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5-1. Use of the Switch in a Communications System

Employment means placing the switch into a communications system. To do this, use all the information presented in the previous chapters about switch capabilities and operation. All the actions involved in employment are part of system planning and network development, which is covered in paragraph 5-6. This chapter also presents additional information to help the planner see his position in the broad sense of overseeing the entire communications system.

5-2. General Concept of Operations

Chapter 1 introduced you to the Army's plan to move to an all digital tactical communications system. This requires the production and fielding of a large amount of new equipment. Other changes will include major increases in capabilities. The Army is making changes in methods of operation and in doctrine which will continue for some time. Figure 1-1 gives an example of this. A major change is that the command communications system will combine with the area or common-user system. The result is a single, cohesive common-user network of telecommunications centers that supports the major commanders in the theater. Each of these centers includes an AN/TTC-39 circuit switch. Paragraph 5-3 gives an example of the makeup of these centers, or nodes.

These changes are concurrent with many other changes in the makeup of Army units. The theater army is now supported by a theater communications command. This in turn has at least two signal brigades. The current corps has one signal brigade for support. All of the brigades include signal telecommunications battalions which are made up of signal telecommunications companies. Those companies assigned to area functions each have one AN/TTC-39 and have evolved from ones that used manual or automatic analog switches (AN/MTC-9 or AN/TTC-38). All of these AN/TTC-39s are of 300-line size. The Army uses 600-line switches only for certain special purposes. These changes apply to a wide range of CE equipment. For example, subscriber equipment, telephone, teletype, data, and facsimile will in time become all digital. Some of this equipment is in use now. New transmission systems include high frequency, troposcatter, and line-of-sight. These, coupled with new digital multiplex devices, will add greatly to the usefulness of the Army's tactical CE systems.

In addition, the Army is applying new concepts in CE management and control. Chapter 1 described the CEMS. The CEMS includes several elements of automation. The CSPE and the CSCE are planned to include some features that will enable users to send orders and reports automatically. This will add to efficiency, responsiveness, and the centralized control of communications. Equipment at the node will also improve planning and control functions.

The conversion from analog-to-digital will take a number of years. During this time, you may see many combinations of equipments and of the organizations that support them. The AN/TTC-39 is a key element in this process of change. Its modular design will enable it to take on a more digital configuration as needed. The switch you work with will most likely be the standard configuration. (See Chapter 2.) You should, however, be aware of its full range of capabilities to help prepare you for the mixed analog-to-digital world that is now upon us.

5-3. Nodal Composition

The major use of the AN/TTC-39 in the theater and the current corps is at an area node. The switch can be an intermediate (tandem) switch (switching between switches), it can serve local subscribers, or it can do both. The last is usually the case. The breakout of services between local subscribers and trunk switching use depends on the needs at the node in question. It will vary from node to node and from event to event. Network planning depends on accurate estimates of nodal requirements. Thus, you must identify these early in the planning cycle. However, the analog-to-digital changes taking place make this task difficult. This is because changes in equipment produce changes in the node's needs. To help clarify all this, this paragraph describes typical area nodes for the AN/TTC-39 in environments that are mainly analog and that are mostly digital.

Current area node.

The nodal arrangement in Figure 5-1 is typical of what you will find in the present stages of AN/TTC-39 use. Except for the circuit and message switches, all of the equipments are basically analog and will in time be replaced, This figure shows a 300-line AN/TTC-39 in the 2/1 configuration which has two SDSGs (analog) and one TDSG (digital). The control facility for the node is the AN/TSQ-84 communications technical control center. This is a replacement for the SB-675 communications patching panel. The AN/TSQ-84 can terminate, patch, and test analog circuits and can terminate and patch individual digital circuits.

The AN/TCC-73 telephone terminal is the standard multichannel system used at the node. It uses pulse code modulation. Its basic building block is 12 channels at 48-kbs per channel for a 576-kbs composite transmission rate. Use of the digital groups of the AN/TTC-39 requires use of 18-channel groups (576 kbs) between AN/TTC-39 switches. This is true even when the GOS requires fewer channels. These trunk groups bypass the AN/TSQ-84 and its limited capabilities for monitoring, testing, and patching. If there is need for additional DTGs, the AN/TSQ-84 can patch individual digital circuits (up to 60) from the AN/TTC-39 to the AN/TCC-73. This reduces the number of digital local subscribers that can connect to the AN/TTC-39. If a switch configured with more TDSGs is used (for example, the 600-line switch in the 3/2 analog-to-digital configuration has 2 TDSGs), up to 8 DTGs are available.

Analog trunks do not have the same restrictions as digital trunks. (See Table 3-1 for maximum terminations.) The major trunking limitation of the available equipment is in its ability to combine multiplex and transmission equipment. Patching of all of the analog circuits and of the individual digital circuits is through the AN/TSQ-84. All local connections (including local subscribers, PBXs, and PABXs) are also through the AN/TSQ-84.

Use of an AN/TYC-39 message switch at the node (as in Figure 5-1) may involve further restrictions. The AN/TYC-39 can operate in the stand-alone mode but if circuit switching is available, it is more efficient, in terms of transmission media, to use the AN/TTC-39. However, the interface between the message and circuit switches will use one of the four (300-line switch) or eight (600-line switch) available circuit switch digital trunk groups. This reduces the digital connectivity even more.

The transmission facilities at this node are the AN/TRC-138 and the AN/TRC-152. The AN/TRC-138 is a microwave radio terminal and repeater using AN/GRC-144 radio sets. The AN/TRC-152 is a newer microwave terminal and repeater set using AN/GRC-103 radio sets. It replaces Radio Set AN/TRC-110. There may also be a satellite terminal at the node. The AN/TSC-85 is a multichannel SHF terminal that can connect to either analog or digital portions of the switch. (For further information about the transmission equipment, see the appropriate technical manuals.)

Objective system area node.

The objective system area node is almost all digital. Thus, the availability of digital equipment influences its composition and operation, and so do the changes in the theater communications system described in paragraph 5-2. Combining the command and administrative systems into one common-user system will affect the user distribution and the traffic load for each node. Figure 5-3 illustrates the objective system area node. This is the equipment arrangement which takes full advantage of digital switching and digital multiplex and transmission systems. However, some analog capability is retained for subscribers who still use older equipment and for emergency use.

The heart of this node is the AN/TTC-39A nodal control circuit switch. This switch incorporates changes which have a major impact on node operations. The digital capability is increased by changing the analog-to-digital matrix relationship from 2 to 1 in the 300-line AN/TTC-39 to 1 to 2 in the AN/TTC-39A. The total number of terminations is also increased. The switch also performs control functions such as channel reassignment and analog line conditioning. (See paragraphs 2-6 and 3-13 for more details.) There is also a control facility as part of the node. This is in accord with the CEMS, and the equipment will be a Control Center AN/TYQ-31. This control center will report to the CSCE at the battalion level and will eventually provide processor-to-processor interface for the nodes and automatic data base entry and updates.

Approximately every third node in the area system has a AN/TYC-39 message switch. Data subscribers can also be served by the circuit switch using a DSVT. Figure 5-2 shows a single subscriber terminal teletype device connected in this way. The node also has a secure digital net radio interface unit (SDNRIU). This enables single channel combat net radio subscribers to enter the switched system. This replaces the radio wire integration devices used in earlier systems.

This node's multiplex and transmission systems are different than those of earlier nodes. Radio equipment will reduce or even eliminate the use of cable to the radio park location. This means that the radios can be placed in the best location for transmitting to other nodes. There are short-range wideband radios (SRWBR) both at the bottom of a hill (where the terrain protects the node), and at the top (where the line-of-sight or other trans-mission characteristics are best). Each SRWBR set includes an AN/GRC-144 radio modified to handle the higher digital group rates. The bottom of the hill set is the AN/TRC-175, which sends the data stream to the AN/TRC-138A at the top of the hill. The older AN/TRC-138 has been modified and this involves not only a modified AN/GRC-144 but also the addition of digital group multiplex equipment. The AN/TRC-138A can patch digital groups to other top-of-the-hill radio equipment for up to eight extension nodal systems and to four other nodes. An extension node is one at a location subordinate to the main node. In Figure 5-2, the AN/TRC-174 radio terminal set transmits to an extension node. An extension node uses either of the radio terminal sets AN/TRC-173 or AN/TRC-174 to terminate the channels at that location. To transmit over longer distances, the system can use a digital tropospheric Radio Terminal Set AN/TRC-170. It may also use a satellite terminal such as the AN/TSC-85A or AN/TSC-93A. Figure 5-3 illustrates the use of digital multiplex equipment at this type node. This diagram shows the part played by each type of equipment at the node.

5-4. Traffic Engineering

Traffic engineering begins after a tactical switched communications network is planned. The planning phase will identify switching node locations and will lay out the interswitch transmission links. It will also determine trunk group types for those links and will make out subscriber lists for the nodes. Traffic engineering is the proper allocation of network assets (principally interswitch transmission channels) to enable the network to carry the calls originated by, and destined for, network subscribers. Initial traffic engineering is done on the basis of estimates made using experience factors from past tactical communications networks. (For a complete description of the traffic engineering process, see FM 11-486-2.)


Below are brief explanations of terms commonly used in traffic engineering.

Holding time. Call holding time is the length of time that a subscriber is off-hook in the process of making a call to another subscriber. It includes the time the network takes to setup and take down the call as well as the time that is actually spent in communication between the subscribers. Call holding times in telephone networks have been shown to follow a negative exponential distribution. The average call holding time in a tactical communications network has been found through experience to be 240 seconds (4 minutes). Use this number in your initial traffic engineering estimates for a new network, or for a new configuration of an existing network.

Busy hour offered load. Communications networks are generally designed to carry, at an acceptable GOS, subscriber traffic offered during the busiest hour of a typical 24-hour day. For tactical communications networks, the measure of traffic is hundred call seconds (ccs). To estimate the subscriber busy hour offered load in ccs for a tactical communications network, or for a particular switch in that network, use the following formula:

      Offered load (ccs) = X x 3,600 seconds x 0.3


      X = number of subscribers to the network or switch

      3,600 seconds = duration of the busy hour in seconds

      0.3 = subscriber average off-hook factor during the busy hour as determined by
      experience (off-hook factor of 30 percent)

      Dividing by 100 converts traffic in call-seconds to traffic in ccs.

Grade of service. GOS, mentioned above, is the measure of acceptable service for a switched communications network. It is the percentage of time that a caller fails to reach the called subscriber (fails to receive either ringback or a subscriber busy signal) when he attempts a call. A particular GOS value is the basis on which you engineer or allocate the assets for a switched communications network. Atypical target value used for automated tactical communications networks is .01. Table 5-1 lists some assumptions about tactical subscriber traffic that were used in designing the AN/TTC-39.

Distribution of calls at a tactical switching center.

Field experience has shown that the calls handled by a major nodal switch, with roughly a 2: 1 ratio of loops to trunks, typically has the following breakdown:

      Local calls

      30 percent

      Nonlocal calls

      70 percent

        Tandem (trunk-to-trunk)

          25 percent
          25 percent
          20 percent

These experience factors, along with the ones previously given, are used in making the initial traffic engineering estimates for a tactical switched communications network. The process is described in subsequent subparagraphs.

Initial traffic engineering.

The objectives of the traffic engineer's initial efforts is to estimate the required sizes of the TGCs between switches so that the network, when actually deployed, will operate close enough to its target GOS to provide effective subscriber service. Once the network is up and running, traffic metering will provide actual data on which to base load balancing and other corrective measures to refine the network's configuration. To obtain size estimates for the TGCs at a switch, the traffic engineer first uses the formula shown above in the subparagraph on busy hour offered load to estimate the local subscriber portion of the switch busy hour traffic load. The local subscriber portion includes local calls, out-trunk calls, and in-trunk calls but not tandem calls. Therefore, the total busy hour traffic load can be found as follows:

      Total traffic load = subscriber portion of traffic load


      0.8 (80 percent) = local calls (30 percent)+ out-trunk calls (25 percent) + in-trunk       (25 percent)

      To get the traffic load on the TGCs, use the following formula:

      Traffic load on all TGCs = total traffic load x 0.9


      0.9 (90 percent) = out-trunk calls (25 percent)+ in-trunk calls (25 percent) + 2 x
      tandem calls (2 x 20 percent)

Tandem calls count twice because a tandem call comes in on one TGC and goes out on another. This traffic load has to be distributed over the various TGCs terminating at the switch. The traffic engineer will distribute it evenly if there is no reason to do otherwise. (For example, if the total TGC traffic load is estimated to be 3500 ccs, and there are five TGCs, then each would carry 700 ccs.) Once the traffic carried by each TGC has been estimated, use the Erlang-B tables in FM 11-486-2 to determine how many trunks the TGC should contain to provide .01 GOS (. 01 GOS means that 1 percent, or 35 ccs, of the offered traffic will not be carried by the TGC because of traffic load). The process above has to be repeated for each switch in the network.


After the network is actually up and running, the traffic engineer can see how close he came with his initial estimates to providing a .01 GOS. He does this by using the traffic metering capabilities of the automatic switches. Adjustments are usually made by adding and deleting individual trunks in TGCs to better match their sizes to the actual loads offered. In combat situations the constraints on available switching and transmission assets, the hostile threat, the continual movement of subscribers, and the resulting network reconfiguration to provide subscriber service combine to provide a real challenge to the traffic engineer.


The AN/TTC-39 will pass more traffic than older switches for the following reasons:

  • CCS reduces the time to set up a call. This in turn reduces the holding time.
  • Combinations of analog and digital trunk requirements make it necessary to use more varied TGCs (analog, digital, and analog-to-digital).
  • Overhead channels handle combined system control, framing, and synchronization. This makes more channels available for traffic.
  • Maximum use of automatic calling without the assistance of an operator reduces call holding times.

5-5. Coordination and Information Flow

It takes a great deal of data, many documents, and much coordination to support the operation of a tactical communication system. With the introduction of automatic switching, coordination and information flow is more complex. Planners and controllers are making management decisions on a real or near real-time basis. All parts of the system are interdependent. You need to understand what information is necessary for planning and you need to know whereto get it. You need to know what documents, directives, and orders you must issue and where to send them.

Figure 5-4 shows some typical documents, displays, and information flows. Not every situation, however, will require all of these documents nor all of these flow paths. The flows shown here mainly reflect the theater level situation. However, most of them apply to other levels as well. The figure can also apply to a variety of switches. The documents shown are specific to the AN/TTC-39, yet you could substitute documents dealing with other switches without affecting the general flow picture. Table 5-2 lists some of the required planning information and tells where to get it.

To follow the whole information flow involved in building a CE network plan, start at the top of Figure 5-4 and read down. The process begins with the commander's initial operations orders. It ends when you fill out the configuration and data base entry worksheets for a specific AN/TTC-39 at a network node. The figure describes only a single planning cycle, but building and refining a communications network never really ends. In actual operation, you would repeat the cycle as often as necessary to refine the plan and to accommodate changes in requirements as they occur.

The documents and information sources you will use include CE operations orders, CEOIs, SOPs, graphic type orders (diagrams, maps, overlays, sketches), Standardization Agreements, and joint publications (Allied Communications Publications and Joint Army-Navy-Air Force Publications). You will also review user requirements and requests for services, as well as troop lists and tables of organization and equipments.

At the CSPE level, you will use these documents to analyze your assigned mission and to prepare a CE estimate on how best to carry out the mission. Your goal will be to prepare a systems concept and a detailed CE plan. The plan must satisfy system requirements as well as possible with available resources. It follows the same format as operations orders and the CE annex. (See FM 24-16.) The CE plan tells how to provide CE support for the command as a whole. It may become the basis for system implementation orders issued by the supporting signal units (at the CSCE level). At the CSPE level, this plan involves several components. Start preparing the plan by filling out planning worksheets P-1 and P-2 as described in paragraph 4-5. These list subscriber and trunking requirements respectively. Then prepare an inventory of the major equipments you will need. Most CE plans will include three elements: multichannel plans, single channel plans, and wire communications plans.

Multichannel systems are either radio (line-of-sight, satellite, or tropospheric scatter) or wire/cable systems that provide more than one simultaneous channel of communications. (Use wire/cable only where radio equipment cannot be used, in rear areas where wire/cable is secure, or to free radio equipment for movement.) Single channel systems may include AM or FM radio, single side band radio, radio teletypewriter, or very high frequency equipments. You will need to determine the specific number and types of radio nets to assign frequencies and to establish station and net radio call signs. Wire communications plans specify the use of field wire, wire laying and recovery equipment, cable, different types of telephones and telephone equipment, teletype-writers, and switches.

With all of the above planning products in hand, you can start preparing a detailed plan for deploying specific AN/TTC-39 switches. For each nodal switch, complete the remaining planning worksheets (P-3 and P-4) and partially fill out the switch data entry worksheets (D-1 through D-27). (See paragraph 4-6 for details.) These worksheets identify class of service marks/features for each switch and for switch subscribers. They will also list PR and SL numbers and subscriber addresses. Next prepare COMSEC keying plans, routing plans, and EUB lists. As the planner, you also direct the composition and release of pre-made data base load tapes and the program library tapes for the automatic switches.

For plans to work, the CSCE must prepare and issue a number of implementation and installation orders. These can include:

  • Telecommunications service orders (TSOs). Special communications authorization requests.
  • Communication system documentation change orders.
  • Fragmentary orders.
  • Standing operating procedures. These are usually prepared and issued through command channels.

Once a system is running, the CSCE must send subordinate nodes a number of other directives to set up its system monitoring and operational control functions. For the AN/TTC-39, these may include:

  • Traffic control directives.
  • Circuit and patching control directives.
  • Transmission control directives.
  • COMSEC data element messages.
  • Directory control messages.

At each of the nodal switches, the switch supervisor will add site-specific information to the AN/TTC-39 data base. He does this by completing the partially prepared data base worksheets D-1 through D-27 received from the CSPE/CSCE. The supervisor will also carry out the switch configuration or strapping instructions spelled out in his orders. He does this by completing worksheets S-1 through S-9. Once the switches have been initialized, tested, and are operating, the nodal control facility will provide detailed local communications planning, direction, and control. This will require the switch supervisor to monitor switch performance, to modify or correct circuit configuration/operation, to activate and deactivate circuits, and to coordinate these changes with the appropriate CSCE. The CSCE will maintain records and reports received from subscribers and from the switching equipments. These will cover data on such matters as resource commitments, trouble conditions, traffic status, diagnostics, and metering. The switch VDUs may display some of this information and some may be printed out automatically or prepared by hand.

To maintain system control and to improve the overall system performance, the switch crews must compile various reports and send them back up the chain of control. These reports provide feedback information, which helps in controlling day-to-day operations. In addition, periodic reports on the system's operation are needed. The Army uses these for long-range planning, for research and development, for programming procurements, for planning personnel needs and for training, and for developing and modifying doctrine. Coordination and information flow in current tactical CE systems is a continuing cycle of planning, replanning, adjustment, and day-to-day direction. Its purpose is to keep the system running smoothly in the face of rapidly changing technical and field conditions.

5-6. System Planning and Network Development

So far, this manual has described only partially the place of the circuit switch in a network. To make the picture complete, this paragraph guides the planner through the planning process. It also discusses the numerous factors that the planner must consider when putting together a circuit switching network. The previous discussion on coordination and information flow is the basis for much of the planning process. It will be helpful to refer back to this discussion and to Figure 5-4 as the process develops.

Planning guidance.

The circuit switch may be used at a headquarters or to provide local subscriber service within its home area at a non-headquarters location. In the latter case, your major planning function will be to allocate equipment and calling services to those subscribers. This also can be the situation when a switch supports a task force headquarters where most of the staff and subordinate units are in the close vicinity. However, it is more likely that the AN/TTC-39 is part of a complex network of circuit switches. The major example is within the rear areas controlled by a theater headquarters. Paragraphs 1-1 and 5-2 provide some insight into the general organization of these areas. The geographical area size can be very large (such as the whole European theater). Such size increases the complexity and the potential problems.

The circumstances surrounding the procurement, introduction, and use of the AN/TTC-39 compound this already complicated situation. This is a factor of the transition from analog-to-digital communications and of the organizational changes involved. The organizational changes are the result of more efficient use of equipment and personnel. This in turn results from the use of digital systems, from changes in training needs, and from the doctrinal changes involved with the new capabilities. As the planner, you must function in this environment. You must deal with both the new and the old while these changes are taking place.

The title planner as used here describes the responsibilities more than it does the position. The senior communicator is responsible to the commander for communications. He will have charge of planning, engineering, and operating staffs and organizations. Planning is a function at all levels, down to the switch itself. Direction and guidance, however, must come from the top level. This seems to imply centralized planning (and control), and this is indeed a fact of the analog-to-digital trend. There are several reasons why CE networks can no longer be operated piecemeal. These include the complicated mix of old and new equipment, the changes in capabilities, the greater speeds at which systems work, and the increased need to control the system. They also include the increased interconnection of systems (other services, Allied, AUTOVON), and the fact that automation demands more precise information.

However, there is a paradox in the centralized direction and control we now have. In the older, manual systems, the communicator had complete control of the operation of the system. For example, all calls passed through switchboard operators who made connections, monitored and tested the system's workings, and reported problems. The users run our new system. Users are able to make their own connections of all kinds. This increases the need for detailed planning. It also increases the need to train the user. No longer can a subscriber simply pick up his handset and tell the operator to route him to a distant switch. He must know how to do it himself by proper dialing. The operator is available to handle certain calls, but does not have the time to handle routine calls. Thus, part of the planning effort must be to determine user training needs. The planner must also set up checks to prevent untrained users from degrading any part of the system by improper use. The user is not the only one who may need additional training. The planner's increased role increases his need for training also. He (and his staff) must be adept in communications and traffic engineering and in network operations. Knowledge of equipment, automated processing, real-time reporting, and technical administration is vital. Planner training requires extensive instruction in each of these areas. All network planners should have this training.

Earlier it was mentioned that planning takes place at all levels including the switch or node. Of course, planning varies from level to level and depends on guidance from the next higher level. It is within this framework of the CE management system that the planner's work focuses. (See paragraph 1-3). As paragraph 5-3 described, reporting and certain operational functions are automatic. However, this automation is a result of direction from the CSCE, which resides at the signal telecommunications battalion controlling the switches in its area. There are planners at the CSCE and also at the CSPE who handle broad, long-range planning. The CSPE resides at the corps and theater headquarters. The function at the CSCE involves operations, logistics, and administration. The planner at the CSPE handles the detailed system planning and resource and management function.

Network development principles.

The development of any CE system must follow certain logical procedures that rest on standard principles of communications use. These procedures fall into phases:

  • Determine the mission.
  • Compile and evaluate requirements.
  • Assess the resources.
  • Configure the network.
  • Develop routing.
  • Allocate services to subscribers.
  • Establish control and reporting.
  • Issue the orders.
  • Test the system.

These procedural phases are, of course, interdependent. Each has an effect on the others. They apply generally in the order listed and represent a logical progression. However, at the completion of each phase, the planner must look at all of the other phases to see what adjustment is needed. In other words, the use of these procedures is an iterative process. We will use them hereto illustrate the planning needed for a switched network.

Determine the mission. The usual method for announcing a mission is the operations order. Paragraph 5 of the operations order, Command and Signal, provides some information for the CE planner. There may also be a CE annex to the order. If the planner helped write these portions of the order, he has already done part of this task. In all cases the planner should be a part of the order-writing process. If the network is automated, he must participate. The operations order defines the mission of the CE organization. The planner then does a mission analysis to find if the mission is feasible and to find its cost. This is a general, preliminary estimate but it is necessary. It lets the commander know if the CE system will be overextended.

Compile and evaluate requirements. Once the mission has been assigned, the details are put together. The first step is the CE estimate which provides a logical way to decide on a course of action. The CE estimate involves some analysis of requirements and the possible effects of enemy capabilities. The next step is to see what the pattern of needs will be. The best way to do this is to make a rough sketch of a possible network. This sketch will eventually be your multichannel systems diagram. Remember to base this sketch on the mission, the estimate, and your own SOP. This SOP is important because it shows you how the standard system is usually put together. For a corps, the rough sketch may look like Figure 5-5. Such a sketch will become your worksheet as you progress. It is best to combine this with a map overlay so that you can analyze actual locations later.

Now list each major headquarters in the corps area and in each division area. These are the critical supported activities. Then show all other concentrations of supported units. At first, list only the nodes to support these headquarters and units. Paragraph 4-5 describes the use of planning worksheets. You should start with worksheet P-1 (subscriber list) for each circuit switch at each node so far listed. This will give a preliminary idea of your resource needs. Fill in most of the items on this worksheet later on. In the same way, start a worksheet P-2 (trunk list) for each node. List only the rough information and show only the simple connections between each node. This is the first compilation of requirements. Adjust it at each step. It is the first part of your CE plan.

Assess the resources. You have already addressed the question of resources in the estimate. This is, of course, the primary issue in the design of any system. You must first determine that you have enough resources to establish a doctrinal or a standard system. This done, you will then be able to tailor one to meet your mission requirements. The first step here is to make (or review) a list of communications units and to inform yourself on what they can do. It is important to try to maintain unit integrity when assigning equipment. This means that, when a unit is responsible for communications in an area, it should have control over all equipment that affects the level of service. For example, one unit should supply and operate all equipment at a node, if possible. Your next item is a list of all major items and of the units to which they are assigned. This will be your running record of what you have and of where it is to be used. Careful use of this will keep you from overcommitting resources and will enable you to keep track of them.

Configure the network. You should now have all the information you need to develop a complete network. The first step is to go back to your network plan and assign other nodes. You will also assign equipment to those nodes. Base your assignments on the following factors:

  • Support of lower priority users.
  • Redundancy for restoral and for alternate routing.
  • Capability of transmission media.
  • Electronic and physical security.
  • Expected unit displacement frequency.
  • Nodal equipment limitations.

You should apply the rest of the capability to support the other users of the network. Generally, you would have assigned AN/TTC-39s to the higher priority nodes. Now you must portion out the rest of the AN/TTC-39s and the other high capacity switches. These include such items as AN/TTC-38s and other automatic and manual switches. If you need interface devices of any type, list them also and allocate them in your equipment inventory. (See Chapter 3, Section III, for interface information.) When you assign nodes, arrange them so that destruction of any one will not isolate any part of the network. The key to this is in your routing plan. This will also provide the basis for alternate routing under both normal and emergency conditions. (See paragraph 5-8).

The capability of the transmission media is, of course, critical to the network arrangement. For example, if enough repeater equipment is available, the network will be able to handle rough terrain. But using terminal sets for repeaters will reduce this ability, The need for electronic and physical security because of the tactical situations can place severe limitations on your configuration. Where possible, you should not direct antennas toward enemy territory. You should also protect node and transmission locations from enemy fire and from enemy incursions.

You can form an estimate of unit displacement frequency from the nature of the mission. In a swift moving advance, units will not stay in one place very long. (See paragraph 5-7 for displacement information.) You must then plan to locate more nodes forward or along an axis so that support is available for units moving in and out of an area. To keep up with the forward movement, you may also have to save nodal communications for your own nodal jump capability. This would decrease the number of possible nodes in your network. The factor nodal equipment limitations refers to the maximum number of subscribers and trunks the node can handle. It also refers to limitations on connections to other nodes due to the nature of the control and multiplex equipment. Your network plan should now look like Figure 5-6.

Develop factors affecting configuration.

During the configuration phase and also during the next two phases (routing and subscriber services), you must deal with many details in the use of the circuit switch. This will enable you to complete the planning and to issue the communications order. Refer back to paragraphs 4-5 and 4-6, which tell how to use the worksheets for AN/TTC-39 operation. Your orders should include a set of these worksheets for each switch. This will ensure that the switch supervisor knows exactly what the configuration and trunking are to be. You have already started P-1 and P-2 but have not filled in all of the blanks. The switch supervisor will complete those having to do with specific resources of the switch. However, you must know these resources for each switch so that you can allocate them. Keep copies of worksheet P-5 (circuit card inventory) for each switch in your file and require a return copy for each switch that goes into operation. Include instructions of this nature in your SOP so that you do not have to repeat these instructions.

Now complete worksheets P-1 and P-2 (subscriber list and trunk list). From these, develop P-3 and P-4 (loop terminations and trunk terminations). The planner will not usually fill out strapping worksheets (S-1 through S-9). However, the information you need for them should be on either your planning or data entry worksheets. Now partially fill out the data entry worksheets (D-1 through D-27). They will be completed at the switch. Gathering information and filling out worksheets may seem like a big job. However, you will probably only do it once. As an operation continues, you should only be issuing changes. At this point, the decisions you have made to configure the network have given it its major shape. Your future decisions will continue to affect it, however, and will add to the data elements that will become the data bases for each switch. Some of these decisions follow:

Extension nodes. Your network may have to serve areas of deployment that are too far from a major node to be served by long locals. These deployments will help conserve your switching capacity, but they will be a drain on transmission and multiplex capacity.

Traffic load. Paragraph 5-4 has information on traffic loads. When first planning a network, you will have to estimate the amount of the load. Draw on system records, history, and your own experience. One of the reasons you started P-1 worksheets for each switch is to provide a pattern of use as an aid to this estimate. Add to this from your information on extension nodes. The key part of the traffic estimate deals with large headquarters subscriber traffic, which probably involves the highest proportion of users. Once you have a picture of the traffic, you can determine the number of trunks. Now use this figure to plan the amount of equipment to use. When you plan the routing, you may want to alter this somewhat. Use the information in paragraphs 3-3 and 5-4 to design the network for GOS.

COMSEC. The first consideration in COMSEC planning must be compatible keys and COMSEC equipment. You must designate circuit switches as cryptonet control stations for COMSEC key distribution. See FM 24-27A for further instructions. TB 380-40 provides additional information on key management and distribution.

Switch loop rate. Paragraphs 3-2 and 4-6 describe the loop rate (or channel rate). You can set each switch for either all 16 kbs or all 32 kbs. If your network contains any analog switches, use the 32 kbs rate for the entire network. You can use the 16 kbs rate only when the all digital network is a reality. Use of the 32 kbs rate will minimize path degradation which could occur with multiple analog-to-digital and digital-to-analog conversions.

Essential user bypass. As the network planner, you control the allocation of EU. The operations order and your SOP give you guidance for this. However, the telecommunications battalion planners should verify your assignments because they are closer to the served units. Use worksheets D-7 and D-27 for the switches involved.

Glare. See paragraphs 3-7 and 4-6. Glare is classmarked by TGC. Assign classmarks on worksheet D-6 for every switch so that one switch accepts and one rejects glare for each TGC.

Numbering plan. Each switch can be classmarked for either 3/4 or 4/3 plans but cannot use both at the same time. See paragraph 4-4. The format will be decided at the theater level, but if possible, use the 4/3 plan for your network so that the standard TTNP applies. Use worksheet D-1/D-4 for each switch, At the same time specify whether the switch will accept abbreviated dialing. You can authorize this for all switches that have local subscribers. Use worksheet P-1 to assign PR, SL, and XXX numbers. Also use this worksheet to make fixed directory assignments. The PR is assigned by theater, and SL and XXX are assigned at lower levels.

Data traffic. There is no analog-to-digital or digital-to-analog conversion for data subscribers. In your path planning, you must assign digital paths for digital data calls and analog paths for analog data calls.

Gateway switches. Review paragraph 4-7. Use worksheet D-1/D-4. The use of this classmark minimizes some analog-to-digital conversions that are permitted at gateways but not at intermediate switches. A gateway may do unlimited conversion for all calls, but if the switch is classmarked gateway NO, there are limits. When the switch is a tandem or intermediate switch for a call, it will not do conversion on alternate routes if the primary TGC is an interswitch trunk (AN/TTC-39 to AN/TTC-39). But it will allow analog-to-digital conversion if the primary TGC is to any switch other than an AN/TTC-39. This requires the planner to designate gateways only where needed to assure that calls being switched through intermediate switches will be completed. Only two conversions are permitted. The goal is no more than one.

Traffic limits for precedence traffic. (See worksheet D-6.) Use this restriction to assure that incoming calls can get through to a switch. Do this on a network-wide basis so that incoming as well as outgoing traffic can be balanced.

Other restrictions. Use zone restriction, call inhibit, and load controls based on operational experience. It is very difficult to impose these restrictions unless there is some network experience to show the need. It is probably wise not to use them until a significant problem shows that they are necessary.

Develop routing (network routing plan).

Review paragraph 4-7 for a description of the routing schemes for the AN/TTC-39. Also review the calculations you made for the traffic load. Trunk requirements based only on the traffic load will reflect minimum needs. The basic factor here is alternate routing. Each set of routes to a distant switch consists of one primary and up to five alternate TGCs. TGCs are a set of trunks between two switches, including both analog-to-digital types. The composition of these TGCs reflects the traffic load estimated between two switches (local plus tandem traffic) and the transmission characteristics needed. Each TGC should be some combination of the types of trunks listed in paragraph 4-6. This mix enables the switch to search each trunk type in an order determined by the type of call. The switch will then match the trunk to the caller's equipment characteristics. (See paragraph 4-7 for details.)

Return now to the network plan (as per Figure 5-6). Make sure that every node has at least two connections so that the elimination of one node will not isolate another. You may also want to add connections in certain areas of heavy usage. For guidance, draw on your experience and on the traffic engineering data. This will provide additional alternate routes. However, it is important that you keep alternate routing to the minimum needed to meet GOS requirements. An excess could cause uncontrolled multiple routing resulting in system degradation.

Now develop the network routing plan. You must list each switch that can receive calls in the routing tables of every other switch. These tables show primary and alternate routing. However, you must also plan routing for the entire network. Figure 5-7 is a simplified sample theater routing diagram showing a theater with three corps and four divisions each. Each of the nodes is numbered (SL) and is within a PR zone. Two area codes cover the theater and corps and division areas. The area code and the PRSL identify each node. Each connection (TGC) has a number. Figure 5-8 is the form of a sample network routing plan. To use this form list each node vertically and horizontally. Then for each switch list the primary and alternate routes to every other switch. The sample has switches 601 72 10, 701 83 18, and 701 92 24 filled in to show how this is done. Note that you must read from the originating switch at the top.

Choose the routes according to the size of the TGC, the most direct connection, and the amount of traffic expected. In most cases, the primary connection should be the most direct one. If switches connect directly through a TGC, use this as the primary route. If routes must go through other switches, use the shortest path as the primary unless traffic or TGC size dictates otherwise. The example in Figure 5-8 shows only one alternate route. However, the AN/TTC-39 is able to provide up to five, depending on the number of routes serving each switch. Now extract from the routing plan the information for the data entry worksheets for each AN/TTC-39. Use the ANY entry (worksheet D-11) for each route between areas. Use the APR entry (worksheet D-12) for primary zone routing. Use the ASL entry (worksheet D-17) for switch locations. The rest of the routing entries are for variations of these basic routing decisions. (See paragraph 4-6 for further discussion.) A complete routing plan may contain further information about numbering plans, fixed directory routing, subscriber numbers, area codes, and switch codes. On the other hand, the CE order may include this information.

Allocate services to subscribers. You have already started worksheet P-1 (subscriber list). You should refine your preliminary estimate of the subscriber needs at each switch. You should also check your estimate with the telecommunications battalion that serves the switch area. The essential user bypass and the fixed directory list are especially important. Give them close attention. (See Chapter 3, Section I, for additional information.)

Establish control and reporting. The purpose of the CEMS is to manage the communications system of which your circuit switched network is a part. (See paragraph 1-3.) The CEMS is the principal tool for controlling the system and for receiving reports on its operation. However, the uses of this tool will vary considerably, depending on whether automated devices are present. If they are, you will have much more timely information available and may even be able to automate changes to the network. In either case, at this phase of network development you must inform all subordinate elements as to how you will manage the system. Inmost instances, your SOP will cover this. In writing the SOP consider all of your needs as the planner and all of the needs of the operating elements. The CEOI can also be helpful. It will provide much of the technical information and instructions you will need. However, you will usually have to supplement it with the SOP and by the CE order.

Issue the CE order. At this point, you are ready to issue the CE order. The format for this is in FM 24-16. As already noted, this may be part of the command's operations order. For the corps planner, it will more likely be a signal brigade order. The CE order includes the worksheet already described, diagrams and map overlay showing the switching network, and a multichannel systems diagram showing the technical layout of the entire system. Your network plan may well be the basis for this multichannel systems diagram. (See paragraph 5-5 for further information.)

Test the system. The best way to test the communications system is to operate it. However, it is much less painful to try it out before operations begin and to iron out any bugs that are present. If you can, test the circuit switched network as soon as it is operational. Do this by having network personnel place random calls from each switch. If you have access to a computer, you can develop a simulation program to check parts of the network by sending out data streams that represent traffic. In any case, you must require technical control personnel to run local tests between switches. Although these test should be part of your SOP, you must specify the extent, duration, and report procedures for each operation.

5-7. Displacement

Unit movements.

No matter how stable a combat operation is, the communicator must always be prepared to move. Communications is always critical during a movement. The same is true during the early stages of setting up after a movement. But movement of all or part of a supported unit does not always mean that you must move everything. In a circuit switch planner's ideal situation, the units can move from one switching node to another. You would only have to worry about maintaining contact while the unit is mobile. If the planner is able to configure the network so that it lies along the axis or direction of movement, connection will be simplified. Units needing the area system for support will tie into the nearest node. Remember that these moving units may be part of the fixed directory system.

Nodal movements.

Changes in CE demands, such as shifts in unit concentrations, may require you to move a node. If you were able to keep enough equipment as spares or in reserve, you can use a phased displacement. This permits you to setup a switch and the other needed nodal equipment at the new location and then gradually phase down the old. Phasing may also mean simply turning power on at the new site and shutting it off at the old. But remember that a circuit switch does not operate by itself. You must plan for the use and the phasing of transmission equipment, multiplex equipment, and technical control. You must, in other words, plan for the total node.

If the switch movement is from one of the major headquarters, you may already have jump equipment allocated. In most such cases, the new switch will move with the advance headquarters, It can also move ahead of it so that it will be partly operational when the headquarters arrives. Again, the other nodal equipment will move with the switch. Whether the movement involves anode or a headquarters support switch, you may have to reconfigure your network. If only a few nodes are moving, this task is fairly simple. It grows complex when there is constant network movement. However, both cases involve the same procedures. Below are the advanced planning tasks for moving a node:

  • Estimate the need for using jump capability (if any). If you have another AN/TTC-39 available, you can position it to start up as soon as the old one shuts down. You may have to use a temporary manual switch or a smaller automatic switch to fill in while the AN/TTC-39 moves.
  • Determine the subscribers at the new location and those to be discontinued at the old location. Some of the old subscribers will move also. Others may be served by another node. You may have to activate the EUB for those critical users at the old location who must continue to receive service.
  • Use already reported metering data to estimate the amount of trunking needed for the new location. Review your assets to see if you have enough. Set priorities if necessary.
  • Look at the network plan (Figure 5-6 is an example) to determine a location for the new node in the network. Revise the plan.
  • Look at the routing (Figure 5-8 is an example) to see what changes are needed. Assign primary and alternate TGCs.
  • Estimate the time needed to tear down, move, and set up the node. Also estimate the times needed to connect new subscribers.
  • Engineer the transmission paths. Take into account terrain, security, frequency problems (EW and friendly), and logistics.

After you finish the advance planning, issue the CE order. The order will specify:

  • Times required to tear down the node and to make it operational at the new location.
  • Lists of subscribers and when to connect them.
  • Changes to the data bases of each switch in the network. Each must adjust its data base to cover routing to the new location.
  • New subscriber services, especially EUB and fixed directory lists.
  • Instructions to technical controls about cutover and patching of circuits. Include orders to discontinue and reestablish circuits. Require reports on each.
  • Unit responsibility for the node and the switch, for transmission control, and for MUX equipment.

These procedures are similar to those for planning a new network (paragragh 5-6). This is because displacement procedures are really procedures for changing a part of your network.

Network movements.

In a fast-changing situation, you may need to move more than one node at a time. You may have a combination of nodes closing out, being jumped (putting replacement nodes in place before closing out the old one), and being established. But whether the situation is simple or complex, the basic things you must do are the same. Complexity means that a number of things are happening in a very short time. In such cases, you will in effect be planning your network over and over. The procedures set forth in paragraph 5-6 provide a basis for this.

Switch capabilities for displacement.

  • Setup time for the AN/TTC-39 is 80 minutes with 6 fully trained personnel. This includes initialization. It does not include subscriber instrument installation.
  • Teardown time is 60 minutes. Again, this does not include removal of subscriber instruments.
  • Because the switch is truck-mounted, it can start moving as soon as it is powered down and the cables disconnected.
  • The EUB and fixed directory procedures can assist in the displacement operations.

5-8. Reconfiguration and Restoral

Reconfiguration is a general term that describes changes made to a circuit switching network. Sometimes this term also describes a change to the physical arrangement of the switch. Chapter 2 uses the term configuration to show the number of matrices, cards, and other equipment for a particular switch model. Paragraph 5-6 uses the same term to describe the circuit switching network arrangement. In this paragraph, reconfiguration refers to network changes. Restoral refers to the efforts made to return communications service to the subscriber, to a node, or to the network in general.


It is possible to reconfigure a network with no change in the service provided. For example, you can discontinue one node and serve its subscribers from another node. If the trunk switching remains adequate, the service will stay the same. Paragraph 5-7 showed how displacement can result in network reconfiguration. On the other hand, displacement might not affect the network at all, especially if you can use jump equipment. There are four reasons to reconfigure a circuit switching network:

  • To meet a new mission.
  • To increase the efficiency of the network.
  • To conserve equipment.
  • To adapt to transmission media.

When the mission changes, you must begin a new planning cycle to meet the new requirements. The transition from one configuration to the next is very critical. You will probably have to do it in a short time. In this case, you must plan for scaled-down communications while the equipment is in transit. Sometimes this will consist only of a mobile radio until the critical part of the system is in place. This critical part will generally be the backbone of the network from theater headquarters to the corps. Figure 5-5 is a sample backbone network. Such a network could be even more austere with even fewer nodes. If you are fortunate, you may have enough time to phase the transition. In this case, you can leave some circuit switching nodes in place until the supported units move or until other nodes must be in new locations.

It is always important to increase the efficiency of your circuit switching network. This is true even when there is constant movement of part or all of the communications units. Look for ways to save transmission capability or to identify and retire unneeded nodes. This will leave you better prepared for the next change or displacement. You should be doing a constant analysis of the network. (This is called configuration analysis.) Base this procedure on your information input. Your traffic analysis should also be continuous. It will tell you whether or not your preplanned trunking is adequate. Consult the metering information from each switch in the network. (See paragraph 3-13.) (Establish the reporting of this by SOP.) If the percentage of blocked calls on any trunk is higher than your planned GOS you must make adjustment. Examine also the precedence call figures to see if the subscriber services need adjusting. Bear in mind that if no blocking occurs you may have allocated too much trunking. Also stay alert to subscriber comments. Commanders at all echelons should have satisfactory service. Thus, your SOP should include a system to find out whether they are getting it.

Conserving equipment is essential if you are gathering your resources for a change of mission or a displacement. You can reconfigure by withdrawing equipment from service, by rerouting certain trunks, and if necessary, by providing reduced service to area subscribers. Part of your constant configuration analysis will focus on adapting to transmission media. This is a key factor in improving network efficiency. Different types of transmission equipment have differing capacities and you may at times have to change from one to another. Take for example a very extended network in which a node moves beyond the transmission equipment's line-of-sight. In this case you may have to install a troposcatter or a satellite system. If the use of the satellite system reduces the availability of trunks, you may have to reconfigure to provide more trunks from another node. You may even have to degrade the service. On the other hand, the use of the troposcatter system may add trunks. Again, you may want to reconfigure this time to take advantage of the added capacity. You could use these extra trunks to send traffic to a node connected to the remote one, which in turn might allow you to move or to eliminate a node. Remember that the AN/TTC-39 functions most often as a tandem switch. Any change of capability that changes the need for trunk switching is a valid reason for reconfiguration.


FM 24-22 sets up a way of assigning priority indicators to circuits and systems. There are five categories: 1, 2, 3, 4, and 0. The letters A through I show subpriorities. The combination of the two shows the times and the order in which restoral of a circuit or system should take place. For example, a 1 means restore service immediately; a 2 means restore in 10 minutes, 3A in 20 minutes, 3B in 1 hour, 3C in 6 hours, 4A in 24 hours, and 4B in 72 hours. A 00 means restore after all others. DOD Directive 4605.2 (CONFIDENTIAL) tells how to use this system. Designations used in tactical systems are 1A, 1C, 1D, 1E, 1F, 1G, 2C, 2D, 2H, 2I, 3A, 3B, 3C, 4A, 4B, and 00. As the planner you can extract the needed portions so that CSCEs will have this guidance. Use this method to assign a priority to each trunk and to each system. This will tell all controllers, operators, and planners which outages to work on first.

Bear in mind, however, that restoral of network means more than simply tying in and turning on equipment. Paragraph 5-6 showed how to plan the network so that a loss of one node would not isolate any portion of the network. This meant providing alternate routing to at least one other node. The combat situation may dictate that you plan for even more alternates so that multiple node failures will do the least harm. If a node is out temporarily, the network will route around it if trunks exist. If the node failure is permanent, you should change the data bases of all switches to show the new routing and then delete the switch. Failure to do this will lessen the network's efficiency because it will always be trying to transmit on a primary route that is not there. Thus, restoral can mean trying to replace the node, meanwhile depending on alternate routing. If you cannot do this, you will have to reconfigure the network. The procedure is the same even if several nodes are in trouble. If you have unused nodal assets, you can apply them according to the above priority system.

5-9. Interoperability

Tactical communications networks must be designed to exchange information and to interconnect, signal, and use the facilities of other networks. The AN/TTC-39 can fulfill this important interoperability requirement for all planned networks. Switch design factors that support this involve:

  • The ability to handle various signaling schemes.
  • The use of various analog and digital transmission formats.
  • The ability to substitute for nontactical switch-es such as AUTOVON.
  • The ability to act as a gateway switch to other systems.
  • The use of interface units such as the NATO interface unit.
  • The ability to connect to most types of commercial switches.
  • The ability to handle multiple numbering schemes.
  • Use of net radio interface unite to connect various types of radio users to switched subscribers.

Table 3-5 shows the AN/TTC-39 connections to the networks described below.

AUTOVON (Department of Defense communications system).

The AN/TTC-39 design conforms to the AUTOVON analog network. The AN/TTC-39 can connect to the AUTOVON network and acts as a PABX. It provides AUTOVON telephones with subscriber access. It can also substitute for an AUTOVON switch, This means that the switch design includes AUTOVON signaling, supervision, and trunking procedures. The AN/TTC-39 also recognizes the numbering system of the AUTOVON switch. It can use all AUTOVON preemptions and protocols.

NATO and other Allied communications systems.

The AN/TTC-39 connects to NATO networks through Telephone Signal Converters CV-3478/TTC-39(V). These are commonly known as NATO interface units (NIU). Each NIU can connect up to 8 NATO circuits in accordance with NATO Standardization Agreement (STANAG) 5040. AN/TTC-39 subscribers can call:

  • NATO subscribers
  • US elements under the command of a non-US NATO force
  • NATO subscribers under the command of US tactical forces. Any allied system conforming to STANAG 5040 can communicate with the AN/TTC-39 system.

Commercial networks.

The AN/TTC-39 provides equipment, signaling, and supervision to connect to commercial 2-wire, 4-wire, and 8-wire trunks from commercial offices. All types of commercial telephone terminal sets can be connected to the switch.


The AN/TTC-39 is a universal switch in the tactical, strategic, and commercial world. It can carry information across network system boundaries. It handles a wide variety of signaling and numbering schemes. It can be used as a gateway switch and can substitute as a strategic switch, It uses the latest CCS format. It can operate with both present and future tactical modulation, multiplexing, and transmission equipment, and it includes built-in COMSEC capabilities.

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One Billion Americans: The Case for Thinking Bigger - by Matthew Yglesias