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 com- mittee 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 up- grades to a system that is nearing the end of its technical life cycle. This appen- dix 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 distrib- uting 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.1.1 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 frame 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.
TABLE B.1 Characteristics of Networking Schemes
TADIL A TADIL C TADIL J TADIL J
Characteristic Link 11 Link 4A Link 16 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 Radio TDMA TDMA
broadcast point-to-point
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 Opera- tions (N62G), Washington, D.C.
APPENDIX B 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 them- selves. 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 pre- paring 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 termi- nals 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-
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 pat- terns; 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 num- bers 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 simulta- neously 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 chan- nels 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 sur- rounded by the dead times of synchronization patterns and propagation allow- ances.
B.2 ASSESSMENT OF PMW 159’S PLANNED IMPROVEMENTS