Transmission Protocols

Transmission Protocols Since Link-16 exchanges much of the same data that is used in both Link-11 and Link-4A, a brief comparison of the architectures, the capacities, and the data rates of the three systems is useful. During normal operation, Link-11 operates using the protocols of the Roll Call mode. In this mode, each participating unit is polled by the NCS to transmit data. On completion of data transmission, the unit returns to the receive mode and the next unit is polled until all units have been polled. This cycle is continuously repeated. Link-11 messages are called M series messages. Link-4A uses the time-division multiplexing principle with a command-and-response protocol to enable the operator to control multiple aircraft independently on the same frequency. Link-4A messages sent to the controlled aircraft are referred to as V series messages and messages received from the controlled aircraft are called R series messages. Link- 16 uses the Time-Division Multiple Access (TDMA) principle of data communications. Using this architecture with time interlacing provides the system with multiple and apparently simultaneous communications nets. Instead of assigning each unit a PU number, Link-16 assigns each unit a JTIDS Unit number, or JU. The JU identifies the units and determines a preassigned set of time slots that designate when the unit transmits and receives data. Each time slot is 1/128 of a second, or 7.8125 milliseconds, in duration. When a JU transmits data, the frequency that the data is transmitted on is changed every 13 microseconds (µsec), according to a predetermined pseudo-random pattern. Link-16 uses 51 different frequencies for data exchange. This frequency hopping adds to the security and integrity of the system by making it nearly impossible to jam. Link-16 Nets Link-16 has the capability to handle multiple nets. A Link-16 net is a group of participants sharing mutually beneficial tactical information. For example, using the Link-11 system, a net is formed by a group of participants. These participants operate on the same frequency. A separate net is formed when another group of participants operates on a different frequency. The second net would be used by participants involved in a fleet exercise that wouldn’t want the exercise data to interfere with the normal tactical net. The controlling station and aircraft using Link-4A is also a net. Link-16 has the ability to form multiple nets. The Link-16 system has 128 numbers used to designate particular nets (00-127). Net number 127 is reserved to indicate a stacked net. A stacked net is formed by setting up the time slots so that they have the same set, initial slot number, and recurrence rate. When the system is initialized, the use of net number 127 indicates a stacked net is to be used and the operator can then specify locally which net to use for operations. Figure 7-5 illustrates the concept of a stacked net used for air control. Net 1 is a group of aircraft controlled by the ship, while Net 3 is a group of aircraft controlled by an E-2. If the E-2 requires additional aircraft, the ship can direct the aircraft under its control to the E-2. As the aircraft approaches the E-2, the pilot can switch to Net 3 and immediately become an active participant in the new net. Even though the operator has several nets available to monitor or use, a single terminal can transmit or receive on only one of them for each time slot. Stacked nets are possible because the frequency- hopping pattern is different for each net. Examples of stacked nets are voice nets and control nets. Figure 7-5.—Stacked nets using Link-16. 7 - 5