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SECTION 2 COMMUNICATIONS BACKGROUND TELEPHONE SYSTEM In discussions of tha intarconnection situation, it is convenient to consider separately the exchange and long~distance parts of the telephone plant. In its very simplest f orm this consists of a telephone, a "loop" to the central office, the automatic telephone exchange, and, perhaps, trunks running from the nearest central office either .(a) to other central offices nearby; or (b) extending into the to11~te1ephone network, The Te1eph6ne The user interfaces with the telephone system at the telephone instrmnent, From the network~contro1 viewpoint, the user performs as a highly adaptable logic and memory system that responds to incoming calls, initiates calls, and reacts reasonably predictably to a variety of situations encountered in using the telephone, The mechanisms he uses to exert this control in the simplest form of telephone are the switch hook and the dial, Lifting the handset closes electrical contacts in the switch hook to signal the central office. These switch~hook signals play an important role in subsequent operations. (One of them is establishing, for charging purposes, the times at which the call was initiated and terminated.) On receiving dial tone (which the user distinguishes from several other tones produced for his use), the user responds by operating the rotary dial or set of push buttons to correspond to the nmnber he desires to reach, as read from the telephone directory or taken out of his memory (not always accurately). The user takes certain actions depending on whether, subse~ quent1y, he hears the voice of the wanted party, receives busy tone, continues to hear an unanswered audible ringing signal, reaches the wrong telephone, etc. At times he may hear a voice~recorded announcement and react accordingly, or he may reach an intercept operator with whom he converses. The user, in short, by manipulating the telephone instrument, plays a crucial role in the network~contro1 signaling function of the telephone system. The telephone instrmnent, its controls, and the various signals from the system beyond the instrmnent to which the user responds are chosen in recognition of experience with the user's capabilities, limitations, and behavior patterns. The same is true of the quantity of SWitching equipment at the central office, which is chosen to fit traffic patterns as to calling frequency, duration of message, etc. 15 - ~
, r i - 16 - i i I' The various systems' solutions arrived at for the user/telephone combination at the point of access to the network may not necessarily be the same and certainly are not necessarily optimal where the combination· i is replaced in part or completely by machine or computer. Such a machine " or computer, with or without interface devices, must reproduce most or all of the logical and memory operations now performed by the user. It may be conjectured that the machine is more accurate and more rapid, though not necessarily as versatile. The primary function of the telephone instrument, of course, is to transmit and receive speech. The statistical distributions of levels and waveforms sent into the telephone system depend on the characteristics of both the user and the telephone instrument. The loops and long-distance trunks are designed to handle the range of levels encountered, without introducing crosstalk between pairs in multi-paired cable, or overloading the long-distance multiplexed system with its common amplifiers. To this end, there are limits both as to the output at the user station and the input to the trunks. The telephone instrument is being used with increasing frequency to handle signals other than human voice, the telephone user's voice and ear being replaced by an acoustically or inductively coupled data set, cardiograph machine, facsimile machine, etc. Again, replacement of the user by a machine implies compromises. Specifically, the machine-generated signal levels and waveforms must be chosen to be both effective and non- interfering. This is accomplished in part by specification in the appropriate tariffs. In acoustic coupling, the signals are first converted to specified audible sounds and the telephone handset is fitted into a specified holder where these sounds are picked up. In inductive coupling, the electrical circuits within the handset pick up electromagnetic signals from the attached device. In both cases, the exact details of telephone-instrument design are important. Small changes in the telephone instrument may take obsolete acoustic and inductive coupling arrangements. Coordination between the designers of telephone instruments and the designers of acoustic and inductive couplers is required to avoid this. A third function of the user's telephone installation is to protect the user, telephone employees, and the rest of the telephone system against harm. The telephone instrument and installation are insulated against contact with electric power sources. The telephone instrument con- tains a "click reducer" to eliminate the hazard of acoustic shock (a dangerously high level acoustic impulse to the listener's ear) etc. It is designed to maintain careful balance to ground on both sides of the telephone line, avoiding noise and cross-talk effects. It contains non- linear devices that limit energy levels, particularly on short loops to the central office. Where the user telephone system is replaced by a machine, with or without interfacing equipment, the three basic functions of network control,
- 17 - transmission, and protection must all be preserved. Finally, these basic functions must be handled without mutual interference. Specifically, the network-control signaling function must be protected against interference from speech or other signals. As will be pointed out later, this consideration sets additional limits on the level and waveform of signals that can be transmitted throughout the system from the telephone. The Loop The "loop" connecting the telephone to the central office (or "trunk" connecting the PBX to the central office) is one of the major elements of total telephone-plant investment. The loop, for our purposes, includes the interior wiring in the users' premises, the "dropll from the premises to the point of attachment to the cable running to the central office, and a selected pair of wires in that cable, either assigned wholly for the use of a single user or shared with other users. Important characteristics of the loop are its length and the size of the copper conductors. Since a minimum of direct current, at least, must be drawn over these conductors to supply the microphone in the telephone and 20-cycle alternating current must be fed over them to ring the telephone bell, there are upper limits on length of loop and fineness of conductor gauge. Similarly, there are limits connected with the attenuation of voice signals, and the distortion to the direct-current signals used for switch-hook supervision and the detection at the central office of the fact that the called party has answered so that ringing may be "tripped." If considerations of limiting loop length and gauge are identical when the user/telephone subsystem is replaced by a machine, there need be no changes in loop design and layout. If not, some changes may eventually be indicated. Loops and short-haul trunks are derived from copper-wire pairs in cables carrying several hundred or several thousand pairs. To hold cross talk between services carried over these pairs to a minimum, there must be strict control in cable manufacture to avoid structural imbalance. The effect of this careful control can be destroyed if improperly designed or improperly installed equipment is connected to the ends of the pairs. One of the basic requirements for any device connected to the telephone network, therefore, is that it not introduce imbalance l in impedance to ground from the two wires of the pair at the point of connection. Cross talk (undesired coupling of signals from one channel to another) can also be created if excessively high signal levels are applied. To avoid cross talk from this source, limits are set on the output levels from the user station. Finally, cross talk in cables increases with lSection 3
- 18 - frequency. Since paired cables are used increasingly to handle communica- tions involving higher frequencies (e.g., PICTUREPHONE), the limits on levels into these cables are set differently for frequency bands above the voice range. Key Telephones and PBX's Not all telephone instruments are connected directly to the central office over loops, particularly non-residence telephones for business, government, or professional use. In this case, additional switching systems are interposed between the telephone instrument (extension telephone) and the central office. These are manually operated key telephone systems and automatic (or sometimes manual) PBX's or PABX's (two acronyms for essentially the same thing). Some of these systems are of a size and complexity comparable to a telephone central office. PBX's are sometimes, but not always, located on the user premises. In recent years there has been increasing use of Centrex service. In Centrex service, the PBX's sWitching may be done either on the customer's premises, or in the telephone central office. PBX extensions are reached directly by dialing from the telephone network (direct inward dialing). The telephone extension number becomes part of the nationwide numbering system. On outward calls from approved extensions, the called telephone is reached without the intervention of the PBX operator (direct outward dialing). The extension in some cases is identified automatically for billing purposes. The Central Office Dial central offices are of the step-by-step progressive-control type, or of the common control type (crossbar and most recently electronic switching). In a step-by-step office, the user more or less directly controls the switches in the central office when he operates the dial mechanism. Since these switches are mechanical devices with definite speed limitations, the dial-return mechanism is equipped with a speed governor, as a kind of buffer against an impatient user. In common-control offices, operation of the dial controls the condition of groups of relays or solid- state electronic circuits, which are made available for the user's sole use, when he gets a dial tone. These relay or electronic-circuit combina- tions then control the central office switches to set up the desired connection. In general, these latter arrangements are faster. In some cases this is taken advantage of by doubling the speed of the dial mechanism from 10 to 20 pulses per second. Except for this, however, the same type of telephone instruments are used for all types of dial central offices. Push-button or Touch-Tone control will be referred to later.
- 19 - Exchange and Toll Trunk Catrier Systems Telephone sw:ttching offices are interconnected into a nationwide sWitching plan or hierarchy in which the local central office is at the lowest hierarchical level. The switching centers of the hierarchy are interconnected over short~ and long-haul trunk circuits. These circuits are of voice-bandwidth (approXimately 3,200 cycles) and handle two kinds of signals; 1) the message signal itself -- voice, data, etc.; and 2) the network control signals used in setting up and taking down connections, controlling SWitches, start of billing, and, in general, what is known as interoffice "bandshakfng ,". (e;xcllllIlge of. call statllsinformationbetween switching offices by single_frequency IS.F.] signaling). It is important to good service that the message signals not produce false network_control signals. This can happen. For example, "talk-off" is a condition in which an unusual voice sound can be interpreted by the signaling equipment as an indication that the subscriber has hung up. When the system is used for other than voice, restrictions on energy level and waveform are imposed to avoid similar adverse effects. In certain trunk systems, a separate channel is used for network control-signals and these precautions are not required. The majority of trunks, however, use a single channel for both purposes. Restrictions on energy level and waveform are also required to avoid cross talk and noise among services sharing the same facilities. Multi-channel carrier systems carry twelve to many thousands of voice channels through common amplifiers over paired cables, coaxial cables, microwave radio relay and (internationally) submarine cable and satellites. These common amplifiers can handle only limited signal power without over- loading. The effect of overloading is to introduce noise and cross talk into many voice channels. The total available load capacity of the amplifiers are designed to be shared evenly by all the channels, whether they are handling voice or other communications. Specifications for individual channel loading have been established to be an optimum comprise between low levels where underlying system noises dominate and the higher levels where intermodulation noise and cross talk prevail. Other Uses of the Telephone System One of the uses of the telephone network for purposes other than switched message telephoning has been mentioned -- acoustic or inductive coupling to the telephone instrument for handling data, picture transmission, etc. This is only one of many non-telephone uses. Services Private~Line Uses of the telephone system fall into two broad categories: 1) private-line services, and 2) serVices provided on the switched network. Private_line channels may be terminated in either carrier-provided or customer-provided terminal equipment.
- 20 - The loops and trunks of the telephone network have been made available to other services operated by large users of communications: Western Union, the railroadl> , la'l'ge:lndustries, government, etc, In some cases, thesea'l'rangements have involved interconnection between leased lines and equipment owned and maintained by the telephone companies, but operated by the user (for example,the 8l~type teletype store-and...forward i." switching system), In other cases, Westerri Union for example, circuits ~. i only are provided -~ the user attaches his own equipment, I Touch-Tone Services There are over 1,000,000 telephone installations in which the rotary dial has been replaced by a 10- or l2-button "touch-tone" combination. The touch-tone signals, unlike the rotary~dial signals, can be used, not only to control the setting up of the connection, but also to transmit data once the connection has been set up,
