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LESSON 3
NET RADIO INTERFACE OPERATION
CRITICAL TASKS: S1-5878.04-9001, 01-5704.04-9001, 01-5704.07-0001
OVERVIEW
LESSON DESCRIPTION:
In this lesson you will learn the definition of NRI systems. You will learn how these systems are placed in the division and how they are established. You also learn the procedures for operating an NRI station and what equipment is used to establish them.
TERMINAL LEARNING OBJECTIVE:
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| CONDITION: | You will be given information from this lesson. |
| STANDARD: | To demonstrate competency of the terminal learning objective, you must achieve a minimum score of 70% on the subcourse examination. |
| REFERENCES: | The material contained in this lesson was derived from the following publications: FM 11-37, FM 11-50, and FM 24-18. |
You have already learned how the commander uses the wide variety of tactical radios and telephone systems available to him to command and control his unit's operations in the modern battlefield. Sometimes, however, he may have need of both telephone and radio systems simultaneously. Thus he has need of some method of interconnecting tactical radios with wire telephone systems. The NRI system provides this interface. Figure 3-1 illustrates a basic NRI system.
Figure 3-1. Basic NRI system
1. Establishment of NRI.
NRI is established and maintained by the division signal battalion. NRI stations may be set up wherever they are required, but the signal battalion establishes certain NRI stations as a matter of routine. At the division level, these stations are located at:
a. Division main CP.
b. Division support command.
c. Division area signal centers (ASCs).
d. Division artillery.
The operator of the NRI system is responsible for maintaining both the radio and wire portions of the system in working order. His responsibility does not stop at the facility. He must also be able to locate problems at their source and quickly correct them even if they are not in the actual NRI facility itself. Because of this, the NRI operator must be familiar with the operation of the telephone switchboard and the radio site as well as the NRI facility.
2. Uses of NRI.
Just as the name implies, NRI is a set of equipment and procedures that provides an interface between wire telephone networks and tactical radio systems by connecting a radio transmitter to the telephone system at one end of the communications chain and connecting a radio receiver to the telephone network at the other end. An obvious advantage is that unit commanders can communicate with their units via wire telephone even when they are mobile. Some other uses of NRI (at the division level) are:
a. To establish emergency communications between mobile FM radio stations and elements connected to the division area telephone system by telephone.
b. To establish communications between FM radio stations separated by distances that are beyond the direct operating range of their FM radio sets.
c. By the commanding general, division staff, and other designated key personnel in the division when operating from a mobile CP to contact division elements connected to the division area communications system.
d. For initial establishment of telephone service from the division area communications system to using units.
e. For voice communications between mobile combat elements in the division forward area and supporting division logistics elements in the rear.
f. For communications between low-flying Army aircraft operating in distant parts of the division area and airstrips of flight control elements connected to the division area communications system, when directed FM radio cannot be maintained.
g. For communications between forward air controllers and the air liaison officer's communications facilities when these facilities are connected with the division area communications system.
h. To keep commanders and staffs in contact with subordinate and higher headquarters, as required, during displacement of CPs.
i. To connect two switchboards and to span a break in a wire line between units.
j. For communications during river crossings.
3. NRI Equipment.
The equipment used for NRI varies according to the mission of the unit using NRI. The type of radio and telephone system employed also dictates the type of NRI equipment. With the advent of SINCGARS and MSE, the different types of NRIs available have expanded. Standard NRIs will connect the regular two-wire telephone service and either the AN/VRC-12 series radios or SINCGARS equipment. You can also establish NRIs between SINCGARS and MSE equipment. The following paragraphs will introduce you to some of the equipment used for an NRI.
a. AN/GSA-7. The AN/GSA-7 is the main component used to provide NRI services. It is a small remote switching device that can control the AN/VRC-12 series radios remotely at distances of up to 16 km (10 miles). The circuitry in the GSA-7 allows the operator to connect the two-wire telephone system and the four-wire push-to-talk FM radio system. It automatically converts the radio signal to a telephone format and vice versa. The GSA-7 also converts the 20 Hz telephone ring signal to a 1600 Hz radio attention tone so that the operator will be aware of incoming calls. You do not have to use proper radio communications procedures when you are talking on a regular phone system. Once the phone system is connected to the radio system, however, anyone using the interface must observe strict radio communications procedures regardless of which portion of the system he is using. To remind the users of this, the GSA-7 produces a 2000 Hz beep tone when it is keyed by the remote telephone operator. The operator can hear this tone at five second intervals while he is talking. This is to remind him of the need to use proper radio procedures. The GSA-7 can be mounted in a vehicle or installed in a fixed location. Figure 3-2 shows the front of the AN/GSA-7.
Figure 3-2. Front of AN/GSA-7
b. AN/GRA-39. If you do not have an AN/GSA-7 unit, or if your unit malfunctions, you can use the AN/GRA-39 as a substitute for it. The GRA-39 is a bit more limited than the GSA-7. It can only control the radio set remotely from up to 3.2 km (2 miles) away. It will, however, provide the NRI interface and allow you to continue your NRI operations. The GRA-39 consists of two units; the C-2329/GR, used at the radio site, and the C-2328/GR, used at the remote site.
c. Telephone Switchboards. You can use a wide variety of telephone switchboards to provide NRI services. Manual switchboards SB-22/PT, SB-86/P, and SB-3082/GT; and manual switchboard configurations AN/MTC-1, AN/MTC-3, AN/MTC-7, AN/MGC-9, AN/TTC-35, AN/TTC-23, and AN/TTC-29. The automatic switchboard AN/TTC-38 will also interface with NRI equipment. The basic connection for the NRI system is to connect the switchboard to the two LINE terminals on the GSA-7.
d. Auxiliary Equipment. Establishing an NRI station requires a few pieces of auxiliary equipment not yet mentioned. If you are using the GSA-7, you will also need a portable telephone set TA-312/PT, a cable assembly CX-7474, and an installation kit. If you are using a GRA-39 to provide NRI you will need a switchboard SB-22/PT (if that is not the primary switchboard you are using) to provide the interface between the GRA-39 and your primary switchboard. The GRA-39 also uses the CX-7474 cable assembly. You also require a radio set to complete the opposite end of the NRI system. Figure 3-3 shows the connections for an NRI system using the AN/GSA-7 and the AN/VRC-49 radio set.
Figure 3-3. Connections for NRI system using AN/GSA-7 and AN/VRC-49 radio sets
e. Security Equipment. Since you can use NRI to pass important information, it is desirable to provide communications security for the NRI system. The VINSON and NESTOR secure devices are both compatible with NRI equipment. The principal difference is that the VINSON device (KY-57) can provide security for all portions of the NRI system while the NESTOR device (KY-38) will only provide secure radio. Thus if you use NESTOR to secure your NRI system you must ensure that the wire portions of your system are in a secure area and not subject to being tapped.
4. NRI Procedures.
Because NRI connects two different communications systems with different communications procedures, it is logical that a procedure should be established for communicating on an NRI network. Regular wire telephone system communications procedures are substantially less formal than radio procedures.
Since the NRI system provides access to both, any party using NRI should always observe formal radio communications procedures. This includes the correct use of call signs, procedure words, and authentication procedures. The call procedure is essentially the same for any NRI equipment. The connection procedures may change depending on what type of NRI equipment you are using.
a. Call Originating from a Telephone Subscriber. When a telephone subscriber desires to make an NRI call, he must first contact his switchboard operator unless he can call the NRI operator directly. This depends on the type of switchboard used to provide the NRI interface. If the subscriber contacts the switchboard operator first, he will request to be connected to the NRI operator. Once he has contacted the NRI operator, the subscriber identifies his station and tells the NRI operator what radio station he wishes to contact. The NRI operator then looks up the call sign for the subscriber and the radio station he is contacting and contacts the radio station using his radio set. He contacts the desired station's radio net and requests permission to enter it. Since he is a new station entering a net, both he and the net control station (NCS) for the net he is contacting must authenticate. Once the NRI operator has entered the net, he contacts the desired radio station and informs the operator that there is an NRI call for him. He then tells the telephone subscriber that he has contacted the desired party. He also informs the subscriber of his call sign (since the subscriber does not have a communications-electronics operation instruction (CEOI)) and of the call sign of the radio station, and admonishes the subscriber to use proper radio procedures. He then tells the subscriber what level of security is on the line and tells him to proceed with his call. If the NRI interface is provided by the GSA-7, a beep tone will continually remind the telephone subscriber that he is talking on a radio net and should use proper radio procedures.
b. Call Originating from a Radio Station. When a radio station operator desires to place an NRI call, the first thing he must do is contact the NRI operator. He may have to leave his net if the NRI frequency is different from his own. Once he has contacted the NRI operator, he requests the call sign of the party he wants to communicate with. The NRI operator looks up the station designation of the telephone subscriber and contacts the subscriber directly or through the switchboard operator. As with the previous example, once the NRI operator has contacted the telephone subscriber, he informs the subscriber that there is a radio call for him. He gives the subscriber the necessary call sign information and again admonishes the subscriber to use proper radio procedures. He then contacts the radio station operator and tells him to proceed with his call. Appendix B contains an extract from FM 24-18 that illustrates the procedure of placing an NRI call from both the subscriber and radio operator ends using the AN/GSA-7.
5. Radio Set Control C-6709/G.
The radio control set C-6709/G is a modern radio control set which will replace the AN/GSA-7 and AN/GRA-39 for NRI services with an automatic switchboard. It can control a radio set from a distance of up to 2.5 miles (4 km). It is equipped with circuits to automatically process NRI calls so that the operator does not have to be so involved in the call process. With the AN/VRC-12 series radios, the C-6709/G is compatible with both the VINSON and NESTOR devices. The C-6709/G is also used to provide NRI between CNR radios and the MSE telephone system. When it is used in this application, the KY-90 security device is used to provide secure communications. The procedure for placing NRI calls with the C-6709/G is similar to that for the AN/GSA-7 and AN/GRA-39. Appendix B provides an extract from FM 24-18 that illustrates this procedure and the operation of the C-6709/G unit itself. You should review this information now.
6. Summary.
In this lesson you have learned about the NRI system, including its uses and equipment used to provide it. You have also learned how to place an NRI call as a telephone subscriber or as a radio station operator. Now use this information to complete the following practice exercise.
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