National Academies Press: OpenBook

A Technical Analysis of the Common Carrier/User Interconnections Area (1970)

Chapter: Section 3-- Transmission and Protection Considerations

« Previous: Section 2-- Communications Background
Suggested Citation:"Section 3-- Transmission and Protection Considerations." National Academy of Sciences. 1970. A Technical Analysis of the Common Carrier/User Interconnections Area. Washington, DC: The National Academies Press. doi: 10.17226/13320.
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Suggested Citation:"Section 3-- Transmission and Protection Considerations." National Academy of Sciences. 1970. A Technical Analysis of the Common Carrier/User Interconnections Area. Washington, DC: The National Academies Press. doi: 10.17226/13320.
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Suggested Citation:"Section 3-- Transmission and Protection Considerations." National Academy of Sciences. 1970. A Technical Analysis of the Common Carrier/User Interconnections Area. Washington, DC: The National Academies Press. doi: 10.17226/13320.
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Suggested Citation:"Section 3-- Transmission and Protection Considerations." National Academy of Sciences. 1970. A Technical Analysis of the Common Carrier/User Interconnections Area. Washington, DC: The National Academies Press. doi: 10.17226/13320.
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Suggested Citation:"Section 3-- Transmission and Protection Considerations." National Academy of Sciences. 1970. A Technical Analysis of the Common Carrier/User Interconnections Area. Washington, DC: The National Academies Press. doi: 10.17226/13320.
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Suggested Citation:"Section 3-- Transmission and Protection Considerations." National Academy of Sciences. 1970. A Technical Analysis of the Common Carrier/User Interconnections Area. Washington, DC: The National Academies Press. doi: 10.17226/13320.
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Suggested Citation:"Section 3-- Transmission and Protection Considerations." National Academy of Sciences. 1970. A Technical Analysis of the Common Carrier/User Interconnections Area. Washington, DC: The National Academies Press. doi: 10.17226/13320.
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Suggested Citation:"Section 3-- Transmission and Protection Considerations." National Academy of Sciences. 1970. A Technical Analysis of the Common Carrier/User Interconnections Area. Washington, DC: The National Academies Press. doi: 10.17226/13320.
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Suggested Citation:"Section 3-- Transmission and Protection Considerations." National Academy of Sciences. 1970. A Technical Analysis of the Common Carrier/User Interconnections Area. Washington, DC: The National Academies Press. doi: 10.17226/13320.
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Suggested Citation:"Section 3-- Transmission and Protection Considerations." National Academy of Sciences. 1970. A Technical Analysis of the Common Carrier/User Interconnections Area. Washington, DC: The National Academies Press. doi: 10.17226/13320.
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Suggested Citation:"Section 3-- Transmission and Protection Considerations." National Academy of Sciences. 1970. A Technical Analysis of the Common Carrier/User Interconnections Area. Washington, DC: The National Academies Press. doi: 10.17226/13320.
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Suggested Citation:"Section 3-- Transmission and Protection Considerations." National Academy of Sciences. 1970. A Technical Analysis of the Common Carrier/User Interconnections Area. Washington, DC: The National Academies Press. doi: 10.17226/13320.
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Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

SECTION 3 TRANSMISSION AND PROTECTION CONSIDERATIONS rNTRODUCTI:ON In this section we discuss the factors behind the carrierfs tariff restrictions on the power and waveform of signals sent over the telephone networks (signal criteria), THE :PANEl HAS CONCLUDED THAT THE SIGNAL CRITERIA rN THE TARrF:fS ARE REASON- ABLE. SrGNALS WHICH VIO- LATE THESE CRITERIA CAN CAUSE HARM BY INTERFERING WITH SERVICE TO OTHER USERS We discuss next the sources and effects of harmful voltages on personnel and plant, the exposures of the telephone system to these . voltages, and the additional risks introduced by user-provided equipment. THE PANEL CONCLUDES THAT INCREASED EXPOSURE TO HAZARDOUS VOLTAGES CAN RE- SULT FROM UNCONTROLLED INTERCONNECTION Finally we discuss the subject of cross talk, and how this undesirable effect may be produced by unbalanced (to ground) attachments to telephone lines. THE PANEL CONCLUDES THAT THE MAINTENANCE OF LINE BALANCE IS IMPORTANT TO GOOD SERVrCE. LrNE BALANCE CAN BE IMPAIRED IF POORLY DESIGNED OR IMPROPERLY IN- STALLED AND MAINTAINED EQUIP- MENT IS ATTACHED TO THE SYSTEM - 21 -

- 22 - The following paragraphs introduce the technical background appropriate to the later, mora datailed·discussion of signal criteria, protection criteria and line Unbalance, TECHNICAL FACTORS TO BE CONSlDERED IN THE INTERCONNECTION OF USER-OWNED TERMINALS TO THE ~UBLIC NETWORK The public telephone network has been engineered, on a statistical basis, to provide a variety of services to a large number of residential, commercial, military, and 9th~ USer& with diff~ent s~icere~uir~ents, The numbers and duration of the calls placed by these users cover a wide range, Users are served by many types of telephone facilities at a range of distances from their serVing central offices~ The trunks that tie these offices into the 10ngMdistance portions of the network also vary statistically in type and length; Resultant ranges in transmission parameters of the loops and trunks produce variations in the overall end-to-end characteristics of switched connections through the network. The alternate routing of calls, which allows the automatic adjustment of traffic patterns to meet changing load re~uirements,can increase or decrease the number of links used in setting up successive calls between the same two locations. In short, both the service and the plant have been designed and can only be understood and treated on a statistical basis. Because the numbers involved in telephone network are large, it is always possible to prOVide service to a small number of identified users whose re~uirements depart from the statistics in terms, for example, of the nature of signals to be transmitted. Special treatment might, for example, involve the selection of suitable pairs in local cables to minimize cross talk. It is clearly not economic, however, nor in some cases even possible, to provide special treatment to a very large portion of the total subscribers since the bulk of the service provided must match the capabilities of the bulk of the serving facilities. If, in addition, users whose signals depart from normal are not identifiable, there is no way to provide them with special treatment. If the network is to accommodate large numbers of user-owned terminal equipment, it follows that signal amplitude, wavefoPm) and energy distribution introduced by this e~uipment must continue to conform to the parameters used in the overall netWork design. Even a single user, whose signals are such as to cause cross talk or interference in multi-pair cable systems or cause overload in broadband carrier systems, can cause serious deterioration of service to a group of users.

- 23 - Data and Voice ~otivation is one ~actor in the determination o~ the likelihood that generated signals will·e:xceed·the spectral power'-handling capability of telephone facilities. Whei"e voice transmission is involved, there is generally no -mo t tvat fon f or exceeding design limits· since the network components have been das Lgned to accOmmodate the range of talker volumes and network links that will be. experienced, with no advantage to excessive levels. In data communication, however, it is to the user's advantage to increase the signal-transmission level in order to improve his own error performance, albeit at the expense of degraded performance of other users, It is necessary ,therefore,inthiscaseto· ensure that signals applied to the network do not exceed the transmission capabilities of the telephone facilities, In addition to control of signal levels and wavefo·rms, the interconnection of user-provided terminals involves other considerations. The first of these is the risk of voltages hazardous to personnel and to the network, The most important problem, of course, is the danger to telephone installation and maintenance personnel. Installation and maintenance must be carried on without interruption of existing service, It is the practice for cable and exchange plant workers to work barehanded on cable pairs and junctions in the immediate proximity of hundreds of other pairs and junctions in normal use, There is pptential hazard in this activity due·to the adjacency of the telephone system to electric power systems. However, over the years the two systems have evolved effective measures to avoid injury. Similarly, effective measures must be evolved where there is interconnection of user- owned devices, to ensure that additional· harmful voltages do not reach the telephone network from this source. Due to interconnection with the anticipated increase in user-owned terminal devices using 115 V AC and/or high DC voltages, the possibilities of harm due to poor initial design, improper installation, and/or inadequate maintenance are significant and must be faced in the interconnection of user-owned equipment. Another situation in which service to other subscribers may be impaired is where the telephone line, normally well balanced, becomes unbalanced when a poorly designed, installed, or maintained device is attached to it. Telephone cables are very carefully manufactured to minimize unwanted pickup of interference -- either from other telephone circuits or from nearby power systems. It is necessary to maintain this longitudinal balance at all times on all pairs, If this balance is degraded by some attached equipment, not only will interference be present on the unbalanced pair, but also other pairs in the same cable will be disturbed. Again, adequate provision must be made to ensure that user-owned terminals meet and maintain the longitudinal balance that is fundamental to maintaining the quality of network service, as do carrier-provided terminals.

- 24 - S~gnal c~~te,~a, p~otection, l~ne a~e d~scussed and balance in ~n pa;rag~aphs,' detail the f ollowing SIGNAL CRITERIA The Panel has examined,thebas~s of the signal c:rite;ria (as specified in the ta;riffs)' that set l:1:mits on both "in_band'" and Hout_ of-band" power, Criteria. fo;r in~band (below 3,995 Hz) signal-power levels ar e set to load the freqilencY""dtvision1llll1t:tp1ex ca;rrier systemswh~ch fu;rnish~ost long-haul ~oice-grade serVices, so as to optimize the signa1-to-noise ratio for all users, The criteria for out-of-band signa1- power levels are set to avoid ,interference to other paJ;rs in the same cable, at frequencies above 3,995 Hz, Such cross talk between cable pairs increases at higher frequencies. A third category of signal criteria sets 1im~ts on signal power in a specific region of the in-band range (2,450 to 2,750 Hz). These restrictions safeguard the6periition of the 2,600 Hz in-band signa1~ng system, which is a1~ost universally used in long-distance telephone service, False operation of the in-band signaling system has serious results: improper billing, intermittentinter'i;uptions, insertion of a band-elimina- tion filter in the transmission path, or evencrnnp1ete disconnection of a call, As a part of this study, the Panel has examined the structure of the te1ephone-crnnpany plant and has determined'that it does not provide protective mechanisms by either level 1imite;rs or filters to co;rrect for signals exceeding criteria limits, We have also examinedtheoperiition of the te1ephone-crnnpany plant and have determined that the system is designed to operate in accordance with the criteria. The derivations of the three classes of signal criteria, as set forth in the tariffs, are discussed under the following three subsections. In~BandSigna1~P6wetCriteria The tariff requirements on in-band power 1 are as follows:, FCC 259, FCC 263 -- the power of the signal at the cent;ra1 off~ce not exceed 12 dB below one milliwatt when averaged ove;r any th;ree-second inte;rva1. 2 FCC 260 -- the power of the signal 'that may be applied by theuser~provided equipment, to, the Telephone, Company interface located on the user's premises lIn-Band power is defined as the total power in the band'be1ow 3,995 Hz, 2Ther e is also a requirement that the signal applied to the loop plant not exceed OdBm.

- 25 - will be specified by the Telephone Company for each application to be consistent with the signal power allowed on the telecommunications network. The above requiremeuts on iu-band power are based on interference considerations of 10ng-hau1 3 frequency division multiplex carrier systems. These systems include cable carrier systems with capacities ranging up to 3,roo channels and microwave radio carrier systems with up to 1,8J0 channel capacity. Virtually all voice~grade services longer than about 200 miles use these types of facilities, These .. systems are designed t o- handle a per-icharmej, Load- of -16dBm long-term average power measured at.a network reference transmission level point. This -16dBm power is the maximum average power per 4kc channel that can be permitted without incnrring a noise penalty (increase in total system noise power). Below the -16dBm per channel average signal power, the noise is predominantly thermal (or random) noise. In addition to this thermal noise (which is independent of total signal power), the broadband systems are also subject to intermodulation noise due to non-linearity of the carrier amplifiers. At these low levels, this increases with signal power and at the -16dBm average signal power per channel, the intermodulation noise and thermal noise are equal. At signal power above -16dBm, the noise is predominantly intermodulation noise, this increases at a faster i rate than the signal power. Maximum signal-to-noise ratio is obtained with average signal power at -16dBm. Since both directions of transmission normally are not used simultaneously and not all channels are active at the same time, it has been determined that an average power limit of -13dBm applied to all users of a system is consistent with the long-term loading objective of -16dBm. In developing the tariff criteria, this -13dBm three-second average power limit was translated to refer to a specific physically identifiable location. The selected location was the serving central office and the usual loss between this point and the equivalent network reference transmission level point is ldB. Thus, the maximum signal power that may be permitted at the central office is -12dBm when measured over any three-second interval. When this power level is exceeded, the effect on other users of voice and data services is increased noise and interference. Depending upon the nature and number. of the excessive signals, this noise and inter- ference may appear in the following forms: (a) Increased background noise or hiss on the channel (b) Crackling or static on the channel (c) Cross talk of other users' conversations into the channel. This cross talk may be either 3Section 2, p. 19.

i ntelligible or merely bursts of garbled slleech er~or (d) Increased rates'on data channels (e) Comlllete loss of serVice caused by catastrophic overload of Une facilities, The netWork of 10ng"distancefaci1ities to which the in"band power criterion is applicable is used on a1~ost all long"distance connections (over 200' miles' in length), 1'h±.s network provides ~any diverse paths over which voice and data calls ~ay be carried, NetWo:tk""lllanagement techniques plus dynamic alternate Touting plans vary the specific path (and specific broadband facility) that a particular.point"to"point call will use, Similar changes in routing also occur on prtvate"line services, particularly when a facility failure requires an alternate facility for serVice restoration. This need for facilttyflexibiUty necessitates that all chaime1s be operated at equal signal levels, Hence,' an equal apportionment of system power;'" handling capability to all channels is appropriate, ,t I'" .•....••. \' .. '0 ,. \\ '_I \'" " \' \',' I'" ,-. Out';'cif-'Bartd 'Signa1,;.Power'Ct'iteria , The ta't'iff requirements on out"of"band 4 power' a't'e as follows: FCC 259, FCC 260, FCC 263 "" the signal that is app1ied'bytheeustomer" provided equipment to the Telephone Company interface located on the customer's premisesmeet'the following limits: (a) The power in the baiidfrom 3,995'Hz to 4,005 Hz shall be'at least 18dB below, the stipulated maxtmum in"band signal power. (b) The power in the band from 4,000 Hz to 10,000 Hz shall not exceed l6dB below one milliwatt. The power in the band from 10,000 Hz to 25,000 Hz (c) shall not exceed 24dB below one milliwatt. The power in the band from 25,000 Hz to 40,000 Hz (d) shall not exceed 36dB below one milliwatt, The power in the band above 40,000 Hz shall not (e) " exceed ,50dB,below"one lItilliwatt, 4The out"of-band region is defined as those frequencies greater than 3,995 Hz.

- 27 - Criterion (3,995-4,005 Hz) The limitation on power in the band from 3,995 Hz to 4,005 Hz is based on potential interference in N3 carrier systems. This is an intermediate-range cable carrier system used to provide intercity circuits of 50 to 200 miles in length. By the end of 1968, there were almost 4,000,000 circuit miles of N3 carrier in the Bell System, which accounted for about 15 percent of all intercity circuits in the 50-200 mile distance range. The interfering effect caused by power in excess of the criteria isa gain variation 01' flutter in ilnother user's channel. In order to meet the overall system-flutter objective, it is necessary that the power of the interfering signal be 56dB below the power of the 4kHz carrier at the input to the carrier system's gain regulator. Based upon this reuqirement, the criterion for the 3,995 Hz to 4,005 Hz band is calculated as follows: Spurious signal-to-carrier ration -56dB + 8dB Carrier to maximum signal Average 4kHz suppression in +30dB channel filters -18dB Allowable 4kHz to in-band power ratio Criterion (4-l0kHz) The criterion for power in the band from 4,000 to 10,000 Hz is based on interference considerations in audio braodcast (radio and television) services. The most critical of these services, from a noise standpoint, is FM broadcast, which has an overall peak signal-to-noise requirement of 60dB. In order to meet this overall requirement, the studio-to-transmitter allocation of peak signal-to-noise is 65dB. Based on this signal-to-noise requirement and a peak transmitting level of +18dBm, the maximum channel noise permitted is -47dBm. Using this limit, the 4 to 10kHz criterion is calculated as follows: -47dBm Maximum. noise Correction for measuring techniques 5 and allowance for maintaining margin -10dBm Correction for multiple disturbers - 3dB System equalization -25dB Cross-talk coupling loss at 8kHz 6.9dB Allowable 4 to 10kHz power on disturbing pair -16dBm 5Broadcasters normally use nonweighted noise measurements and align their equipment at 400Hz, while the Telephone Company uses weighted noise measurements and aligns audio channels at 1,000 Hz.

- 28 - The interference mechanism in the case of these channels is cable cross talk. The resulting user effect is noise or tones heard in the channel. Due to the large number of·u1timate users affected by interference with audio broadcast services; it is very important to avoid such effects. (10~25kHz) Criterion The criterion for the10·to 25kHz band is based on considerations of interference into theU1'Cartier system, which uses the 14 to 22kHz band for transmission from the user to the central office. , '. The U1 subscriber carrier system is a relatively new system and is not widely used at present. However, . 1ooking ahead to increased copper costs and reduced electronic costs, it is expected that loop systems operating in this frequency range will likely be used to an increased extent. To meet noise objectives for this system, the minimum carrier-to- interference ratio in this band is set at 75dB. Based upon this requirement, a maximum signal of 21dB below a milliwatt would be permissible on a single disturbing pair based upon cable cross-talk coupling characteristics alone. Because other noise and cross-talk sources can exist in a given cable, the criterion was set 3dB lower than the limit for a single disturbing source. This provides assurance that the system-noise objective will be met under most conditions. The criterion is computed, as follows: Interference-to-carrier (18kHz) ratio for 15dBm noise at subscriber terminals -75dB -29dBm Carrier level Correction for multiple disturbers - 3dB Cross-talk coupling loss at 18kHz 83dB Allowable 10-to-25kHz power on -24dBm disturbing pair Criterion (25-40kHz) The criterion for the 25~0-40kHz band is also based on inter- ference into the U1 carrier system. The U1 system uses the 26-t0-34kHz band for transmission from the central office to the user. The required carrier-to-interference ratio for this band is 77dB. To meet this requirement the criterion of 36dB below one milliwatt was established. It reflects consideration of both the increased cable cross-talk coupling and the greater transmission loss at these higher frequencies and also makes allowance for' other. noise and cross talk in the cable. The criterion is calculated as follows:

- 29 _ Interference-to-carrier (30kHz) ratio for l5dBm noise at subscriber terminals -77dB Carrier level -34dBm Correction for multiple disturbers - 3dB Cross-talk coupling loss at 30kHz 78dB Allowable 25-to-40kHz power on disturbing pair -36dBm Criterion (Above 40kHz) The criterion for power iri the barid above 40kHz isbased6h potential interference into PICTUREPHONE service and into cable carrier systems operating in that frequency range. The effect of interference to PICTUREPHONE service on the user is snow in the picture or herringbone patterns superimposed on the desired picture, due again to cable cross talk. Signal Criteria (Criteria for Distribution of In-Band Power) The tariff requirements concerning distribution of power within the transmission band are: FCC 259, FCC 260, FCC 263 -- to prevent the interruption or disconnection of a call, or interference ~nth network control signaling, it is necessary that the signal applied by the user- provided equipment to the Telephone Company interface located on the user's premises at no time have energy solely in the 2.450-to-2,750 Hz band. If sign~l po~er is in the 2,450-to-2,750 Hz band. it must not pxceed the power present at the s~e time in the 800 to 2,450 Hz band. Tn the 2,600 Hz single-frequency (SF)6 signaling system, the SF receivers respond to signal power in a relatively narrow band nominally centered on 2,600 Hz. However, factors such as manufacturing tolerances, aging of components and ambient-temperature differences produce some variation in both the nominal bandwidth and the center frequency of the receiver-response band~ In addition, ~ form of distortion termed "carrier shift," which ID8Y be encountered on cer~ain types of transmission systems, causes small frequency changes in the signal and is another source of acco~nt, ~e fi~d variation. When factors such as these are tRken jute that the effective SF response band lies between 2,450 and 2,750 Hz. The receivers are designed, however, not to respond to power in this band when an equal or greater amount of power is present at the same time in the 800-2,450 Hz portion of the voice band. This criterion applies at the user's terminal and includes allowances for the sources of variation cited before as well as differences ~n transmission loss for different frequencies in the voice band, over regular telephone connections. 6Section 2, p. 19.

- 30 - [-- ~ Harmful Voltages In this section we discuss sources of harmful voltages !' r- appropriate to the interconnection· issue network, exposures to these voltages, and effects produced by· them •. The major hazard of significance is to maintenance personnel. Equipment hazards are considered minor since i I only the single termination associated with each loop would be harmed in , r case of excessive voltage. I Hazard to Personnel This involves; (1) the effects of electric shock on human beings and (2) the extent to which network personnel may be exposed to such shock as a result of the connection of user-provided equipment and/or systems. 1. Effects of Electric Shock. Harmful effects are determined by the amount of current passing through the human body. The amount of current depends on several factors: the voltage on the electric conductor to which the body is exposed, the source impedance of the voltage, and the highly variable body resistance. In many ways, the most dangerous source of potentially fatal currents is 110 or 220 volt AC. The major danger of this source stems from its ubiquity around users' premises and the fact that the protective devices that are presently connected to telephone lines will usually not operate if the line is exposed to 110 volts. Yet the presence of the voltage is potentially lethal to personnel who come· in contact with that line. 2. Extent of Personnel Exposure. As explained, the telephone companies provide service to customers by means of physical conductors in the exchange plant. Each time service is installed, removed or repaired, telephone servicemen make physical contact with wire pairs and terminals at one or more points in the station equipment or at the terminal appearances of the wire pairs on customer premises in outside manholes or on poles, and in the central office building. In general, the work operations require a hands-on type contact. The size of the wires, the terminal sizes and spacings, and the dexterity required, generally preclude the use of protective clothing or devices such as rubber gloves. This is not to say that rubber gloves are never worn. They are prescribed for many construction operations, particularly when working on joint-use poles shared with power companies. But they are inappropriate for such tasks as splicing together multi-conductor, fine-gauge cables.

- 31 - The conductors that fan out from a wire center (or central office building are carried in densely packed cables, ranging from as few as 6 to 2,700 pairs of conductors per cable, and they are spliced together and terminated on closely spaced terminals in cross-connection boxes and in sealed splices along the routes. Therefore, servicemen working on a single pair are exposed not only to that one pair at terminal field appearances, but also to additional pairs that are connected to adjacent terminals. Effects of Interconnection on· the· Harmful-Voltage· Problem The direct electrical connection of user-provided equipment and communications systems to telephone company lines adds an add it iorta1 source for the introduction of potentially harmful voltages into the telephone plant. This can come about by a faulty equipment design or manufacture, or a faulty installation, both of which could cause 110 V AC or higher to appear on the loop. This potential hazard is also unique in that it is perhaps the E1asiest source to protect against in that the telephone-line exposure occ·rs specifically at the point of interface with the user equipment. sured protection at the interface can provide suitable protection in both directions, i.e., protect the user from possible voltages on telephone lines and protect the telephone personnel from high voltages introduced by user-provided equipment or systems. In Section 5 we discuss protective mechanisms for this need. Loop Balance Connections between customer premises and central offices are normally made by individual wire pairs in multi-paired cables. The wires, because of the close proximity to each other, have mutual capacitive and inductive coupling effects. Mutual coupling results in cross talk b~tween adjacent pairs, which, if not controlled, increases the noise level on all circuits concerned. Cross talk, in aggravated instances, can produce interfering signals of an intelligible nature, which violates, or appears to violate, the privacy of one or more users. Cross Talk in Cables To minimize electrical interactions among individual wire pairs within the cable, the pairs are twisted and balanced to ground. Twisting of the wire pairs reduces the effects of magnetic coupling to an insignifi- cant factor. Capacitive coupling is, however, still a factor and has to be carefully controlled.

- 32 _ The longitudinal balance in cables is controlled in manufacturing so that the coupling tOSS between pairs is generally well over 100dB with about one percent of pairs having coupling losses of 80dB·or less at Since this coupling is primarily capacitive, the coupling loss 1,rnO Hz. will decrease (hence cross talk will increase) with· increasing frequency at the rate of 6dB per octave. Tests have shown that if one conductor of one pair is grounded; cross talk will be worsened by 20dB, and if one conductor of each of two pairs is grounded, it will be worsened by as much as 60 dB. Therefore central-office circuits and telephone-station equipment and wiring in the telephone network are designed; installed, and maintained to ensure a high degree of balance to ground. While cables are designed and controlled in manufacture. to maintain balance and reduce cross talk, these controls become ineffective if equipment attached to the cable pairs is itself improperly designed, installed,or maintained. Cross talk will result if user-provided equipment is unbalanced to ground. This can occur if: (a) Equipment is poorly designed initially. Terminating the telephone pair in an unbalanced circuit is a common error. (b) Equipment is improperly installed so as to apply a ground to one side of the line. This may occur accidentally through insulation being scraped away or with nails or staples cutting through wires. (c) Equipment can fail in service. A component can break down and cause unbalance on the line. Cross talk can be insidious and difficult to locate because. the malfunction is partial rather than total. The user mayor may not be aware that he is causing trouble to other partie~ especially if his service appears normal. Thus, the deteriorated performance can exist for a long period before diagnosis and correction. It should be noted that, with multiple party-line operation, one side of the line is grounded through the ringer. However, the ringer impedance is high enough to avoid unbalance at voice frequencies.

Next: Section 4-- Network-Control Signaling »
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A Technical Analysis of the Common Carrier/User Interconnections Area assesses the technical factors affecting the common carrier/user interconnection area of public communications. This book develops technical and background information that might be useful to common carriers, users, and equipment manufacturers in reaching and implementing solutions to immediate problems. This includes a technical evaluation of various contending points fo view regarding the common carrier/user interaction area, the various problems to which these views relate, and the various technical and policy alternatives for responding to these problems in the near future.

A Technical Analysis of the Common Carrier/User Interconnections Area addresses questions of the propriety of the telephone company-provided network control signaling requirements and various alternatives to the provision thereof by the telephone company; the necessity and characteristics of telephone company-provided connecting arrangements and various alternatives to the provision thereof by the telephone company; and basic standards and specifications for interconnection and the appropriate method to administer them.

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