Need for Electronic Switching
In the foregoing chapters, two basic types of automatic electromechanical switching systems have been studied: direct control, in which the subscriber dial directly controls the equipment which establishes a connection, and common control in which the switching equipment completes calls through the shared use of a small quantity of sophisticated equipment.
The direct control system is typified by the step-by-step (SxS) central office. Currently is serves nearly all the lines of the Independent Telephone companies in the United States and about 44 percent of the Bell Systems' lines.
Common control systems serve the remaining 55 percent of the Bell System's lines and can be divided between 48 percent crossbar and 7 percent panel dial systems.
At the present time, Electronic Switching Systems (ESS) serve only 1% of the telephones in use, but it may be anticipated that almost every central office will be equipped with electronic equipment by the year 2000. The capabilites of electromechanical direct and common control equipment to handle the increasing volume of traffic provide for the expanding service features are severely limited by the slow switching speeds of these systems. As a result, research and development efforts have been directed for many years towards producing a workable and economical electronic switching system. The recent development of solid-state devices, such as transistors and integrated circuits, have greatly increased switching speeds, leading to the development of electronic switching.
Essentially it is speed that justifies the need for greater complexity of an electronic switching system. For example, it is now possible to switch, or change circuit conditions at speeds of a few nanoseconds, one billionth of a second (10-9 seconds). Several milliseconds are usually required for switching - relay operations - in the electromechanical system. The higher speeds of the electronic devices make it possible for a single common control element in an electronic switching office to serve as many as 65,000 lines. The No. 5 Crossbar office, on the other hand, is normally capable of serving only 10,000 lines and it requires a number of duplicate groups of control equipment.
Electronic Switching Concepts
The electronic switching systems that have been deployed differ materially from the electromechanical systems. Instead of merely replacing relays, selectors, and crossbar switches of the electromechanical systems with transistors or other electronic circuitry, many new concepts have been envolved. Two principal types of electronic systems, constructed and placed in service, employ a different control and transmission approach although both use similar techniques. They are designated the Time Division Multiplexing (TDM) and the space division categories.
The TDM electronic switching system may be pictured as comprising a common highway over which all coversations take place. This highway is time-shared by all connecting subscriber lines and trunks through a series of high-speed electronic gates. Initial developments by Automatic Electric Company and the Bell Telephone Laboratories utilized this method. The Automatic Electric Company's EAX and the Bell System's No. 101 ESS employed TDM by means of electronic switching. This particular type is more suitable for PBX installations and small central offices, although recent improvements have increased the capacity of the No. 101 ESS to 3,000 lines. The space division method establishes an individual path between the calling and called lines which is the basis of the Bell System's No. 1 ESS.
TDM uses a common transmission path or highway which may be compared to the frequency division method of telephone carrier systems whereby several voice channels are stacked on the same conductors. A different frequency band is used for each voice channel in this type of carrier system. In the TDM system, on the other hand, a speech signal is sampled on a repetitive basis and transmitted in a definite time sequence with respect to the samples of the other voice channels. This method, which also is utilized in the Pulse Code Modulation (PCM) carrier system, is illustrated in Figure 1.

In order to faithfully transmit and reproduce the original voice signals, they must be sampled instantaneously at regular intervals and at a rate which is at least twice the highest significant frequency of the signal. Because voice channels normally cover a 200 - 3,000 Hz range or a nominal 4,000 Hz bandwidth, a sampling rate of 8,000 Hz is generally employed in order that two samples can be taken during each half-cycle of the speech signal. These samples are then transmitted as a series of pulses over the common transmission path. In this manner, the same tranmission facility may be time-shared by many calls.
Referring to Figure 1, follow a call in a TDM electronic switching office where subscriber A is talking to subscriber B which the control and memory circuits remember. Line A is connected to the common highway in its assigned time slot. At the same time, an idle link also is connected to the highway and a sample from A is transmitted into it and temporarily stored. When the time slot of line B is connected to the highway by its electronic gate, the link will also be connected. Thus, the sample stored in the link will be transmitted to B during the slot period. This same process occurs on a two-way basis from B to A. The control and memory circuits are required to remember that subscribers A and B are talking to each other, and to send appropriate controlling pulses to the electronic gates at the proper intervals.
The transmission paths of the TDM electronic switching system generally follow the concepts of the space division type. This chapter will concentrate primarily on the principles, techniques, and equipment that have been designed for the Bell System's No. 1 Electronic Switching System (ESS). It should be understood, however, that they also are applicable in many respects to other electronic switching systems such as the EAX type of Automatic Electric Company. The new electronic switching systems, which eventually will replace the electromechanical types, make use of the stored program incorporating logic, memory, and a central control unit to govern operations. The No. 1 ESS is composed of two main parts designated the central control and the switching network; each part functions separately.
The central control unit has the following five principal elements associated with it: line scanner, program store, call store, switching network, and administration and maintenance center. These elements and their locations are shown in Figure 2. Note that there is a continuing bilateral exchange of information between central control and its related components. This interchange mainly concerns the status of subscriber lines and calls in progress. The teleprinter is the communication device used between central control and the administrative and maintenance personnel.

Many new nomenclatures, as would be expected, are linked with electronic switching techniques which have no counterpart in electromechanical systems. A number of them relate to computer and data processing procedures. For instance, memory and logic play important roles in all switching systems; memory contains the stored instructions, while logic makes the decisions on how to use the instructions. Recall that in the No. 5 Crossbar system the originating register provides the memory and the maker makes the logical decisions.
A memory provides a storage space for data and instructions: it stores the program of instructions as well as the data words upon which the instructions operate. Information may be programmed into the memory and extracted from it; therefore the information stored in the memory of the No. 1 ESS may be temporary or semipermanent. Temporary storage of memory may be regarded as an electronic slate. For example, the call store unit is a temporary memory that records the instantaneous state of a subscriber's line, registers the digits dialed and notes other transitory data during the progress of the call. When the call is completed, this electronic memory slate is wiped clean of the information.
Any material or device that possesses at least two stable states has a memory. The ordinary light switch is an everyday example of a memory device because it has two stable states, on and off, and it "remembers" its position as the result of a manual operation. Relays also can remember an electrical command; they have functioned as memory devices in electromechanical offices for many years. Their operating and release speeds, however, are too slow for general use in electronic switching systems. The memory devices developed for the No. 1 ESS may be classified as the permanent magnet twistor type used for semipermanent storage in the program store, and the ferrite sheet mechanism for temporary storage purposes in the call store unit.
An additional departure from electromechanical switching systems is the omission of the line (L) and cutoff (CO) relays from all subscriber lines. Instead, each subsciber line connects to a saturable-core transformer called a ferrod sensor. This device, shown in Figure 3, indicates whether the line is on-hook or off-hook. Each ferrod sensor is scanned about five times every second by electronic circuits in the No. 1 ESS to determine if a change of state has occured; that is, from on-hook to off-hook or vice-versa.

Another essential device is the ferreed crosspoint switch. Figure 4, which performs the switching operations in the line-link and trunk-link networks. It may be compared to the crossbar switch. The ferreed consists of two magnetic reeds sealed in a glass envelope mounted between plates of a two-state magnetic alloy. This alloy can be switched very quickly from one state to another with very short pulses of current. It remains magnetically saturated until another current pulse switches the alloy back to the original state. The ferreed also may be compared to the codelreed type of relay that was described in Chapter 8, except that it operates in fractions of a millisecond. The ferreed crosspoints only switch the T (tip) and R (ring) of the talking paths. The selection of idle paths and trunks is accomplished by other electronic means.

Electronic Gates and Logic Circuits
Basically, the memory devices in the No. 1 ESS store the binary digits or bits in a memory cell and recall them on command. Millions of bits may be recalled after only a few microseconds while others may be stored for years and recalled repeatedly as often as required.
The binary system is based on the powers of two in contrast to the decimal system which is based on the power of ten. A bit (an abbreviation for binary digit) represents one of two conditions or states. In electronic switching systems, the binary digit 1 can represent the presence of a signal, or a true or yes condition. The 0 binary digit can indicate the absence of a signal, or a false or no condition.
Digits in the decimal system may be converted to binary form by the use of code groups of binary bits to represent each of the ten decimal digits. A four-bit binary code, used in the EAX system for this purpose, is shown in Figure 5. Each bit in this table has a value or weight. The bit at the extreme left has a weight of 1 and the bit at the extreme right has a weight of 8. The decimal digit represented by the code group is obtained by adding the weights of the positions in which a binary 1 appears. For example, the decimal digit 6 is composed of binary bits 0110 in that order. By adding the weights of the positions in which 1 appears, 2 + 4, one obtains a total of 6.
Logic circuits are very important elements of electronic switching systems. Electronic gates, mentioned in connection with TDM electronic switching systems, are specimens of logic circuits. In reality, the electronic gate or gating circuit is a relatively simple electronic switching circuit employing solid-state devices, such as transistors and diodes. It allows information, which may be in the form of electrical pulses, to flow through if certain signals are present. If other than the desired signals are present, it will prevent the flow of information.
The transistor generally is used as the gating or switching device in logic circuits because it is capable of providing current, voltage, or power gain to drive succeeding circuits. In addition, the transistor can invert the input signal and can match output to input impedance for optimum power transfer.
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Code Weights
Decimal Digit 1 2 4 8
0 0 0 0 0
1 1 0 0 0
2 0 1 0 0
3 1 1 0 0
4 0 0 1 0
5 1 0 1 0
6 0 1 1 0
7 1 1 1 0
8 0 0 0 1
9 1 0 0 1
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Figure 5 - 4-Bit Binary Code
These logic or gating circuits furnish certain logical relations which, by convention, have been classified as AND, OR and NOT gates. For instance, in the AND gate, electrical pulses appearing simultaneously on all its input leads would produce a pulse on the output lead. However, if the pulses do not appear simultaneously on all the input leads, no pulse will be present on the output lead. Another and more commonplace example is a reading lamp plugged into a wall socket which is controlled by a wall switch. The lamp will not light unless both the lamp switch AND the wall switch are turned on.
The OR gate is a multiple-input circuit having an output that can be energized only when one or more of any of the input circuits are in a specified state. A simple illustration of the OR logical function is the action of the interior lights in an automobile. They will light only if one front door OR another door is opened.
The NOT gate actually is a simple input circuit whose output is energized only if its input is not energized. This type of gate circuit is also an inverting amplifier. For example, when its input is grounded a negative potential will appear on the output lead. Similarly, when a negative voltage is applied to the input of the NOT gate, approximately zero volts will be present at its output. The corresponding logic operations and logical symbols of these gates are illustrated in Figure 6 in addition to the AND, OR, and NOT logic operations represented by the indicated mathematical signs.

The input and output levels of these logic elements may be referred to by binary digits 1 or 0 instead of in terms of voltage. A binary 1 can be regarded as representing a positive potential and 0 as being zero volts or ground potential. In this manner, two or more input leads of an AND gate, for example, can be compared to give a 1 (true or yes) conclusion or a 0 (false or no) result on the output lead.
The AND, OR, and NOT gates comprise three of the basic logic elements used for electronic switching functions. The switching roles of these gates depend upon adequate signal levels so that amplifiers usually are included in these circuits. Time is another very important element. Its importance necessitates the use of storage devices. Thus, in summary, we can state that the basic logic elements are comprised of switching, amplification, and storage devices, all of which are essential components of the No. 1 ESS and EAX electronic switching systems.
Stored Program Function
The use of a stored program system to control operations contributes great flexibility to the No. 1 ESS since the stored program system offers a changeable memory and logic. A measure of the flexibility that is possible by this means can be realized by comparing the logic methods employed in the electromechanical system.
In electromechanical systems, the connecting copper wires "contained" the logic and each circuit was wired to perform a specific programmed operation. As a result it usually was necessary to add or replace equipment and to rewire circuits in order to effect a change in operations. Moreover, it often would cost more to modify the existing equipment for new features than to provide these features with a new system. In contrast, the stored program in the No. 1 ESS is contained on plug-in cards which are inserted in the memory. Consequently, it only is necessary to change a memory card to alter a logical operation in the stored program. This facility enables future design concepts or additional services to be readily added to existing No. 1 ESS offices.
In the electromechanical system, each operation is the switching sequence triggers the operation that follows it. In the No. 1 ESS, the program store is the trigger. For instance, it may initially direct that dial tone be sent to a subscriber originating a call, next a connection could be made to the switching network for another call followed by the release of the connections for a third call and, only then, to return to the first call to record the dialed digits. Any one step in processing a single call is segregated from any other step. From the subscriber's viewpoint, the call appears to continue in an unbroken sequence until the connection is completed. Only one call at a time may be handled by the system but its enormous operating speed makes it seem as though all calls are being handled simultaneously.
The actual circuit actions that accomplish each step in the process take place in central control as directed by the program store. Every few microseconds one of the approximate 100,000 instruction words in the stored program directs the central control in some basic operation of call processing or automatic maintenance. While each step may have a different duration, the flow of actions follows an exact schedule since only one instruction at a time can be executed by central control. For example, the condition of each subscriber line normally is scanned once every 200 milliseconds. However, during dialing the line must be scanned at a faster rate to avoid missing any digit pulses. The switching network operates at a slower rate so that instructions to make connections in it are issued at correspondingly slower rates. These different time cycles are reflected in the formation of the stored program. Some of the essential components of the program store and the call store, which comprise the memory and logic of the No. 1 ESS, are outlined in Figure 7. The stored program contains five functional groups of programmed logic. Each controls a particular phase in handling a call. The following is a brief description of the functions of each group:

During a call, each of the first four programs assumes and relinquishes direction over central control in accordance with a schedule governed by the executive control program. In order to assist this process, use is made of the call store as a clearing house of information between these four functional groups. The call store is divided into various sections, each with a number of memory slots. Information is deposited and withdrawn as needed from the temporary memory stores in the call store. They are called registers, hoppers, and buffers is accordance with their functions. Each call being processed has one or more registers associated with it. The input programs fill the hoppers with information that is processed by the operational programs. The buffers are stocked with data by the operational programs and the subroutines for use by the output programs. The processing of any call, consequently, entails a constant interchange of data between the program store and the call store.
Line Scanning Operations in No. 1 ESS
Lifting the handset signals the origination of a call for electronic as well as electromechanical signaling systems, but from that point on the operations involved in processing a call in the No. 1 ESS are entirely different and not analogous to electromechanical systems. For instance, in the step-by-step and crossbar central offices, a line (L) relay is associated with every subscriber line. This relay operates whenever the handset is lifted to initiate a call. Its operation starts the electromechanical switching process. In the No. 1 ESS there are no line relays. Instead, each subscriber line is connected to a ferrod sensor, a saturable ferrite-core transformer which is composed of a retangular ferrite stick surrounded by four windings. Two windings are connected in a balanced circuit to each tip and ring conductor of the subscriber lines. The other two windings connect to the ferrod scanning circuit as shown in Figure 8.

The ferrod sensor indicates the state of its subscriber line, that is, whether it is on-hook or off-hook. If the subscriber handset is on-hook, no current will flow in the line conductors. Therefore, a current pulse applied to one winding of the ferrod sensor will produce a corresponding pulse in the other winding. When the handset is off-hook, as in originating a call, current will flow in the line conductors and the ferrite stick of the ferrod sensor will become saturated. In this case, when a current pulse is applied to the "interrogate" winding from the ferrod scanner circuit, the ferrite core already will be saturated. This condition will prevent the generation of an output pulse in the "pulse out" winding, thereby denoting an off-hook state. Thus, the on-hook and off-hook states are indicated, respectively, by the absence of presence of current flow in the tip and ring conductors of the subscriber line.
The battery supply for the balanced windings of each ferrod sensor in controlled by the crosspoint contacts of a bipolar ferreed switch on the line-link network. This ferreed switch is operated on originating calls to connect the calling subscriber line to the line-link network for subsequent connection to service or other trunks. Similarly, on incoming calls, the ferreed switch functions like the crosspoints of a crossbar switch to connect the line-link network to the called line. Thus, the ferreed switch may be considered to function in a similar fashion as the cut-off relay in electromechanical switching systems.
The ferreed sensors of all subscriber lines are scanned at least once every 200 milliseconds by central control as directed by the line scanning program (see Figure 9). The "present" state of the line is reported to central control by the line scanning program and the "last" state by the memory in call store. Whenever a change of state of any subscriber line is found by central control, the line scanning program temporarily stops the scanning action. At the same time, the equipment number of the particular line on the line-link network is recorded in call store denoting, for example, the start of the call. The action initiated on one line is completed before the next line is scanned. When all ferrod sensors have been scanned, the line scanning program returns control to the executive control program.

State of Line Logic Operations
The logic circuits enable central control to determine whether or not a change of state has occurred in any subscriber line. Suppose that central control checks the condition or state of line 6789 and has requested reports from the line scanning program and the memory in call store concerning the line's respective "present" and "last" state. Recall that a binary 0 digit can represent zero voltage or no-pulse output, and that a binary 1 can indicate a voltage or pulse output. If line 6789 is on-hook, its ferrod sensor when scanned will produce a pulse. This action will cause a 1 bit to be sent back to central control. If this line should be off-hook, no pulse would be generated during the scanning process, transmitting a 0 bit to central control as an indication of the line's "present" state. Concurrently, the call store memory will be advising central control of the "last" state of line 6789.
The "present" and "last" conditions of line 6789 are simutaneously received by the logic circuit in central control as shown in Figure 10. This logic circuit comprises AND, OR, and NOT gates. If both "present" and "last" states are represented by bits 0 or 1, the output signal in central control will be 0, signifying that the condition of line 6789 has not changed since the last scan. However, if there has been a change, the output signal in central control will be 1.

Assume that a call is being originated by line 6789. The off-hook condition will cause current to flow in the line and its ferrod sensor will not return a pulse when scanned for its "present" state. This will cause a 1 bit to be transmitted to the logic in central control. The essential elements of this logic are four electronic gates, OR, AND #1, NOT, and AND #2 as shown in Figure 11. First, the ferrod line scanner reports on the "present" state of 6789 by sending a 1 bit as mentioned above. At the same time, the call store memory advises that the "last" state was 0 (on-hook condition). Bits 1 and 0 are now applied in parallel to the respective inputs of gates OR and AND #1. Since an OR gate will produce an output signal only when any one or more of its input circuits are energized, the output of OR will be 1 in this instance. On the other hand, pulses must appear simultaneously on all of its input leads for an AND gate to produce an otput pulse. Since AND #1 has a 0 and 1 input, its output will be 0. The NOT gate may be considered an inverted amplifier. Therefore, the 0 bit from AND #1 will become a 1 at the input to AND #2. Because both inputs to AND #2 are 1, an output pulse will be produced and 1 will be sent to central control as an indication that a change of state has occurred in line 6789.

As another example, suppose that the "present" and "last" states of line 6789 are the same, namely, on-hook. The inputs to gates OR and AND #1 in this event will be 0 from both the line scanner and call store memory. The outputs of gates OR and AND #1 likewise will be 0. The NOT gate will invert the 0 from AND #1 to a 1 for connection to an input of AND #2. However, since the other input to AND #2 (from OR) is 0, the output of AND #2 also will be 0. This 0 signal will serve to inform central control that the state of line 6789 has not changed.
Central Control
The most important role in the processing of a call is played by central control. It is a binary digital and computing instrument of the syncronous type that performs very complex actions, one at a time. Clock pulses generated from a 2 MHz crystal oscillator, which provide cycles of 5.5 microseconds, govern its stepping from one cycle of operations to the next. Because of its complexity and the very high reliability requirements, the entire control unit of the No. 1 ESS is duplicated. That is, there are two units of central control, program store and call store. The two central controls actually process all data and operations simultaneously and the results are compared at key points in the No. 1 ESS to check against any errors.
Understanding the operations of central control and its associated program store and call store would require a comprehensive knowledge of data processing and computer engineering. These explanations are outside the field of this book so that it only will be practical to summarize the principal actions performed. Three major classes of instructions are received by central control. The first comprises orders to sense the state or condition of lines or trunks. For example, central control may be directed to examine the ferrod line scanning circuits associated with subscriber lines to detect requests for service, as indicated by a change of state. This scanning operation serves to detect inputs to central control.
A second class of instructions processes the input data. In this case, central control may process the input data, deposit the results temporarily in call store, and later recall them when they are needed. Additional data may be subsequently obtained from program store. These operations do not necessarily progress from one step to the next in sequence. Central control, on encountering certain conditions during any stage in the program, may decide to transfer to another processing action instead of continuing through the particular program.
A third class of instructions refers to outputs produced by central control which operate, for example, relays in trunk circuits and close ferreed switches on line-link or trunk-link networks. In general, a single instruction controls a single operation. However, individual instructions can be combined in various ways as required for control purposes.
Central control processes instructions by first requesting information from program store and call store. In this respect, it may obtain service requests from call store and then ask for processing instructions that are stored at a particular address in program store. In return, program store may send certain instructions to central control that were previously deposited at that address. One part of the instruction tells central control what to do with the information being processed and the address in the second part of the instruction tells where to do it.
Transmission of data and control signals between central control and other units of the No. 1 ESS is handled over a peripheral bus system which is a special multipair cable that interconnects the major subsystem elements used in processing calls. Figure 12 is an elementary representation of the peripheral bus arrangement. Note that two to six program store units and up to fourteen call store units may be provided depending upon the size of the particular No. 1 ESS central office.

Memory Devices
The memory devices in the No. 1 ESS are designed to store a bit in a specific location and to recall it on command. Millions of bits can be stored in a single memory and some bits may be stored for years and recalled as often as needed. Other bits may be recalled within a few microseconds. Both the permanent magnet twistor memory used in program store and the ferrite sheet memory employed in call store comprise magnetic cores.
A bistable ferrite material of uniform composition is used for the magnet core of a memory cell. Its main magnetic characteristic is a square-loop magnetization or hysteresis curve. Thousands of such cores are assembled in a coordinate frame of wires to form the memory. Each ferrite core has two wires which intersect at right-angles to each other, that is, in the X and Y directions. The core can be magnetized in either a clockwise or counterclockwise direction by passing a sufficient current through these wires in the proper direction. Clockwise magnetization of the core represents a binary 0 digit, and counterclockwise magnetization, a binary 1. Figure 13 shows a memory cell in its simplest form.

To store or "write" a binary digit into the core memory necessitates that current be sent through the X and Y wires in the proper direction. This is accomplished by applying half of the current value needed to switch or change the magnetization of the core to the X wire. The other half of the current value is put on the Y wire. The current in each wire, therefore, is less than that needed to change the core's magnetization; that is, from clockwise to counterclockwise or vice-versa. Moreover, the magnetization of a core will not be changed if only one of the wires passing through it carries current. The result is that only the particular ferrite core at the intersection of the current carrying X and Y wires will be magnetized in either a clockwise or counterclockwise direction. This core then will remain magnetized without further application of current.
The same current values are applied to the ferrite core memory cell to read-out the binary digit that had been stored. As an example, assume that the core already is magnetized in a clockwise direction. Now, if the applied currents also tend to magnetize the core in the same direction, the core's magnetization will not be changed. However, if the core has been magnetized in a counterclockwise direction, the applied currents will cause a change in the core's flux. This reversal of the magnetic flux will induce a voltage pulse in the "sense" wire which is used to read-out the memory. A read-out of the memory, in effect, consists of sensing these resultant voltage and no-voltage patterns.
The ferrite sheet memory used in call store is of modular construction. Each module contains 192 separate ferrite sheets arranged in three adjacent stacks of 64 sheets. Four such modules comprise one call store. A single ferrite sheet contains 256 holes on a 16-by-16 grid. The material surrounding each hole is a memory cell. It is similar to the single core memory cell. The X, "inhibit," and "sense" wires are run through all 64 sheets in each stack of the module. The Y wires are formed by interconnecting plated conductors on three ferrite sheets on each level of the module. Figure 14 shows a typical memory cell on a ferrite sheet.

The ferrite sheet memory in call store functions similarly to the simple core cell in the figure. In operation, the X and Y conductors carry the currents to switch the magnetization of the core material at their junction. The "inhibit" wire is utilized to prevent the memory cell from switching during the "writing" of a binary digit. For this purpose, a pulse is sent over the "inhibit" wire simultaneously with, but in the opposite direction to, the pulses over the X and Y conductors. The "sense" wire serves to read the memory by checking for voltage pulses or no pulses.
The twistor memory is employed in program store. Its basic element in an aluminum card containing 2,816 tiny rectangular magnetic spots of vicalloy which is a special magnetic alloy similar to that used in tape recorders. These vicalloy spots are the basic memory cells. A magnetized spot represents a binary 0 digit and an unmagnetized spot represents a binary 1. The condition of each memory cell (vicalloy magnetic spot) is sensed by a twistor wire, a copper wire that has been helically wrapped with a very thin peralloy tape. The twistor wire is run along under the vicalloy magnetic spots. Unlike the ferrite sheet memory in call store, the twistor memory can be read an unlimited number of times. To change the stored information, it is necessary to temporarily remove the particular twistor aluminum card and then magnetize or demagnetize its vicalloy spots as required. See Figures 15 & 16.


No. 1 ESS Switching Network
In the step-by-step system, the selector switches, directly controlled by the dialed information, pick a network connection by testing the control (C) or sleeve (S) leads for an idle path or trunk. In crossbar switching systems, the marker tests the S leads of various links to find a set of idle switching paths for connections. In the No. 1 ESS the ferreed switch performs all switching network connections.
The line-link and trunk-link frames comprise the switching network in the No. 1 ESS. The line-link network connects through junctors to establish paths for intraoffice, interfoffice, and tandem (trunk-to-trunk) calls. Intraoffice calls, however, bypass the trunk-line network by utilizing an intraoffice junctor circuit which provides talking battery and supervisory signaling on intraoffice calls. All other traffic is routed through the trunk-link network to the respective trunk and service circuits for necessary control and supervision.
Figure 17 shows the general arrangement of the switching network and its associated electronic processing units. An interoffice outgoing call from subscriber A, for example, would be routed through the line-link network, the junctor grouping frames and the trunk-line network to the designated outgoing trunk on the trunk frame. A local call from subscriber B to C would be connected only through the line-link network to a junctor circuit on the junctor grouping frame. These calls are illustrated in the diagram.

When fully equipped, a line-link network contains four line switch frames and four junctor switch frames. Each line switch frame incorporates the ferrod sensors and bipolar ferreed switches associated with the subscriber line that is serves. Normally, 64 subscriber lines have access to 16 links or a 4-to-1 line concentration ratio. A total of 16 line concentrators with control circuits, ferrod line scanners and bipolar ferreed switches usually are provided to serve up to 1,024 subscriber lines on each line switch frame. Therefore, one line-link network would have a capacity of 4,096 lines and 1,024 junctors. The crosspoints of the ferreed switches on the line switch and junctor switch frames perform all path interconnections for a call under direction of central control. Note that only two-wire paths, the tip and ring leads (talking circuits), are switched.
A trunk-link network usually contains four junctor switch and four trunk switch frames. The number of these frames can be increased if the traffic warrants a larger concentration ratio. For the usual 1-to-1 concentration rate, one trunk-link network will have a capacity of 1,024 trunks, the same as the number of junctors. Figure 18 shows the general arrangement of the line-link network, junctor frames and the trunk-link network.

The switching network, with its many possible links and paths available for any call, requires some means to indicate which paths or trunks are idle and which are busy at any given time. In electromechanical switching systems, use is made of the control or sleeve leads as a memory device for this means. Selectors in the step-by-step and panel offices test these leads when hunting for idle paths. In the No. 5 Crossbar, the marker performs this function. The No. 1 ESS makes use of memory in call store to select idle links or paths in an unique way.
Call store maintains in one area of its memory a network map. This network map contains a record of the idle or busy states of all links or paths in the central office. An idle state is indicated for any link or path involved in a call by the binary digit 1 and a busy state by the binary digit 0. Consequently, call store and not the switching network is consulted for idle paths in completing a call. For instance, when a call is in progress, every link or path used is identified by a group of binary digits called path memory word and stored in call store. These busy links are marked by the binary digit 0 in the network map of call store. When the call is disconnected and the same links become idle, they will be marked by the binary 1 in the network map. At the same time, the associated path memory word will be erased from call store. A simplified diagram, Figure 19, illustrates the possible paths that may be recorded in the network map of call store on a call from subscriber A to B. A 1 indicates an idle link and 0 a busy link. Thus, there are only two idle paths available, as indicated by arrows, in this particular example.

Intraoffice Call
A brief review of how a call is handled in the No. 1 ESS will help to explain the total system more clearly. The last scan of line 6789, in an off-hook state, indicated that a call was being initiated. This information, sent by the line scanning program, is recorded in a line service request hopper of call store. At the same time, other information in received from program store concerning the kind of station equipment in use; that is, rotary dial or Touch-Tone. Assuming that line 6789 is equipped for Touch-Tone dialing, central control will direct the line-link and trunk-link networks to connect the line's termination to a service trunk equipped for Touch-Tone digit reception. Dial tone will be sent to the calling line by this service trunk.
Next, the digit scanning program will function and central control directs that that subscriber line be scanned at 10 millisecond intervals in order to detect and record the Touch-Tone pulses. These pulses are temporarily stored as input data in an assigned register in call store. No further action is taken until dialing is completed. After all digits are dialed, the program store is consulted for the next step. Central control is informed as to the type of call, intraoffice in this instance, and it will refer to the network map in call store for idle links and paths available for connection to the called line. Upon receiving this data, central control will direct the line-link to make the necessary interconnections between the calling and the called line through a junctor circuit.
If the called line is busy (off-hook state), the ringing connection program will direct that busy tone be returned to the calling party from the junctor circuit. If the line is idle, the junctor circuit will connect ringing current and an audible ringing tone will be sent back to the calling subscriber.
When the called party answers the call, the resultant change of state is recorded by call store. Central control then directs that the calling line be scanned at the regular 200 millisecond intervals. No further action is taken until the parties disconnect. At that time, as indicated by a change of state (from off-hook to on-hook), central control will direct the line-link network to release the connections. Interoffice calls are handled in a similar manner. In this case, both the line-link and trunk-line networks are interconnected and directed by central control to connect the calling line to an outgoing trunk. For tandem calls (trunk-to-trunk), only the trunk-link network is utilized.