What is Link 11?

Link-11 is a netted, half-duplex, digital data link used by NATO and the US armed forces for exchanging digital tactical information between land-based, airborne, and seaborne platforms. This digital data link allows the exchange of radar track information and written messages between ships, shore installations, and aircraft. Link-11 is identical to the US armed forces’ Tactical Digital Information Link A (TADIL-A) and is also known as MIL-STD-6011.

Link-11 uses two frequency bands, a high-frequency (HF) band and another in the ultra-high-frequency (UHF) band. In the HF band, it has a ground wave range of up to 300 nautical miles and may reach a sky wave range of 1,000 nautical miles. In the UHF band, it can cover the Line-Of-Sight range of 20 to 30 nautical miles for ship-ship connections and covers 150 nautical miles for ship-air connections. The link-11 system uses QPSK modulation and can broadcast packets of 30 bits length, with 6 bits of error correction and 24 bits of payload data (encrypted). 

The Link-11 network can operate at two different data rates, the fast data rate operates at 75 frames per second (2250 bps - for UHF), and the slow data rate operates at 45.45 frames per second (1364 bps- for HF). Up to 61 participants (e.g., ships, submarines, aircraft, shore facilities) can be hosted on a Link-11 network at any one time. The system normally operates in a roll call mode under the control of a Net Control Station (NCS). This article discusses roll call mode in the following section.

Link 11 is now being replaced by Link 22 Tactical Data Link, boasting noticeable increases in capability, e.g., up to 125 participants can be hosted, and it can achieve data rates of 4kbps for an HF network and data rates of 12kbps for a UHF network. But, Link 11 is expected to remain the chief naval tactical data link for another ten years; it will be gradually replaced by Link 22 from about 2024.

Link-11 Architecture

A Link-11 network consists of a Network Control Station (NCS) and a number of Participating Units (PU) or Picket stations. The NCS is a participating unit in the link 11 architecture that act as a common control station that permits access to the net and maintains net operational discipline.

Onboard system architecture of the Link-11

The basic onboard system architecture of the Link-11 system is shown in figure above. Each participating unit has an onboard system that contains essential hardware components such as Tactical Data System (TDS) computer, an encryption device, a data terminal set (DTS), and a radio set.

Tactical Data System (TDS): It is a computer that receives data from sensors, such as radar, navigation systems, and operators. This collects this information in digital data bases.  The TDS computer features a memory buffer that stores incoming and outgoing messages. This computer manages a database that consists of information on enemy position fixes, tracks, information on own PUs, vessels, aircraft, sonobuoys, etc.

The key functions of the TDS include supplying tactical digital information to the net participants, retrieving and processing incoming digital tactical information received from other net participants, managing the display functions for the operator, performing track updates on contacts, and replying to operator inquiries.

Encryption Device: It is a device that encrypts the data from the TDS computer when transmission and sends it to the DTS (see below figure, transmission). When the PU (participating unit) is receiving, encrypted data is received from the DTS, then decrypted and sent to the TDS computer (see below figure, receiving).

Data Terminal Set (DTS): The data terminal set (DTS) is the heart of the Link-11 system that acts as the system modulator/demodulator (MODEM). The TDS computer sends 24 bits of data to the DTS via the encryption device.

The DTS adds 6 bits of data (called hamming bits) for error detection and correction. Now, the 30 bits of data are phase-shift modulated into 15 audio tones. These 15 data tones and a Doppler correction tone are combined to generate a composite audio signal, which is then sent to either the UHF or HF radio equipment for transmission. While receiving, the DTS receives the composite audio signal from the radio and separates the 15 data tones and the Doppler correction tone, demodulates the 15 data tones into 30 data bits, checks the 6 hamming bits for errors, and sends 24 bits to TDS computer via the encrypted device. DTS's normal mode of operation is in the half-duplex mode when it can either transmit or receive.

The key functions of the DTS include error detection and correction, audio signal generation, link protocol control, NTDS (Navy TDS) interface control, and digital/analog conversion.

Link-11 offers the following six modes of DTS operation:

Net Synchronization: This mode of operation permits the terminal of each picket station to synchronize its internal timing to that used by the net control station (NCS) terminal. When the task force is formed, the picket stations inform the NCS of their readiness to establish link operations. Upon establishing communication with all picket stations, NCS transmits Net Synchronization (Net Sync).

Net Test: This mode of operation is used to confirm connectivity between the Link-11 units. Upon completion of the Net Test, all picket stations report their status information to the NCS. After that, the NCS directs all PUs to switch to the Roll Call mode and initiates link operations.

Roll call mode: This is the normal mode of Link-11 operation that provides full utilization of force area surveillance and tracking capabilities. In the roll call mode, one unit operates as the Net Control Station (NCS) while the other units operate as the picket unit. The NCS initiates the roll call by addressing and transmitting an interrogation message to a specific PU that then responds by transmitting its data. The NCS then callup the next PU address for interrogation. The roll call mode is used to provide all PUs with continuous, near real-time exchange of tactical information.

Broadcast: In his mode, one PU will continuously send a series of short broadcast messages to all the members of the net. Once manually initiated, the transmission will continue to be sent automatically until the operator stops it manually.

Short Broadcast: In this mode, a picket station or the NCS sends a single message to all members of the net. The data transmission is initiated by the operator and is terminated automatically when the computer has finished sending the DTS data.

Radio silence: This mode allows a unit to receive Link-11 data but does not allow the unit to transmit data. This mode is typically used just prior to joining an active net in order to sync its TDS with the NCS.

Link 11 is expected to be replaced by Link 22 in 2024. Click here to learn about Link 22.