What is Link 16?

Link 16 is an encrypted, jam-resistant Tactical Data Link (TDL) network used by U.S. and NATO Allies to create situational awareness among dispersed battle elements by sharing information, over a common communication link. This enables the command and control centers to create Common Operating Pictures (COP), which allows friendly forces to electronically observe the battlespace, identify threats, and acquire targets.

Link 16 communications are used to transfer real-time combat data, voice communications, imagery, and relative navigation information in the battlefield. This network uses JTIDS-compatible communication terminals to transmit and receive data messages. Via Link 16 network, messages can be broadcasted simultaneously to as many users as needed. Link 16 is a nodeless network i.e., it does not depend on any one terminal to act as a node, instead all Link 16-capable terminals act as nodes.

This Tactical Data Link (TDL) network was developed by ViaSat Inc. and Data Link Solutions (DLS) LLC to improve interoperability and support joint operations of land, sea, and air forces.

The main application of Link 16 is as an air and missile defense command and control system. This network is being used by various countries for national air defense, linking their sea- and land-based vessels, ground-based sensors, and surface-to-air missile systems. This helps them to protect their airspace by identifying threats and neutralizing them. This link network has been credited by the US Air Force as a key factor for saving lives in multiple contested environments, owing to the high degree of situational awareness the network provides.

Link 16 uses Time Division Multiple Access (TDMA) techniques to provide multiple, simultaneous communication paths through different networks by assigning each user with a unique time duration slot. The network can be used to broadcast messages between two users or from a single user to multiple users at a time. It is referred to as a nodeless network i.e. a single Link 16 terminal does not act as the node for other terminals and thus, every terminal act as a node in the network.

Link 16 data links operate in the radio frequency (RF) band from 960 to 1215 MHz as allocated by the International Telecommunications Union (ITU) radio regulations. It supports data exchange by supporting any of the three data rates: 31.6, 57.6, or 115.2 Kbps. However, by incorporating more advanced high-performance radio architectures along with physical layer techniques such as frequency hopping spread spectrum (FHSS), Link 16 network can deliver data rates of more than 1 Mbps. It supports up to 128 time slots per second, 127 networks, and is appropriately assigned among the JTIDS radio units.

The information exchanged within the Link 16 network is typically coded based on J-series messages, a set of message formats that use binary data words with well-defined meanings. The J-series messages include network management-related information such as communication control, time slot reallocation, radio relay control, connection status, network time update, acknowledgement, and route establishment. Depending on the situational scenario, the data words are grouped under different functional areas and allocated to different network participation groups (NPGs) that include:

1. Precise Participant Location and Identification
2. Real-Time Surveillance
3. Airborne Communications Control
4. Mission Management, and
5. Electronic Warfare and Coordination

The Link 16 network consists of multiple terminals that are located in various ground, marine, and air defense vehicles. These terminals are military-grade radio equipment designed according to the IPO office in San Diego, California. Different governing bodies formulate policies, standards, plans, and requirements for radio equipment to be used by land, sea, or airborne platforms. For instance, in the US, the lead Air Force command for the MIL-STD-6016 military standard is the Air Force Global Cyberspace Integration Center located in Boston, Massachusetts.

Types of Link 16 Network

A Link 16 network consists of three types of communication modes that determine which network type is suitable for the intended network member. All members of the network should select the same mode so that all the nodes can communicate with each other. Mode 1 refers to the normal mode of Link 16 where the network nodes or members use FHSS technique to hop between different frequencies within the 960-1215 MHz band to transmit and receive tactical messages. This hopping mechanism allows multiple Link 16 networks to transmit and receive over the channel. This type of network is called a Multi-Net. In Modes 2 and 4, members can exchange messages without using frequency hopping scheme. Modes 2 and 4 allow only a single network to share messages at a time. This type of network is called a Single-Net. Mode 3 exists and is a valid mode, but not used in practical applications.

Single Net: A Single Net indicates that each network member in a network has the same net number, security scheme, and the same frequency. Each network member is allocated a single time slot during which the messages can be sent and received. In this network, the nodes cannot perform frequency hopping as only a single frequency is used by the nodes of this network. A typical Single Net Link 16 network is shown in the figure given below.

                      Single Net Link 16 Network, Image Credit: Link 16 Model Architecture for Multiple Nets Simulation, IEEE

Multi Net: In a multi-net, different networks are available for different cluster platforms and each network contain its unique security authentication scheme, frequency band, and frequency hopping patterns (as there are multiple bands). This topology is used to isolate several NPGs that share the time slot across different networks, thereby eliminating or significantly reducing interference among multiple users. Frequency hopping pattern technique along with other signal processing techniques together improve the network performance of Link 16. A multi net design of Link 16 is shown in the figure given below.

 Multi Net Link 16 Network, Image Credit: Link 16 Model Architecture for Multiple Nets Simulation, IEEE 

Architecture of the Link 16 Network

The architecture of Link 16 defines the protocol stack that combines different network layers, algorithms used to organize the messages, encode, decode and perform various other operations to transmit and receive the messages over the tactical link effectively.

 Components of Link 16 Network, Image Credit: Link 16 Model Architecture for Multiple Nets Simulation, IEEE

The protocol stack architecture for Link 16 is similar to other networking protocols (for example, OSI model) that are developed for computer networking applications. Initially, the application layer present is responsible for generating the desired tactical messages along with added security authentication schemes. The network layer receives these messages and simultaneously checks the optimal routes for the packets, and also determines whether other nodes in the network are active. The packets are accordingly addressed in this layer and then sent to the data link layer – which encodes, decodes and organizes the data as frames. It is also responsible for handling the messages to and from the physical layer and determines how nodes recover from potential interference that may occur during communication. The physical layer is the final layer of the Link 16 network and performs other signal processing-related functionalities such as channel coding and modulation of the message. The propagation delay is nothing but the wireless channel via which the message signals propagate to and from multiple nodes in the network. When the node/nodes on the other end receive the packet, they perform the reverse operation of decoding and organizing the message as shown in the above figure.

Applications of Link 16

Airborne Platforms: Link 16 network is currently used in several defense platforms in the land, sea, and air. For airborne purposes, popular aircrafts and fighter airplanes such as the B-2 Spirit, F/A-18 Hornet, F-22 Raptor, and F-35 Lightning II use Link 16 terminals on-board to communicate between them and with other gateways that extend the network to friendly forces beyond the contested region.

Maritime: Link 16 is being used in various US carrier ships, French carrier, Royal Navy ships, Japan maritime, MILGEM project class, and Swedish authorities.

Ground-based Platforms: VESTA (verification, evaluation, simulation, training, and analysis) is a minivan that mounts a radio station on top of it. This radio station supports Link 16 capabilities, which allows it to communicate with military platforms in the battlefield.

Read about other link networks - Link 11 and Link 22.