Link-22 will be able to host an increased number of users on a single network, including air assets such as naval support helicopters, compared to the legacy Link-11 TDL. (Thomas Withington)

A quiet revolution is occurring in naval communications. The stalwart Link-11 Tactical Data Link (TDL) is being replaced by Link-22. How does this new communications architecture work, and what might it offer to naval operations?

The title of Joseph Heller’s seminal novel published in 1953 refers to a double bind, the dilemma of pilots from the United States Army Air Force’s (USAAF) fictitious 256th Squadron deployed to Italy during the Second World War. The ‘Catch-22’ is the dilemma that anyone pilots who appeared insane were not obliged to fly missions, while any pilots who petitioned to stop flying out of a concern for their safety was performing a rational act and therefore deemed sane by USAAF medical staff and fit to fly. Link-22, the North Atlantic Treaty Organisation’s (NATO’s) future naval TDL is an attempt to solve a similarly vexing quandary for communications at sea. The quandary in question is the fact that NATO’s Link-11, can receive or transmit, but cannot do both simultaneously, and is limited in the quantity of data it can handle.

To understand the limitations of Link-11, one must first examine the provenance of this TDL. According to the 2006 publication Tactical Digital Information Link-Technical Advice and Lexicon for Enabling Simulation, published by ASRC Communications, a firm providing US government military and civilian agencies with intelligence, surveillance and reconnaissance support, Link-11 can trace its lineage back to 1982 when the US Department of Defence (US DOD) published Military Standard-188-203. This specified the interoperability and performance standards for the Tactical Digital Information Link-A (TADIL-A), as Link-11 is also known. Military Standards (Mil-Std) are published by the US DOD to help achieve standardisations of equipment, protocols and practices. Although Mil-Std-188-203 was drafted by the US DOD, Link-11 has since proliferated through NATO, and allied nations. For example, the Royal Australian Navy’s (RAN) ‘Canberra’ class amphibious assault ships are outfitted with Link-11 compatible communications, according to the RAN, as are the ‘Baynunah’ class corvettes operated by the United Arab Emirates Navy, open source reports note. 

Maritime patrol aircraft, such as this French Navy Breguet/Dassault ATL-2 Atlantique may carry a number of TDLs including Link-16 for air operations, and Link-11 to support maritime operations. (Thomas Withington)

The Workings

The raison d’etre of Link-11 is fairly uncomplicated, while its operation is anything but. Essentially, Link-11 provides a communications network across NATO and allied forces, predominantly in the naval domain for the exchange of tactical information. According to the ASRC Communications publication, Link-11: “employs netted communications techniques and standard message formats for the exchange of digital information among airborne and land-based, submarine and shipboard tactical data systems.” These netted communications are performed using High Frequency (HF) and Ultra High Frequency (UHF) wavebands of two megahertz (MHz) to 29.9MHz, and 225MHz to 400MHz respectively. These two frequency choices serve two distinct purposes: Openly available information regarding Link-11 has stressed that when using the TDL for HF communications, it provides a range of circa 860 nautical miles/nm (1592.7 kilometres/km), thanks to the ability of HF transmissions to ‘bounce’ off the ionosphere; an area of the atmosphere at an altitude of between 40.5nm (75km) to 540nm (1000km). The ionosphere acts as a naturally-occurring ‘satellite dish’ across which transmissions can skip to reach intercontinental ranges. When being used for UHF communications, however, Link-11’s range dramatically reduces to circa 25.8nm (47.8km) given the inability of UHF transmissions to bounce off the ionosphere, restricting them to line-of-sight communications. Unsurprisingly, HF Link-11 can be used for long-range communications, with UHF Link-11 being utilised for short range communications, such as between ships in a naval task group.

At the heart of the Link-11 and Link-22 TDLs are their ability to exchange tactical information, particularly radar track information, between platforms. (Thomas Withington)

As Link-11 is used for the exchange of tactical messages it works as a computer-to-computer link. As computers form the heart of the network, messages must have the same meaning to the stations transmitting tactical information, as those receiving it. For example, a civilian cellphone receives a stream of data which contains information that this data should be presented to the user as a SMS (Short Message Service) text message and hence depicts it thus on the users’ cellphone screen. Digital information contained in this message informs the phone that the data traffic is a text message and not a ‘tweet’, for instance. Link-11 communications work in a similar fashion by employing messaging standards. Fundamental to Link-11 communications are the ‘M’ series messaging protocols used by the system, with 43 different M-series codings corresponding to the message’s function. According to the ASRC Communications publication, M3 messages will relate to the position of a radar surface track, while M6D messages relate to electronic warfare command and control. Fundamental to a Link-11 system onboard a ship (or any platform or at any installation, for that matter) is the Tactical Data System Computer (TDSC). The TDSC collects data from a ship’s equipment, such as its naval surveillance radar or its electronic warfare sensors and converts this information into the M-series messaging format. As an official statement provided to Armada by BAE Systems notes: “Within NATO, Link-11 is primarily used (to) support the exchange of air, surface and subsurface tracks.”

Roll Call

The modus operandi of managing the Link-11 network uses a so-called ‘roll call’ approach, according to BAE Systems’ statement. Each Link-11 network is controlled by a Data Net Control Station (DNCS) which is embedded in a Link-11’s communications ensemble. A network is comprised of the DNCS, and each Link-11 equipped platform, be that a surface ship, submarine, aircraft or shore installation known as a Participating Unit. When the network is running, each DNCS ‘polls’ each Participating Unit, requesting it to transmit any data that it may wish to share with other Participating Units on the network. After each Participating Unit has transmitted their data, they switch to a receive mode, allowing them to then receive data which has been transmitted by other units, for which they are the intended recipient. As long as the network is running, the DNCS continues to poll the Participating Units and they continue to transmit their information, before switching to receive information. However, this roll call system has shortcomings: One confidential source highly familiar with the workings of Link-11 and Link-22 told Armada that the roll call systems means that “technically there is no congestion (on a Link-11 network), but the net cycle turnaround (the time taken to transmit and receive by all Participating Unit) is extremely long, so the tactical data refresh rate is unacceptably long and you endanger the mission.” Furthermore, Link-11 is a point-to-point communications system which means that it cannot perform ad hoc networking by which a transmission can reach its intended recipient by hopping across several Participating Units which act as relays to get the transmissions to their intended recipient.

Put simply, when a Participating Unit with Link-11 communications joins a Link-11 network it is given a particular ‘slot’ by the Link-11 Net Control Station (NCS), which manages the overall network, for the transmission and reception of information. The NCS allocates a particular timeslot to the Participating Unit once it joins the network. A Link-11 network can host up to 61 units capable of transmitting and receiving Link-11 messages at any one time, but an unlimited number of Link-11-equipped platforms operating in a receive-only mode can be hosted on the network.

A thesis published in 1992 by Thor A. Simensen, a student at the US Navy’s Naval Postgraduate School in Monterey, California provides a simple description of how Link-11 messaging works: A ships’ sensor, such as her naval surveillance radar, generates some information, such as a track for a potentially hostile aircraft. This information is sent from the radar to the TDSC and converted into a Link-11 format message, in this case an M2 message relating to an air track position. This message is then encrypted using a KG-40/A encryption device. This now-encrypted message is sent to the Link-11’s Data Terminal Set (DTS), essentially a modem allowing computer-to-computer communications, and is changed from binary information into audio tones. These audio tones are then transmitted through a standard ship-borne HF/UHF tactical radio when it is that Participating Unit’s particular turn on the Link-11 network. For the reception of this transmission, the process works in reverse. Once the Participating Unit has sent its message, it’s intended recipient, perhaps another ship, receives the intended message. This message is received by the ship’s radio, converted from its audio tone into a digital signal using the DTS, decrypted by the KG-40/A and processed using the TDSC, which will recognise this message, due to the M series format used, as detailing radar track information, and then update the displays of the ship’s combat management system with this new information regarding the hostile aircraft track.

As noted above, Link-11 has been going strong since the early 1980s. Over that timeframe, a large number of companies have become involved in the provision of equipment to enable Link-11 communications. This includes DRS Technologies, Wavecom, Northrop Grumman, Thales, Data Link Solutions and Ultra Electronics to name just a few. Several of these companies are now involved in the provision of equipment for Link-11’s successor, Link-22. Despite Link-11’s all-important role of handling tactical messages between platforms, it has weaknesses. The statement provided to Armada by BAE Systems stressed that Link-11 is “based on 1960s technology.” As noted above, no more than 61 Link-11 transmitting and receiving Participating Units can be hosted on a single network. Moreover, the network has a single point of failure , should the platform hosting the NCS be lost or cease to function, the “entire link goes down.” It is possible to reconstitute the network by configuring another DTS in that network to act as the NCS should this eventuality occur, and despite this process taking seconds, it could result in precious seconds lost during high tempo naval operations.

Enter Link-11

Link-22 commenced life in the 1980s as the NATO Improved Link-11 (NILE) initiative which, you’ve guessed it, aimed at bettering the performance of Link-11. Six nations stepped up to the mantel regarding the development of Link-22; Canada, France, Germany, Italy, the UK and the US and, although the Netherlands was an original member, it was replaced by Spain. In 1992, NATO drafted the baseline agreement and requirement for Link-22’s interoperability, and on 26 January 2006 NATO’s Standardisation Agency in Brussels, which synthesises equipment and procedure standards across the alliance, published STANAG (Standardisation Agreement) 5522 which articulated the basic requirements for Link-22 communications. All NATO members are now obliged to ensure that their naval communications will handle Link-22 traffic over the coming decade as Link-11 is progressively phased out. Nevertheless, as Thomas Bogel, head of technologies and studies at Rohde and Schwarz’s secure communications division noted that: “Link-11 is not only used by nations which have or will have access to Link-22. Therefore it is expected that Link-11 will stay in service for many years and a continuous process of moving from Link-11 to Link-22 will happen.” The Link-22 initiative is managed by the NILE Programme Management Office (PMO) based at the US Navy’s Space and Naval Warfare Systems Command in San Diego, California. Beyond the NILE PMO, the NILE Communications and Interoperability Working Group ensures that Link-22 communications equipment provided by industry meets NATO cryptographic and interoperability standards.

Link-22 will be able to host an increased number of users on a single network, including air assets such as naval support helicopters, compared to the legacy Link-11 TDL. (Thomas Withington)

Advantages

Publicly available reports speak of Link-11 remaining in service until circa 2023. The chief advantages offered by Link-22 include its ability to employ the same HF/UHF tactical radios utilised for Link-11 communications, despite requiring a new Data Link Processor (DLP) to generate and receive Link-22 messages, and a new System Network Controller (SNC); a computer employing either the Linux or Windows-XP operating system to view and draft Link-22 messages. Both the DLP and SNC are linked to a Link Level Communications security system which handles all of the Link-22 encryption and decryption. In fact, this latter point is an understandable area of concern. One confidential source close to the Link-22 programme told Armada that the Link-11’s KG-40/A cryptographic unit is no longer considered secure enough for current and future military operations. Furthermore, a Link-22 network will be able to host 125 Participating Units (as opposed to 61 for Link-11). Finally, Link-22 offers major improvements in bandwidth. Presently, Link-11 can handle written messages at speeds of 1.8 kilobits per second (kbps). As BAE Systems’ statement articulated: “Data rates are slow and in a military environment where data must be kept from the opposition, there is a lack of jam resistance.” This resistance to jamming forms an integral component of Link-22’s architecture, and regarding data rates, these increase to four kilobits-per-second for HF Link-22 messages and up to 12.6kbps for messages transmitted across UHF.

Moreover, unlike the roll call architecture at the heart of Link-11, discussed above, Link-22 uses a Distributed Time Division Multiple Access (DTDMA) approach. Whereas the Link-11 architecture depends on a DNCS this is not the case with a Link-22 network. This means that there is “no single point of failure,” the confidential source continued. Each of the Participating Units or NILE Units (NUs) are assigned a unique address on the Link-22 network. Put simply, the DTDMA approach takes a channel which has a specific HF or UHF frequency. Each of these channels is divided into specific timeslots and each timeslot is assigned to a specific NU, with the specific NUs all transmitting in their assigned time slots one after another. This allows a large number of users to inhabit a specific channel without using it the whole time, and hence making it unavailable for other NUs.

Much as each NCS possesses a DTS in a Link-11 network enabling it to act as the network manager, in a Link-22 network, one NU is assigned to act as the Network Managing Unit (NMU), should the NMU be lost for whatever reason, a standby NMU, assigned when the network was activated, is ready to take up the task. A Link-22 so-called ‘super network’ can split itself into eight specific radio frequency channels which form networks and each NU can be a member of several of these networks and hence act as a gateway between them. The net effect of this, notes BAE Systems is that up to 125 NUs can be hosted on a Link-22 super network; a major improvement from the 61 Participating Units which can be hosted on a Link-11 network.

Whereas the Link-11 architecture uses M series messages (see above), Link-22 employs, according to the BAE Systems statement, ‘F’ series messages: “which have been designed for Link-22 and which have no exact counterpart in other data links (these messages) provide the same dictionary elements and 70 percent message format commonality to Link-16, in order to facilitate data forwarding.” Link-16 is a NATO and allied TDL primarily used for supporting air operations by the exchange of track and other tactical data between aircraft, vessels and land platforms/installations. This TDL is examined in more detail in the author’s Let’s Get Linky article in the October/November 2016 edition of Armada.

Theory into Practice

Major activity is now underway across NATO navies, led by the US Navy, to ensure that their respective platforms are ‘Link-22 ready’ for the new TDL’s entry into service early next decade. (Thomas Withington)

In development terms, Mr. Bogel notes that: “the development of the components of the tactical data link itself (are) already finished. Beyond that, there is individual work to be done to integrate Link-22 as a system,” at the heart of this will be ensuring that both legacy and new vessels, and other air and land platforms/installations, are equipped with the Link-22 ensemble as part of their communications. The hard work now focuses on rolling Link-22 out across NATO and allied nations. A statement provided by Thales, which is heavily involved in Link-22, noted that the full commissioning of the Link-22 architecture is expected to occur in 2020/2021, with the United States planning to perform interoperability testing with in-service Link-22 equipment in the 2020 timeframe and France expected to perform similar work a year earlier in 2019.

by Thomas Withington