Tactical Data Link 16
In this second of a series of articles, ADBR examines joint force integration through the data link lens. We analyse LINK 16 in detail and provide an explanation of how it works and why it remains a key enabler for operational capability and coalition interoperability.
We highlight some of the challenges associated with Link 16 as part of a broader network, and the need to build a sovereign capability with the expertise to manage an increasingly complex network environment.
Tactical data links (TDL) provide resilient and trusted networks that enable the ADF to generate joint force capability. Yet the complexities involved within each network, added to the continuous introduction of new networking technologies, significantly challenges the ADF in achieving its ambition of a truly interconnected joint data network (JDN).
A JDN is important because it is a network of communications and electronics systems that carry TDLs, multi-sensor early warning information, and intelligence data to support joint force operations. The ADF JDN is primarily composed of data from multiTDL networks (MTN), through to intelligence networks such as the Integrated Broadcast Service (IBS). The ADF MTN currently uses Link 11, Link 16, satellite Link 16 (SAT-J), and the Joint Range Extension Application Protocol (JREAP).
Link 16 remains the ADF’s primary TDL and is essential for tactical co-ordination across the five operational domains of air, land, maritime, space, and information and cyber.
LINK 16 FUNDAMENTALS
‘Link 16’ is the term generally applied to both the message standard used for exchanging information as well as the physical bearer system, commonly known as the terminal. Link 16 possesses many advantages over legacy data links such as Link 11.
Link 16 uses a system architecture known as Time Division Multiple Access which allows users to be assigned an opportunity to transmit or listen to data over a fixed period of time, known as a time slot (TS). At the operational level, there are 1,536 TSs spread across a 12-second frame that are then divided between users based upon their capabilities and information exchange requirements.
The result is that some users get more TSs than others. Dependent upon the network design, some TSs may be shared between users while others are dedicated to a single user. Link 16 terminals operate within the ultrahigh frequency band 969-1206MHz and hop over 51 frequencies spaced 3MHz apart. However, this band is shared with other civilian aeronautical radio navigation systems and consequently terminal transmissions could potentially interfere with national air navigation and the safety of flight.
To avert any detrimental impact on these systems, every Link 16 terminal has an interference protection feature. Each user nation then decides how Link 16 shall be used by approving a frequency clearance agreement.
There are a variety of Link 16 terminals in existence, ranging from the Joint Tactical Information Distribution System (JTIDS) Class II, variants of the Multifunctional Information Distribution System (MIDS), through to those that are software defined, specifically MIDS-Joint Tactical Radio System (JTRS), and small form factor radios.
Both the JTIDS Class II and MIDS variants are hardware defined with the latter the current terminal of choice for many users. However, there is no doubt that while the software-defined radios provide the user with more flexibility and added capability, that capability carries with it the significant risk that terminal interoperability issues will soon affect the joint use of Link 16 in the battlespace.
WHAT IS A NETWORK?
One element of Link 16 that continues to cause confusion is the use of the terms network and nets. To begin with, a network is a number of synchronised participants that know where the 12-second frame begins and ends. It is critical that all users correctly synchronise in order to successfully exchange data. To support the synchronisation process a common Network Time Reference (NTR) must be established between all users.
To support this, each terminal has a clock and users can only establish and maintain synchronisation by alignment of all user clocks. The network time can be provided in two ways, either by a single user operating within the network or via an external source (External Time Reference). The first method is where a single user is allocated the duty of being the NTR and, as a result, the clock time within their terminal is used as the system time of the Link 16 network.
This user NTR then periodically transmits a net entry message to help other units join the network, align their own terminal time and thus maintain synchronisation. The second is where users utilise time from an external source, most commonly co-ordinated universal time (UTC). Some terminals are ETR-capable and, as the time source is external, a Link 16 network can have multiple NTRs as they are all using and subsequently transmitting the same time.
It is important to note that even if a user is not ETR-capable they can still operate in an ETR network, they simply need to be given the time.
WHAT IS A NET?
A net is defined by a discrete frequency-hopping pattern (FHP) and within each network there are 128 numbered 0-127. These 128 individual patterns are derived from the crypto-transmission security (TSEC). A Link 16 terminal hops over the 51 frequencies based on the net upon which it has been designed to operate. To add flexibility, a terminal can jump between nets at the end of each TS. Thus, for a user to maintain synchronisation within the network, and correctly align their terminal to an FHP, they must align and maintain system time.
RIGHT PLACE AT THE RIGHT TIME
Given the many variables involved, there can be a number of reasons why units are unable to join a network. The most common mistake is that users load the wrong crypto (TSEC) and consequently fail to be on the right FHP even if they have the right Link 16 network and time. With a Link 16 network using 128 different Frequency Hopping Patterns spread across the 1,536 TS, the art is to ensure a user’s terminal is in the right place (TS) at the right time (FHP) to exchange data. Having established a TS as the basic unit of access to the Link 16 network, we need to understand how much capacity each TS affords the user.
Our 12-second frame consisting of 1,536 TS equates to each TS measuring a period of 7.8125 milliseconds. This period is further sub-divided and it is the data portion that allows users to transmit information. Once again, the network design process is fundamental, as it is this process which determines the capacity (packing) available to the user within the data portion of the TS. As we explained in the November-December 2019 edition of ADBR, Link 16 uses J-Series messages to exchange data.
There are various J-Series messages defined from messages that convey situational awareness information through to digital control and position, location and identification information. All these messages are subsequently allocated a label and sub-label, for example the J3.2 is the Air Track message where 3 is the label and 2 is the sub-label. However, it is how these messages have been defined that we need to understand in order to identify how many J-Series messages a user can fit within each TS.
United States (US) Military Standard-6016 explicitly describes how each J-Series message is constructed and transmitted. Each J-Series message consists of a number of 70-bit words and these words are described as initial, extension and continuation. Every J-Series message starts with an initial word, and one or more extension words may be required depending upon the purpose of the message, and finally, one or more continuation words may be necessary.
In the case of the J3.2 Air Track message, this has one initial word, one extension word and five continuation words defined. The rules within the US standard dictate that a minimum of two words shall be transmitted for each Air Track message.
Therefore, if the network design process had packed the data portion of a TS to support only three words, a user would be limited to a single Air Track. Whereas, if a TS was packed to support 12 words a user could potentially transmit up to six Air Tracks. When packing a TS this way there is a considerable difference to the capacity of the TS, and while this is an obvious advantage it also carries disadvantages such as reduced electronic countermeasure (ECM) resistance and range.
Another disadvantage of Link 16 is that it operates over ultra-high frequency and thus communications is line-of-sight (LOS) only. Considering the extent of many operational Link 16 networks, there is a potential that users are unable to exchange data. Joint Range Extension Application Protocol (JREAP) enables a user to overcome this, but not all users are equipped with this capability.
Furthermore, Link 16 was in use well before JREAP and so to overcome the issue of beyond LOS (BLOS) a feature known as Relay can be employed within any Link 16 network, as long as it is included within the network design process. Unfortunately, like many features of Link 16, there is always a downside: relay costs, and it costs the network design process TS.
We already know that a user is allocated TS to transmit their data but what if the receiver is BLOS? Basically, a third party known as a relay unit has to be in LOS with both the sender and receiver and operating on the right net at the right time. Added to this, the relay unit has to be allocated a relay TS for every TS of data the sender needs relaying, otherwise known as paired slot relay. The result is that the 1,536 time slots can very quickly be used up, and they certainly will if every unit wants to relay all of their data.
Accordingly, network planning and design ensure that a balanced approach is applied across all users within the network while accommodating each user’s individual information exchange requirements.
DESIGN AND PLANNING
Simply viewing Link 16 in terms of technology underplays the importance of building the team of people able to plan, design and manage the Link 16 network. On top of that there are the other TDLs within the MTN and that is only part of a broader JDN. The TDL community has well-established procedures in place to meet the challenges involved in establishing the MTN. However, the major challenge for the ADF and other nations using the TDLs is how to ensure the right data reaches the right people at the right time across the whole of the JDN.
In Australia, we can achieve this by continuously cooperating with our allies, enhancing knowledge delivery, and building upon the positive relationship that already exists between the Commonwealth and industry. In our next article, we will continue to explore how Link 16 will fundamentally change over the next 10 years. We will discover that the introduction and operational use of software defined radios will add another level of complexity to the Link 16 network. This will require careful management and a much broader working knowledge of data links across the Australian defence and industry community.