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Page 151 B Analysis of the Capabilities and Limitations of Link 16 The Navy is currently supporting programs that are dedicated to improving the performance of Link 16 tactical radio network technology. These programs are managed by the ATDLS program office (SPAWAR PMW 159). The committee finds that these programs have technical merit and are likely to be of substantial benefit to the Navy. However, they are best viewed as late-life upgrades to a system that is nearing the end of its technical life cycle. This appendix contains the detailed technical analysis of Link 16 that has led the committee to its conclusions. Table B.1 shows the characteristics of networking schemes currently used in the Navy's tactical arena. JTIDS and its slightly more modern variant, the multifunctional information distribution system (MIDS), are the Navy's chosen radio subsystems for distributing force control messages. As such, these radio subsystems implement the Link 16 (TADIL-J) networking scheme and message set. The messages include surveillance tracks, weapons coordination, air control, target information, PPLI, and even digitized voice networks. JTIDS radios—or their MIDS variants—will be installed on a variety of aircraft, surface ships, and submarines over the next 7 years, as well as in Patriot and THAAD forces. B.1 JTIDS CHARACTERISTICS Certain technical characteristics of the JTIDS waveform that have important effects on the types of networks that can be built with JTIDS radios are briefly described in the following paragraphs.1 1 The material in this section is based on Naval Studies Board, National Research Council. 2000. Network-Centric Naval Forces: A Transition Strategy for Enhancing Operational Capabilities, Na-
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Page 152 TABLE B.1 Characteristics of Networking Schemes Characteristic TADIL A Link 11 TADIL C Link 4A TADIL J Link 16 TADIL J Link 22 Antijam No No Yes No Crypto-secure Yes No Yes Yes Data rate (kbps) 1.3 to 2.25 5.0 28.8 to 115.2 2.4 Message standard M series V/R series J series J series Participants 20 4-8 128+ 40 Critical nodes Yes Yes No No Voice circuits No No 2 No Architecture Radio broadcast Radio point-to-point TDMA TDMA Frequency HF/UHF UHF UHF/Spread HF/UHF Spread SOURCE: Welch, LCDR David, USN, “TADIL Comparison” in “U.S. Naval Tactical Data Links,” briefing to the Tactical Network Panel of the Committee on Network-Centric Naval Forces on February 17, 1999, Command and Control Systems Directorate, Office of the Chief of Naval Operations (N62G), Washington, D.C. B.1.1 Waveform JTIDS operates in the L-band. It divides the spectrum into 51 channels between 969 MHz and 1209 MHz, with a channel spacing of 3 MHz. Certain portions of the spectrum are also used for identification, friend or foe (IFF), tactical air and navigation (TACAN), distance measuring equipment (DME), and Mode S, which excludes two subbands and imposes some restrictions on exactly how JTIDS can be used in noncombat situations. In particular, time-slot duty cycles for JTIDS must be restricted to no more than 20 percent under normal conditions. Exercise conditions do not have duty-cycle restrictions, and full combat conditions have no restrictions. JTIDS uses a TDMA waveform. Every 24-hour day is divided, in the JTIDS waveform, in 112.5 epochs. Each epoch lasts 12.8 min and is subdivided into 64 frames of 12 s apiece. Each flame is further subdivided into 1536 time slots. tional Academy Press, Washington, D.C. Information on JTIDS has been derived from two sources: (1) Welch, LCDR David, USN, “U.S. Navy Tactical Data Links,” briefing to the Tactical Network Panel of the Committee on Network-Centric Naval Forces on February 17, 1999, Command and Control Systems Directorate, Office of the Chief of Naval Operations (N62G), Washington, D.C., and (2) U.S. Army Program Executive Office Air and Missile Defense and Life Cycle Engineering Center, Missile Research Development and Engineering Center, U.S. Army Aviation and Missile Command, “Introduction to JTIDS,” Redstone Arsenal, Ala.
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Page 153 Each time slot is thus 7.8125 ms long. Time slots within frames are organized into three distinct sets labeled A, B, and C. Time slots within a frame are identified as A-0, B-0, C-0, A-1, B-1, C-1, . . . A-511, B-511, C-511. A given radio (“terminal”) may have up to 64 blocks of time slots assigned to it. Each time-slot block is defined by a triplet called a time slot block (TSB): set (A, B, or C); index (0 to 511); recurrence rate (0 to 15). Each assignment for a given terminal is designated as transmit, receive, or relay. A JTIDS net is a group of terminals that exchange messages among themselves. In other words, it is a group of terminals whose time slots have been defined so that when one member of a net is transmitting, every other member of the net is receiving. Obviously this requires careful planning to ensure that indeed all the other members are receiving at that time, that only a single radio is granted a transmit time slot at a given time, and so forth. The JTIDS architecture allows 127 different nets (numbered 0 through 126) to be active simultaneously within the same RF spectrum. Since JTIDS is a frequency-hopping radio, each net is made mutually exclusive by assigning a unique frequency-hopping pattern for transmissions. B.1.2 Other Technical Characteristics Access modes. As defined, JTIDS provides three distinct access modes for a terminal that needs to transmit: dedicated access, contention access, and time slot reallocation access. Dedicated access. In this mode, the network planners ensure—by preparing the corresponding time-slot plan for a given network—that a given JTIDS terminal has exclusive use of an assigned TSB. This mode has the advantage that the terminal is guaranteed access to the network at regular intervals; it also has the corresponding disadvantage that the time slot is wasted if the terminal has nothing to say at a given moment. Contention access. In this mode, which is quite different, a given net provides a pool of time slots for any terminal's use. Any terminal that needs to transmit will randomly select a time slot from this pool and transmit in that time slot. This mode has a number of advantages: it is easy to plan, makes it quite simple for terminals to enter or leave the net while the net is in operation, and provides some of the traffic efficiencies of statistical multiplexing for traffic that is bursty or hard to predict. Its main disadvantage is that multiple terminals may transmit during the same time slot, which can result in lost messages and/or some terminals hearing one transmitter while others hear a different one. Time slot reallocation access. In this, the most complex mode, all terminals share a single pool of time slots, as is also true for contention access. Rather than transmit at will, however, the terminals perform a distributed algorithm to apportion the time slots. Each terminal transmits its bandwidth needs periodical-
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Page 154 ly, and every terminal performs identical algorithms to ensure that the pooled time slots are apportioned according to the needs. The committee believes that this access scheme has not yet been implemented in practice, but—as will be seen below—it is one of PMW 159's projects. B.1.3 JTIDS Data Rates Each JTIDS time slot has the following components: The time slot begins with a variable-start jitter delay; then, synchronization and time-refinement patterns; the payload (message header and data); and, finally, dead time to allow for RF propagation. This discussion will concentrate on the message data portion of a time slot. Each data portion can contain 3, 6, or 12 75-bit words, depending on the exact encoding of the message. Thus, each time slot can carry anywhere from 225 to 900 bits of data payload, giving an aggregate data rate for a given JTIDS net of between 28,800 and 115,200 bps. Some of this raw capacity is used for housekeeping and so is not available for tactical traffic, but these numbers give an idea of the approximate capacity of a JTIDS net. By comparison, current commercial phone-line modems run at roughly 53,000 bits per second in the downstream direction. Thus, one JTIDS net has a raw capacity ranging from one half to twice the capacity of a phone-line modem. Since JTIDS divides its available L-band spectrum into 51 channels, the extreme upper bound on the number of bits per second that can be transmitted simultaneously from all JTIDS terminals in a tactical arena is 51×115,200, or 5,875,200 bps. This assumes that all available spectrum is devoted to JTIDS, that all terminals use the maximum possible data rate, and that all time slots in all channels are used for transmission, and it ignores the overhead of housekeeping bits. Working from the previous calculation, JTIDS achieves 5,875,200 bps in 51× 3 MHz of RF spectrum, for an aggregate spectral efficiency of 0.0384 bps/Hz. Partly, of course, this is driven by the tactical need for very robust antijam features. To a noticeable extent, though, it is driven by the basic short-frame TDMA structure of the JTIDS waveform, where rather short payloads are surrounded by the dead times of synchronization patterns and propagation allowances. B.2 ASSESSMENT OF PMW 159'S PLANNED IMPROVEMENTSTO LINK 16 Table B.2 presents the improvements in tabular overview form. Subsequent paragraphs describe and assess each of the improvements in more detail. 2 2 Information in this section is derived from McCloud, Kenneth L., “PMW 159 Advanced Tactical Data Link Systems (ATDLS) Program Office,” briefing to the committee on July 26, 2000, Space and Naval Warfare Systems Command (PMW 159A), Arlington, Va.
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Page 155 TABLE B.2 Planned Improvements and Potential Benefits Planned Improvement Potential Benefit Dynamic network management system (DNMS) for Link 16 Incremental increases in the flexibility of Link 16 networks, perhaps coupled with greater ease of planning and configuring for such networks Enhanced throughput Higher bandwidth communications across Link 16 radio channels Optimized relative navigation More accurate relative position and time information for Link 16 platforms Joint range extension, S-TADIL J Increased ability to transmit J-series messages across non-JTIDS radio channels Link 16/JVMF advanced concept technology demonstration Gateways between Link 16 radios and their messages, on the one hand, and the Army's messaging system on the other Link 16 missile and tactical terminal (LMT2)/TacLink weapons Tactical command and position/location links to guided munitions B.2.1 Dynamic Network Management System for Link 16 These are a set of interrelated changes to the JTIDS channel access protocols that should allow more flexible use of JTIDS networks. Key technical features are improvements in the methods used for late net entry, for reallocating time slots as demand changes, and for varying the throughput rate dynamically. In passing, it is noted that many of these techniques have already been used in a wide variety of other radio systems and hence pose relatively little technical risk. Assessment. These incremental improvements will probably succeed and will make JTIDS somewhat more flexible than with its current (highly rigid) architecture. As a result, there is potential to make JTIDS easier to use in practice. This is important and useful work and should be supported. On the other hand, to a large extent these improvements are merely Band-Aids for a fundamentally unsuitable network architecture. They will allow more flexibility in the use of JTIDS networks, but the improved system can be considered “flexible” only in comparison with classic JTIDS; it is by no means as flexible as modern commercial systems. In addition, of course, JTIDS will remain a closed system. The final judgment, therefore, is that this work should be supported—it is certainly better than classic JTIDS—but that it will not in the end provide the degree of flexibility required for today's or tomorrow's tactical communication needs.
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Page 156 B.2.2 Enhanced Throughput This program aims to increase JTIDS's bandwidth by employing modern channel encoding techniques to achieve more bits per second per hertz. The upper bound on the improved speed is claimed to be 1.1 Mbps, which is nearly 10 times the current maximum rate. Assessment. This program is a low-risk incremental improvement to JTIDS that may well have practical utility. As such, it should be supported. It would be unwise, however, to assume that the new maximal rate of 1.1 Mbps will in fact be achieved often in practice. Maximum rates for wireless communications are usually achieved only for stationary objects that are quite close to each other in a clear RF environment, because performance degrades quickly with Doppler effects, distance, and interference. Since JTIDS is generally employed between mobile platforms across relatively long distances, the actual data rates may be well below maximal. In addition, since most JTIDS time slots are received by a number of different platforms, the transmitted data rate must reflect the lowest common denominator among the receivers (e.g., the farthest away, the fastest moving, the one with the oldest equipment). Again, the reader is reminded that the commercial wireless world is in a creative foment at the moment and that a large number of very-high-speed wireless technologies are now appearing in the market. On the whole, these technologies are likely to deliver significantly higher overall throughput than enhanced JTIDS since they do not suffer from JTIDS' very short time slots, which ensure that a very high percentage of potential transmission time is in fact sacrificed to dead time between bursts. This is, therefore, a good incremental enhancement to JTIDS, but the Navy should also look elsewhere for high-bandwidth wireless technology. B.2.3 Optimized Relative Navigation This program plans to improve the relative navigation capabilities of Link 16 so that it will deliver position/location information with an accuracy that equals or exceeds that of the current GPS system (≤3 m circular error probability) and time synchronization to the nanosecond level. It will do so by transmitting raw, uncorrected pseudo ranges and employing new algorithms on these data. Assessment. Higher-level functions of battle management such as the SIAP rely on highly accurate position and time information. Thus, any effort to improve these data could have a significant payoff. This particular method has the additional virtue of being independent of GPS and thus providing a robust backup capability for position and time services and should be supported.
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Page 157 B.2.4 Joint Range Extension, S-TADIL J Joint Range Extension, S-Tadil J combines two distinct programs, both of which aim to add a capability to transmit J-series messages across non-Link-16 communications channels. Assessment. This is a highly desirable goal, but the approach is fundamentally misguided. The key problem here is that JTIDS has confounded the distinct problems of message formats and RF channel architecture. The proper solution—and one that has been universally adopted in the commercial communications world, both in the Internet and in all telephone technology, for decades—has been to use a layered protocol stack so that any type of message can flow across any type of communications medium. Rather than sort out how J-series messages should be conveyed across any type of medium, these programs are attempting minor incremental “kludges” to work across satellites and so on. The committee believes that this entire approach will ultimately reach a dead end. A program that determined how J-series messages could sent using the Internet Protocol suite would better serve the Navy.3 They could then be transmitted across virtually every known type of communications channel with no additional effort on the Navy's part. B.2.5 Link 16/Joint VariableMessage Format ACTD This advanced concept technology demonstration (ACTD) will show that the J-series messages conveyed across Link 16 can be translated to the messages formats employed in the Army's digitized battlefield. Assessment. From a high-level viewpoint, it is unfortunate that the Army message formats are not compatible with those used in Link 16, but since this is the case it is clearly better to gateway the two systems together with translators than to have no connection between them at all. B.2.6 Link 16 Missile and Tactical Terminal/TacLink Weapons This concept envisions a small tactical radio, based on Link 16 technology, that can be installed in cruise missiles and other guided munitions to give them (1) a precise positioning system, (2) a command link for updates on mobile target locations, and (3) improved potential for battle damage assessment. This concept would also enhance overall situational awareness since it would allow all missiles and guided munitions to be included in the SIAP. Assessment. Judgment is reserved on this concept. It seems to be a forward-thinking idea that is well aligned with the necessary future direction of 3 There have been some limited experiments along these lines.
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Page 158 BMC3. And JTIDS certainly does have good antijam properties, which would be essential in such tasks. On the other hand, JTIDS provides a poor starting point for this concept, partly because JTIDS networks have proven extremely difficult to plan and configure, but mainly because thus far JTIDS radios are extremely expensive. It is certainly possible that both problems could be overcome, but the solution would certainly be much easier if a different starting point were adopted.
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