A data link protocol sets the rules for transferring data across a directly connected link. It defines how a receiver recognizes frames in the transmitted stream and may also coordinate addressing, sequencing, error handling, or flow control. In the OSI reference model, these functions belong to Layer 2, between the physical and network layers.
What is a data link protocol?
A data link protocol is a set of rules for transferring data across a directly connected link. It explains how the receiver identifies data-link units—usually called frames—and, depending on the protocol, how the endpoints coordinate sequencing, errors, and flow.
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It is a role rather than one universal protocol. Different link technologies use different protocols, and their particular rules and services can vary.
Where does the data-link layer fit?
In the OSI reference model, the data-link layer is Layer 2. It sits above the physical layer, which carries bits or equivalent physical signals, and below the network layer, which uses data-link services to move data across networks.
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For example, a network-layer protocol can provide network-to-network delivery, but a device still needs a protocol for communicating across the link directly attached to it. The two layers solve different parts of the communication problem.
What does a data link protocol do?
Frames the transmitted data
A physical stream needs boundaries so a receiver can tell where one data-link unit begins and ends. A framing protocol provides a way to mark those boundaries. A frame is the data-link transmission unit; it may contain a header or trailer, as well as the data being carried.
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May coordinate sequencing and flow
Some data-link protocols use sequence control to organize units or flow control to regulate how quickly data is sent. These are possible services, not requirements shared by every protocol.
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May detect or recover from errors
A protocol may include error detection, error recovery, or both. These functions are also protocol-dependent. Do not assume that every data-link protocol detects errors in the same way or retransmits damaged data.
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What are LLC and MAC in IEEE 802 LANs?
In the IEEE 802 LAN architecture, the data-link layer is divided into two sublayers: Logical Link Control (LLC) above Media Access Control (MAC). IEEE 802.2 addresses LLC functions, services, protocol, protocol data unit structure, and its interfaces with the network layer and MAC.
This relationship applies to the IEEE 802 architecture; it is not a universal description of every data-link technology. The details depend on the applicable standard.
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How does “Link Layer” differ from the OSI Network Layer?
Terminology can be confusing because Internet documents use “Link Layer” for the layer below the Internet Layer and above the Physical Layer. That Internet usage is not the same as the OSI Network Layer. In the OSI model, the data-link layer is Layer 2 and the network layer is Layer 3; in Internet terminology, IP is an Internet-layer protocol and a directly connected network also uses its link protocol.
What are examples of data link protocols?
- IEEE 802 LANs: IEEE 802.2 concerns the LLC sublayer within the IEEE 802 LAN architecture.
- ISDN: ITU-T Q.920 describes LAPD as a data-link-layer protocol that conveys information between Layer 3 entities across the user-network interface using the D-channel. This is a context-specific example.
- Point-to-point links: RFC 1547 discusses point-to-point protocol requirements and uses Layer 2 frame and framing terminology. A requirements document should not be read as proof that every listed behavior is required in every deployed protocol.
How should two data link protocols be compared?
Use the applicable standards for each protocol rather than assuming that all links provide the same services. Useful comparison questions include:
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- RELIABLE - IEEE 802.3x flow control provides reliable data transfer
- What link or medium and topology does it support?
- How does it mark frame boundaries, and what is the frame format?
- How are endpoints identified or addressed?
- Is operation connection-oriented or connectionless?
- What error detection or recovery behavior does it provide?
- Does it use sequencing or flow control?
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