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Planning Factors


3-1. Loop and Trunk Capacities

The loop and trunk capacities of the AN/TTC-39 vary, depending on the specific configuration of the switch. The AN/TTC-39 is modular in both hardware and software. Because of this, it can provide a whole family of compatible switching configurations. Table 3-1 shows the standard terminal allocations for the five basic configurations. Figures 3-1 and 3-2 depict line termination capacities for two AN/TTC-39 switch sizes and configurations: The 600-line (V)2 and the 300-line (V)3, respectively. These are the configurations you will find in the field until more digital equipment is in service. Review Chapter 2 for equipment descriptions. See Table 3-5 for terminal descriptions. Table 3-2 provides more detailed information on the types of terminations. It shows a typical issue of loop and trunk terminations of each type.

3-2. Digital Operating Rates

The AN/TTC-39 can operate at either a 16- or 32-kilobits per second (kbs) digital channel rate. All trunks within a trunk group and all loops terminated on a given switch must operate at the same rate. However, a switch operating at 32-kbs can accommodate trunk group traffic at a 16-kbs channel rate. This traffic must flow on designated trunk groups terminated on the TDMX. You must replace or strap certain circuit cards to change TDMX trunk groups from a 32-kbs to a 16-kbs rate. Table 3-3 identifies the cards you must strap to change the rate of an AN/TTC-39. Paragraph 4-4 gives specific instructions for strapping each card.

3-3. Traffic Handling Capacity

The AN/TTC-39 can accommodate a number of tactical and strategic applications. This gives it a broad capacity to handle a variety of traffic. Its tactical applications include use in various type of node configurations (command, base, and area), as an access of trunk/tandem switch and as a stand-alone switch. It can also function in combined circuit switch/message switch configurations. Call rates and traffic loads will vary with the various uses and configurations of the switch. In analyzing traffic handling capacity, you must consider how certain factors affect the switch traffic handling capacity. These include subscriber/loop traffic loading, off-hook factors (percentage of time the circuit is in use), and average call holding times. Trunk group traffic loading, total busy hour traffic load, and voice/data traffic mix are also factors. Refer to paragraph 5-4 for further information on traffic engineering.

You should plan the switch network so that there is a grade of service (GOS) goal of 0.100. This means that, during the busy-hour, no more than one call (at the Routine and Priority precedence levels) per 100 attempted calls will fail to go through (or be blocked) for lack of an available path. In critical situations this GOS goal can be degraded up to 0.20 with the approval of the senior CE officer at the corps or theater level depending on the level at which the planning was done. The AN/TTC-39 will provide nonblocking service for Immediate, Flash, and Flash Override precedence levels. It does this by preempting lower level calls on a line. The SDMX of the AN/TTC-39 is designed for a grade of service of .001 and the TDMX is designed for nonblocking. Analog-to-digital calls use intermatrix units (IMU) that can cause blocking at equal precedence levels.

3-4. Secure and Nonsecure Operation

The AN/TTC-39 provides automatic switching and trunking service to both secure and nonsecure analog and digital subscribers. System security features can function at all traffic modes. This provides for mixed secure and nonsecure traffic and data. The switch can connect to and be part of military switched networks of all types, both secure and nonsecure. It can also connect with commercial switches and terminal equipment and with Allied and NATO communication systems. The AN/TTC-39 will provide for compatible signaling and supervision. This means that no changes to existing equipments are necessary when such additional equipments as NATO interface units and SF adapters are used.

Secure operation.

The AN/TTC-39 provides secure operations by means of encryption and the use of protected wireline distribution systems (approved circuit). Refer to AR 530-2 and to FM 24-27A for modes of secure operation. A COMSEC module is contained within (built-in) each AN/TTC-39. This module provides the required loop and trunk security devices for digital voice, data, and record traffic. A digital terminal security device encrypts and decrypts transmissions to and from the switch. The subscriber uses the DSVT (TSEC/KY-68) for this function. Although the KY-68 is digital, its subscriber may place and receive calls to and from analog terminals. When the call is to an analog terminal, not classmarked for Security Required, the KY-68 must be changed to the nonsecure mode. A nonsecure warning red light shows on the instrument panel and a nonsecure warning tone sounds in the earpiece when an off-hook terminal is operating in a nonsecure connection. The call will then be conducted in the nonencrypted mode. If the analog user has been classmarked for Security Required, the call will remain encrypted until it reaches the destination switch. If a KY-68 user calls the CSP at the circuit switch, an LKG will remain connected for the duration of the call.

Nonsecure operation.

The AN/TTC-39 can provide automatic, nonencrypted, voice only switching access for nonsecure subscribers with a TA-954, digital nonsecure voice terminal (DNVT). The circuit switch also provides access to a variety of analog subscriber equipments that use the normal wideband, common battery 20-Hz, 1600-Hz, and AUTOVON equipment via LTUs. Any voice subscriber connected to the circuit switch can communicate with any other voice subscriber. This can be either directly or via a trunk. The exception is when security devices or classmarks of either subscriber prohibit the connection. It is also possible to connect two analog subscribers with each other via digital trunks. The circuit switch provides analog-to-digital and digital-to-analog converters. These can provide connections between a subscriber using an analog voice terminal and a subscriber using a digital telephone set. The converters also can connect approved analog voice subscribers routed over digital trunks. The TA-954 provides nonsecure subscriber service for uncleared areas and secure service using protected distribution systems. These are done by a combination of physical security and transmission link encryption. The DNVT employs CVSD modulation that converts analog voice signals to a 16- or 32-kbs digital bit stream. It can operate in the nonsecure circuit switched mode with the KY-68. It can also send a single frequency control (seize) tone for use with net radio interface devices to allow for half-duplex voice transmissions.

As mentioned, the circuit switch can also accommodate a variety of analog subscriber equipments. Five of the seven LTUs of the switch directly terminate a variety of analog subscriber types and service terminal equipment. They provide the proper connection with the SDMX. These five types of LTUs are:

  • Normal wideband (NW).
  • Common battery (CB).
  • 20 Hz.
  • 1600 Hz.
  • AUTOVON telephone.

The five units are interchangeable (two per card), Any SDMX terminal can be adapted for use by an analog subscriber. A termination on the SDMX can be connected to any termination on the TDMX and vice versa.

Normal wideband line termination unit (NWLTU). The normal wideband LTU connects the A crosspoint matrix with either a direct current or tone-supervised loop, trunk, or adapted line. The inlet and outlet transformers connect the termination adapter to the NWLTU. The connection is then made in the silicon-controlled switch (SCS) crosspoint matrix completing the path from the termination adapter to another subscriber through the LTU and SCS.

Common battery. The common battery LTU connects the SCS crosspoint matrix and a common battery-supervised, 2-wire loop or trunk.

20 hertz. The 20-Hz LTU interfaces the SCS crosspoint matrix and loops and trunks. It provides two-way 20-Hz ringdown with idle tone on trunks.

1600 hertz. The 1600-Hz LTU connects the SCS crosspoint matrix with the 4-wire trunks. It provides two-way 1600-Hz ringdown with idle tone.

AUTOVON telephone. The AUTOVON telephone LTU interfaces the SCS crosspoint matrix and overseas AUTOVON telephones. The AUTOVON telephone is a 4-wire subset that employs DTMF signaling. It employs DC loop supervision on the subset transmit terminal with DC ringing on the receive terminal.

3-5. Timing of Switches.

A primary and secondary network timing source will be designated, and a network timing diagram, similar to Figure 3-3, will be included in the operations plan or operations order. All networks should have a designated master timing source and at least one alternate source in case of failure of the master source. Any network AN/TTC-39 may be designated as a master source. All other digital switches shall be slaved to the master for synchronization. A timing path for a given switch should not traverse more than one intermediate switch. The master source may derive its timing from a source external to it (for example, AN/TRC-170 or AN/TSQ-111), but is still the source for all timing in the network.

3-6. Numbering Plan Format

The AN/TTC-39 uses the following numbering format or portions of it as described in this paragraph. There are also prefixes for special features. (See paragraph 4-4.)

9YX MYX NNXXXXX    where M = 2 thru 8
                                                    N = 2 thru 9
                                                   Y = 0 and 1
                                                  X = 0 thru 9.

The first three digits, 9YX, identify the NATO nation (military unit) being called. (See STANAGs 5046 and 4214.) The next three digits, MYX, represent a set of national area codes. The last seven digits, NNXXXXX, identify a combination of switch codes (similar to commercial telephone exchanges) and subscriber numbers. The seven digits are then defined by one of two numbering plans, the 4/3 or 3/4 plans. These plans group the seven numbers NNXX XXX or NNX XXXX. The switch can function with only one numbering plan at a time. However, it can serve calls that use an alternate numbering plan. It does this by routing such calls over an interconnecting trunk that carries all routing indicators of the calls. Numbering plans that the AN/TTC-39 can accommodate fall into two categories:

  • The current Army tactical telephone numbering plan (TTNP).
  • Certain nontactical numbering plans.

Tactical telephone numbering plan.

The TTNP places the basic NNXXXXX seven digits in the 4/3 format. The TTNP consists of a 2-digit primary area code, a 2-digit switch location, and a 3-digit subscriber code in the following format:

PR-SL-XXX     where PR = 72 through 98
                            (except 80,81,90,91)
                            SL = 00 through 99
                            X = 0 through 9.

You can use portions of this numbering scheme as abbreviations. For example, you can use PR routing to specify routing to a primary zone or area, SL routing for routing to a switch location, and XXX routing for routing to a subscriber or loop.

Nontactical numbering plans.

In a nontactical system, the AN/TTC-39 uses a numbering plan similar (but not identical) to the AUTOVON numbering plan. This type of plan uses a 3-digit switch code and a 4-digit subscriber code in the 3/4 format:

NNX XXXX     where N = 2 through 9
                                      X = 0 through 9.

Again, you can use portions of this numbering plan as abbreviations. For example, NNX routing can specify routing to a switch and XXXX routing can specify routing to a subscriber or loop. You can also use NNXX numbering to conserve NNX codes on calls to a private branch exchange (PBX) or an expanded switch. The NNXX refers to the switch code NNX plus the first digit of the subscriber code. This enables up to 10 PBXs or expanded switches to share the same NNX code. (See paragraphs 4-4 and 5-6 for a detailed discussion of numbering and how it applies to the AN/TTC-39. FM 24-26 provides further information about numbering plans.)

3-7. Services and Classmarks

The basic service of the AN/TTC-39 is to establish connections between subscribers on a direct dialing basis. However, the switch also provides supplementary services, and it imposes restrictions on subscribers through application of modular hardware and activation of specific software. You can specify these supplementary services and restrictions by switch data entries called class-of-service marks, or classmarks. Listed below are the more significant of these.

Subscriber services.

Specialized subscriber services include the following:

  • Precedence and preemption.
  • Conferencing.
  • Direct access.
  • Automatic line hunting service.
  • Compressed dialing service (switching system wide).
  • Abbreviated dialing service.
  • Fixed directory service.
  • Call transfer service.
  • Data service.
  • Commercial office.

Precedence and preemption. Subscribers, if properly classmarked, can use five levels of precedence in making calls. In ascending order, the five levels of precedence are: Routine (R), Priority (P), Immediate (I), Flash (F), and Flash Override (FO). The originator of a call sets its precedence level regardless of the precedence levels authorized for other parties in the call. The switch supervisor, through the use of classmarks and by direction of the planner, can easily assign and change a maximum precedence level for any subscriber line. Calls routed to other switches employing no precedence level or noncompatible precedence levels are assigned the desired level within the AN/TTC-39 switch. However, the desired precedence converts at the destination switch to the level compatible with that switch serving the called terminal. Preemption occurs when a call in process or equipment necessary to process a call is of lower precedence than a preempting call. A call can not preempt a line, trunk, or switching center function handling another call of equal or of higher level precedence. The precedence indicator precedes all other dialed digits. If preempting occurs, a tone burst will notify both parties on the preempted call. Upon request, the operator can provide a higher precedence on a call-by-call basis. The precedence reverts as soon as the call ends, however.

Conferencing. The AN/TTC-39 uses analog conference bridges to establish progressive and preprogrammed conferences. You can classmark the switch to handle up to six simultaneous five-party conferences per 600 external lines (in the (V) 1 and (V) 2 configurations). The (V) 3 configuration can handle up to four conferences. The maximum conference sizes (including the originator) at any switch are 20 parties for the 600-line switch and 14 for the 300-line switch per progressive or preprogrammed conference.

  • Progressive conferencing enables the subscriber originating a conference to dial the other subscribers in any sequence. He must wait and verify whether he has connected each party before calling the next one.
  • Preprogrammed conferencing establishes conferences between predesignated subscribers. It functions via a software programmable list of conferees. The originator sets up the conference by keying in the program code (6C) and the conference list designation (NX). The central processor of the switch then looks up a table that supplies the conferees' addresses. This enables the switch to connect each listed party to the conference bridge and to notify the conferees by recorded announcement. Each conference list is classmarked as either secure or nonsecure. If a nonmember tries to dial a conference list, he gets an error tone. If there is no bridge available for the conference, the originator gets a busy tone. The bridge is released only if all parties are unavailable.

Direct access service (DAS). This feature enables one subscriber to access another subscriber by simply going off-hook. The connection between points is made automatically. A telephone classmarked as a DAS phone cannot call any parties other than the ones he is classmarked to call. A DAS phone, however, can receive incoming calls from other DAS phones that are classmarked to call him. DAS phones can operate in two ways: both ways or only one way. Both ways mean that either party can directly access the other by going off-hook. This allows a temporary hotline to be set up between these two DAS classmarked phones. This only works if the planner does not allow other DAS phones in the network to access either of them. One way means one party can be class-marked to directly access another specified party by going off-hook. You can classmark up to 60 lines for DAS service.

Automatic line hunting grouping. When a user dials the address of a subscriber who has been classmarked as a member of an automatic line hunting group, the switch local to the group tries to contact the desired party. If the called party is busy, the switch tries to contact the next listed member. It in fact calls each member of the group via a predesignated search list until it finally contacts one. If the priority of the call is higher than Routine and if all lines are busy, the search begins again at the Priority level. The switch repeats the cycle until it can preempt a lower precedence call. Should all the lines remain busy, the originator receives a busy tone. A maximum of 32-line hunting groups with up to five parties in each group may be programmed in the AN/TTC-39.

Compressed dialing. This feature lets a sub-scriber save time by dialing fewer numbers and getting faster connections. It also reduces the number of digits that he needs to memorize. Compressed dialing lets a subscriber dial a 2-digit number (NX) plus C to reach the called party. The switch translates the 2-digit code via a table lookup and routes the call to the specified party. There are two categories of compressed dialing: common pool and individual.

  • The common pool holds 80 subscribers. This pool may be accessed by those subscribers classmarked for this. A switch can handle up to five common pool lists of 80 subscribers each.
  • Individual compressed dialing provides same type of service as common pool. The difference is the manner in which it is programmed. There is one lookup table with a capacity of 80 subscriber listings. These 80 listings can be assigned into 8 groups of varying size of 2 to 80 subscribers.

Each switch can handle five common pool lists and one individual dialing list that can be subdivided into eight smaller lists. A subscriber with access to an individual compressed dialing list cannot have access to a common pool lists.

Abbreviated dialing. Abbreviated dialing is like dialing between extensions in a PBX. The switch supervisor classmarks each switch programmed for the 3/4-numbering plan for either abbreviated or not abbreviated dialing. A switch classmarked for the 4/3-numbering plan is automatically classmarked for abbreviated dialing. Abbreviated dialing enables a subscriber to place a call by dialing only the last three or four digits of each others subscriber's address on the same switch. The number of digits depends on whether the switch uses the 4/3- or 3/4-numbering code. To call other switches, the caller must prefix the 7-digit or 10-digit address with a nine.

Fixed directory. The AN/TTC-39 provides the fixed directory list feature to both roving subscribers and roving units. There are two lists; a fixed directory subscriber list (FDSL) and a fixed directory unit list (FDUL). The processor retains the fixed numbers for both types of list. When the calling subscriber dials, the switch looks to the auxiliary routing table, translates the fixed number to a normal directory number (up to 7 digits), and applies normal routing procedures. The caller dials one constant 7-digit number (the first 2-digits are 99) regardless of the subscriber's location within an area code. The format for FDSL numbers is 99 PXJXZ (P = 7 through 9, X = 0 through 9, J = 7 through 9, and Z = 0 through 3). The FDSL consists of 3,400 entries. In the format for FDUL numbers the first 2 digits after the 99 refer to the unit and the remaining three to a specific party within the unit. They are in the form XXIXX (X = 0 through 9 and I = 0 through 6). The FDUL consists of 100 entries.

Call transfer. This feature enables an operator or a subscriber classmarked as such to transfer calls automatically to another AN/TTC-39 sub-scriber's telephone. Call transfer is not available to data subscribers or to line-hunting group members. Only 40 subscribers per switch can use this service at any one time. It applies only to incoming calls. Call transfer can be initiated only by subscribers with keypad telephones.

Data service. The AN/TTC-39 is designed to provide real-time data switching to both analog and digital telephone subscribers within their own communities. Terminal equipment includes teletype, paper tape and card devices, mobile data terminals, weather facsimile, low speed video scanners, printers, magnetic tape terminals, and imagery devices. To avoid signal degradation, an analog data subscriber should request analog required service by keying 3C as an access code. Digital subscribers need not key the access code. If a data call between an analog subscriber and a digital subscriber at another switch is attempted, the circuit switch returns an error tone.

Commercial office. Each DTMF subscriber may be permitted direct access to a commercial network (ATS entry). If routing has been assigned (ACN worksheet), the access code 5C is dialed to acquire the commercial network dial tone. (See the subparagraph on commercial network access in paragraph 4-4.)

Other classmarks.

Other classmarks deal not so much with subscriber services as with network and switch operations. Among these are loop and trunk classmarks such as spill forward, switch operating channel rate (16 and 32 kbs), digital trunk group channel rate, data service, data equipment type, secure call privilege, security required, key net identification (long loops), and satellite link identification. Some of the more significant of these are listed and described below:

  • Zone restriction/call inhibit.
  • Traffic load control.
  • Secure call/key conversion.
  • Glare.
  • Automatic intercept.

Zone restriction/call inhibit. The circuit switch uses classmarks to prevent classmarked terminals from completing calls to certain restricted zones. Each switch has eight zone restriction tables. Two of the eight contain up to 101 entries each. The rest contain up to 33 entries each. You must designate each of the zone restriction tables as either permissive or restrictive. If permissive, a terminal classmarked for that table can call only the listed codes. If restrictive, the terminal cannot call the listed codes. Classmark trunk or loop terminations either for one of the eight zone restriction tables or for global. Those with global classmarks can call anywhere. The AN/TTC-39 returns a recorded announcement to any attempt to call a restricted zone. This is applicable to both 3/4- and 4/3-numbering plans. Calls inhibit is like zone restriction except that it prohibits all calls to listed numbers. Each switch maintains a list (local subscribers and PBX trunks) containing area codes that can receive no calls. There are no classmark exceptions. If a subscriber or PBX trunk tries to dial a call inhibit area, the switch returns a recorded announcement.

Traffic load control. The switch can limit calls during high traffic periods to make optimum use of available switching and transmission resources. The switch does this by classmarks assigned to each directly connected subscriber and lower level switch in two ways:

  • By restricting access to trunks (transmission resources).
  • By restricting access to the switch (switching resources).

When traffic load control is done by trunk restriction, restricted subscribers (and lower level switches such as PBXs and unit-level switches) attempting to make trunk calls are returned a busy signal. These subscribers, however, are permitted local call access. (For purposes of traffic load control, a line(s) from an AN/TTC-39 to a PBX or unit-level switch is considered to be an access trunk that can be classmarked for traffic load control.) When traffic load control is done by call restriction, restricted subscribers attempting to make calls do not receive dial tone. Any one of the following five classmarks (1-5) may be assigned to each directly connected subscriber and lower level switch to obtain various levels of traffic load control:




Most essential (no restriction)
More essential (trunk restriction only)
Essential (trunk restriction only)
Less essential (switch access restriction only)
Least essential (switch access restriction only)

The circuit switch automatically activates traffic load control when traffic on trunks and calls to the switch reaches certain preset thresholds. You may also activate traffic load control by setting or changing the preset traffic thresholds through readily changeable software programming. Traffic load control for trunk restrictions (levels 2 and 3) may be implemented independently from traffic load control for switch access restrictions (levels 4 and 5). No traffic load control is imposed on those subscribers and lower switches classmarked for level 1. When trunk traffic restriction is desired, level 3 is implemented first; if additional trunk traffic restriction is desired, level 2 is implemented. Similarly, when switch access traffic restriction is imposed, level 5 is implemented first; if additional switch access restriction is desired, level 4 is implemented. See Table A-6 for a summary of classmarking for load control levels.

Secure call key conversion. In a circuit switched network, a number of cryptonets are established. Each switch uses one or more traffic (switch net) keys for its own subscribers. Key conversion thus becomes necessary to allow subscribers from different areas of the network to establish secure calls. The AN/TTC-39 at the originating end of the call controls the key distribution process so that two subscribers can be provided with the same per call key. This process of key transfer and conversion is covered in detail in FM 24-27A.

Glare. Glare condition occurs when two switches try to access each other at the same time over the same trunk. Without the proper software, neither switch would get access to the trunk. This could result in a lost call if only that trunk were available. Antiglare software will recognize the condition and will assign the use of the trunk to one or the other of the two switches, depending on the classmark. To set this up, enter classmarks into the switch data bases to designate each connected trunk group cluster (TGC) as either ACCEPT (glare) or REJECT (glare). When ACCEPT has been assigned to a switch TGC, that switch prepares to accept the glare signal and will either reroute the call or return a busy signal to the subscriber. When REJECT has been assigned, the switch will reject the incoming glare signal and will proceed with the call it placed on the trunk. Be careful not to assign the same classmark to both switches for the same TGC. Work out glare classmark rules on a network basis. (See paragraph 5-6.)

Automatic intercept. A caller will now and then dial a number that is nonexistent, unassigned, marked disabled, or otherwise unavailable. In these cases the AN/TTC-39 will intercept the call and will inform the caller of the condition via a recorded message. The switch sends calls intercepted from-20-Hz ringdown subscribers to the operator, who then completes the call. The recorded messages are:

  • The number you are calling is not assigned or out of service.
  • Area called is restricted at this station.
  • Standby for conference.
  • Precedence is being downgraded to highest allowed this station.

3-8. Essential User Bypass

The function of the EUB is to make sure that priority users retain service even if both switch CPUs fail. This feature uses a bypass function to transfer up to 60 designated digital subscribers from their home switch to a distant AN/TTC-39 switch. The switch EUB selector can prestore up to 60 from/to addresses in its random access memory. Use the assign and display EUB configuration (AEU) command (Figure 4-35) to load the EUB connection information from the CPU into a random access memory in the EUB selector. The VDU will display this information. When needed, you will activate the EUB function from the EUB control position of the patch and control panel. Upon activation, the EUB selector transmits the 60 prestored connection commands. The EUB selector can also accept manual from/to connection addresses. Key these manual addresses via the EUB control position of the patch and control panel. Bear in mind, however, that the maximum number of designated digital EUB subscribers, both manual and prestored, cannot exceed 60. You can also designate analog telephones to be essential users (EU). However, in order to activate these, you must strap them through the switch manually just as you would on a patch panel. You must also use analog trunks because the conversion capability is bypassed. Each AN/TTC-39 can accept up to 60 subscribers from two separate failed switches.

When setting up a communications network, you must take great care in assigning EUB subscribers to their backup switches. The number of trunks between the two switches limits the number of bypass subscribers that a distant switch can accept. You must also instruct distant switch personnel to enter your list of EUB subscribers into the tables of their switch. To do so, will use the ARB command (Figure 4-54). In addition, the distant switch must plan for the use or reuse of subscriber functions (classmarking), including rekeying actions, to accommodate your bypass subscribers. This is because your users will also become subscribers at that switch.

3-9. Signaling and Supervision

The AN/TTC-39 employs a full range of signaling and supervision modes to transfer call and network-related status, control, and information bearing message among terminals, subscribers, and switching nodes. Supervision refers to line status and control-type signals like off-hook, dial tone, ringing, and ringback. Signaling refers to information bearing signals, such as the addresses of called and calling parties. These signals are call-related. They initiate, set up, and complete calls between subscribers connected to the circuit switch. The switch also uses network-related signals. These perform such functions as maintenance testing, equipment failure identification, and periodic routine checks. See Table 3-5 for a summary of the AN/TTC-39 signaling modes and of the interface equipments compatible with these modes. In paragraph 4-2, the subparagraph on analog and digital call processing describes how signaling is used in typical call processing sequences for analog and digital subscribers.

The AN/TTC-39 uses two basic types of signaling and supervision modes: in-band and out-of-band. In-band signaling carries control and status information into the same channel (or frequency bandwidth) that carries the voice (or message) information. Out-of-band signaling uses a channel (or frequency bandwidth or time slot allocation) separate from the voice (or message) channel. This band transmits only control and status information. For analog transmissions, in-band signaling uses either an SF or DTMF within the nominal 4-kHz voice bandwidth. AN/TTC-39 in-band digital signaling inserts special digital signaling codes into the same channel that carries the digital voice (or message) bit stream, It then uses digital logic in the receiving terminals to decode signaling information. The switch also uses digital in-band trunk signaling (DIBTS). A DIBTS buffer provides in-band signaling between the AN/TTC-39 and subscriber switches such as the SB-3865. Out-of-band analog signaling in the AN/TTC-39 uses one designated trunk of an analog trunk group to carry signal and control data for trunks in that trunk group. This signaling channel operates at a 2.4-kbs rate. Outside the switch, the signal is combined with other members of the transmission group and is then transmitted to the distant switch. This is known as CCS or as common channel interswitch signaling. For the AN/TTC-39, digital out-of-band signaling is similar to the analog CCS.

3-10. Magnetic Tape

The AN/TTC-39 uses magnetic tape cartridges to store data, operational programs, and diagnostic and maintenance routines. Two cartridges contain scratch tapes for temporary storage. The program library tape contains the OLCOP programs and the others contain off-line diagnostic and maintenance programs. Two MTTs may be used to load the programs into the processor memory. Both communicate with the central processor units through the MTC. Either MTT can be selected to perform storage and retrieval functions. Data base information directly entered via the VDU keyboard and stored in memory can be written on one of the scratch tapes. This tape is then referred to as the current data base tape. This tape may be used to load and/or reload the processor in restoring operation in the event memory is lost. When online changes are made to the database, you can use a scratch tape to update the database information, keeping the data base content current and on tape.

3-11. Satellite Connectivity

All AN/TTC-39 satellite trunk connections must be classmarked to provide variable time delays consistent with trunk lengths. Each switch through which a call passes adds a time delay path penalty to the outgoing trunk. Place echo suppressors in the circuit when the total delay path penalty exceeds 40 milliseconds. (The time delay path penalty classmark is a programmable item from 0 to 40 milliseconds in 5-millisecond steps.) Because of their extreme path lengths, satellite trunks require echo suppressors to setup clear connections. You can mark all outgoing trunks as either satellite or nonsatellite (Y or N). (See Figures 4-30 through 4-32.) You can also specify the maximum time delay path penalty (40 milliseconds). Each switch adds its own satellite path delay penalty to all outgoing satellite trunk calls. The total penalty reflects the number of satellite trunk links involved in the connection.

Certain analog switches, like the AN/TTC-38, cannot provide their own satellite path delay penalty classmarks. This creates a problem when calls from such switches pass through the AN/TTC-39. In such cases, the AN/TTC-39 must increase the satellite trunk link count by one to account for the incoming satellite trunk link. The switch then forwards this signaling data item to the next switch. The switch/supervisor can control the maximum number of tandem satellite links through which a connection may be routed. The choices are 1, 2, 3, or unlimited. If the supervisor does not set a threshold in the routing table, the switch automatically sets an unlimited threshold.

3-12. Operator/Machine Interface

The operator/machine interface consists of peripheral equipment by which the circuit switch personnel communicate with each other, with subscribers, and with the central processors. The AN/TTC-39 uses the following equipments for the functions:

  • A call service position.
  • The digital subscriber voice terminal (KY-68).
  • Telephone set (TA-838).
  • A teletypewriter (AN/UGC-74).
  • A control and alarm panel.
  • An intercommunication station (LS-147).

Digital subscriber voice terminal (KY-68).

The dual shelter circuit switch uses two KY-685. The single shelter circuit switch uses one. In the dual shelter switch, one KY-68 resides in the control shelter and one in the switching shelter. The KY-68s provide secure voice communications for the operator.

Intercommunications system.

The Intercommunications System LS-147/FU for the circuit switch consists of a nonsecure system of two or three networks. These networks are as follows:

  • Local switch net. Only the dual shelter switch uses this net. Terminals in the two shelters connect to each other through the intershelter cabling.
  • Colocated switch net. There is a terminal in each switch shelter. These connect to each other through the intershelter cabling and connect to other colocated circuit or message switches through the outside plant.
  • Nodal control net. There is a terminal in each switch shelter. These connect to each other through the intershelter cabling and connect to nodal control through the outside plant.

Two intercommunications systems are employed for use on the three networks described above. This is done by using an applique which permits the use of one intercom on any of the above two nets. Normally, the applique provides a switched connection to one of the two terminating networks. When activity in the form of electrical signals is detected on the unselected network, an alarm sounds in the applique of the intercom system. This alarm alerts the operator to switch the network selector on the applique to the other net. Interface to the outside plant is through the SDSG and TDSG patch panels and the SEPs.

Visual display unit/keyboard.

The VDU/KB consists of a visual display monitor, a keyboard, and a visual display controller. The VDU/KB is the primary operator/machine interface with the CPG. The VDU/KB uses a menu-type display of all mnemonic commands along with their meanings. From this, the operator can select the proper commands. Software responds to a command with a set of questions along with a set of fixed fields for the answers. Based upon the answers, the VDU/KB may display additional questions or may display additional data. Below each question is a set of possible responses. The operator selects from a range of choices and can choose the information elements needed from a set of some 60 interactive displays.

The software checks each response to determine that it is valid and that it has no adverse ramification. To be valid, a response must be correct in terms of field size and parameter range. A ramification check looks at all the possible consequences of a data entry. If an entry would cause discrepancies in the data base, the switch will void the command. For each command the switch runs a comparison check for compatibility with existing data.


The UGC-74 TTY is both an input and an output device. It consists of a keyboard for data entry and a page printer. Each shelter has one TTY. Its prime user is the supervisor, and its prime use is for switch maintenance. The TTY also provides hard copies of all changes to the data base, of all detected faults, and of all traffic monitoring data. The page printer can print at 60 characters per second and uses an 8-level ASCII code (seven information bits, one parity bit). When performing maintenance, the supervisor uses the MY to request special exercise test routines from the processor. The software accepts a limited number of special inputs from the TTY keyboard and issues a specific set of messages as hard copy output.

Call service position.

Each circuit switch contains one local CSP. It can also have up to three remote CSPs at a distance not greater than 100 meters from a shelter. The electrical designs of the local and remote CSPs are the same. They differ only in that the remote positions are in cases while the local position is mounted on a table top. The CSP assists subscribers who are having trouble making calls. It also serves subscribers who do not have dial or keyset telephone instruments. Through the CSP, the operator can provide the following specific services:

  • Help complete local calls.
  • Give directory and routing information.
  • Respond to trouble.
  • Help complete trunk calls.
  • Verify busy signals and numbers that fail to answer.
  • Establish conference calls.
  • Respond to verbal precedence and preemption requests.
  • Hold calls.
  • Split calls.
  • Establish secure calls.

The CSP interacts with the SDSG and TDSG to provide signaling and voice access to the switching matrices. The CSP voice ports can terminate on either the SDMX or the TDMX, but in both cases subscribers from the other matrix can be served through intermatrix units. The CSP accommodates voice communications through a three-party bridge. This enables the operator and two subscribers to hold a three-way conversation. Most calls to the operator involve two steps: First, subscribers request operator intervention and go into the queue. Then the operator uses his push buttons to bring the calls out of the queue and to deal with the request. There are three basic ways in which subscribers can get the services of the operator. Two of these involve placing the subscribers in a queue.

The standard method for reaching the operator is to simply go off-hook and dial 0 (following precedence, if desired). The queue has a capacity of twenty calls and is shared by up to four CSPs. If there is room, the calling subscriber goes into the queue at the keyed precedence level and receives a ringback until the operator answers. If there is no room and preemption is not possible, he receives a busy tone. To answer a call in the queue, the operator simply depresses the QUEUE ANSWER push button. Certain subscribers, such as 1600-Hz and 20-Hz ringdown lines and trunks and TA-312 subsets, may lack a dial-up capability. If they are classmarked for CB, these subscribers go into the queue automatically when they go off-hook. Each CSP has a unique directory number. By dialing that number (following precedence, if desired), a subscriber causes the ANSWER push-button indicator (PBI) at the operator's position to flash. To answer, the operator depresses the ANSWER PBI.

Control and alarm panel/control transfer logic.

The CAP/CTL panel provides a visual summary of current status and configuration of the system. The CAP portion is a summary status display of the circuit switch. The CTL portion has switches to manually select the processor/controller/peripheral configuration. Another section on the panel shows the status of the redundant processors. The CAP/CTL provides the following functional capabilities:

  • Automatic initial start-up processor selection.
  • Manual call processing initiation.
  • Automatic processor switchover control and processor status indication, both visual and audible.
  • Automatic initial start-up controller selection.
  • Automatic switchover and configuration control for the SCGs, COMSEC controllers, SBCs, and controller status indicators, both visual and audible.
  • Manual controller configuration control. Automatic and manual selection control, plus visual and audible status indications for the peripherals.
  • Subsystem summary fault indications, both visual and audible. System traffic load restriction, both visual and audible.
  • Alarm acknowledge and test capabilities.
  • Control and alarm panel lamp test.
  • Processor to processor interface on/off indications and manual selection control.

3-13. AN/TTC-39A Capabilities and Functions


Table 3-4 is a summary of the capacity of the AN/TTC-39A and a comparison with the capacities of the 300-line and 600-line AN/TTC-39 switches. Note the substantially increased digital capability and the lower analog capability.

Channel reassignment function.

This new capability adds an electronic patch panel to allow the supervisor to combine and decombine channels/trunks. Both single channel to single channel and group to group reassignments can be done. The patching is made through the TDMX and data bases can be created on-line or off-line. Several new commands are used to assign the channel reassignment function and to display transmission groups, channel reassignment, and individual channels being reassigned. (See Chapter 4.)

Engineering orderwire.

Both digital and analog orderwire capabilities are added. Orderwire capability can be used on all 30 DTGs. Twelve 16-kbs DVOW can be used on a diphase DTG at 256 kbs or higher. Six AVOW can be used on diphase and dipulse DTGs from 72 to 4608 kbs. There is added an orderwire control unit to connect the AVOWs and DVOWs to the DTGs. One KY-57 allows orderwire encryption. One of the DTGs can carry a group of 12 multiplexed 16-kbs DVOWs for connection to a CSCE.

Automatic frame synchronization.

If the processor fails to synchronize a DTG, frame synchronization will be initiated automatically. This eliminates the manual push buttons which were on the TDSG patch panel in the AN/TTC-39.

Processor-controlled strapping.

This new capability reduces the number of printed circuit boards (PCB) that have to be strapped manually. The functions which were most often manually strapped are now done electronically by the processor. The cards involved are:

  • Group modem.
  • Transmission group module.
  • Nine channel multiplexer/demultiplexer.
  • Time division memory module.
  • Trunk signaling buffer.

Processor-controlled strapping is done at the VDU by using the commands assign and display switch initialization (ASI), assign digital transmission group (ADT), assign trunk group cluster (ATG), assign on-line diagnostics (AOD). These commands were already used in the AN/TTC-39. (See Table 4-21.)

Analog line conditioning.

Line conditioning for analog circuits is provided by using an analog line conditioning patch panel. Gain and attenuation can be applied to as many as 24 circuits at onetime. Equalization can be applied to 2 circuits at one time.

Analog DTG interface.

A TDMX interface is provided for analog AC supervised loops using a DTG. This allows up to 40 TA-341s, TA-838s and TA-720s operating in the 4-wire, local battery mode, using DTMF signaling to use CVSD modulation through a DTG traffic channel. DTMF receivers and the digital signal generator are used for signaling.


3-14. Switch Control

Switch control involves functions that manage and impose effective control on the AN/TTC-39. This includes the surveillance capability of the switch, which provides statistical information on the overall use of the system to planners and supervisors at all levels. It also includes the technical control features of the switch. Finally, control deals with the relationship of the switch to the nodal and system facilities controls; with the reconfiguration of the switch in response to tactical situations or failures; and with the communications between the supervisor, the operating positions in the shelter, a colocated message switch, and nodal control.


The AN/TTC-39 has a built-in metering capability that monitors selected loops and trunk groups. This tool provides the switch performance information needed to maintain top performance. The assign traffic metering (ATM) command enters the traffic metering requirements into the data base. Paragraph 4-6 describes the data entry commands for the switch and the procedures for data input. The switch can monitor given loops and TGCs for periodic traffic reports. You can select the time interval between reports. To make sure that all the desired data elements get into the data base, you should maintain detailed records. These records can be in worksheet form. They must address the following data base elements:

Loop number = A-BB-CC or DD-EE. This identifies the loop's matrix address. The numbers must be compatible with entries in the assign terminal services (ATS) and the ATG worksheets. (See paragraph 4-6 and Tables A-3 and A-4.)

TGC number = XXX (1-127). (See ATG worksheet.)

Report interval. One to four digits specify the interval between reports in minutes. The switch generates reports automatically at the specified time. Use the design frequency for network reporting (AFR) worksheet to assign these intervals for each type of report.

Type of Report. (See below.) Use worksheets (paragraph 4-6) to help you gather this information. All trunks and switched loops in the AN/TTC-39 can be metered. Traffic metering begins at the switch supervisor's console. The supervisor uses SOPs, plans, and orders for guidance to identify the loops and trunks to be metered. The planner or CSCE will provide specific direction. There are two methods for acquiring the data from traffic metering. One involves automatic printing at fixed intervals. In the other, the supervisor requests specific printouts. The automatic method is set by entering metering requirements and time intervals into the data base via the VDU. In the other method, the supervisor specifies the trunk group or loop to be metered and then selects the type of metering he wants. He then initiates the metering using the VDU. The circuit switch can meter 10 subscriber loops and 28 trunk groups at one time. Report intervals can be set for 15, 30, 60, 240, 480, or 1,440 minutes. The AN/TTC-39 makes eight traffic metering reports available to the supervisor. Below are descriptions of each. The R number identifies the report code used on the VDU.

  • Operator report (R3). This meter counts each time an attempt is made to call the operator. It also counts each time a call is removed from the operator queue and connected to the operator. It does not count calls initiated by the operator or extended for a subscriber.
  • Total calls by precedence (R4). The number of outgoing calls by precedence is counted. The meter also counts the number of all trunks busy, by precedence, for each TGC. This is the number of times a TGC cannot complete a call offered over an idle trunk within the TGC. It counts more than once per call if the condition exists at primary and alternate TGCs.
  • Trunk group cluster status (R5). The average number of trunks busy per TGC is counted by precedence. The meter also counts total calls completed per TGC and primary route attempts. The latter is for each time an attempt is made to route a call over the TGC as a primary route.
  • Calls preempted per TGC (R6). This meter counts by precedence each time a trunk in a designated TGC is preempted.
  • Digital transmission group error rate (R27). This reports the approximate bit error rate for up to 12 DTGs.
  • Total traffic count (R44). Attempted calls are counted for these categories: network to network, network to subscriber; subscriber to network; and subscriber to subscriber. For example, a network-to-network call is a call coming into the switch over a trunk and going out over a trunk.
  • Loop report (no R-number). This meter counts calls originated by designated loops by precedence.
  • Call offered to a remote switch (R47). This meter is not yet active.

Technical control.

The term technical control or tech control usually applies to a central facility through which all or most circuits pass. We need technical controls for testing, patching, troubleshooting, circuit conditioning, status reporting, and other technical management functions. Some of these functions can be performed at the switch or at the transmission equipment, but the more complex CE systems are likely to need central facilities. The AN/TTC-39 is designed to work with such facilities.

Automatic tech control. This concept of technical control is a basic element of the CEMS. Review paragraph 1-2 to see how the nodal control performs this function. Bear in mind that the equipment for automatic control will not be available for some time. In the meantime you will use manual equipment. This will continue even after many functions become automatic. This also means that certain capabilities will not be available. You will have to plan for this and to understand these limitations. Paragraph 5-3 shows how to use some of the tech control facilities.

Manual tech control. With the AN/TTC-39, you will probably use the AN/TSQ-84 communications technical control center. This equipment can interface, patch, and test analog circuits only. The AN/TSQ-84 cannot interface with the digital group multiplex capability of the AN/TTC-39. (See paragraph 5-3 for further explanation.) These limitations will affect your planning because they restrict the number of digital circuits and the connectivity to the switch with other AN/TTC-39s. You can control individual digital circuits of the AN/TTC-39 through the AN/TSQ-84, but you cannot control digital trunk groups. They feed directly to the transmission systems and the directly-connected multiplex without providing the ability to monitor, test, or patch. Under this arrangement, the switch supervisor must coordinate these circuits. He must also spend more time working with the tech control.

Equipment configuration.

Chapter 2 showed how the capabilities of the AN/TTC-39 vary according to the number of analog or digital modules it contains. This flexibility is consistent with the change to all digital systems spelled out in Chapter 1. The switch also can vary according to the kinds and numbers of circuit cards (or PCBs) in use. These PCBs include LTUs, adapters, scanners, interface devices, controllers, senders, receivers, drivers, MUX/DEMUX units, buffers, and other support electronic components for the switch. Paragraph 4-6 describes the circuit card inventory that you will make when first using the switch. All switches may not have the same number of cards. They may also have different spares due to repairs and breakdowns. Thus, each switch has its own unique configuration of equipment and cards within the specified limits. Anything that changes the capabilities of the switch is a reconfiguration. All this is significant because such resources as circuit cards may be limited. You may have to apply them throughout a network in ways that reflect the needs of the network. For example, a switch serving a large headquarters may need more LTUs than a tandem switch. You may need to reconfigure both switches to provide enough cards (including spares) for both.

The AN/TTC-39 is flexible enough to permit this rearrangement of equipment. In addition, certain card nests can accept several different kinds of cards. This depends on the kinds of interfaces involved and on the sizes of the signaling and MUX groups. (See TM 11-5805-681-12 for further information on card nest locations and options.)

3-15. Diagnostics

Diagnostics enables the AN/TTC-39 to refine fault isolation to a card or card group. The processor can evaluate the switch either automatically on a periodic basis, in response to a fault indication, or on demand.

The maintenance concept for the AN/TTC-39 is to allow the maximum on-site repair with available skills, tools, and test equipment. This requires a family of spares that can be carried without compromising mobility. Organizational personnel will fault isolate to a least replaceable unit and will replace the faulty unit from organizational spares. For switching logic, diagnostics and maintenance aids will provide fault isolation to the card level. Not covered are faults involving the central processor and peripheral device controllers. All switching logic is subject to workable repair by replacement. Diagnostics will also refine fault isolation for failures of the central processing subsystem. The maintenance concept specifies additional aids to continue rapid fault isolation to the card level. These include the fault catalog, the module test set, and the VDU/KB. The switch supervisor conducts most of the initiated diagnostic testing on the circuit switch via the VDU/KB.

Maintenance and diagnostic software.

Fault isolation and repair activities will involve a combination of on-line maintenance and diagnostic sofware and built-in test equipment (BITE). The supervisor's access to the software depends on the function to be performed and on the frequency of use of each program. The programs reside in off-line memory storage on magnetic tape. They can be recalled to main memory (core) storage when needed. On-line maintenance involves detecting and isolating circuit switch fault conditions and repairing or restoring malfunctioning equipment. Again, the two basic parts of the process are the BITE and the diagnostic software. BITE is an integral part of the circuit switch used to monitor and detect equipment fault conditions. It consists both of specific equipment used for test purpose only and of general circuitry integral to other functional circuit switch elements. Software on-line diagnostic routines work in conjunction with the BITE to periodically assess the status of the system and to detect and isolate faults. The supervisor can augment the periodic tests by using the VDU/KB to request special or operator indicated testing.

Visual display unit/keyboard.

The VDU/KB provides the primary interface between the switch supervisor and the processor. The supervisor uses it for data base updates, system status reports, and as a maintenance aid in requesting special maintenance and diagnostic routines. These routines include:

  • Display of failure responses to software initiated tests.
  • Initiation and display of supervisor initiated tests.
  • Initiation and display of all resets and switchovers.
  • Display of call processing fault messages.
  • Display of major equipment status.
  • Request and display of traffic metering.
  • Erasure of computer memory during node overrun.
  • Request for data base transfers between core and magnetic tape.

The supervisor uses the VDU/KB to assign, change, delete, and display the switch data base. The VDU/KB also handles requests to execute special on-line diagnostic routines, to alternate system time-outs, and to impose traffic load control. Its operation involves a menu with question and answer type fixed format displays. The menu displays all the mnemonics along with their meanings. This makes it easy for the supervisor to select the proper command mnemonic. When the supervisor enters a command, software responds with a set of questions along with a set of fixed fields for the answer. The switch supervisor has access to equipment loop, trunk, and group status indicators through his VDU and automatic printout .

VDU/KB initiated tests.

The switch supervisor uses the VDU/KB to request specific tests to aid in performing fault isolation. To initiate these tests, use the AOD command. A list of all tests, resets, switchovers, and the test numbers associated with each AOD command can be requested and displayed. Tests are requested by entering the AOD command at the VDU/KB. When the command is entered, a list of tests is displayed on the VDU screen. If the desired test is listed on the screen, you can run it by entering the AOD command and the proper test number. If the testis not listed on the initial AOD screen, other screens with test listings can be obtained by keying the NEXT key. Repeated keying of NEXT will display all the screens with the list of all possible tests, resets, and switchovers for the AN/TTC-39.

For certain tests, ancillary equipment may be required. After the test is complete, the software will display its results. Figure 3-4 shows the general format for display of test results. All tests result in one of the four following states:

  • Unit good.
  • Unit failed.
  •  Unit busy.
  • Unit undefined.

Test results of good and failed are self-explanatory. If the equipment that you want to test is in use, the test will not run. The software will then display a busy message. If you try to test apiece of equipment that does not exist or is out of range, the software will display an undefined message.

Circuit switch tests.

Through the VDU/KB, you can test circuit switch equipment in either active or standby modes. You can also test via either the control shelter TTY or the switching shelter TTY. The TTY in both shelters connects with the circuit switch processors. There are three types of circuit switch tests.

  • Periodic initiated tests. These tests run on a scheduled or automatic basis to meet availability requirements. You can also initiate them via the VDU/KB or TTY.
  • Fault initiated tests. Software runs these tests in response to a detected fault.
  • Switch supervisor initiated tests. The switch supervisor initiates these tests manually via either VDU/KB or TTY.

Table 3-5 provides a complete list of these tests. Any software initiated test will provide a VDU output only when the test results indicate a problem condition. There will be no display if the test results are positive. Any fault display will appear in the protected region of the VDU.

TTY operation.

You can request all maintenance and diagnostic commands, as well as the assign equipment in/out-of-service command, from either the control shelter TTY or the switching shelter TTY. The control shelter TTY will log all failure responses. The VDU and TTY output messages are essentially the same. However the TTY output will contain the time and date the message is logged. Both the VDU/KB and the TTY interface with each of the processors via one channel of an input/output extender. You can switch the TTY manually from one redundant processor to another via a select switch located on the control and alarm panel. The TTY is the main device for logging the various messages that must be displayed to the switch supervisor.


3-16. Connections to Subscribers

Connections to subscribers and to other switches make a switch part of a network. An interface consists of the electrical and mechanical connections and the procedures necessary to make these connections functional. As the planner, you must understand the conditions under which the interface can be made. The physical connections to the switch are through multipair cables, coaxial cables, and fiber optic cables. The electrical interface is more complicated. Switches connect to subscribers and to other switches by way of circuits (which are called channels), lines, or trunks. Below are functional definitions of these connections:

  • Loops -- single channel connections to sub-scribers (may be grouped for transmission purposes).
  • Trunks -- channels between switches.
  • Trunk group cluster -- a collection of both analog and digital trunks between two switches.
  • Digital transmission group -- a set of trunks and trunk group clusters which share the same transmission, facility.
  • Interswitch trunk -- trunk (or trunk group cluster) between the AN/TTC-39 and a switch using CCS.
  • Extraswitch trunk -- trunk (or trunk group cluster) between the AN/TTC-39 and a switch not using CCS.

Planning for or installing a switch in a network involves several major tasks. You must connect the proper equipment to each channel or line. You must also program the switch to recognize the characteristics of each of these lines or channels. In addition, you must setup the correct circuit card configuration and card strapping and prepare the switch data base. Each of these tasks requires a knowledge of interface procedures, capabilities, and limitations. This section contains detailed descriptions of the AN/TTC-39 interface. It also describes the signaling, supervision, and special characteristics of these interfaces. Finally, it briefly discusses interface security.

3-17. Trunk and Telephone Interface Equipment

The AN/TTC-39 works with a wide variety of switches, converters, and telephones. This paragraph describes the equipment used within the switch to process information.

Analog access.

Analog lines and trunks end in an LTU before they are routed to the SDMX. These units provide the SDMX with standard control signals derived from various different line characteristic signals. There are five types of LTUs.

  • NW -- normal wideband for direct current or tone supervised loops or trunks.
  • CB -- common battery supervised 2-wire loop or trunk.
  • 20 -- two-way 20-Hz ringdown on loops and two-way 20-Hz ringdown with idle tone on trunks.
  • 1600 -- two-way 1600-Hz ringdown with idle tone on 4-wire trunks.
  • AV -- 4-wire overseas AUTOVON telephone loops with DTMF signaling and DC loop supervision.

Loops or trunks that use 2600-Hz single frequency, DC closures, or E&M supervision require special adapters. These adapters convert special signals to signals that the NWLTU will accept. There are three types of special adapters.

  • SF-- single frequency (2600-Hz) supervision and control adapter.
  • DC -- direct current closure adapter that provides access to 2-wire commercial central offices and PBXs.
  • EM -- E&M adapter for use on 6-wire trunks to PBXs and commercial central offices.

Analog signaling and supervision.

The typical tactical network channel contains the signaling and supervision features needed to complete a call over analog lines and trunks. The AN/TTC-39 has a special feature that enables suitably equipped switches to exchange signaling and supervision over a separate analog signaling channel. This channel operates at 1200 baud. The term for the function is common channel signaling.

Digital access.

Digital trunks and subscriber loops terminate on the digital TDMX. The signals come into the AN/TTC-39 in a digital stream or through an individual diphase or CVSD modulated subscriber loop. There are three types of group modems that can connect these signals:

  • DIGPM -- The diphase group modem transforms a diphase signal to a digital baseband bit stream. It is used for 72 channels or less.
  • DISGM -- The diphase supergroup modem also transforms a diphase signal to a digital baseband bit stream and vice versa. It is used for channel modularities of 128 and 144.
  • DPLSM -- The dipulse group modem is used to interface with existing (pulse code modulation) multichannel transmission equipment.

There are also two types of loop devices that are used for digital access:

  • DILPA -- The diphase loop modem A transforms a 4-wire full duplex diphase modulated signal (from a subscriber with a digital terminal) to a digital signal and vice versa.
  • CVSD -- The continuously variable slope delta unit provides 4-wire, full-duplex interface for analog subscribers entering the digital switching group.

Digital signaling and supervision.

Digital signaling and supervision consist of digital signals either mixed in with the information bits (in-band) or carried on a separate channel (out-of-band). There are three signaling and supervision types:

  • DIBTS -- digital in-band trunk signaling controlled by a buffer. DIBTS provides in-band digital signaling and supervision between the AN/TTC-39 and a subscriber switch such as the SB-3865.
  • CCS -- common channel signaling using a separate subchannel in a digital transmission group. CCS provides signaling and supervision information for TGCs.
  • LOOP -- digital in-band signaling for a loop by means of digital code words in the data stream.

3-18. Switch, Trunk, and Telephone Line Interface Table

Information on line and trunk interface equipment characteristics is very important to planners and operators. Table 3-6 summarizes and tabulates this information. It also presents an overall picture of all the equipments and services that the AN/TTC-39 supports. The table shows the equipments involved in planning the initial design or in making additions after a switch is in use. Below is a key to the table headings:

  • EQUIPMENT OR SERVICE. Table 3-7 gives nomenclatures.
  • SIGNAL AND SUPERVISION OPTIONS. These are variations normally requiring a separate assignment of terminal equipment type.
  • TYPE. This refers to the assigned terminal equipment type. These are type numbers assigned to equipments or services. The numbers inform the switch of the line or trunk characteristics of the equipments/services. (See Table A-1.)
  • ADAPT. This refers to the adapter required, if any.
  • LTU. This identifies the LTU to be used.
  • SIGNAL. This identifies the type of digital signaling scheme.
  • MODEM. This identifies the modem access.

Use this table to complete the planning and the data entry worksheets. Figure 3-5 will help you use it. This figure illustrates eight (A through H) interfacing schemes. These represent the most common connections of loops, trunks, and switch equipment. The relationships of each to the table are as follows:

A - Telephone and local net radios that can be routed directly to the switch via junction boxes, such as J-1077/U. These connections require no adapters or interface devices.

B - Switchboard and switched trunks and access lines that require no special adapters. These are normally routed to the switch from the nodal technical control. Table 3-6 uses a blank in the column labeled ADAPT to indicate these.

C - Switchboard and switched trunks and access lines requiring special adapters. These have an entry in the ADAPT column of Table 3-6.

D - An analog trunk to and from another AN/TTC-39 or any switch using CCS. (See note 3 of Table 3-6.)

E - Analog telephones and local net radios routed directly to the switch with access to the TDMX. The MODEM column of Table 3-6 shows these as CVSD.

F - Analog loop and access lines accessing the TDMX through a technical control. Once again, the MODEM column of Table 3-6 shows these as using a CVSD modulator in the switch.

G - Digital telephones using diphase modulation transmission to enter the switch. The MODEM column of Table 3-6 shows these as DILPA.

H - Digital trunks using the group modem to enter the switch. These are shown in Table 3-6 as digital group or supergroup modems (diphase or dipulse).

The cable or wire shown in Figure 3-5 is WD-1 field wire, WF-16 field wire, WM-130 cable, CX-4566 cable, or coaxial cable, as appropriate. The LTU column of Table 3-6 shows the type of line termination unit required. The TYPE shown in Table 3-6 is the classmark number the switching system uses to identify the line or trunk. The SIGNAL SUPERVISION OPTIONS column shows why there is a change in classmark for different trunks running from the same switch.

3-19. Communications Security

This paragraph describes the security relationships and functional interface of the integrated COMSEC equipment. FM 24-27A has specific functions and capabilities. Below are brief descriptions of the COMSEC equipments and their interface characteristics.

COMSEC controller.

The AN/TTC-39 COMSEC controller links the COMSEC equipment with the CPG. It contains all  the circuitry necessary to perform data conversion and to operate COMSEC Interface Control Unit HGX-84/TSEC.

COMSEC interface control.

The HGX-84/TSEC connects the COMSEC equipment with the rest of the switch equipment. It reports errors in command signal transmission and malfunctions in the COMSEC equipments to the switch processor. It also provides an encrypted or black control channel interface between the circuit switch processor and the following COMSEC equipment:

  • Trunk Encryption Device TSEC/KG-81.
  • Loop Key Generator/Common Unit HGX-82/TSEC.
  • Loop Key Generator TSEC/KG-82.
  • Automatic Key Distribution Center HGX-83/TSEC.
  • Key Generator TSEC/KG-83.


3-20. Operation Under Emergency Conditions

The switch can operate satisfactorily under a wide range of conditions. Some of these conditions are the result of the combat environment, some the result of weather, and others the result of the terrain and climate. Paragraph 3-21 shows how the switch can operate under extreme conditions. It also gives the design tolerances of the switch. If any of these conditions are exceeded or if combat actions cause damage, the switch must operate under emergency conditions.

Power during emergencies.

The power system is one of the most critical systems of the switch. (Refer to Chapter 2 for a description of this system.) The circuit switch power subsystem (CSPS) is one of the four subsystems of this power system. It helps assure that the critical AC and DC loads (those needed to keep the switch functioning) can get power if the prime AC power source fails. It does this by switching to a bank of batteries that can maintain the critical loads for 15 minutes. The batteries can be recharged in 24 hours after a complete discharge. This recharging begins automatically as soon as the prime AC power is restored. The CSPS keeps the batteries fully charged during normal operations. There are also a series of circuit breakers that will trip if the power fluctuates or falls below minimum voltages. If this occurs, check the prime AC power source. This will usually be a trailer-mounted generator. You may have to switch to batteries while you replace or repair the generator.

Essential user bypass.

If your switch fails, the EUB capability lets you transfer your most important subscribers to another switch. (Refer to paragraph 3-8 for an explanation. Paragraph 4-6 shows you how to do this.) Use the AEU and ARB worksheets and coordinate the lists with both switches in advance. Up to 60 digital subscribers can transfer in this way. (If your switch is accepting EUB subscribers, it can accommodate 60 from each of two switches.) These subscribers must be connected to the TDMX, and there must be enough digital trunks to carry them. You can also transfer analog EUB subscribers by manual patching using analog trunks.

Emergency patching.

With manual patching, you can maintain emergency service for subscribers not on the EUB lists. If the switch fails, use the patch panel to connect subscribers directly to each other or to trunks. Make sure that the equipment at both ends is mutually compatible. You should make out both the EUB and the manual patching lists before an emergency occurs. Include both in your SOP so that there is no question about who gets service. Because of the confusion likely when a switch is bypassed, it is important that the planner controls both the bypassing and the restoral of the switch.

Switch abandonment.

There are four things you must do if a switch must be abandoned or if a node is in danger of enemy capture:

  • Erase memory and zeroize key variables.
  • Erase or destroy magnetic tapes and records.
  • Destroy classified components of the switch.
  • Disable the switch.

Erasing memory deprives the enemy of information that could be used for intelligence purposes. The order to do this may come to the node or switch from higher headquarters. However, if the node is about to be overrun, you must make the decision on your own. To erase memory, enter the PUNT command. (See Table 4-18 for a list of all commands,) This command will not appear on the screen. You must then reconfirm the command on the keyboard. This will start a 5 minute time-out period. You can abort at any time within those 5 minutes. It may take too long to erase the magnetic tapes. If possible, take them with you. If you cannot, chop them up or burn them. Burn all paper records. If you have time, disable the switch by cutting its wires and breaking the circuit boards. (Disconnect all power first.) (See TM 750-244-2 for approved destruction methods.)

3-21. Operation Under Extreme Conditions

The AN/TTC-39 can operate very effectively and very efficiently under extreme environmental conditions. The shelters are fully insulated and weatherproofed for tactical operations in all types of climates. However, you should take special precautions for the following extreme conditions.

Low temperature.

The switch heater and electronics equipment can start running at -50 F without damage to the equipment. However, at low temperatures, the switch does not reach full operational capability until the equipment has been in operation for 2 hours. External heaters can reduce this 2-hour warm-up time. Extreme cold causes cables to become hard, brittle, and very difficult to handle. When handling and connecting cables to shelters, take the following precautions:

  • Remove unnecessary loops and links.
  • Free all connectors of frost, snow, or ice.
  • Replace covers on receptacles and close entrance box covers when they are not in use.
  • Open hood shields and lower the covers when the entrance boxes are open.
  • Replace connector covers when a cable is disconnected.
  • Keep open connectors out of the snow. By no means should you drag an open connector through the snow.

High temperature.

The AN/TTC-39 electronic equipment and ECUs can start at +120 F external ambient temperature. An added safety factor involves solar radiation. The switch can reach full operation after it has run for 45 minutes. This includes the extra 15 minutes that the master timing unit needs for stabilization. Under abnormal or emergency conditions, the switch can operate satisfactorily for at least 2 hours in extreme heat with only one of the two ECUs working. In hot, dry climates, connectors and receptacles are subject to damage from dust and dirt. Be sure to replace covers on connectors and receptacles and to close covers on entrance boxes when they are not in use. Never place or leave open connectors on the ground.


The circuit switch can operate without degradation during and after prolonged exposure to humidity extremes. This includes relative humidities as high as 100 percent at all ambient air temperatures up to +80 F, high humidity corresponding to a dew point of +86 F at an ambient temperature of +100 F, and a low relative humidity of 5 percent at +120 F. The switch equipment is always subject to damage from moisture and fungus in warm, damp climates. Make sure that all equipment is checked periodically and that all moisture and fungus is wiped from the equipment.


The switch will sustain no physical damage or degradation in performance when subjected to the wind and rain conditions of an extreme tactical environment. You should, however, protect it from long-term exposure to dirt and water. Conduct periodic visual checks of the equipment to ensure that:

  • Connector covers are replaced when a cable is disconnected.
  • Open connectors are not placed near or in the water.
  • Covers on receptacles and entrance box covers are closed when they are not in use.
  • All moisture and possible dirt and fungus are wiped from the equipment.

Sand and dust.

The AN/TTC-39 is fully weatherproofed. This means that it can perform at full capacity under all adverse conditions. This feature protects the switch from the effects of fine sand and dust particles at wind speeds of up to 40 miles per hour. It also protects against the dust that can build up within the enclosure as a result of operator activities. However, connectors and receptacles are susceptible to damage from fine sand and dust. As a result, you should conduct periodic inspections of exposed switch equipment. Make sure that covers on connectors and receptacles are replaced and that covers on entrance boxes are closed when not in use. Keep all open connectors off the ground.


The AN/TTC-39 shelter can withstand 40 pounds per square foot of snow loading on the top of the shelter. (See subparagraph above titled low temperature for precautionary measures.)

Salt fog.

The circuit switch in its shelters can withstand prolonged exposure to a salt-laden atmosphere without any operational degradation. Check external equipment periodically to make sure that all moisture is wiped from the equipment.


The AN/TTC-39 equipment is resistant to fungus and should not be adversely affected by it. (See subparagraph above titled sand and dust for precautionary measures.)

Electromagnetic compatibility.

The AN/TTC-39 contains shielding, bonding, and grounding protection for electromagnetic compatibility. The openings in the shelter for ventilation, air conditioning, heating, or any other purpose use a screen or honeycomb filter. This acts as a waveguide cutoff for the highest frequency that the shelter processes. Multiple powerline radio frequency interference filters are on the inside of the shelter at the input power panel. The shelter, with a modification kit (MOD-1079), has a 60-db shielding effectiveness to electrical fields and to plane waves over the frequency range of 14 kHz to 100 MHz. It also has a 60-db shielding effectiveness to magnetic fields over the frequency range of 200 kHz to 1000 MHz. External cables are shielded. Coaxial cables are double shielded.

3-22. Reliability and Maintainability

You must understand how to keep the AN/TTC-39 working reliably. You also need to know the factors that affect the maintainability of the switch. This knowledge will help you:

  • Minimize maintenance actions at the organizational level.
  • Reduce downtime spent in corrective maintenance.
  • Increase the availability of the switch during operations.
  • Produce data feedback for use in corrective actions and in evaluating the work of supporting organizations.


Reliability is the measure of how the switch performs under all types of conditions. Paragraph 3-21 described some extreme environmental conditions. These will cause reliability to fall off. So will the emergency conditions described in paragraph 3-20. The most critical factor is temperature. The switch meets the following standards:

  • In the range of +32 F to +100 F ambient (outside) temperature, there should be no more than .2 percent lost calls.
  • At +25 F to +32 F, there should be no more than .3 percent lost calls.
  • At +100 F to +120 F, there should be no more than .4 percent lost calls.

In general, the switch should be available for use 99.9 percent of the time.


Maintenance of the switch must conform to the 99.9 percent availability goal. Thus, you must plan maintenance for times when the switch is in use. This includes work at the organizational and intermediate levels. Depot level work requires that the switch be taken out of use. You should not need to do this until the switch has been running for 3 years.

Corrective maintenance. Organizational level personnel can correct at least 95 percent of all failures. The mean corrective maintenance time should be 30 minutes. The maximum corrective maintenance time must not be greater than 60 minutes.

Preventive (scheduled) maintenance. You can do preventive maintenance on the AN/TTC-39 without interrupting its operation. This takes an average of 15 minutes per day. There may be times when you have to shut down the equipment for reasons of safety. In these cases, mean preventive maintenance time should be only 5 minutes.

Operational maintainability. This has to do with such external factors as outside maintenance, supply, and administrative actions. The mean downtime for these items is 45 minutes. In no case should the switch be down for more than 90 minutes. Include these losses of operating time only in long-term planning. This means planning for periods of 30 days or more.

Intermediate level maintenance. Fewer than 5 percent of switch failures should need intermediate level maintenance actions. The mean corrective maintenance time for this is 1 hour.

Maintenance levels.

All maintenance, as determined by the logistic support analysis and level of repair, will be performed at three levels: organizational, intermediate, and depot.

Organizational maintenance. At the organization, 95 percent of the AN/TTC-39 failures are corrected by removing and substituting the lowest replaceable units (LRU). These include major assemblies, subassemblies, modules, and PCBs. You should also replace minor components, such as fuses and knobs, and do minor repair of cables on site. The replacement of connectors and pins in the HGF-82 and the HGF-85 are also included. The capability exists to detect a fault and isolate it to an LRU by using BITE. Thus, by using BITE with maintenance diagnostic programs for troubleshooting, LRU replacements will be the only action required, other than routine nontechnical preventive maintenance.

Intermediate maintenance. Work at this level uses the common support and test equipment to service the 5 percent of repairs/faults that cannot be restored by replacement of LRUs. These failures consist primarily of problems in chassis components, wiring, wire and cable connectors, and patch panel connections. COMSEC maintenance at this level consists of fault isolation to the circuit boards, the replacement of faulty boards, and verification testing of all repair actions. Switch COMSEC equipment (except for the KG-81, the KG-82, and the KG-94) is tested at this level with the ST-34 test set. The KG-81 and its replacement, the KG-94, are tested with the STX-34 test set. Nodal maintenance facilities are equipped with shelterized maintenance facilities for on-site intermediate level and for stockage of repair parts. No intermediate COMSEC maintenance capability is provided at the node, however.

Depot maintenance (special repair activity). Depot maintenance consists of both software and hardware support for the AN/TTC-39. The hardware support consists primarily of the testing, fault isolation, and repair of those LRUs forwarded to the depot from intermediate maintenance. The depot also has the capability to repair and overhaul end items whose maintenance requirements exceed the intermediate maintenance capability. The software support encompasses the maintenance of the existing software configuration. It also includes any software modifications resulting from changes in operational requirements. Processor controlled test equipment, such as the AN/USM-410 or TSEC/ST-51, is used for hardware support for all repairable LRUs and boards. Fault isolation is accomplished automatically under processor control to the greatest extent possible. The test procedures will isolate the fault to a piece part or circuit node of the LRU or board. Computer aided diagnostic software is used to automatically determine computer guided probing instructions.

3-23. Survivability and Vulnerability

The switch has certain built-in features to help it withstand enemy attack. TM 11-5805-681-12 describes these. SOPs, the CEOI, and operations orders provide instructions on coping with the effects of enemy action. Also review paragraph 3-20 on emergency operations. Simple physical damage to the switch requires repair or replacement of the damaged components. Causes of such damage can include explosives, small arms fire, or such natural causes as storm or lightning. Other threats may be more complex. These include nuclear attack, chemical attack, and electronic warfare.

Nuclear attack.

Repair physical damage from nuclear blast and heat by replacing components. You might also use components as spares for less damaged equipment. (See TM 11-5805-681-12 for repair procedures.) Radiation will not normally affect the equipment, but it will affect personnel. The electromagnetic pulse (EMP) from a nuclear detonation can damage electronic components by inducing high voltages in their wiring. The electromagnetic compatibility kit, MK-1079, provides some protection. In some cases, EMP may not cause damage but could interrupt calls. A well-trained subscriber will recognize the problem and reinitiate the call.

Chemical attack.

Chemical attack affects personnel. The main protection consists of individual protective clothing and protective mask. Use of these will keep switch personnel -- and the switch -- in operation. The shelter also provides some protection. The ECU and shelter air filters will help remove some liquid agent particles. There is also a kit available that includes collective protection equipment. This consists of a protective entrance and air filter. Rely mainly, however, on the mask and clothing. Decontamination procedures are in your unit SOP.

Electronic warfare.

Enemy electronic warfare (EW) is an important combat factor. Its goal is to deprive us of our electromagnetic systems. When you consider how much military equipment depends on electronics, you can see how big the problem is. For practical purposes, the AN/TTC-39 switch does not radiate. Thus, the switch itself is not likely to be a prime target for intercept or jamming. More likely targets are the transmission equipments supporting the switch. You should consider the vulnerability of the switch in the total communications system and apply measures to reduce this vulnerability system-wide. In most EW situations consider the following countermeasures:

  • Use COMSEC equipment to the maximum possible. This reduces the amount of information available from intercepted telephone calls.
  • Try to avoid establishing calling patterns.  
  • Limit transmission power of transmission equipment. This reduces intercept possibilities.
  • Orient antennas of radios away from hostile territory. This will be a factor in network planning. (See paragraph 5-6.)
  • Consider the use of alternate transmission facilities. This keeps the circuit switch from being connected to one electronic signature or location. In practice this would place the switch in one location, possible protected, with two or more other radio locations used alternately for trunking to the switch.

3-24. Physical Security

When deployed, the AN/TTC-39 will reside in a restricted area. This refers to a secure area set up for safeguarding materials. Entry is subject to special controls and restrictions. Restricted access to the AN/TTC-39 will safeguard the integral COMSEC equipment, key lists, and classified COMSEC material. Specific protection requirements will be found in your unit physical security SOP. Your unit security officer will have this information. Examples of appropriate security controls are:

  • Fences combined with on-site or perimeter guards.
  • Fences combined with ID and security check procedures.
  • Fences combined with alarms.

Each shelter of the circuit switch functions as an exclusion area. Access to the shelter is restricted to those who need to go in. There will be a visitors' register for those with temporary access. All switch personnel must have at least a SECRET clearance. Only personnel with the required security clearances can operate the switch in the field. Personnel with less than a SECRET clearance can enter and remain in the shelter only with a properly cleared escort. This includes those who perform occasional noncryptographic maintenance, repair, or housekeeping functions. In all cases, a need to know must be verified. That is, persons entering the shelter must have a valid reason for being there. Clearance is necessary for access to equipments and fill devices, keyed or unkeyed, and to supporting documents. This will include military or civilian employees of the US Government or of its contractors. Guards and security patrols who provide area protection for the shelters need not be cleared. However, such uncleared guards cannot enter the restricted area. Guards who are cleared can provide more flexible service. They can, for example, serve as escorts for uncleared visitors.

3-25. Siting

In the press of network planning, it may be easy to overlook the siting needs of the AN/TTC-39. You can avoid this by including in your SOP requirements for node and equipment sites. This forces all concerned to consider these during planning. It is also important to make a reconnaissance of the command post (CP) and node sites. Base this on the SOP requirements for sites and on the network's operational needs. Follow it with a terrain analysis. This will help you determine how to use your line-of-sight radio systems. It will also help you verify the selection sites for all the network's equipment.

Since the AN/TTC-39 does not radiate it does not have an electronic signature. You can locate the switch near a CP with no signature related restrictions if you specify the use of cable to the transmission equipment. Power units may be a factor, however, and restrictions on heat (for infrared detection) and sound may apply. Camouflage capability is important. You should look for natural cover and concealment to conceal the size of the system and its cable network. Use natural terrain features such as ravines or tree lines for cables. Bury cable when possible.

CP and nodal sites must have access to trans-mission media. If you use such radio links as the short-range wideband radio, you must provide for line-of-sight to the top-of-the-hill radio. Note that use of the SRWBR will probably create the electronic signature mentioned above. If you use cable to the transmission media, you must consider whether cable can be installed over the terrain or if it is impractical. The advantage of separating the switch and other nodal elements from the transmission equipment is that you can locate the node in a more protected and concealed location. Review paragraph 3-23 for EW siting considerations.

You must make a careful study of the actual site for the switch. The site should have a dry surface with good drainage. It should have a good electrical ground or be near a spot (such as a wet area) where the ground is good. If this is not available, you may have to dig a hole and construct a ground. The surface of the switch should be level, with no more than a 10 percent slope. When possible, use natural camouflage, such as trees. Augment this with netting. Do not cut foliage for camouflage, as this is easy to detect.

Locate power units at least 30.48 meters (100 feet) away to protect the switch from fire and to minimize noise. Bury power cables and communications cables when possible to hide them from detection and to protect them against damage from vehicles and artillery. Keep power and communications cables as separate as possible.

You should always put strong emphasis on equipment and personnel survivability. If possible, locate the switch in protected terrain, such as below a cliff or in a canyon. Sometimes you may wish to dig the switch in to help protect it against small arms fire or artillery. If the threat is this great, however, moving the node might be a more prudent measure. (For further information about displacement, see paragraph 5-7.)

*There are other PRs in use for overseas AUTOVON.

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