A data link is any communication channel that carries digital information between two points. The term shows up in several different fields, and its exact meaning shifts depending on context. In computer networking, it refers to a specific layer of technology that moves data between devices on the same local network. In aviation, it describes systems that let pilots and controllers exchange messages digitally instead of by voice. In military operations, it refers to encrypted tactical networks that share real-time battlefield information between ships, aircraft, and ground units. All of these share a core idea: a reliable path for data to travel from one point to another.
The Data Link Layer in Computer Networking
In the OSI model, which is the standard framework for how networks are organized, the data link layer is Layer 2. It sits between the physical layer (the actual cables and radio signals) and the network layer (which handles routing across different networks). Its job is node-to-node delivery of data within the same local network.
When data arrives from the network layer as packets, the data link layer breaks them into smaller units called frames and sends those frames bit by bit to the physical layer beneath it. Each frame includes the sender’s and receiver’s MAC addresses, which are unique hardware identifiers assigned to every network device. A network switch is the key piece of hardware at this layer. It reads MAC addresses on incoming frames and forwards each one to the correct device on the network.
The data link layer also handles error detection. Before a frame leaves the sender, the layer adds extra bits that act as a mathematical fingerprint of the data inside. The most common method is called a Cyclic Redundancy Check, or CRC. The sender performs a calculation on the data and appends the result to the frame. When the frame arrives, the receiving device runs the same calculation. If the result doesn’t match, the frame was corrupted during transmission and gets rejected. Simpler methods like parity checks also exist, where a single extra bit is added to flag whether the number of 1s in a data block is even or odd, but CRC catches far more types of errors.
Common Data Link Protocols
Ethernet is by far the most widely used data link protocol. It governs how devices on wired local networks package, address, and transmit frames. Wi-Fi operates at the same layer but uses radio signals instead of cables. For point-to-point connections, such as a computer dialing into an internet provider over a phone line (or certain VPN tunnels), the Point-to-Point Protocol (PPP) handles the same framing and error-checking tasks. Each of these protocols solves the same fundamental problem: getting data cleanly from one device to the next device on the same link.
Data Links in Aviation
In aviation, a data link replaces or supplements voice radio communication between pilots and air traffic controllers. The two most important systems are ACARS and CPDLC.
ACARS (Aircraft Communications Addressing and Reporting System) has been around since the 1970s and handles routine operational messages: gate assignments, weather updates, fuel data, and maintenance reports. It works like text messaging for airlines, automating information that would otherwise clog up voice frequencies.
CPDLC (Controller-Pilot Data Link Communication) goes further. It lets controllers send clearances, altitude changes, and routing instructions directly to the cockpit as digital text messages, which pilots can accept or reject with a button press. This eliminates the frequency congestion and misunderstandings that are common with voice communication, especially over oceans where radio coverage is spotty. A related service, CPDLC-DCL, automates departure clearances so pilots can receive and amend their clearances before takeoff without waiting on a busy radio frequency.
Satellite-based data links are now expanding what’s possible in the air. Recent measurements of Starlink’s aviation service show a median download speed of about 152 Mbps per aircraft terminal and upload speeds around 24 Mbps, with latency hovering around 50 milliseconds. Individual passengers on a connected flight typically see around 65 Mbps download speeds. That’s fast enough to stream 4K video with zero buffering, a dramatic leap from the slow, expensive connections that defined in-flight Wi-Fi for years.
Military Tactical Data Links
Military forces use specialized data links to share real-time tactical information, such as the positions of friendly and hostile units, across ships, aircraft, ground stations, and command centers. The most widely used is Link 16, the standard tactical data link for NATO forces.
Link 16 is a secure, jam-resistant digital system that operates on radio frequencies between 960 and 1,215 MHz. It uses a time-division technique where each participant on the network is assigned specific time slots to transmit, preventing collisions and making the signal harder to intercept or disrupt. Because it operates at these frequencies, direct communication is limited to line-of-sight distances. To extend range beyond the horizon, Link 16 data can be relayed through satellites or bridged onto internet-based protocols for long-haul transmission.
What makes tactical data links valuable in combat is automation. Rather than a pilot verbally relaying the position of a radar contact, Link 16 automatically shares that track with every other platform on the network. Everyone sees the same picture at the same time.
Industrial and IoT Data Links
In factories, power plants, and building automation systems, data links connect sensors, controllers, and machinery. The dominant physical standard since 1983 has been RS-485, a wired interface designed for harsh industrial environments. Unlike the single-ended signaling used in older serial connections, RS-485 sends data as a differential signal across a pair of wires. This makes it far more resistant to electrical noise from motors, welders, and other heavy equipment that would corrupt a weaker signal.
A single RS-485 bus can connect up to 32 devices in parallel, using a nominal cable impedance of 120 ohms with matching termination resistors at each end to prevent signal reflections. Most industrial setups use a two-wire, half-duplex configuration where devices take turns sending and receiving. Four-wire, full-duplex setups allow simultaneous transmission in both directions when higher throughput is needed. Many of the higher-level industrial protocols that engineers work with daily, including Modbus, Profibus, and DMX512 (used in stage lighting), rely on RS-485 as their underlying physical data link.
Drone and UAV Data Links
Unmanned aerial vehicles depend on data links for two critical functions: command and control (flying the aircraft) and payload data (streaming video or sensor feeds back to the operator). These typically run on separate frequencies to prevent interference between them.
A common configuration uses the 430 MHz UHF band for flight control commands and the 2.4 GHz band for payload data like camera feeds. With airborne transmitters running at about 500 milliwatts of power, this setup can maintain reliable communication out to roughly 10 kilometers in normal atmospheric conditions. Higher-frequency bands in the 2.7 to 5.0 GHz range are also used for specialized applications, often requiring coordination with aviation authorities since these overlap with airborne radar and navigation frequencies. Longer-range military drones use satellite data links to maintain control and stream sensor data from thousands of miles away.