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A data link protocol sets the rules for exchanging data across a directly connected link. It defines how a receiver recognizes frames in the physical signal and may also coordinate sequencing, error handling, or flow control. In the OSI model, this is the data-link layer, or Layer 2, between the physical and network layers.
What does a data link protocol do?
A data link protocol is a set of rules for transferring data over a link between directly connected devices. It describes how data is organized into units the receiving device can identify and how the two ends of the link interact. There is no single protocol used on every link: different network technologies and interfaces use different rules.
The protocol operates at the data-link layer. The physical layer carries bits or equivalent signals; the network layer uses data-link services to send network-layer information across the link. The data-link layer bridges those roles by defining the link-level units and, depending on the protocol, additional coordination between endpoints. The OSI layer description is set out in ITU-T Recommendation X.200.
How does framing work?
Framing is the central job readers should associate with a data link protocol. A physical signal arrives as a stream; the receiver needs a way to determine where one data-link unit ends and another begins. A framing method marks or otherwise enables recognition of frame boundaries so the receiver can interpret the stream as separate units.
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A frame is the data-link layer’s transmission unit. It can carry data along with a header, a trailer, or both. The header and trailer are not necessarily identical across protocols: each protocol defines its own format and boundary method. RFC 1547 describes frames in terms of the data-link transmission unit and a framing protocol that marks a frame’s beginning and end. X.200 calls delimiting and synchronization functions “framing.”
Which functions vary between protocols?
Framing does not mean that every data link protocol provides the same reliability or coordination features. Standards describe functions such as sequencing, error detection, error recovery, and flow control, but a particular protocol may provide some, all, or different mechanisms. Do not assume that a Layer 2 protocol retransmits damaged frames or controls the sender’s rate unless its specification says so.
- Sequence control: May help identify ordering among transmitted units.
- Error detection: May let a receiver detect corruption in a frame.
- Error recovery: May specify what happens after an error, such as recovery behavior; its presence and details depend on the protocol.
- Flow control: May coordinate the pace of transmission between endpoints.
X.200 lists these among data-link functions, while RFC 1547 discusses point-to-point protocol requirements. Neither should be read as proof that every deployed data-link protocol implements every listed function in the same way.
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How do LLC and MAC fit into IEEE 802?
In IEEE 802 LAN architecture, the data-link layer is divided into two sublayers: Logical Link Control (LLC) above the Media Access Control (MAC) sublayer. IEEE describes LLC through its functions, protocol, services, protocol data unit (PDU) structure, and interfaces with the network layer and MAC. See the IEEE 802.2 information page and the IEEE catalog entry for ISO/IEC/IEEE 8802-2:1998.
This relationship is specific to the IEEE 802 architecture. It is too broad to say that LLC always handles every logical communication function while MAC always handles every physical detail; the applicable technology and standard define the actual responsibilities. IEEE 802 is a family of standards addressing LAN and MAN physical and data-link layers.
Is “Link Layer” the same as the OSI Network Layer?
No. The words can be confusing because “Link Layer” is used in Internet technical documents for the layer below the Internet Layer, while the OSI model has a separate Network Layer above the data-link layer. In RFC 1812, the Internet “Link Layer” is below the Internet Layer and above the Physical Layer; the RFC explicitly warns that this is not the OSI Network Layer.
In practical terms, IP handles Internet-layer addressing and forwarding, but a host sending to a directly connected network also needs that network’s link protocol. When comparing terminology, check whether a source is using the OSI model or Internet architecture rather than assuming “Link Layer” and “Network Layer” have the same meaning across both.
Examples of data link protocols and standards
IEEE 802 LANs
IEEE 802.2, also identified as ISO/IEC/IEEE 8802-2, addresses LLC within the IEEE 802 LAN architecture. It is an example of a data-link sublayer specification, not a universal protocol for every type of network link.
ISDN LAPD
For Integrated Services Digital Network (ISDN), ITU-T Recommendation Q.920 describes LAPD as operating at the OSI data-link layer. It conveys information between Layer 3 entities across the user-network interface using the D-channel. This is a context-specific example rather than an Ethernet example. See ITU-T Recommendation Q.920.
Point-to-point links
RFC 1547 discusses requirements for a point-to-point protocol and uses Layer 2 frame terminology. It is a requirements document; its discussion should not be treated as a checklist of behaviors that every deployed point-to-point protocol must provide.
Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.How to compare two data link protocols
When comparing specific protocols, use their applicable specifications rather than assuming they share one frame structure or reliability model. These are useful comparison dimensions:
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- Frame boundaries and format: How does the receiver find a frame, and what fields make up its format?
- Addressing or endpoint identification: How are the intended sender, receiver, or link endpoints identified?
- Connection model: Is operation connection-oriented or connectionless?
- Error behavior: Does the protocol detect errors, recover from them, or leave recovery to another layer?
- Sequencing and flow control: Does it order units or regulate transmission, and by what mechanism?
These dimensions reflect the range of functions and service modes described in X.200 and IEEE 802 materials. Specific answers must come from the standard for the protocol being compared.
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