Transmission ProtocolsSince Link-16 exchanges much of the same datathat is used in both Link-11 and Link-4A, a briefcomparison of the architectures, the capacities, andthe data rates of the three systems is useful.During normal operation, Link-11 operates usingthe protocols of the Roll Call mode. In this mode,each participating unit is polled by the NCS totransmit data. On completion of data transmission,the unit returns to the receive mode and the next unitis polled until all units have been polled. This cycleis continuously repeated. Link-11 messages are calledM series messages.Link-4A uses the time-division multiplexingprinciple with a command-and-response protocol toenable the operator to control multiple aircraftindependently on the same frequency. Link-4Amessages sent to the controlled aircraft are referred toas V series messages and messages received from thecontrolled aircraft are called R series messages.Link- 16 uses the Time-Division Multiple Access(TDMA) principle of data communications. Usingthis architecture with time interlacing provides thesystem with multiple and apparently simultaneouscommunications nets. Instead of assigning each unita PU number, Link-16 assigns each unit a JTIDSUnit number, or JU. The JU identifies the units anddetermines a preassigned set of time slots thatdesignate when the unit transmits and receives data.Each time slot is 1/128 of a second, or 7.8125milliseconds, in duration.When a JU transmits data, the frequency that thedata is transmitted on is changed every 13microseconds (µsec), according to a predeterminedpseudo-random pattern. Link-16 uses 51 differentfrequencies for data exchange. This frequencyhopping adds to the security and integrity of thesystem by making it nearly impossible to jam.Link-16 NetsLink-16 has the capability to handle multiple nets.A Link-16 net is a group of participants sharingmutually beneficial tactical information. Forexample, using the Link-11 system, a net is formed bya group of participants. These participants operate onthe same frequency. A separate net is formed whenanother group of participants operates on a differentfrequency. The second net would be used byparticipants involved in a fleet exercise that wouldn’twant the exercise data to interfere with the normaltactical net. The controlling station and aircraft usingLink-4A is also a net.Link-16 has the ability to form multiple nets. TheLink-16 system has 128 numbers used to designateparticular nets (00-127). Net number 127 is reservedto indicate a stacked net. A stacked net is formed bysetting up the time slots so that they have the sameset, initial slot number, and recurrence rate. Whenthe system is initialized, the use of net number 127indicates a stacked net is to be used and the operatorcan then specify locally which net to use foroperations. Figure 7-5 illustrates the concept of astacked net used for air control. Net 1 is a group ofaircraft controlled by the ship, while Net 3 is a groupof aircraft controlled by an E-2. If the E-2 requiresadditional aircraft, the ship can direct the aircraftunder its control to the E-2. As the aircraftapproaches the E-2, the pilot can switch to Net 3 andimmediately become an active participant in the newnet. Even though the operator has several netsavailable to monitor or use, a single terminal cantransmit or receive on only one of them for each timeslot. Stacked nets are possible because the frequency-hopping pattern is different for each net. Examples ofstacked nets are voice nets and control nets.Figure 7-5.—Stacked nets using Link-16.7 - 5
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