DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

�

NONRESIDENTTRAININGCOURSE

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July 1997

Fire Controlman

Volume 6—Digital Communications

NAVEDTRA 14103�

DISTRIBUTION STATEMENT A: Approved for public release; distribution is unlimited.

Although the words “he,” “him,” and“his” are used sparingly in this course toenhance communication, they are notintended to be gender driven or to affront ordiscriminate against anyone.

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PREFACE

By enrolling in this self-study course, you have demonstrated a desire to improve yourself and the Navy.Remember, however, this self-study course is only one part of the total Navy training program. Practicalexperience, schools, selected reading, and your desire to succeed are also necessary to successfully roundout a fully meaningful training program.

COURSE OVERVIEW: In completing this nonresident training course, you will demonstrate aknowledge of the subject matter by correctly answering questions on the following subjects:

• types of communications systems;• the decibel system of power measurement;• synchronous and asynchronous communications as used in data communications systems;• methods of data modulation and demodulation used in various types of data networks;• the operation of modems used in data communications;• methods of multiplexing data in communications networks;• equipment associated with and the operation of the Link-11 data communications system;• equipment associated with and the operation of the Link-4A data communications system;• equipment associated with and the basic operation of local area networks.

THE COURSE: This self-study course is organized into subject matter areas, each containing learningobjectives to help you determine what you should learn along with text and illustrations to help youunderstand the information. The subject matter reflects day-to-day requirements and experiences ofpersonnel in the rating or skill area. It also reflects guidance provided by Enlisted Community Managers(ECMs) and other senior personnel, technical references, instructions, etc., and either the occupational ornaval standards, which are listed in the Manual of Navy Enlisted Manpower Personnel Classificationsand Occupational Standards, NAVPERS 18068.

THE QUESTIONS: The questions that appear in this course are designed to help you understand thematerial in the text.

VALUE: In completing this course, you will improve your military and professional knowledge.Importantly, it can also help you study for the Navy-wide advancement in rate examination. If you arestudying and discover a reference in the text to another publication for further information, look it up.

1997 Edition Prepared byDSCS(SW/AW) Robert M. Maynard

FCCS(SW) Edwin L. Rodriguez

Published byNAVAL EDUCATION AND TRAINING

PROFESSIONAL DEVELOPMENTAND TECHNOLOGY CENTER

NAVSUP Logistics Tracking Number0504-LP-026-7710

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Sailor’s Creed

“I am a United States Sailor.

I will support and defend theConstitution of the United States ofAmerica and I will obey the ordersof those appointed over me.

I represent the fighting spirit of theNavy and those who have gonebefore me to defend freedom anddemocracy around the world.

I proudly serve my country’s Navycombat team with honor, courageand commitment.

I am committed to excellence andthe fair treatment of all.”

TABLE OF CONTENTS

CHAPTER

1

2

3

4

5

6

APPENDIX

I

II

PAGE

Fundamentals of Data Communications . . . . . . . . . . . . . . . . 1-1

The LINK-11 System . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-1

LINK-11 Fault Isolation . . . . . . . . . . . . . . . . . . . . . . . . . . 3-1

LINK-4A . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4-1

New Technology in Data Communications . . . . . . . . . . . . . 1-1

Local-Area Networks . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-1

Glossary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . AI-1

References Used to Develop the TRAMAN . . . . . . . . . . . . AII-1

INDEX . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .INDEX-l

NONRESIDENT TRAINING COURSE follows the index

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INSTRUCTIONS FOR TAKING THE COURSE

ASSIGNMENTS

The text pages that you are to study are listed atthe beginning of each assignment. Study thesepages carefully before attempting to answer thequestions. Pay close attention to tables andillustrations and read the learning objectives.The learning objectives state what you should beable to do after studying the material. Answeringthe questions correctly helps you accomplish theobjectives.

SELECTING YOUR ANSWERS

Read each question carefully, then select theBEST answer. You may refer freely to the text.The answers must be the result of your ownwork and decisions. You are prohibited fromreferring to or copying the answers of others andfrom giving answers to anyone else taking thecourse.

SUBMITTING YOUR ASSIGNMENTS

To have your assignments graded, you must beenrolled in the course with the NonresidentTraining Course Administration Branch at theNaval Education and Training ProfessionalDevelopment and Technology Center(NETPDTC). Following enrollment, there aretwo ways of having your assignments graded:(1) use the Internet to submit your assignmentsas you complete them, or (2) send all theassignments at one time by mail to NETPDTC.

Grading on the Internet: Advantages toInternet grading are:

• you may submit your answers as soon asyou complete an assignment, and

• you get your results faster; usually by thenext working day (approximately 24 hours).

In addition to receiving grade results for eachassignment, you will receive course completionconfirmation once you have completed all the

assignments. To submit your assignmentanswers via the Internet, go to:

http://courses.cnet.navy.mil

Grading by Mail: When you submit answersheets by mail, send all of your assignments atone time. Do NOT submit individual answersheets for grading. Mail all of your assignmentsin an envelope, which you either provideyourself or obtain from your nearest EducationalServices Officer (ESO). Submit answer sheetsto:

COMMANDING OFFICERNETPDTC N3316490 SAUFLEY FIELD ROADPENSACOLA FL 32559-5000

Answer Sheets: All courses include one“scannable” answer sheet for each assignment.These answer sheets are preprinted with yourSSN, name, assignment number, and coursenumber. Explanations for completing the answersheets are on the answer sheet.

Do not use answer sheet reproductions: Useonly the original answer sheets that weprovide—reproductions will not work with ourscanning equipment and cannot be processed.

Follow the instructions for marking youranswers on the answer sheet. Be sure that blocks1, 2, and 3 are filled in correctly. Thisinformation is necessary for your course to beproperly processed and for you to receive creditfor your work.

COMPLETION TIME

Courses must be completed within 12 monthsfrom the date of enrollment. This includes timerequired to resubmit failed assignments.

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PASS/FAIL ASSIGNMENT PROCEDURES

If your overall course score is 3.2 or higher, youwill pass the course and will not be required toresubmit assignments. Once your assignmentshave been graded you will receive coursecompletion confirmation.

If you receive less than a 3.2 on any assignmentand your overall course score is below 3.2, youwill be given the opportunity to resubmit failedassignments. You may resubmit failedassignments only once. Internet students willreceive notification when they have failed anassignment--they may then resubmit failedassignments on the web site. Internet studentsmay view and print results for failedassignments from the web site. Students whosubmit by mail will receive a failing result letterand a new answer sheet for resubmission of eachfailed assignment.

COMPLETION CONFIRMATION

After successfully completing this course, youwill receive a letter of completion.

ERRATA

Errata are used to correct minor errors or deleteobsolete information in a course. Errata mayalso be used to provide instructions to thestudent. If a course has an errata, it will beincluded as the first page(s) after the front cover.Errata for all courses can be accessed andviewed/downloaded at:

http://www.advancement.cnet.navy.mil

STUDENT FEEDBACK QUESTIONS

We value your suggestions, questions, andcriticisms on our courses. If you would like tocommunicate with us regarding this course, weencourage you, if possible, to use e-mail. If youwrite or fax, please use a copy of the StudentComment form that follows this page.

For subject matter questions:

E-mail: n311.products@cnet.navy.milPhone: Comm: (850) 452-1503

DSN: 922-1503FAX: (850) 452-1370(Do not fax answer sheets.)

Address: COMMANDING OFFICERNETPDTC N3116490 SAUFLEY FIELD ROADPENSACOLA FL 32509-5237

For enrollment, shipping, grading, orcompletion letter questions

E-mail: fleetservices@cnet.navy.milPhone: Toll Free: 877-264-8583

Comm: (850) 452-1511/1181/1859DSN: 922-1511/1181/1859FAX: (850) 452-1370(Do not fax answer sheets.)

Address: COMMANDING OFFICERNETPDTC N3316490 SAUFLEY FIELD ROADPENSACOLA FL 32559-5000

NAVAL RESERVE RETIREMENT CREDIT

If you are a member of the Naval Reserve, youmay earn retirement points for successfullycompleting this course, if authorized undercurrent directives governing retirement of NavalReserve personnel. For Naval Reserve retire-ment, this course is evaluated at 6 points. (Referto Administrative Procedures for NavalReservists on Inactive Duty, BUPERSINST1001.39, for more information about retirementpoints.)

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Student Comments

Course Title: Fire Controlman, Volume 6—Digital Communications

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NETPDTC 1550/41 (Rev 4-00

CHAPTER 1

FUNDAMENTALS OF DATA COMMUNICATIONS

INTRODUCTION

Although you, as a Fire Controlman, may not be directly involved in datacommunications, you definitely need to be aware of how data communicationsaffects your ship’s mission. This training manual introduces and explains thebasics of data communications. Computer data frequently must be transmitted fromone point to another. The distance involved maybe a few feet, or it may be hundredsof miles. Data transited over long distances often must be converted to a formcompatible with either landline or radio wave transmission and reception. Thischapter explains how such conversion occurs and techniques used in the conversionand transmission procedures.

After completing this chapter, you should be able to:

State the types of communications systems.

Describe the decibel system of power measurement.

Explain asynchronous and synchronous communications as used in datacommunications systems.

Describe the methods of data modulation and demodulation used invarious types of data networks.

Describe the operation of modems used in data communicationsnetworks.

Describe the methods of multiplexing data in communications networks.

COMMUNICATIONS SYSTEMS

The devices used to transfer digital data makeupwhat is known as a communications system. In itsmost basic form, a communications system consists ofthe three components shown in figure 1-1. They are

Figure 1-1.—Communications system.

the transmitter, the receiver, and a communicationschannel that connects the two units.

The transmitting equipment converts the data ofthe sending system into a form that can be sent overthe communications channel, accepted by thereceiving equipment, and converted back into usabledata by the receiving system. Data sent over acommunications system is in one of the following twoforms: analog or digital.

An analog signal used in data communicationsvaries continuously between a minimum and amaximum value. As the signal varies, it assumes aninfinite number of specific values between the two

1-1

digital data pulses into a form acceptable to thevarious types of communications channels. Theequipment most often transmits digital data over adistance by varying a continuous analog signal inamplitude, frequency, or phase.

Communications channels that can pass data intwo directions (transmit and receive) are known asduplex channels. Single-direction channels aresimplex channels. Duplex channels may operate inone of the following two modes: half-duplex orfill-duplex. Half-duplex channels transmit data inone direction, pause, and then receive data comingfrom the opposite direction. Full-duplex channels, onthe other hand, can transmit and receive datasimultaneously.

TYPES OF COMMUNICATIONS CHANNELS(TRANSMISSION MEDIA)

In the fleet and at shore activities, you willencounter several forms of communications channels.The most common channels are landlines and radiocommunications.

Landlines

Landlines are physical lines or cables that connectthe digital equipment. Originally, landlines referredto telephone lines and were limited to carrying analogaudio frequencies (voice frequencies). For digitalinformation to be carried over these lines, the

limits. The signal can be varied in amplitude(amplitude modulation), frequency (frequencymodulation), or phase (phase modulation) to conveythe data. We will discuss each type of modulationlater in this chapter.

A digital signal has a limited set of values (1 or 0,true or false, etc.). A limited number of discretepulses can be transmitted in a fixed period. Theunique sequence of the bits represents the data.

Digital equipments (computers and peripherals)within a system normally communicate with eachother in pure digital pulses (serial and parallel).Transmitting digital information over a distancerequires the use of special equipment to convert

characteristics of one or more tones or carriers in theaudio-frequency range had to be modified inamplitude, frequency, or phase.

Today, telephone lines are commonly used inmany network applications. Bulletin boards, such asBUPERS ACCESS, use existing telephone lines; butmany landline-based systems use dedicated lines.Dedicated lines are common in local area networks(LANs). In a LAN system, several computers arejoined together to share information with all the userson the system. System connections are made usingcoaxial, dual-coaxial, fiber-optic, or twisted-paircable. The type of cable depends on several factors,such as the number of users on the LAN and themaximum distance between workstations.

The device used to convert the digital data into aform usable by the communications channel and backto digital data is known as a modem.

Modem is an acronym for MOdula torDemodulator. The modulator function converts thedata of the transmitting system into discretemodifications of the tone or carrier signals. The de-modulator converts the data-carrying tone or carriersignal into digital data for the receiving system.

Radio

Radio waves have been used for teletype andvoice communications for many decades. Theadvantages of radio-based systems are that they aremore mobile and can communicate over barriers suchas large bodies of water. Tactical information links,like those we will cover in chapter 2, are almostexclusively radio-based.

Radio communications are based on frequencyranges or radio-frequency bands. The frequencyrange of the carrier frequency determines theoperational characteristics of the system. Table 1-1illustrates the international frequency bands and theiruses. The tactical digital information systems used bythe Navy generally use portions of the hf and uhfbands.

1-2

Table 1-1.—Frequency Bands and Their Applications

In the radio transmitter, the data signals (discreteor tones) are modulated (impressed) on to the carrierfrequency and transmitted into space when thetransmitter is keyed. A receiver tuned to the carrierfrequency picks up the signal and demodulates thedata-carrying signals from the carrier. The datasignals can then be converted to digital data by theappropriate devices. For more information on radiooperations, refer to Navy Electricity and ElectronicsTraining Series (NEETS), Module17— Radio-Frequency Communications Principles.

THE DECIBEL MEASUREMENT SYSTEM

Technicians who deal with communicationsequipment often speak of the gain of an amplifier ora system in units called decibels (dB). Decibels areused as an indication of equipment performance;therefore, you need a basic understanding of thedecibel system of measurement.

As the actual calculation of decibel measurementis seldom required in practical applications, theexplanation presented in this text is somewhatsimplified. Most modern test equipment is designedto measure and indicate decibels directly. This design

eliminates the need for complicated mathematicalcalculations. Nevertheless, because many data linksystem alignment procedures center around dBreadings and references, you need to understand thesignificance of an equipment gain rating as expressedin decibels.

The equipment used in communications systemsconsists of several components, such as amplifiers,communications lines, antennas, couplers, andswitches. Each component in the system will affectthe signal by introducing a signal loss or gain. Theselosses and gains can be described by a ratio of thepower input and output by the equipment or cable.The ratio can be calculatedformula:

Output power =Input power

by using the following

Power ratio

If a communications system has four components,the gain or loss at each component must be calculatedand these ratios multiplied. The following is anexample of the gain/loss calculation of afour-component system:

1-3

In this system, the output of the signal is twice asstrong as the input to the system.

As you can see, this constant multiplication of theratios can be wearisome, and the products can beextremely small or large. Therefore, the discoverythat adding the logarithms of the numbers would yieldthe same result as this calculation led early scientiststo develop the unit of measure called the bel.

The bel, named in honor of Alexander GrahamBell, expresses the logarithmic ratio between the inputand output of any given component, circuit, orsystem. The bel maybe expressed in voltage, current,sound levels, or power. The formula is as follows:

The gain of an amplifier can be expressed in bels(N) by dividing the output (P1) by the input (P2) andtaking the base 10 logarithm (log 10) of the resultingquotient. Thus, if an amplifier doubles the power, thequotient will be 2. When you consult a logarithmtable, you will find that the base 10 logarithm of 2 is0.3; so the power gain of the amplifier is 0.3 bel.

Experience has shown that the bel is a rather largeunit that is difficult to apply. A more practical, easierunit to apply is the decibel (1/10 bel). Any figureexpressed in bels can be converted to decibels bymultiplying the value by 10. Thus the ratio of 0.3 belis equal to 3 decibels.

The reason the decibel system is used to expresssignal strength is shown in table 1-2. For example,saying that a reference signal has increased 50 dB ismuch easier than saying that the output has increased100,000 times.

The basis of the decibel measuring system is theamount of increase or decrease from a reference level.Whether the input power is increased from 1 watt to

1-4

100 watts or from 1,000 watts to 100,000 watts, theamount of increase, or gain, is still 100 times or20 dB. Examine table 1-2 again, taking particularnote of the power ratios for source levels 3 dB and 6dB. As the table illustrates, an increase of 3 dBrepresents a doubling of power. The reverse is alsotrue. If a signal decreases by 3 dB, half of the poweris lost. For example, a 1,000-watt signal decreased by3 dB will equal 500 watts, while a 1,000-watt signalincreased by 3 dB will equal 2,000 watts.

Table 1-2.—Decibel Power Ratios

When you speak of the dB level of a signal, youare actually speaking of the logarithmic comparisonbetween the input and output signals. The inputsignal is normally used as the reference signal. Insome instances, a standard reference signal must beused in place of the input signal. The most widelyused reference level is a 1-milliwatt signal (600-ohmload). When the 1-milliwatt reference is used, thestandard decibel abbreviation of dB is changed todBm; dBms are used as an indication of power, whiledBs are used to indicate the ratio between the inputand output.

A signal level of +3 dBm is 3 dB above 1milliwatt, and a signal level of –3 dBm is 3 dB below1 milliwatt. Whether you are using dB or dBm, a plussign (+) or no sign indicates that the output level is

Figure 1-2.—Asynchronous character code.

greater than the reference (power gain), while a minussign (–) indicates that the power level is less than thereference (power loss). The value 0 dBm indicatesthat the output power is equal to the 1-milliwattreference. It is also used to express a definite amountof power (1 milliwatt). The value 0 dBm equates to1 milliwatt.

DIGITAL DATA COMMUNICATIONSTECHNIQUES

Data signals transmitted over communicationschannels need to follow specific protocols to ensurethey are synchronized. In normal I/O data exchanges,this process is accomplished by the system of requestsand acknowledges. In addition, the data signals haveto be properly formatted for the receiving computer todecode them properly.

ASYNCHRONOUS AND SYNCHRONOUSCOMMUNICATIONS

Two major data-formatting methods are used tomake sure the transmitting computer and the receivingcomputer(s) are synchronized: asynchronous(character framed) and synchronous (messageframed). Both methods are used to identifyintelligence transmitted in the form of serial bitstreams.

Asynchronous Transmission

Asynchronous transmission of data is commonlyfound in landline communications systems and someforms of teletype communications. Generally,asynchronous, or character-framed, transmission isused to transmit seven- or eight-bit data, usually inASCII character format. Each character has a specificstart and end sequence—usually one start bit and oneor two end (stop) bits. Figure 1-2 illustrates thetransmission format of an asynchronous data stream.A parity bit (even or odd) maybe included to ensurethe accuracy of the transmitted data. Asynchronouscharacters may be transmitted one at a time or as astring of characters; however, each charactertransmitted will have start and end bits. When datasignals are transmitted in this format, synchronizationoccurs on a character-by-character basis between thetransmitting and receiving devices and provides someallowance for timing inaccuracies. Any inaccuracy intiming is corrected with the arrival of the nextcharacter.

Synchronous Transmission

Most tactical digital information linkscommunicate using synchronous messages.Synchronous transmission is a more sophisticatedmethod of data transmission. It sends data in longuninterrupted streams, with a predefine start and stopsequence. The start sequence is generally referred to

Figure 1-3.—Synchronous message format.

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as the preamble. The principal function of thepreamble is to alert the receiver of incoming data andprovide a reference to synchronize the receiver withthe transmitted signal. Following the preamble is astart code that informs the receiving equipment of thebeginning of the message data. The basic format ofthe synchronous data message is shown in figure 1-3.The incoming bit stream is then used to synchronizethe receiver or demodulator timing. A stop codefollows the message data to indicate the end oftransmission.

MODULATION/DEMODULATION

Modulation modifies a signal so it can carry dataover the communications channel. The demodulatorremoves the data from the carrier. For most datacommunications applications, the carrier is acontinuous sinusoidal waveform (sine wave). Thefrequency of the carrier varies, depending on theapplication. Landline transmission generally uses theaudio-frequency bandwidth signals (300 to 3,000 Hz).Radio channels use audio-frequency tones as datacarriers modulated to a radio-frequency signal, or theymodulate the radio-frequency signal itself to conveydata.

The three basic modes of modulation areamplitude modulation, frequency modulation, andphase modulation. Each of these modes modifies thecarrier signal in some manner to convey data.

Amplitude Modulation

When amplitude modulation is used for digitaltransmissions, the amplitude of the carrier signalrepresents the two discrete data states (1 or 0). Thesignal represents a logic 1 when the amplitude(peak-to-peak), at the same frequency, is greater at adifferent time, as shown in figure 1-4. The decreasein signal amplitude, below a predetermined threshold,indicates a change from a logic 1 to a logic 0.

Frequency Modulation

The frequency of the carrier signal or audio tonesmodulated to the carrier signal can be modified toindicate the two discrete states. As shown in figure

Figure 1-4.—Amplitude modulation.

1-5, a selected frequency can be used to indicate the1 state of a bit, and another selected frequency can beused to indicate the 0 state. The change in frequency,or frequency shift, indicates the same relationship asthe change in amplitude did in amplitude modulation.

Figure 1-5.—Frequency modulation.

Shifting the frequency of the carrier signal iscalled frequency-shift keying (FSK) or binaryfrequency-shift keying (BFSK). FSK usuallyinvolves shifts to frequencies above or below aselected center frequency. Transmission of thefrequency above the center frequency indicates abinary 1; the frequency below the center frequencyindicates a binary 0. The center frequency is nottransmitted. FSK is used in systems such as link 4A.

Another method of using frequency shifts involvesaudio-frequency tones. Two discrete audio tones maybe modulated to a constant frequency carrier signal.One of the tones is used to indicate a mark, or binary1, the other a space, or binary 0. This method offrequency modulation is called audio-frequency toneshift (AFTS).

Phase Modulation

Phase modulation ismodulation. It is based

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a more complex mode ofon the relationship of the

360-degree carrier sine wave to the baseline of thesine wave. The carrier signal starts on the baseline, asillustrated in figure 1-6, and continues to form a curvecalled the sine wave. When the sine wave reaches itsmaximum positive amplitude, it is at the 90-degreepoint. When it returns to the baseline, it is at 180degrees. When it reaches its maximum negativeamplitude, it is at 270 degrees; and when it returns tothe baseline, it is at 360 degrees or the 0-degree pointfor the start of the next cycle. This process occursover a period, with the number of full cycles persecond (Hz) being the frequency of the signal. A fullcycle is the transition from the 0-degree point to the360-degree point.

Figure 1-6.—Carrier sine wave,

For a particular frequency this process continueswithout interruption. Phase modulation involvesinterrupting the cycle at one or more degree pointsand instantaneously changing the direction oramplitude of the sine wave. Figure 1-7 shows how a180-degree phase shift is used to indicate two discretestates. The third cycle of the carrier is interrupted atthe 180-degree point. Instead of continuing in thenegative direction, the sine starts at the 0-degree pointagain. The resultant signal has the same frequencyand amplitude as the original signal but is 180 degreesout of phase. This phase shift can be directly relatedto a digital input at a modulator in which one

particular phase represents the 0 bit and the otherphase represents the 1 bit.

Multibit Modulation

While the 180-degree phase shift can be used toindicate two discrete states, many points on the sinewave can be defined to represent different bitconfigurations. Individual phase changes of 0degrees, 90 degrees, 180 degrees, and 270 degreesfrom a reference phase can each represent twoseparate data bits. For example, a 0-degree phaseshift or no phase shift could indicate a binary 00; a90-degree phase shift, a binary 01; a 180-degree phaseshift, a binary 10; and a 270-degree phase shift, abinary 11. This type of modulation is known as amultibit, or quadrature (four-state) phase-shiftmodulation, as shown in figure 1-8. Keep in mindthat only one continuous frequency and amplitudesignal is being phase-modulated to transmit two bitsof data for each phase shift.

Figure 1-8.—Multibit phase modulation.

A modification of the quadrature phase-shiftmodulation, called differential quadraturephase-shift keying, uses the difference between aphase-shifted signal and its preceding sine wave torepresent data. When a phase shift is detected, thecurrent signal is compared with the previouslytransmitted phase signal. The difference between thetwo signals is computed to determine the amount ofphase shift. The previously transmitted signal is usedas the reference phase for demodulating the data bits.Two binary digits are represented by phase changes of-45, -135, -225, and -315 degrees. The -45 degreeshift indicates a binary 11; the -135 degree shift, abinary 01; the -225 degree shift, a binary 00; and the-316 degree shift, a binary 10.

Figure 1-7.—Phase modulation.

1-7

Figure 1-9.—Full-duplex modem.

MODEMSTransmitter Section

Modems come in a variety of configurations.Their design dependsincluding the following:

Asynchronoustransmissions

on a number of factors,

or synchronous data

Simplex, half-duplex, or full-duplexcommunications

Type of communications channel

Type of modulation/demodulation used

Modems may be stand-alone devices with theirown power supplies and indicators. They may also beintegrated into the design of larger equipments inwhich the modulations or demodulations are only oneof the functions performed by the device.

A functional block diagram of a modem is shownin figure 1-9. A full-duplex modem consists of twosections: the transmitter and the receiver sections.These two sections are functionally separate fromeach other.

The transmitter section consists of a data encoder,the modulator, the band-pass filter, and the transmitcontrol logic. The data encoder takes the digital datasignal to be transmitted, and when necessary, convertsit into the bit pattern acceptable to the modulatorcircuit. The modulator converts the data into thecarrier signal. The most popular forms of modulationare frequency-shift keying (FSK), phase-shift keying(PSK), and quadrature phase-shift keying. After thedata signals are modulated, they are fed to theband-pass filter circuitry. The band-pass filter thenallows only the desired frequency to pass through thecommunications channel. The transmit control logicprovides the timing signals necessary for thetransmission of data to take place.

Receiver Section

The receiver section consists of a band-pass filter,a demodulator, a data decoder, and the receivercontrol circuit. The band-pass filter allows only thedesired carrier signal to be received from thecommunications channel. The demodulator removesthe data from the carrier signal and feeds the data to

1-8

Figure 1-10.—A time-division multiplexer (TDM) system.

the decoder. The decoder reassembles the data into a be designated in a single modulation change. If twoform compatible with the receiving system. In thereceiver section, the incoming signal is often fed tothe receiver timing logic to control the receiver timingcircuitry.

MULTIPLEXING

One requirement of a data communications systemis for it to transmit as many intelligent signals aspossible in a fixed period using a single-communications channel. The rate of datatransmission is measured in the number of bits persecond (bps) transmitted. The bps rate is oftenconfused with the baud rate. Baud refers to the rate atwhich a modulated signal between two deviceschanges in 1 second. For example, if the signalbetween two modems changes frequency or phase ata rate of 2,400 times per second, the baud is 2,400. Ifyou are using a modulation method in which a singlemodulation change carries one bit, the 2,400 baud isalso 2,400 bits per second. Using more sophisticatedmodulation methods, several bits of information can

bits of data are transmitted with each modulationchange, the data transfer rate is 4,800 bits per secondat 2,400 baud.

The data signals being transmitted are normallymultiplexed to increase the transmission rate of dataover the communications channel or to increase theefficiency of the channel by allowing multiple usersof the same channel. The two methods commonlyused to multiplex communications channels aretime-division multiplexing and frequency-divisionmultiplexing.

Time-Division Multiplexing

Time-division multiplexing (TDM) grants eachuser full channel capacity, but assigns time slots toeach user. Each user is connected to a time-divisionmultiplexer. Data signals from the user are fed to thetime-division multiplexer buffer, and the time slotsare rotated among the users and scanned for data.Figure 1-10 illustrates the typical construction of a

1-9

time-division multiplexer system. The data from eachuser can be in the form of bits, bytes, or blocks. Thedata signals from all users are compiled into framesf o r t r a n s m i s s i o n o n a single, high-speedcommunications channel.

Transmit and receive frames are used forhalf-duplex communications. Transmit frames aresent and a receive time slot is enabled for returninformation. In this manner, a single carrierfrequency and modem may be used to transmit andreceive information at a fairly high rate of speed.

Since time slots are preset and assigned, if a userhas no data to transmit, the time slot is wasted.Advantages of a TDM system include the following:its ability to handle devices with varying speeds, itseffectiveness when used with devices that transmitdata almost continuously, and its simpleimplementation.

Frequency-Division Multiplexing

Frequency-division multiplexing (FDM) dividesa band of frequencies into several distinct channels ortones. Each tone carries a portion of the data beingtransmitted. FDM devices can be complex because aseparate modulator/demodulator circuit is required foreach tone used. The composite tones are thenmodulated to a single carrier frequency for radiotransmission.

FDM allows for the parallel transmission of dataover a single communications channel. For example,the Link-11 communications system uses 15 audiotones to transmit 30 bits of parallel data. Each tonetransmits two bits of differential quadraturephase-shift keyed data.

SUMMARY—FUNDAMENTALS OF DATACOMMUNICATIONS

This chapter introduced you to the building blocksof a data communications system. The followinginformation summarizes the important points youshould have learned.

COMMUNICATIONS SYSTEMS— Digitaldata devices that exchange data over distances areknown as communications systems. A basiccommunications system consists of the followingthree components: a transmitter, a receiver, and acommunications channel. The transmitter convertsdigital data into a form (digital or analog) useable bythe communications channel. The receiver acceptsdata from the communications channel and convertsthe data back to its pure digital form.Communications systems that can transmit andreceive data are known as duplex systems, whilecommunications systems that are limited to transmitonly or receive only are simplex systems. Duplexsystems that transmit data, pause, and then receivedata are half-duplex systems. Full-duplex systemscan transmit and receive data simultaneously.

COMMUNICATION CHANNELS— Severaltypes of communications channels are in use today.The most common are landlines and radiocommunications. Landlines are physical cables thatconnect computers; they are common in local areanetworks. Radio communications use theradio-frequency bands to exchange information. Themost common bands used in the Navy are the HF andUHF bands.

DECIBEL MEASUREMENT SYSTEM— Thedecibel measurement system is used to measure thegain or loss of amplifiers, antennas, communicationslines, and other types of communications equipment.A gain of +3 decibels (dB) indicates that the outputpower of the circuit, compared to the input power, hasdoubled. Each +3 dB gain indicates a doubling ofpower. For example, a signal that has a gain of 6 dBis twice as strong as a signal that has a gain of 3 dB.

A S Y N C H R O N O U S T R A N S -MISSION— Asynchronous transmission refers todata sent without the use of timing pulses. Datasignals are sent a byte at a time, with start, stop, andparity bits added to each byte.

S Y N C H R O N O U S T R A N S -MISSION— Synchronous transmission refers to thesending of long, uninterrupted streams of data with apredefined start and stop sequence.

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MODULATION/DEMODULATION— Modul-ation is the modifying of a signal to carry intelligentdata over the communications channel. Several typesof modulation are available, depending on the systemrequirement and equipment. The most frequentlyused types of modulation are amplitude modulation,frequency modulation, and phase modulation.Demodulation is the act of returning modulated datasignals to their original form.

AMPLITUDE MODULATION— Amplitudemodulation refers to modifying the amplitude of asine wave to store data.

FREQUENCY MODULATION— Frequencymodulation refers to changing the frequency of asignal to indicate a logic 1 or a logic 0. Onefrequency indicates a logic 1, and the other frequencyindicates a logic 0.

PHASE MODULATION— Phase modulation ismore complex than amplitude modulation orfrequency modulation. Phase modulation uses asignal frequency sine wave and performs phase shifts

of the sine wave to store data. A modification ofphase modulation involves the use of several discretephase shifts to indicate the state of two or more databits.

M O D E M S — A modem is a device thatMOdulates and Demodulates data in a digitalcommunications system. Modems are available in avariety of types, with various speeds and capabilities.A modem consists of two functionally separateareas—the transmitter section and the receiversection. The transmitter section prepares, ormodulates, the data for transmission. The receiversection demodulates, or returns, incoming data to itsoriginal form.

MULTIPLEXING— Multiplexing refers toprocesses used in digital communications systems tomake the most efficient use of system time.Multiplexing can involve time-sharing of thecommunications channel by several users or assigningseveral frequencies for the parallel transmission ofdata.

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CHAPTER 2

THE LINK-11 SYSTEM

INTRODUCTION

Tactical data links are usually limited to a specific area of operation and are usedfor command and control of specific forces. Link-11 is the U.S. Navy shipboardversion of NATO’s Tactical Data Information Link “A” (TADIL A). The Link-11system is used to provide high-speed, computer-to-computer exchange of digitaltactical information among ships, aircraft, and shore installations, as shown in figure2-1.

Figure 2-1.—Tactical digital information links.

Link-11 data communications can operate with either high-frequency (HF) orultra-high-frequency (UHF) radios. In the HF band, Link-11 provides gaplessomnidirectional coverage of up to 300 nautical miles from the transmitting site. Inthe UHF band, the Link-11 system is capable of line-of-sight omnidirectionalcoverage, approximately 25 nautical miles between ships and 150 nautical miles forship-to-air links.

To understand the operation of the Link-11 system fully, you must be able toidentify the hardware components that compose it and the functions they perform.Keep in mind that although the specific equipment used on board your ship maydiffer from the examples used in this chapter, the purpose of your Link-11 setup isstill the same, that is, to pass tactical data to other units.

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After completing this chapter you should be able to:

Describe the composition of a typical Link-11 system.

Describe the operation of the Link-11 transmission and receive cycles.

Describe the six operating modes of the Link-11 system.

Describe the function of the Link-11 encryption (security) device.

Describe the audio tones generated by the Link-11 Data Terminal Set.

Describe the word formats used to transmit Link-11 tactical data.

Describe the message formats used in the various Link-11 operatingmodes.

Describe the operation of the Link-11 Data Terminal Set.

LINK-11 FUNDAMENTALS

To monitor the operation of and performmaintenance on the Link-11 system, you mustunderstand how the different pieces of equipmentinteract with each other. Let’s take a look at a basicLink-11 system.

LINK-11 SYSTEM OVERVIEW

A typical shipboard Link-11 communicationssystem (figure 2-2) consists of the followingcomponents: the CDS digital computer, acryptographic device, the Link-11 data terminal set,the communications switchboard, and the HF or UHF

radio set transceivers (transmitter/receiver), anantenna coupler, and an antenna. The data terminalset is the center of the Link-11 system and is coveredin detail later in this chapter. The communicationsswitchboard is used to select the desired HF or UHFtransceiver. An external frequency standard is alsopart of many Link-11 systems. Additionally, theShipboard Gridlock System (SGS) is installed onmany ships. On SGS-equipped ships, an AN/UYK-20is placed in the line between the CDS computer andthe crypto device.

Figure 2-2.—The Link-11 communications system.

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CDS Computer

The central processor of the Combat DirectionSystem is the CDS computer. Keeping a data base oftracks is among the many functions of the operationalprogram. The information about these tracks can betransmitted to other units over the Link-11 net. Thecomputer sends data to the data terminal set using 24-bit data words. The computer also receivesinformation about remote tracks from other units inthe net and displays these tracks through the displaysystem.

Shipboard Gridlock System

Gridlock is the matching of track positions heldby other ships with the tracks held by your own ship.Gridlock is a procedure for determining dataregistration correction by comparing remote tracksreceived from a designated reference unit to localdata. Ideally, tracks received from remote units thatare also displayed by onboard sensors should betransparent, that is, in the exact same position on theCRT. If the gridlock system does not providecorrelation between local and remote tracks, theremote tracks may be painted twice and overlap eachother, as shown in figure 2-3.

Figure 2-3.—Tracks out of gridlock.

Failure to maintain gridlock maybe the result ofinaccurate positioning data from a ship's sensor, fromthe Ship's Inertial Navigation Systems (SINS), or

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from the ship’s gyro. Failure to maintain gridlockmay also be the result of an inaccurate operator entry.

The SGS computer performs continuousautomatic gridlock calculations. In the event of anSGS computer failure, the flow of Link-11 data to theCDS computer is interrupted. To restore Link-11 dataflow, all SGS installations have switches installed thatallow the technician to bypass the SGS computer untilthe fault is corrected.

Link-11 Security Device

A standard model security device, such as theTSEC/KG-40, commonly referred to as the KG-40, isused with the Link-11 system. When the DTS istransmitting data, the KG-40 receives parallel datafrom the CDS computer, encrypts the data, and sendsit to the DTS. When the participating unit (PU) isreceiving data, the TSEC/KG-40 receives encrypteddata from the DTS, decrypts, and sends to the CDScomputer.

Because of the specialized training and securityrequirements of cryptographic equipment, we will notcover the internal operation and controls of thesecurity device.

Data Terminal Set (DTS)

The data terminal set (DTS) is the heart of theLink-11 system. The DTS is the systemmodulator/demodulator (MODEM). The CDScomputer sends 24 bits of data to the DTS via theSGS computer and the encryption device. The DTSadds six bits of data for error detection and correction.These six bits are called hamming bits. The 30 bitsof data are phase shift modulated into 15 audio tones.These 15 data tones and a Doppler connection tone arecombined into a composite audio signal which is sentto either the UHF or HF radio for transmission.

The DTS receives the composite audio signalfrom the radio and separates the 15 data tones and theDoppler correction tone. The 15 data tones aredemodulated into 30 data bits. The six hamming bitsare checked for errors and the 24 data bits are sent to

the CDS computer via the encryption device and theSGS computer.

Link-11 Communications Switchboard

The communications switchboard provides systemflexibility and casualty recovery capabilities byallowing manual switching of the data terminal setand individual HF and UHF radios. A typicalswitchboard will provideinterconnections:

The Link-11 data terminalHF radio sets to provideLink-11 capability

the following

set to one or morethe standard HF

A Link-11 data terminal set to one or moreUHF radios sets to provide UHF Link-11capability

The same communications switchboard may alsobe used for connecting a Link-4A data terminal set toone or more UHF radios to provide standard UHFLink-4A (TADIL C) capability. Link-4A is coveredin detail later in this book.

Radios

The Link-11 system can operate with either an HFradio or a UHF radio. Long-range communicationsare achieved by the use of the HF system. UHFcommunications are limited to line of sight. “Line ofsight” means the radio wave will not bend over thehorizon; therefore, the use of an antenna mountedhigh on the mast will increase the range of UHFcommunications.

Antenna Couplers

Antenna couplers are used to connect a specificradio set to a specific antenna. The coupler providesfor the correct impedance matching of the antenna andthe radio set. For many of the multi-couplers to workproperly, it is extremely important that the correctfrequency spacing be observed. A general rule is toensure a frequency spacing of 15 percent.Frequencies that are too close together can causeinterference and distortion, increasing the signal tonoise ratio and causing bit errors in the data.

Antennas

In oversimplifying the theory of antennaoperation, an antenna is just a piece of wire thatradiates electromagnetic energy from the radio intothe atmosphere and converts atmosphericelectromagnetic radiation into RF current to beprocessed by the radio. As electromagnetic energyfrom the atmosphere passes through this wire, itinduces a current in the wire. This current is fed tothe radio receiver. If the receiver is tuned to the samefrequency as the received signal, the signal can beprocessed. The same wire will radiate anelectromagnetic field if current is flowing through it.

The frequency at which a radio operatesdetermines what size antenna is most suitable fortransmitting and receiving. The higher the frequency,the smaller the antenna will be. Lower frequenciesrequire larger antennas. For example, the full-wavelength of an antenna designed to operate at 4 MHz isabout 250 feet. Since this is too long for shipboardapplication, antennas are designed in submultiplelengths. These include half-wave and quarter-waveantennas.

An antenna can be tuned by introducing acapacitive or inductive load. This loading effectivelychanges the electrical length of the antenna and can beused to extend the frequency range of the antenna.For more information on antenna design andoperation, refer to the Navy Electricity andElectronics Training Series, Module 10, IntroductionTo Wave Propagation, Transmission Lines, andAntennas, NAVEDTRA B72-10-00-93.

Transmission Cycle

The data flow for the Link-11 transmission cycleis shown in figure 2-4. The CDS computer receivesdata from the various ship’s sensors, navigationsystems, and operator entries, and stores this data in adata base. When a Link-11 transmission is required,the computer outputs parallel digital data through theSGS computer to the cryptographic device. Thecryptographic device encrypts the data and sends theencrypted data to the data terminal set (DTS). TheDTS converts the digital data to analog audio tones,

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Figure 2-4.—Link-11 data flow for the transmit cycle.

keys the transmitter using the radio set keyline, andpasses the audio tones, via the communicationsswitchboard, to the transmitter for modulation to theRF carrier signal. The radio set keyline is a signalthat switches the radio to the transmit mode. Whenthe signal is stopped, the radio reverts to the receivemode.

When you are using the HF band, the radiofrequency signal modulation uses amplitudemodulation independent sideband; that is, the uppersideband (USB) and lower sideband (LSB) aretransmitted independently in an effort to overcomepropagation-caused signal losses. The UHF radiouses frequency modulation; therefore, only the USBis used.

Receive Cycle

When a transmitted signal is received, the receiverdemodulates the audio tones from the RF carrier andpasses them via the communications switchboard tothe DTS. The DTS demodulates and demultiplexesthe audio tones into digital data. The digital data issent to the cryptographic device where it is decryptedand sent to the CDS computer for processing.

LINK-11 NET OPERATING MODES

Before we look into the actual operation of thedata terminal set, you need to have some knowledgeof the Link-11 modes of operation and how themessages are formed. Link-11 employs networked(net) communications techniques for exchangingdigital information among airborne, land-based, andshipboard systems. As you have seen, the amount ofhardware required to support Link-11 operations isrelatively small; however, establishing andmaintaining a successful link can be very complex.

Establishing a Link-11 Net

The establishment of a successful link involvesthe interaction and teamwork of the operators andtechnicians of several units working towards thecommon goal. If one unit is having trouble with thelink radio, data terminal set, or other equipment, it canmake the entire link unreliable.

When a task force is about to deploy, the taskforce commander will issue a message that has thenecessary information required to establish Link-11communications. The information in this messageincludes a list of primary and secondary frequencies,designation of the initial net control station, an initialgridlock reference unit (GRU) designation, PUidentification and addresses, an initial data linkreference point (DLRP), and required operatingprocedures. Voice communications are required fornet control and coordination during initialization.

When the task force is formed, the picket stationsinform the net control station (NCS) of their readinessto establish link operations. Upon establishingcommunication with all units, NCS transmits NetSynchronization (Net Sync). If the NCS is usingcorrected timing (normal mode), the Net Sync verifiesthe communications path between NCS and all picketunits. If a picket unit cannot receive Net Sync, itcannot participate in the net. Net Test should followNet Sync. Net Test is used to confirm connectivitybetween the Link-11 units. Units having difficulty inreceiving Net Sync or Net Test should report to NCSthat they are not able to participate in the net and thenbegin corrective action.

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When Net Test is completed, all picket stationsreport their status to NCS. Then NCS directs all PUSto switch to the Roll Call mode and initiate linkoperations. Net Synchronization and Net Test areused in the initialization of the net. The normal modeof operation is Roll Call.

The above scenario has introduced you to severalnew terms and modes of operation. These areexplained in detail in the following paragraphs.

The following are the six modes of Link-11operation:

Net Synchronization

Net Test

Roll Call

Broadcast

Short Broadcast

Radio Silence

Net Synchronization

The Net Sync mode of operation is used toestablish a uniform time base from which all net datacommunications normally initiate. The Net Syncmode is usually initiated when establishing a link netafter all operator entries have been properlycompleted. The Net Sync transmission is manuallystarted by the operator on the NCS platform andcontinuously transmits the Link-11 preamble untilstopped by the operator.

The preamble consist of two tones–the 605-Hztone and the 2,915-Hz tone. During the transmissionof Net Sync, the 2,915-Hz tone is periodically phasedshifted 180 degrees. The time between these shifts isdetermined by the selected data rate and is called aframe.

Each PU is equipped with a very accurate timebase in the form of a frequency standard (internal orexternal). When the NCS transmits Net Sync, each

unit receiving the transmission synchronizes itsindividual time base with the Net Sync signal. If thepicket station is operating in the corrected sync mode,as is normally the case, the picket will check to seethat it can recognize the Net Sync signal as a means ofverifying that a good radio link has been established.If a picket is going to operate in the stored sync mode,it will align its stored frame timing to the timing ofthe NCS, using the received Net Sync signal. Sincestored sync timing locks the picket to the time base ofthe NCS, data from other pickets may be lost.Therefore, this mode should only be used duringtimes of poor radio propagation or signal jamming.After the completion of Net Sync, the next operationperformed in establishing a link is usually Net Test.

Net Test

Net Test provides an overall evaluation of the netand equipment performance. When you are operatingin this mode, NCS will broadcast canned test data toall pickets within the net. The data terminal setcontains a code generator that generates twenty-one30-bit data words. Once all the words in the wordtable have been generated, the process automaticallystarts over and keeps running until stopped by theoperator.

Net Test will test the connectivity between allunits and the operation of the DTS. Since it is a localtest, Net Test does not check the interface between theCDS computer and the DTS. Net Testis also helpfulto the technician for setting the audio input and outputlevels of the DTS or radio set.

Roll Call

Roll Call is the normal mode of operation. In thismode, the operator on the NCS platform entersownship’s address and an assigned address (PUnumber) for each PU in the proper switch position.When the link is initiated, each PU is polled for data.Polling consists of sending a call-up message. If thePU fails to respond, the call-up is repeated. If the PUstill does not respond, it is skipped and the next PU ispolled. When a PU recognizes its own address, thePU will transmit its data to all the participants in thelink. When the NCS recognizes the end of the PU

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reply, it automatically switches to the transmit modeand calls up the next PU address. After all the units inthe net have been polled, the NCS transmits its owndata and the process is continuously repeated. TheRoll Call mode provides all PUS with continuous,near real-time exchange of tactical information.

Broadcast

When the Broadcast mode is used, one PU willcontinuously send a series of data transmissions to allthe members of the net. Once manually initiated, thetransmission will continue to be sent automaticallyuntil the operator manually stops it. Through the useof the broadcast mode, other picket stations canreceive real-time tactical information withoutbreaking radio silence.

Short Broadcast

In the Short Broadcast mode, a picket station orthe NCS sends a data transmission to the othermembers of the net. The transmission is initiated bythe operator depressing the TRANSMIT STARTbutton on the DTS control panel and is terminatedautomatically when the computer has finished sendingthe DTS data. This mode is used only when no otherunit is transmitting.

Radio Silence

In the Radio Silence mode, the radio set key lineand the data terminal set audio output are disabled.The receive capability of the DTS is not affected. TheRadio Silence mode is manually initiated andterminated.

BUILDING A LINK-11 MESSAGE

Information transmitted from the DTS originatesfrom two sources. Tactical data always originatesfrom the CDS computer. Other information,including the preamble, phase reference, start andstop codes, and address frames, originates within thedata terminal set. These additional special-purposeframes are added to the data frames to form the propermessages.

For the DTS to control the net properly, strictadherence to the correct message format and netprotocol are required. Every Link-11 message has aspecific format and function. Each Link-11 messagegenerated by the DTS begins with a header consistingof the preamble (five frames) and the phasereference frame (one frame). Control codes, such asthe start code, the picket stop code, and the controlstop code, are also required.

Preamble

The preamble, as previously covered, consists ofa two-tone signal. The two tones are the 605-HzDoppler tone and the 2,915-Hz sync tone. Thepreamble is five frames long and is transmitted at fourtimes the normal power, as shown in figure 2-5. Amore detailed explanation of the preamble tone isprovided later in this chapter.

Figure 2-5.—The Link-11 preamble power levels andframe count.

Phase Reference Frame

The phase reference frame follows the preambleand is shown in figure 2-6. This frame is composedof the normal 16-tone composite signal with the datatones transmitted at 0 dB and the Doppler tonetransmitted at +6 dB. The phase reference frame

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provides the reference for the first frame of data.Each succeeding frame becomes the phase referencefor the following frame.

Figure 2-6.—The phase reference frame added to thepreamble with normal data tone levels.

Information Segment

The information segment of the Link-11 messageis composed of control code frames and message dataframes. Control code frames consist of a start code,a stop code, and an address code. Each control codeis two frames in length and performs a specificfunction. Control codes are not sent to the CDScomputer.

START CODE.— The start code is a two-framecode that follows the phase reference frame, as shown

Figure 2-7.—The start code added to the Link-11transmission.

in figure 2-7. When sensed by the DTS, the start codecauses the DTS to send a prepare-to-receive datainterrupt to the CDS computer.

MESSAGE DATA FRAMES.— Message dataframes contain the tactical data being disseminatedand follow the start code, as shown in figure 2-8. Thenumber of message data frames depends on theamount of tactical information the unit transmits. The24 bits of data contained in each frame is sent to theCDS computer.

Figure 2-8.—The message data frames added to theLink-11 transmission.

STOP CODE.— The stop code is a two-framecode that follows the data message in a Link-11transmission and is shown in figure 2-9. There aretwo types of stop codes: the control stop code and thepicket stop code. The control stop code is used inmessages originated by NCS (NCS report) andindicates that a picket address code follows the stopcode. The picket stop code indicates to the NCS thatthe picket unit has completed its messagetransmission. Both the control stop code and picketstop code cause the receiving DTS to send the End-of-Receive interrupt to the CDS computer.

LINK-11 MESSAGE FORMATS

The formats of the messages transmitted by theLink-11 system vary with the mode of operation.

Roll Call Mode Messages

In the Roll Call mode, the unit designated as thenet control station sends out two types of messages.These are the NCS call-up message (interrogation)and the NCS report (message with interrogation). Athird message, the picket reply message, is sent bypicket unit in response to interrogation messages.

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Figure 2-9.—The stop codes added to the Link-11transmission.

C A L L - U P ( I N T E R R O G A T I O N )MESSAGE.— This message shown in figure 2-10consists of the five-frame preamble, the phasereference frame, and the two address frames. Thecall-up message does not use start and stop codes.

Figure 2-10.—The NCS call-up message.

NCS REPORT AND CALL-UPMESSAGE.— This message shown in figure 2-11consists of the five-frame preamble, the phasereference frame, the two-frame start code, the dataframes containing the NCS report, the two-framecontrol stop code, and two frames containing theaddress code for the next PU.

Figure 2-11.—The NCS report message.

PICKET REPLY MESSAGE.— The picketreply message shown in figure 2-12 consists of the

two-frame start code, the data frames, and the two-frame picket stop code.

Figure 2-12.—The picket reply message.

Short Broadcast Messages

The Short Broadcast is a single data transmissionto all members of a net by a station that may be actingas either picket or NCS. It is the same format as thepicket reply message shown in figure 2-12. The ShortBroadcast message is manually initiated by theoperator at the DTS.

Broadcast Mode Messages

The Broadcast mode messages consist of acontinuous series of short broadcast messages,separated by two frames of dead time, as shown infigure 2-13. The message format is the same as apicket reply message. In the Broadcast mode, onlyone unit will transmit.

Net Test Mode

The Net Test message consists of the five-framepreamble, the phase reference frame, and the Net Testwords generated by the DTS. When all the Net Testwords in the library have been transmitted, thesequence starts over until the operator stops the NetTest.

LINK-11 DATA TERMINAL SET (DTS)

As you have seen, the data terminal set is the heartof the Link-11 system. The DTS performs themodulation, demodulation, and control functionsrequired for proper Link-11 operation. It accepts datafrom the CDS computer in the form of 24-bit datawords, adds six bits of error detection and correction(EDAC) data, and converts all 30 bits into an audio

five-frame preamble, the phase reference frame, the tone package that is sent to the transmitter portion of

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Figure 2-13.—Broadcast mode message format.

the radio set. The key-line signals necessary tocontrol the transmit and receive states of the radio setare also generated by the DTS. Data received fromthe upper sideband (USB) and lower sideband (LSB)p o r t i o n s o f t h e radio set receiver, inthe form of audio tones, is converted into parallelbinary data and sent to the CDS computer.

Currently several design generations of Link-11data terminal sets are used in the fleet. These includethe AN/USQ-59 and 59A, the AN/USQ-63, and theAN/USQ-74. Originally introduced in the early1960s, each successive generation of the Link-11 dataterminal set reflects additional knowledge gainedfrom fleet use and advances in technology. Althoughthe technology used in the different models of theLink-11 DTS may be vastly different, all of themperform the same function.

Normally, the DTS operates in the half-duplex

mode, meaning it can either receive or transmit data,but it cannot do both at the same time. An exceptionis during system test when the DTS operates in full-duplex mode and can simultaneously send andreceive data.

DATA TERMINAL SET FUNCTIONS

The DTS also performs the following functions:

Error detection and correction

Audio signal generation

Link-11 protocol and interface control

Error Detection and Correction (EDAC)

The DTS receives data from the CDS computer inthe form of 24-bit binary data words. The 24-data bits

Table 2-1.—DTS Parity Bit Status Codes

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are expanded to 30 bits by adding six bits for errordetection and correction (EDAC). These six bits arealso called hamming bits. The value of these bits isbased on parity checks of specific combinations of the24-bit data word.

During the receive cycle, the six EDAC, orhamming bits, are examined for errors. There isenough redundancy in the EDAC to allow forcorrection of a single bit error. The operator cancontrol the selection of the error correction mode. Ifthe data word is not a control word, the word isexamined to determine if it is error-free, contains acorrectable error, or contains uncorrectable errors. Ifthe DTS is in the error detection and label mode, adetected error is identified and labeled before the dataword is sent to the CDS computer. In the errordetection and correct mode, the DTS attempts tocorrect an error before sending the data word to theCDS computer. In both modes, the six EDAC bits aredeleted and replaced with two parity error status bits.These status bits are defined in table 2-1.

Audio Tone Generation and Characteristics

The DTS converts the 24-bit data word, alongwith the six EDAC bits, into a composite audio signalconsisting of 16 tones. This composite 16-tone signalis the data frame. The tones range in frequency from605 Hz to 2,915 Hz and are the odd harmonics of 55Hz. The specific frequencies of the tones are shownin table 2-2. The 605-Hz tone is used for Dopplercorrection, and the 2,915-Hz tone is used for data andsynchronization. Each of the data subcarrier tones(tones 2 through 16 in table 2-2) represents two binarybits of differential quadrature phase-shift modulateddata.

The Doppler tone (605 Hz) is not phasemodulated. It is used to correct for Doppler shifts inthe received tones caused by the relative motionbetween the transmitter and the receiver. It is alsoused to correct for the Doppler shift that may occurbecause of differences between the transmitter andreceiver frequency standards.

The 2,915-Hz tone has two separate uses. Duringthe transmission of the preamble and Net Sync, the

2,915-Hz tone is used to identify frame timing. Thistone is phase shifted 180 degrees at the end of eachframe. When detected by the receiving DTS, thephase shift indicates the start of a new frame. Whenthe DTS is in corrected timing, this information isused to set the timing for the data frames that follow.When stored timing is used, the frame timing that wasset during Net Sync is used.

The Doppler and sync tones vary from each otherand the other data-carrying tones in amplitude. TheDoppler tone is 6 dB greater than the other tones.During the Net Sync and preamble frames, theDoppler tone is transmitted at 12 dB and the sync toneis transmitted at 6 dB. The Doppler tone istransmitted at 6 dB during the transmission of dataframes and the sync tone is used as a data tone. Datatones are transmitted at 0 dB.

The audio tones are divided into data frames toidentify the separate parallel groupings of 30 bits. Itis the phase angle shift of each of the 15 data tonesthat conveys the digital information contained in thetone. During each frame, each data tone frequencyhas a particular phase. At each frame boundary, thephase of each data tone is shifted with respect to theprevious frame. The amount of this phase change, orphase difference, determines the value of a two-bitnumber. Two data bits yield the following fourpossible combinations: 00, 01, 10, and 11. Eachcombination is associated with a phase difference ofone of four values: 45 degrees, 135 degrees, 225degrees, or 315 degrees from the previous position.

Each of these angles marks the center of aquadrant, as shown in figure 2-14. Each 90-degreequadrant is assigned a two-bit binary value. Anyphase difference falling within that quadrantrepresents that binary value. This system of dataencoding can tolerate an error in the prescribed phaseshift of up to ±44 degrees before a single bit error willoccur. An error in phase shift that is greater than 45degrees, but less than 135 degrees, will cause thephase angle to fall into an adjacent quadrant. Noticethat the values are assigned to each quadrant in sucha way that if a phase shift error occurs, only one biterror will be introduced as long as the quadrant intowhich it falls is adjacent to the target quadrant.

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Table 2-2.—Tone Library

Figure 2-14.—Link-11 data phase shift encoding.

Link Protocol and Interface Control

In addition to encoding data from the CDScomputer, the DTS generates and recognizes protocoldata that controls the type and number of linktransmissions. These protocol words include codesindicating the start of transmission, the end oftransmission, and the address of the next unit totransmit.

The interface with the CDS computer is under thecontrol of the DTS. The DTS signals the CDScomputer when it has input data or when it wantsoutput data through the use of external interrupts.These interrupts include the prepare-to-transmit,prepare-to-receive, and end-of-receive interrupts.

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Figure 2-15.—The AN/USQ-59 Mode Control panel.

DTS CONTROLS AND INDICATORS

Many parameters that affect the operation of theDTS are under the operator’s control, whether thestation is operating as a picket or as the net controlstation. Both the operator and the technician must befamiliar with the various controls and indicatorsassociated with the DTS. The AN/USQ-59 usesseveral control panels that are usually mounted nextto the operator’s display console. These panels enablethe operator and the technician to control and monitorthe net operation.

The control panels include a Mode Control panel,a TADIL A Control panel, and an Address SelectionIndicator panel. Although the AN/USQ-59 controlpanels are used here to show the controls andindicators of a Link-11 DTS, other data terminal setshave similar controls.

DTS Mode Control Panel

The DTS mode control panel controls andindicators are shown in figure 2-15. The following isa summary of how the controls affect the operation ofthe link and what the indicators mean.

TRANSMIT MODE INDICATOR— Lightswhen the DTS is in the transmit mode.

RECEIVE MODE INDICATOR— Lights whenthe DTS is in the receive mode.

SUMMARY FAULT INDICATOR— Lightswhen a fault in the DTS is detected while the DTS isin the OPERATE mode. There are 27 performancemonitor fault-sensing circuits in the data converter(modem) of the DTS. During the OPERATE mode,14 of these sensors can cause a summary fault. Thefault-sensing circuits monitor areas such as variouspower supplies, signal quality, preamble presence,timing, and audio signal quality. When the DTS is inSELF TEST, the summary fault lamp is lighted whena fault is isolated to a function defined by switchpositions on the fault isolation control and built-intests routines.

LAMP TEST BUTTON— Causes all indicatorson the mode control panel, the TADIL A controlpanel, and the address control unit to light.

FAULT MONITOR/RESET SWITCH— In theMONITOR position, this switch allows the fault-sensing function of the DTS to operate normally andprovide a fault summary signal to the DTS control.When the switch in placed in the RESET position, thefault-sensing circuits of the DTS are reset. TheSUMMARY FAULT lamp is turned off when thefault-sensing circuits are reset.

INTERNAL 100 KHZ/EXTERNAL SWITCH-Allows for the selection of the internal or external100-kHz frequency standard.

DOPPLER CORR ON/CORR OFF SWITCH-Enables the DTS Doppler correction when placed inthe CORR ON position.

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F U L L - D U P L E X / H A L F - D U P L E XSWITCH— In the FULL-DUPLEX position, thisswitch enables full-duplex operation of the dataconverter and the computer I/O adapter. It alsoenables loop back processing of the transmit sidetonedata for input to the computer. In the HALF-DUPLEX position, the DTS operates in the half-duplex mode and the transmit sidetone is disabledfrom being processed and input to the computer.Link-11 uses the half-duplex mode.

SIDEBAND SELECT SWITCH— When theSIDEBAND SELECT switch is placed in the LSB orUSB position, the DTS processes only the lowersideband or upper sideband of the received signal.When the switch is in the DIV position, the DTScombines both the upper sideband and the lowersideband signals to create frequency diversity data forinput to the computer. When the switch is in theAUTO position, the DTS selects the signal with thebest receive quality for processing. The AUTOposition is the normal position of this switch.

DATA RATE SWITCH— Selects the data ratethat the data converter uses. When the switch is in theDUAL 1200 position, the data converter can transmitand receive two unrelated streams of data at 1200 bps.When the switch is in either the 1200 or 2400position, the data converter transmits and receives asingle data stream at 1200 or 2400 bps, respectively.When the switch is in the TADIL A position, the datarate is controlled by the DATA RATE switch on theTADIL A control panel. The TADIL A position isthe normal position for Link-11.

SYNC MODE SWITCH— The SYNC MODEswitch selects the mode of synchronization used bythe DTS receive circuitry and is used in conjunctionwith the TIMING STORED/CORRECTED switch onthe TADIL A control panel. The normal operatingposition for the SYNC MODE switch is in theFAST/CONT position.

When the switch is in the FAST/CONT position,both the fast and continuous synchronization circuitsof the DTS are selected. Synchronization is initiallyobtained during the five-frame preamble andmaintained continuously during the data portion of the

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transmission. The TIMING switch on the TADIL Acontrol panel must be in the CORRECTED position.

When the FAST pos i t ion i s se lec ted ,synchronization is only during the five-framepreamble. If the CONT position of this switch isselected, only the continuous synchronization circuitsare selected. Synchronization is obtained only duringthe data portion of the transmission. The TIMINGswitch on the TADIL A control panel must be in theCORRECTED position for both of these modes.

The INHIBIT position of this switch disables boththe fast and continuous synchronization circuits of theDTS. The DTS will maintain the time base that wasstored when the switch was turned to INHIBIT. Forsynchronization to be held, the unit with its syncmode inhibited must maintain its original geographicrelationship to all other units in the net. This positionis used when the received signal contains interferencethat could cause loss of synchronization.

OPERATE/SELF TEST SWITCH— Thisswitch must be in OPERATE for normal on-lineoperations. When the switch is placed in the SELFTEST mode, the DTS is placed in an off-line modeand the fault isolation circuitry is enabled.

CONTROL ON/OFF SWITCH— When theCONTROL switch is placed to the ON position,+28Vdc is applied to the fault isolation control panel,the mode control panel, the TADIL A control panel,and the address control panel.

TADIL A Control Panel

The TADIL A control panel provides the controlswitches and indicators required to control andmonitor Link-11 operations. Figure 2-16 shows theAN/USQ-59 TADIL A control panel.

XMT DATA ERROR INDICATOR— Thisindicator is lighted when the DTS detects an errorwhile transmitting data in the TADIL A, or Link-11,mode.

RCV DATA ERROR INDICATOR— Thisindicator is lighted when the DTS detects an error inreceived data being sent to the CDS computer.

CODE ERROR INDICATOR— The CODEERROR indicator is lighted when the DTS detects anerror in the received or sidetone (transmit) controlcodes during TADIL A operations.

NET BUSY INDICATOR— The NET BUSYindicator is lighted when the DTS detects that thecommunications net is busy. It is activated when asignal called signal presence is generated by theDTS.

SYNC COMPT INDICATOR— The SYNCCOMPT indicator is lighted continuously, or flashes,when the DTS has achieved synchronization with theNCS data terminal.

TIMING STORED/CORRECTED SWITCH-The TIMING STORED/CORRECTED switchdetermines how the DTS is synchronized. When theswitch is in the CORRECTED position, the fastsynchronization and/or the continuoussynchronization circuitry in the DTS is used. Theposition of the sync mode switch on the mode controlpanel determines whether the fast, continuous, or bothcircuits are used to maintain synchronization. Whenthe switch is in the STORED position, the DTS usesthe time base stored during Net Sync. During normaloperations, this switch should be in the CORRECTEDposition.

OPERATE/RADIO SILENCE SWITCH— TheOPERATE/RADIO SILENCE switch is a two-position toggle switch that allows the DTS to inhibitradio transmissions. When the switch is in theOPERATE position, the DTS operates normally.When the switch is switched to the RADIO SILENCEposition, the radio keyline and transmit audio circuitsare immediately disabled.

NET CONTROL/PICKET SWITCH— TheNET CONTROL/PICKET switch configures the DTSto operate as the net control station or a picket stationin Roll Call mode.

ERROR CORRECT/LABEL SWITCH.— TheERROR CORRECT/LABEL switch determines howthe DTS processes detected errors. When the switchis in the CORRECTED position, the DTS attempts tocorrect detected errors. If a single bit error isdetected, the location of the erroneous bit is detectedand corrected. If an even number of bit errors occurs,the correction circuitry is inhibited. If an odd numberof bit errors occurs, the correction circuitry attemptsto correct the data; however, if an odd number ofmultiple bit errors occurs, an erroneous correction ismade. When the switch is in the LABEL position, theDTS does not attempt to correct detected errors.Instead, the data word sent to the computer is labeledto indicate that errors were detected in the data word.

Figure 2-16.—The AN/USQ-59 TADIL A control panel.

TRANSMIT RESET SWITCH— TheTRANSMIT RESET switch is a momentary contactpushbutton switch. When depressed, this switchcauses any transmission in progress to be terminated.The DTS stops the transmission by inhibiting thegeneration of the output data request, causing a stop

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code to be transmitted. The DTS also resets theaddress control address sequence logic.

NET BUSY INDICATOR— The NET BUSYindicator is lighted when the DTS detects that thecommunications net is busy.

TRANSMIT INITIATE SWITCH— TheTRANSMIT INITIATE switch is a momentarycontact pushbutton switch that causes the DTS toinitiate data transmission when the DATA RATEswitch is in the TADIL A position. The TRANSMITINITIATE switch must be depressed to initiate allDTS transmissions except when the DTS isconfigured as a picket and is in the Roll Call mode.When the net is in the Roll Call mode, only the netcontrol station is required to initiate transmission bydepressing the TRANSMIT INITIATE switch.

MISS CALL INDICATOR— The MISS CALLindicator is lighted when the net control station hasdetected no response from a picket station after twosuccessive interrogations. Once lit, it will remain lituntil a picket responds or the TRANSMIT RESETswitch is depressed.

ADDRESS COMPUTER/CONTROLSWITCH- The ADDRESS COMPUTER/CONTROLswitch determines the source of the address used bythe DTS. When the switch is in the CONTROLposition, addresses are obtained from the addresscontrol unit. When the switch is in the COMPUTERposition, addresses are obtained from the CDScomputer, provided the computer is configured forexternal function operations. The normal position forthis switch is depends on the configuration of thesystem on your ship.

NET MODE SWITCH— The NET MODEswitch determines the mode of operation of the DTS.The modes are BC or broadcast, SHORT BC, ROLLCALL, NET SYNC, and NET TEST.

DATA RATE SWITCH.— The DATA RATEswitch determines the speed and frame timingoperation of the DTS. When the switch is in the1364/9.09 position, the DTS transmits and receives

interval is approximately 9.09 milliseconds. Whenthe switch is in the 2250 position, the DTS transmitsand receives data at a rate of 2250 bps and a frameinterval of 9.09 milliseconds. When the switch is inthe 1364/18.18 position, the data rate is 1364 bps, butthe frame phase shift interval is increased to 18.18milliseconds.

OWN STATION ADDRESS SWITCH— TheOWN STATION ADDRESS switch consists of twothumb wheel switches in which an address is enteredto identify the address the DTS will respond to as itsown. In the Roll Call mode and with the DTSconfigured as a picket station, the DTS will transmitits tactical data when the interrogation messageaddress matches the address entered into the OWNSTATION ADDRESS switches.

RANGE IN MILES SWITCH— The RANGEIN MILES switch also consists of two thumb wheelswitches. These switches are used to select theapproximate distance between the net control stationand the picket station. The range entered into theseswitches causes the DTS to alter the frame timing tocompensate for the signal propagation delay betweenthe picket station and the NCS. The range in milessetting for the NCS is always zero miles.

Address Control Indicator

The address control indicator is used to set theaddress of the picket stations to be interrogated whena unit is configured to operate as the NCS. Theaddress control indicator is shown in figure 2-17. Theaddress control indicator consists of 20 identicaladdress selection modules, which are used to addressup to 20 stations. More than one address controlindicator may be installed in a system to provide theability to interrogate more than 20 stations.

Each address selector module has two thumbwheel switches in which one of 64 octal addressesmay be entered (address 00 and 77 octal are invalid).Also, each address selector module has a power on/offswitch, a power on indicator lamp, and a callindicator, as shown in figure 2-18.

data at 1364 bps. The data frame phase identification

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When a unit is configured as the NCS, theoperator enters all the assigned addresses of the netparticipating units into the address selector modules,and turns on each module with a valid address. Oncethe Roll Call mode is initiated, the DTS will checkeach module sequentially. If the power of the moduleis on and a valid address is entered, the address is sentto the DTS for use in an interrogation message. If thepower switch is in the OFF position, that module isskipped, even if it contains a valid address. Whenenabled by the DTS, the address selector modulesends the address entered in the thumb wheels to theDTS and the call indicator light. The call indicatorwill remain lit until the DTS sequences to the nextaddress module.

CDS INPUT/OUTPUT CONTROL

The data terminal set controls the exchange ofdata with the CDS computer. As describe earlier,input/output communications p r o t o c o l i saccomplished through the use of external interrupts.The prepare-to-transmit data interrupt, the prepare-to-receive data interrupt, and the end-of-receive datainterrupts control the DTS to the computer interface.

CDS Computer Input (Receive) Data Cycle

The input data cycle is initiated by the DTS.When the DTS recognizes the second frame of thestart code, it sets the prepare-to-receive data interrupton the input data lines and sets the external interruptline. The computer acknowledges the receipt of theinterrupt by sending an input data acknowledge (IDA)to the DTS.

Upon receipt of the first message frame, the DTSdemodulates the 24-bit word and places it on the inputdata lines, along with the two error detection andcorrection bits. Once the data is placed on the inputdata lines, the DTS sets the input data request (IDR)line. The computer will sample the data and send anIDA. This process repeats for all frames of themessage. The first frame of the stop code is alsotreated as a message frame and sent to the CDScomputer. When the DTS recognizes the secondframe of the stop code, it will place the end-of-receiveinterrupt on the input data lines and set the interrupt

Figure 2-17.—The Address Control Indicator C9062/U.

line. The interrupt is then processed by the CDScomputer and the input buffer is closed.

If the received stop code is a picket stop code, theDTS simply resets itself. If the stop code is a controlstation stop code, the DTS will compare the next twoframes received with its own station address code.

CDS Computer Output (Transmit) Data Cycle

The output data cycle is initiated when the DTSdetects its own station address, either in an

Figure 2-18.—An address selector module.

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interrogation message or at the end of an NCS reportand interrogation message. When the DTS recognizesits own station address, it starts to transmit thepreamble. During the first frame of the preamble, theDTS sets the prepare-to-transmit interrupt on theinput data lines. The computer samples the interruptand sends an IDA to acknowledge receipt of theinterrupt.

The DTS finishes sending the preamble and phasereference frames. During the second frame of thestart code, the DTS sets the output data request (ODR)active, requesting the first word of the tactical data.The CDS ‘computer responds by placing 24 bits ofdata on the lines and then setting the output dataacknowledge (ODA). The DTS samples the data andclears the ODR. The first frame of data is processedfor transmission and the ODR line is then set torequest the next data word.

This procedure is repeated until all the data wordshave been transmitted. Once the CDS computer hascompleted sending all the data words, it will notacknowledge the ODR from the DTS. If the CDScomputer has not acknowledged an ODR from theDTS in a preset amount of time, the DTS will clearthe ODR line and generate a stop code. Upontransmission of the two-frame stop code, the DTS willreturn to the receive mode.

Net Control Station (NCS) I/O Operations

The station acting as NCS follows the sameprotocols when communicating with the CDScomputer. Some differences exist in the generation ofthe control codes. The net control station isresponsible for interrogating each station. Uponreceipt of a picket stop code, the DTS checks the nextstation address and sends an interrogation message.After the interrogation message is transmitted, theDTS waits to receive a start code from theinterrogated station. If a start code is not recognizedafter 15 frame intervals, the station will bereinterrogated. If a start code is not received afteranother 15 frame intervals, the address control unitwill advance to the next active picket address andrepeat the interrogation process.

The other major difference is when the net controlstation has completed its own tactical datatransmission, a control stop code, followed by thenext station address, is transmitted. Again, if a startcode is not received within 15 frame intervals, asecond interrogation is sent. This secondinterrogation is a normal interrogation messageconsisting of the preamble, phase reference frame, andaddress code.

Modulator/Demodulator

The modulator/demodulator function of the DTSprov