diverse regional rules.

See also