1755.522—RUS general specification for digital, stored program controlled central office equipment.
(a) General.
(1)
This section covers general requirements for a digital telephone central office switching system, which is fully electronic and controlled by stored program processors. A digital switching system transfers information which is digitally encoded from any input port to a temporarily addressed exit port. The information may enter the system in either analog or digital form and may or may not be converted to analog at the exit port depending on the facility beyond. The switching system shall operate properly as an integral part of the telephone network when connected to physical and carrier derived circuits meeting RUS specifications and other generally accepted telecommunications practices.
(2)
The output of a digital-to-digital port shall be Pulse Code Modulation (PCM), encoded in eight-bit words using the mu-255 encoding law and D3 encoding format, and arranged to interface with a T1 span line.
(3)
American National Standards Institute (ANSI) Standard S1.4-1983, Specification for Sound Level Meters, is incorporated by reference by RUS. This includes S1.4A-1985 that is also incorporated by reference. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from ANSI Inc., 11 West 42nd Street, 13th Floor, New York, NY 10036, telephone 212-642-4900. Copies may be inspected during normal business hours at RUS, room 2838-S, U.S. Department of Agriculture, Washington, DC 20250, or at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-6030, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html.
(4)
American Society for Testing Materials (ASTM) Specification B 33-91, Standard Specification for Tinned Soft or Annealed Copper Wire for Electrical Purposes, is incorporated by reference by RUS. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from ASTM, 1916 Race Street, Philadelphia, PA, telephone 215-299-5400. Copies may be inspected during normal business hours at RUS, room 2838-S, U.S. Department of Agriculture, Washington, DC 20250, or at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-6030, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html.
(5)
Bell Communications Research (Bellcore) document SR-TSV-002275, BOC Notes on the LEC Networks—1990, March 1991, is incorporated by reference by RUS. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR Part 51. Copies may be obtained from Bellcore Customer Service, 60 New England Avenue, Piscataway, NJ 08854, telephone 1-800-521-2673. Copies may be inspected during normal business hours at RUS, room 2838-S, U.S. Department of Agriculture, Washington, DC 20250, or at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-6030, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html.
(6)
Bellcore TR-TSY-000508, Automatic Message Accounting, July 1987, is incorporated by reference by RUS. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from Bellcore Customer Service, 60 New England Avenue, Piscataway, NJ 08854, telephone 1-800-521-2673. Copies may be inspected during normal business hours at RUS, room 2838-S, U.S. Department of Agriculture, Washington, DC 20250, or at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-6030, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html.
(7)
Federal Standard H28, Screw-Thread Standards for Federal Services, March 31, 1978, is incorporated by reference by RUS. This includes: Change Notice 1, Federal Standard, Screw-Thread Standards for Federal Services, May 28, 1986; Change Notice 2, Federal Standard, Screw-Thread Standards for Federal Services, January 20, 1989; and Change Notice 3, Federal Standard, Screw-Thread Standards for Federal Services, March 12, 1990. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from the General Services Administration, Specification Section, 490 East L'Enfant Plaza SW, Washington, DC 20407, telephone 202-755-0325. Copies may be inspected during normal business hours at RUS, room 2838-S, U.S. Department of Agriculture, Washington, DC 20250, or at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-6030, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html.
(8)
Institute of Electrical and Electronics Engineers (IEEE) Std 455-1985, IEEE Standard Test Procedure for Measuring Longitudinal Balance of Telephone Equipment Operating in the Voice Band, is incorporated by reference by RUS. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from IEEE Service Center, 445 Hoes Lane, P. O. Box 1331, Piscataway, NJ 08854, telephone (201) 981-0060. Copies may be inspected during normal business hours at RUS, room 2838-S, U.S. Department of Agriculture, Washington, DC 20250, or at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-6030, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html.
(9)
Institute of Electrical and Electronics Engineers (IEEE) Std 730-1989, IEEE Standard for Software Quality Assurance Plans, is incorporated by reference by RUS. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552(a) and 1 CFR part 51. Copies may be obtained from IEEE Service Center, 445 Hoes Lane, P. O. Box 1331, Piscataway, NJ 08854, telephone (201) 981-0060. Copies may be inspected during normal business hours at RUS, room 2838-S, U.S. Department of Agriculture, Washington, DC 20250, or at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-6030, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html.
(10)
RUS Bulletin 345-50, PE-60, RUS Specification for Trunk Carrier Systems, September 1979, is incorporated by reference by RUS. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552 (a) and 1 CFR part 51. Copies may be obtained from the Rural Utilities Service, Administrative Services Division, room 0175-S, Washington, DC 20250. The bulletin may be inspected at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-6030, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html.
(11)
RUS Bulletin 345-55, PE-61, Central Office Loop Extenders and Loop Extender Voice Frequency Repeater Combinations, December 1973, is incorporated by reference by RUS. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552 (a) and 1 CFR part 51. Copies may be obtained from the Rural Utilities Service, Administrative Services Division, room 0175-S, Washington, DC 20250. The bulletin may be inspected at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-6030, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html.
(12)
RUS Bulletin 345-87, PE-87, RUS Specification for Terminating (TIP) Cable, December 1983, is incorporated by reference RUS. This incorporation by reference was approved by the Director of the Federal Register in accordance with 5 U.S.C. 552 (a) and 1 CFR part 51. Copies may be obtained from the Rural Utilities Service, Administrative Services Division, room 0175-S, Washington, DC 20250. The bulletin may be inspected at the National Archives and Records Administration (NARA). For information on the availability of this material at NARA, call 202-741-6030, or go to: http://www.archives.gov/federal_register/code_of_federal_regulations/ibr_locations.html.
(b) Reliability.
(1)
Quality control and burn-in procedures shall be sufficient so the failure rate of printed circuit boards does not exceed an average of 1.0 percent per month of all equipped cards in the central office during the first three months after cutover, and an average of 0.5 percent per month of all equipped cards in the central office during any 6-month period thereafter. A failure is considered to be the failure of a component on the PC board which requires it to be repaired or replaced.
(2)
The central office switching system shall be designed such that the expected individual line downtime does not exceed 30 minutes per year. This is the interval that the customer is out of service as a result of all failure types, excluding dispatch and travel time, i.e., hardware, software, and procedural errors.
(3)
The central office switching system shall be designed such that there will be no more than 1 hour of total outages in 20 years, excluding dispatch and travel time for unattended offices.
(c) System type acceptance tests.
(1)
System type acceptance tests (general acceptance tests) are performed for the purpose of determining whether or not a type of switching system should be added or retained as an RUS accepted system. While general acceptance tests will be required on each system type, they will not be expected to cover every requirement in this section. However, any installation of a system provided in accordance with this section shall be capable of meeting any requirement in this section on a spot-check basis.
(2)
A “completed call” test shall be made part of these system type acceptance tests. There shall be no more than two in 10,000 locally originating and incoming calls misdirected, unsuccessfully terminated, prematurely disconnected or otherwise failing as a result of equipment malfunction and/or equipment failures, or as a result of transients, noise or design deficiencies. This test shall be made with a load box with no less than 10 lines access and 10 subscriber numbers for completion, or equivalent, with no other traffic in the system. If there is a failure in the equipment during this test, the cause shall be repaired and the test restarted at zero calls.
(3)
System type acceptance testing applies basically to factory type testing, and not to owner acceptance testing for individual installations. The overall installed and operating system shall also meet these requirements, except for unusual circumstances or where specifically excluded by this or other RUS requirements.
(d) Types of requirements.
(1)
Unless otherwise indicated, the requirements listed in this section are fixed requirements.
(2)
Optional requirements are those which may not be needed for every office and are identifiable by a phrase such as, “when specified by the owner,” or, “as specified by the owner.”
(3)
In some cases where an optional feature specified in paragraph (e) of this section will not be required by an owner, either now or in the future, a system which does not provide this feature will be considered to be in compliance with this section for the specific installation under consideration, but not in compliance with the entire section.
(4)
The owner may request bids from any RUS accepted supplier whose system provides all the features which will be required for a specific installation.
(5)
The Application Guide, RUS TE&CM 322, provides information about the economic and service factors involved in all optional features, as well as instructions for the completion of appendices A and B of this section.
(e) General requirements.
(1)
The equipment shall provide for terminating and automatically interconnecting subscriber lines and trunks in response to dial pulses (or pushbutton dialing signals, if specified) without the aid of an operator.
(2)
Complete flexibility shall be provided for assigning any subscriber directory number to any central office line equipment by the use of internal programmed memory. Thus, any subscriber line and/or directory number may be moved to another terminal to distribute traffic loads, if the line equipment hardware is compatible with the service provided.
(3)
The system shall be arranged to interface with interexchange carrier trunks and networks using single digit or multi-digit access codes. The system shall be equipped to handle at least 20-digit subscriber dialed numbers. All subscriber directory numbers in the office shall be seven-digit numbers.
(4)
The network and the control equipment shall be comprised of solid-state and integrated circuitry components. Peripheral equipment shall be comprised of solid-state and integrated circuitry components as far as practical and consistent with the state-of-the-art and economics of the subject system.
(5)
The basic switching system shall include the provision of software programming and necessary hardware, including memory, for optional custom calling services such as call waiting, call forwarding, three-way calling, and abbreviated dialing. It shall be possible to provide these services to any individual line (single-party) subscriber. The addition of these services shall not reduce the anticipated ultimate engineered line, trunk, and traffic capacity of the switching system as specified in appendix A of this section.
(6)
The requirements in this specification apply only to single party lines. Although only single frequency ringing is required, other types may be requested in appendix A of this section.
(7)
Provision shall be made for local automatic message accounting (LAMA), and for traffic service position system (TSPS) trunks, or equivalent, to the operator's office when required either initially or in the future.
(9)
The system shall be arranged to serve a minimum of eight All Number Calling (ANC) office codes per office, with discrimination on terminating calls by trunk group, numbering plan, or programmed memory and class mark, if specified in appendix A of this section.
(10)
Busy hour load handling capacity is an important feature when an office approaches capacity. The delays which may occur in call completion during busy hour periods may prove to be excessive in some system designs. Accordingly, each bidder shall provide, in appendix C of this section, data satisfactory to RUS regarding the busy hour load handling capacity and traffic delays of the system.
(11)
Provision shall be made for hotel-motel arrangements, as required by the owner, to permit the operation of message registers at the subscriber's premises to record local outdial calls by guests (see Item 10.5, appendix A of this section).
(12)
Provision shall be made to identify the calling line or incoming trunk on nuisance calls (see paragraph (g)(10) of this section for details).
(14)
Facilities shall be provided to implement service orders, make traffic studies, and perform switching and transmission tests by means of remote control devices if such operations are specified in Items 11.2 and 11.3 of appendix A of this section.
(15)
Provision shall be made for the addition of facilities to record all subscriber originated calls based on dialed directory number, time of day, and duration of conversation. They shall be such that the additional equipment (if any is required) may be added to an in-service system without interruption of service and a minimum of equipment, wiring and software modifications.
(16)
The system shall be capable of distributed switching operation where groups of subscriber lines can be remotely located from the central office. The remotely situated units are known as “Remote Switching Terminals” (RST's) (see paragraph (w) of this section). This does not eliminate the use of pair gain devices such as direct digitally connected concentrators, regular concentrators or subscriber carrier equipment, where specifically ordered by the owner and its engineer.
(17)
The switching system shall have means to synchronize its clock with switches above it in the network hierarchy, when specified by the owner in item 3, appendix A of this section (see paragraph (j) of this section).
(18)
Consistent with system arrangements and ease of maintenance, space shall be provided on the floor plan for an orderly layout of future equipment bays that will be required for anticipated traffic when the office reaches its ultimate size. Readily accessible terminals shall be provided for connection to interbay and frame cables to future bays. All cables, interbay and intrabay (excluding power), if technically feasible, shall be terminated at both ends by use of connectors.
(19)
When specified in appendix A of this section, the system shall be capable of processing emergency calls to a 911 service bureau connected either by a group of one-way 911 lines or a trunk group.
(iv)
If the calling line goes on-hook first, the system shall hold the connection from the called 911 line and return steady low tone to the service bureau. The system shall then begin a 45-minute timeout, after which the calling line is disconnected and an alarm message is printed on a TTY. If the calling line goes off-hook before timeout, the system shall reestablish the conversation path.
(v)
If the calling line does not disconnect, the service bureau attendant shall have the ability to force a disconnect of the established connection with the calling party.
(vi)
When the 911 call is answered, the equipment shall be arranged so that coin lines are not charged for the call. Similarly, if some form of local call charging is used, there shall be no charge for the 911 call.
(vii)
If the 911 service bureau is holding a calling line, it shall be possible for the 911 line to cause the equipment to ring back the calling line. This is done by providing a flash of on-hook signal from the 911 line lasting from 200 to 1,100 milliseconds. The signal to the calling line shall be ringing current if the line is on-hook, or receiver off-hook (ROH) tone if the line is off-hook.
(viii)
Calls shall not be originated from the service bureau via the dedicated 911 lines. If an attempt is made to originate a call, it shall receive reorder tone. After 6 minutes, the system shall print an alarm message.
(ix)
If 911 calls pass through intermediate switching, the forced-hold control, emergency ringback, and calling line status monitoring capabilities are lost.
(f) Line circuit requirements—
(1) General.
The range of direct current (dc) resistances of subscriber loops, measured from the main frame in the central office and including the telephone set shall be at least 0-1900 ohms without loop extension and 1900-3600 ohms with loop extenders, or equivalent. The range when using extension equipment may be significantly reduced for straight line ringers. These limits apply under maximum adverse environmental and manufacturing variation tolerance conditions. Central office voltage shall be stabilized at a value necessary to provide at least a nominal 21 milliamperes current with a nontreated loop of at least 1900 ohms. Minimum loop insulation resistance without loop extenders shall be 25,000 ohms between conductors or from either conductor or both conductors in parallel to ground. Loop insulation resistance for loop extension devices may be 100,000 ohms minimum between conductors or from either conductor or both conductors in parallel to ground.
(ii)
In addition to operating on nonloaded cable pairs and subscriber carrier, the equipment shall function properly with D-66 and H-88 loaded cable pairs, including any provisions the equipment must control for the purposes of proper transmission.
(2) Dialing—
(i) Subscriber dial speed.
The line equipment and central office equipment (COE) in tandem shall operate satisfactorily when used with subscriber dials having a speed of operation between eight and twelve impulses per second and a break period of 55 to 65 percent of the total impulse period.
(ii) Subscriber dial interdigital time.
The line equipment and central office equipment shall operate satisfactorily with subscriber rotary dial interdigital times of 200 milliseconds minimum, and with pushbutton dialing interdigital times of 50 milliseconds minimum.
(iii) Subscriber line pushbutton dialing frequencies.
(A)
The frequency pairs assigned for pushbutton dialing shall be as follows, with an allowable variation of ±1.5 percent:
| Low Group Frequencies (Hz) | High Group Frequencies (Hz) | |||
|---|---|---|---|---|
| 1209 | 1336 | 1477 | 1633 | |
| 697 | 1 | 2 | 3 | Spare |
| 770 | 4 | 5 | 6 | Spare |
| 852 | 7 | 8 | 9 | Spare |
| 941 | * | 0 | # | Spare |
(B)
The receiver shall comply with the operating parameters of the dual-tone multifrequency (DTMF) central office receiver as described in section 6 of Bell Communications Research (Bellcore) document SR-TSV-002275, BOC Notes on the LEC Networks—1990.
(3) Impedance.
For the purpose of this section, the input impedance of all subscriber loops served by the equipment is arbitrarily considered to be 900 ohms at voice frequencies.
(4) Lockout.
(i)
All line circuits shall be arranged for line lockout. When a permanent condition occurs prior to placing a line into lockout, a timed low level warning followed by a timed high level receiver off-hook (ROH) tone (see paragraph (i)(2)(xi) of this section) or a howler circuit (see paragraph (o)(2)(iii)(C) of this section) shall be applied to the line.
(ii)
The line on lockout shall be reconnected automatically to the central office when the permanent off-hook condition is cleared.
(6) Loop extension.
(i)
The number of lines which exceed 1900 ohms will be specified by the owner. When requested by the owner, the bidder shall furnish equipment to guarantee satisfactory operation of all lines.
(ii)
Working limits for subscriber lines with loop extenders are covered in RUS Bulletin 345-55, PE-61, Central Office Loop Extenders and Loop Extender Voice Frequency Repeater Combinations.
(iii)
Ringing from RUS accepted loop extenders, or their equivalent, shall be cut off from the called line when the handset at the called station is removed during the ringing or the silent interval.
(7) Private branch exchange (PBX) lines.
PBX trunk hunting shall be available. It will not be necessary to segregate PBX lines to certain line groups.
(8) Quantity.
A sufficient number of terminations shall be provided, in addition to the quantity specified by the owner for subscriber line service, to meet the requirements of the system for equipment testing, alarm checking, tone transfer, loop around test and other features.
(9) Types.
There shall be provisions for types of lines such as ground start, loop start, regular subscriber, pay stations, etc.
(i)
Provide dial tone in response to origination of a call by a subscriber, except on special lines where the application of dial tone is not applicable, such as manual and hot lines;
(iv)
Register the digits dialed by the calling subscriber where the rotary dial or pushbutton dialing characteristics and the minimum interdigital times are as specified;
(v)
Perform the necessary translation functions when the required number of digits have been registered, and select a channel to a proper outgoing trunk, if one is available, to the designated interexchange carrier;
(vi)
Provide a transmission path from the calling subscriber line to the selected trunk, if an idle one is found;
(vii)
Provide for more than one alternate route to the desired destination when specified by the owner, select an idle outgoing trunk in the first or second choice alternate route trunk group, if all trunks in the higher choice groups are busy, and provide a reorder signal (see paragraph (i)(2)(iv) of this section) to the subscriber if no trunks are available in the last choice alternate route;
(viii)
Translate the proper part of the registered incoming routing data on tandem calls into an identification of an outgoing trunk group, select an idle trunk in that group, initiate the connection of the incoming trunk to the outgoing trunk, set the trunks in the proper configuration for tandem operation, and transmit information as required to permit completion to the desired destination in the distant office;
(ix)
Transmit the proper stored information over the selected trunk to permit completion of outgoing calls to the desired destination by the distant office or offices, and provide multifrequency (MF) outpulsing when specified;
(x)
Register all the digital information on calls incoming from a distant office, when dial or MF pulsing characteristics and interdigital times are as specified;
(xi)
Translate internally a registered directory number into line equipment location, ringing code and terminating class (such as “PBX hunting”) on incoming or intraoffice calls;
(xiii)
Connect the incoming trunk or locally originated call to the called line if the called line is idle;
(xiv)
Permit any type of ringing voltage available in the central office to be associated with any Subscriber Directory Number (SDN), cause the proper type of ringing voltage to be connected to the called line, and remove ringing from the line upon answer whether in the ringing or silent period; and
(xv)
Test and monitor the switching system continually during periods of low traffic using the maintenance and diagnostic subsystem.
(2)
The switching system shall offer at least the following originating and terminating class-of-service indications on a per-line basis to subscribers, as specified by the owner:
(i)
At least one trunk hunting group in each 100 SDN's equipped shall be provided. More may be provided as specified by the owner.
(ii)
PBX groups shall be of a reasonable size commensurate with the ultimate size of the switching system.
(iv)
Each PBX group shall have the capability of being assigned one or more nonhunting SDN's for night service.
(v)
If the called line is a PBX hunting line, the switching system shall test all assigned lines in the hunting group for a busy condition.
(vi)
If the called PBX group is busy, line busy tone, as specified in paragraph (i)(2)(iii) of this section, shall be returned to the originating end of the connection.
(4)
The switching system shall provide pay stations which may be prepay or semi-postpay. The system shall be arranged so that an operator and emergency service (911) may be reached from prepay or semi-postpay coin lines without the use of a coin, when the proper pay station equipment is provided.
(i)
Initiate the line lockout function after a delay, as specified in paragraph (r)(3) of this section, if dial or pushbutton dialing pulses are not received after initiation of a call, preferably routing the subscriber line to a holding circuit for tones and then automatically to lockout;
(ii)
Connect 120 interruptions per minute (IPM) paths busy tone, recorded message, or other distinctive tone to the calling subscriber if an interval longer than that specified in paragraph (r)(4) of this section elapses between dialed digits;
(iii)
Register the standard tone calling signals received from a subscriber station arranged for pushbutton dialing if specified by the owner, provide arrangements to function properly with 12-button pushbutton dialing sets, and return a reorder signal to the subscriber upon receipt of signal from the 11th or 12th buttons if neither of these buttons is assigned functions; and
(iv)
Connect the incoming trunk to the digit register equipment within 120 milliseconds after seizure where direct dialing is received on calls from a distant office, cancel the bid for a register, and return reorder tone to the calling end if dial pulses are received before a register is attached.
(i)
To avoid inefficient utilization of the switching network, that portion of the common equipment that establishes the connection on intramachine calls shall not require more than 500 milliseconds, exclusive of ringing and ring trip, to complete its function under no-delay conditions.
(ii)
The switching system shall provide for duplication in a load sharing or redundant configuration any circuit elements or components, the failure of which would reduce the grade of service of 100 or more lines by more than 25 percent of the traffic carrying capacity.
(iii)
The switching system shall ensure that failure of access to a high choice circuit will not prevent subsequent calls from being served by lower choice circuits, wherever possible.
(iv)
Where only two circuits of a type are provided, circuits shall be designed so that failure of one circuit will not permanently block any portion of the system for the duration of the failure.
(v)
Where more than two circuits of a type are provided, successive usages should be on a rotational or random basis rather than the step-up selection with the possible exception of a last choice trunk.
(vi)
The system shall be designed so that, in the event of a network failure, the system shall immediately or simultaneously use a redundant portion of the network to complete the call.
(7)
The switching system shall provide busy verification facilities with the method of access specified by the owner.
(iii)
The responsibility of restricting subscribers in distant offices from having access to busy verification shall be on the distant office personnel when the toll trunks are used for both toll connecting and verification traffic.
(iv)
When a verification code is used, all digits of the code must be dialed before cut-through to the called line can be accomplished.
(i)
All unused numbering plan area codes, home numbering plan area office codes, service codes and subscriber directory numbers (SDN's) shall be routed to intercept. All intercept administration shall be by changes in memory administrable by telephone company personnel. Maximum machine time to place a subscriber on intercept shall be 15 seconds.
(ii)
Unequipped SDN's intercept shall be effective if the processor memory does not have information concerning the SDN in question.
(iii)
The intercept equipment shall be arranged so that specific SDN's can be routed to a separate intercept circuit for changed numbers.
(iv)
When an intercept call is answered, either by an operator or by a recorded announcement, an off-hook or charge supervision signal shall not be returned, even momentarily, to the originating end.
(v)
When intercepting service is to be handled over the regular interoffice toll trunks, a distinctive identifying tone shall be transmitted when the operator answers. This tone shall be of the frequency and duration specified in paragraph (i)(2)(x) of this section.
(9)
The switching system shall provide nuisance call trap facilities which, when activated, provide a permanent record of the calling and called numbers complete with date and time of day. Where the call originates over an interoffice trunk, the actual trunk number shall be recorded. There shall be provision for the called subscriber to hold the connection and for the positive trace of the call from origination to termination within the office.
(i)
The office shall be arranged so a connection to a terminating channel other than assistance operator shall be released under control of the calling party so that the channel can be reseized, unless the call is to emergency 911 service or other termination arranged for called party control.
(ii)
If the called party disconnects first, the channel used in the originally established connection shall be held until the calling party disconnects or until the timing interval specified in paragraph (r)(7) of this section has elapsed. This feature shall not interfere with the normal operation of calls to intercept, fire alarm, or other special services.
(11)
The switching system shall provide line load control facilities, when specified by the owner, to give preference for originating service to a limited group of subscribers during emergencies.
(i)
These facilities may be activated manually by input-output (I/O) device or automatically after a manual setting of a key (or equivalent) to put line load control into effect, as determined by the bidder. The automatic procedure is preferable.
(ii)
Procedures shall be established to avoid the unauthorized use of the line load control facilities.
(iii)
Where automatic activation is provided, service may be provided to small groups of nonemergency subscribers on limited grade of service whenever the office load becomes low enough to permit this to be done safely.
(h) Interoffice trunk circuit requirements—
(1) General.
The bidder shall supply, as requested by the owner, solid-state technology type trunk and signaling circuits of any of the types described in RUS TE&CM 319, Interoffice Trunking and Signaling, or, with the approval of RUS, any other more recent and desirable types not as yet covered in the manual. For dc signaling, the duplex (DX) and loop types of signaling are preferred.
(iii)
In order to reduce the spares inventory and minimize incidence of improper maintenance replacement of circuit assemblies, the types of trunk circuits shall be kept to a minimum. Variation in assemblies should be mainly limited to variation in signaling modes.
(iv)
Trunk circuits which connect with carrier or 4-wire transmission facilities shall be arranged for 4-wire transmission to avoid an intermediate 2-wire interface between a 4-wire switching system and trunk facilities.
(2) Quantity.
Trunk quantities shall be as specified in appendix A of this section. Sufficient space shall be provided for an orderly layout of trunks. Trunks of a certain type going to the same destination may be grouped together on the original installation.
(3) Requirements for interoffice connections.
(i)
When operator trunks are used in common for both coin and noncoin lines, they shall be arranged to provide an indication to the operator by means of a visual signal or tone when calls are from pay stations. When a tone is used, it shall be of the type specified in paragraph (i)(2)(v) of this section and shall be connected to be heard only by the operator upon answer. It shall be possible to repeat the tone signal.
(ii)
There are no requirements for trunks arranged for manual re-ring by a toll operator, either with the receiver on or off the hook, except to coin stations with the receiver on the hook.
(iii)
On calls from subscribers to the assistance operator, the release of the connection shall be under control of the last party to disconnect. An exception is operator control of disconnect that is used on outgoing trunks to a TSP/TSPS system.