Analog Access to the Telephone Network

Telecommunications Telephony Analog Access to the Telephone Network Courseware Sample 32964-F0

Order no.: 32964-00 First Edition Revision level: 01/2015 By the staff of Festo Didactic Festo Didactic Ltée/Ltd, Quebec, Canada 2001 Internet: www.festo-didactic.com e-mail: did@de.festo.com Printed in Canada All rights reserved ISBN 978-2-89289-541-4 (Printed version) Legal Deposit Bibliothèque et Archives nationales du Québec, 2001 Legal Deposit Library and Archives Canada, 2001 The purchaser shall receive a single right of use which is non-exclusive, non-time-limited and limited geographically to use at the purchaser's site/location as follows. The purchaser shall be entitled to use the work to train his/her staff at the purchaser's site/location and shall also be entitled to use parts of the copyright material as the basis for the production of his/her own training documentation for the training of his/her staff at the purchaser's site/location with acknowledgement of source and to make copies for this purpose. In the case of schools/technical colleges, training centers, and universities, the right of use shall also include use by school and college students and trainees at the purchaser's site/location for teaching purposes. The right of use shall in all cases exclude the right to publish the copyright material or to make this available for use on intranet, Internet and LMS platforms and databases such as Moodle, which allow access by a wide variety of users, including those outside of the purchaser's site/location. Entitlement to other rights relating to reproductions, copies, adaptations, translations, microfilming and transfer to and storage and processing in electronic systems, no matter whether in whole or in part, shall require the prior consent of Festo Didactic GmbH & Co. KG. Information in this document is subject to change without notice and does not represent a commitment on the part of Festo Didactic. The Festo materials described in this document are furnished under a license agreement or a nondisclosure agreement. Festo Didactic recognizes product names as trademarks or registered trademarks of their respective holders. All other trademarks are the property of their respective owners. Other trademarks and trade names may be used in this document to refer to either the entity claiming the marks and names or their products. Festo Didactic disclaims any proprietary interest in trademarks and trade names other than its own.

Safety and Common Symbols The following safety and common symbols may be used in this manual and on the equipment: Symbol Description DANGER indicates a hazard with a highh level of risk which, if not avoided, will result in death or serious injury. WARNING indicates a hazard with a medium level of risk which, if not avoided, could resultt in death or serious injury. CAUTION indicates a hazard with a low level of risk which, if not avoided, could result in minor or moderate injury. CAUTION used without the Caution, riskk of danger sign, indicates a hazard with a potentially hazardous situation which, if not avoided, may result in property damage. Caution, risk of electric shock Caution, hot surface Caution, risk of danger Caution, lifting hazard Caution, hand entanglement hazard Notice, non-ionizing radiation Direct current Alternating current Both direct and alternatingg current Three-phase alternating current Earth (ground) terminal

Safety and Common Symbols Symbol Description Protective conductor terminal Frame or chassis terminal Equipotentiality On (supply) Off (supply) Equipment protected throughout by double insulation or reinforced insulation In position of a bi-stable push control Out position of a bi-stable push control We invite readers of this manual to send us their tips, feedback and suggestions for improving the book. Please send these to did@de.festo.com. The authors and Festo Didactic look forward to your comments.

Table of Contents Introduction... V Courseware Outline Analog Access to the Telephone Network... VII Central Office Operation... IX Private Automatic Branch Exchange (PABX)... XI PABX Analog Trunk... XIII Digital Trunk... XV Sample Exercise Extracted from Analog Access to the Telephone Network Ex. 2-2 Hybrid Function... 3 Sample Exercise Extracted from Central Office Operation Ex. 3-1 Call Processor Functions... 17 Sample Exercise Extracted from Private Automatic Branch Exchange (PABX) Ex. 1-1 Architecture of a Digital PABX... 31 Sample Exercise Extracted from PABX Analog Trunk Ex. 1-2 Analog Trunk Interface... 55 Sample Exercise Extracted from Digital Trunk Ex. 1-2 Digital Trunk Interface... 87 Other Sample Extracted from Analog Access to the Telephone Network Unit Test...117 Instructor Guide Sample Extracted from Analog Access to the Telephone Network Unit 1 The Telephone Set...121 Bibliography III

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Introduction The Lab-Volt Telephony Training System (TTS), Model 8086, is a powerful learning tool that allows students to study the operation of modern telephone networks and digital private automatic branch exchanges (PABX). The TTS is built upon the Reconfigurable Training Module, Model 9431. This module, which uses state-of-theart digital signal processor (DSP) technology, can be programmed to act as different parts of a telephone network. Interface cards that students install in the training module allow connection of real analog and digital telephone sets and trunk lines. A central office (CO) is easily implemented by inserting an analog line interface card into a training module programmed to act as a central office. Similarly, a digital PABX is implemented by inserting a digital telephone interface card into a training module programmed to act as a PABX. Furthermore, simple telephone networks can be set up quickly by adding analog and digital trunk interface cards to COs and PABXs implemented with training modules, and interconnecting the modules with trunk lines. Such telephone networks allow establishment of both intra- and interexchange calls as well as tandem-switched calls. A Pentium-type host computer, connected to the Reconfigurable Training Module through a high-speed data link (Ethernet link with TCP/IP protocol), runs the Lab-Volt Telephony Training System (LVTTS) software. This Windows -based software is used to download programs into the DSP memory of the Reconfigurable Training Module. The LVTTS software is also used to: display the functional block diagram of the telephony equipment (CO, digital PABX, etc.) implemented in the Reconfigurable Training Module, change various system settings and options, such as the telephone ringing cadence, companding type, subscriber names and phone numbers, etc, perform step-by-step observation of call routing sequences, observe real signals throughout the system in both the time and frequency domains using modern virtual instruments, insert faults in the system (password-protected feature) for troubleshooting purposes. The TTS courseware material consists of a series of five student manuals, an instructor guide for each student manual, and a user guide. The following fields of telephony are covered in the TTS courseware: Analog Access to the Telephone Network Central Office Operation Private Automatic Branch Exchange (PABX) PABX Analog Trunk Digital Trunk Each student manual covers one particular subject and is divided into several units. Each unit consists of a series of hands-on exercises dealing with certain aspects of telephony. The exercises contain a clearly stated objective, a discussion, a summary of the exercise procedure, a detailed exercise procedure, a conclusion, and a set of review questions. A ten-question test at the end of each unit allows the instructor to verify the knowledge gained by the student. Each instructor guide provides the measured results as well as the answers to all questions of each exercise in the corresponding student manual. It also provides the answers to the unit test questions. The user guide provides all the information required to set up and use the Telephony Training System. V

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Courseware Outline ANALOG ACCESS TO THE TELEPHONE NETWORK Unit 1 The Telephone Set Introduction to the public switched telephone network (PSTN). Brief description of the central office. Familiarization with the functions and operation of the analog telephone set. Ex. 1-1 Telephone Ringing Telephone ringing. AC ringing voltage specifications. The electronic telephone ringer circuit. Ex. 1-2 The Telephone Switchhook and Handset Operation of the telephone switchhook. The handset and speech circuit. Functions and operation of the speech circuit. Ex. 1-3 Tone Dialing Familiarization with DTMF tone dialing. Frequencies used in DTMF dialing signals. Ex. 1-4 Pulse Dialing Familiarization with pulse dialing. Pulse timing. Pulse dialing with an electronic-type analog telephone set. Unit 2 The Line Interface Role of the analog line interface. Block diagram of the analog line interface. Functions of the analog line interface (BORSCHT functions). Operation of the analog line interface. Ex. 2-1 Battery Feed Power Supply How electrical power is supplied to analog telephone sets. Subscriber loop interface circuit (SLIC) overcurrent protection. Equivalent electrical circuit. Maximum resistance (length) of the telephone line. Ex. 2-2 Hybrid Function Balanced transmitted and received signals on the local loop. Role of the hybrid function in the analog line interface. Implementing the hybrid function with electronic components. VII

Courseware Outline ANALOG ACCESS TO THE TELEPHONE NETWORK Ex. 2-3 Pulse Code Modulation The coding function. Block diagram of a PCM CODEC. Voice digitization and recovery. Conversion of the PCM codes to serial format. Use of companding to improve voice digitization and recovery. Ex. 2-4 Companding Linear quantization and quantization noise. Voice signal-toquantization noise (S/N Q ) ratio versus the voice signal level. Using non-linear quantization to implement companding. Comparing the S/N Q ratio versus the voice signal level, with and without companding. Ex. 2-5 Time-Division Multiplexing Why use time-division multiplexing in telephone systems? Timedivision multiplexing of digitized voice signals. The North American (DS1) and European (E1) multiplexing formats. Time slot assignment. Ex. 2-6 Subscriber Signaling Introduction to subscriber signaling. A typical subscriber signaling sequence. Telephone ringing. Telephone status (on-hook or off-hook) supervision. Denying telephone service to a subscriber. Appendix A List of Equipment Required Bibliography We Value Your Opinion! VIII

Courseware Outline CENTRAL OFFICE OPERATION Unit 1 Signaling Circuit Introduction to the operation and functions performed by the signaling circuit: hook status demultiplexing and storage, digitized DTMF dialing signal-to-data conversion, digitized call progress tone generation, and AC ringing voltage generation. Ex. 1-1 Hook Status Demultiplexing and Storage Familiarization with hook status demultiplexing and storage. Description of how the hook status demultiplexing and storage circuit makes hook status available for the call processor. Ex. 1-2 Dialed Number Detection Familiarization with dialed number detection. Description of how telephone numbers produced using either pulse or tone dialing are detected. Ex. 1-3 Call Progress Tone and Ringing Generation Familiarization with call progress tone and ringing signal generation. Unit 2 Digital Switching Introduction to digital switching circuit. Crossbar switching and step-by-step switching are also introduced. Ex. 2-1 Time-Division Switching Familiarization with time-division switching. Difference between time-multiplexed switching and time-division switching. Implementation of a digital time-division switch. Ex. 2-2 Space-Division Switching Familiarization with space-division switching. Description of stepby-step and crossbar switches. Difference between blocking and non-blocking switches. Implementation of a space-division switch. Description of the control register of the space-division switch used in the Telephony Training System. IX

Courseware Outline CENTRAL OFFICE OPERATION Ex. 2-3 Two-Dimensional Switching Familiarization with two-dimensional switching. Practical considerations that limit the number of interconnections that a timedivision switch or space-division switch can establish. Introduction to space-time-space (STS) and time-space-time (TST) architectures. Unit 3 System Control Introduction to the functions and operation of a call processor. Description of the events that take place in a central office during an intra-exchange call. Familiarization with central office configuration. Ex. 3-1 Call Processor Functions Familiarization with the control functions performed by the call processor during the processing of a call: system supervision, signaling, dialed telephone number reception and processing, connection control. Ex. 3-2 Intra-Exchange Call Routing Sequence Familiarization with a call routing sequence of control actions performed by the call processor during the processing of an intraexchange (local) call. Ex. 3-3 Central Office Configuration Familiarization with central office configuration. Configuration of central office equipment so that it complies with the telephone standards of the country where it is installed. Unit 4 Supplementary Services Familiarization with the various supplementary services offered by today's telephone companies. Ex. 4-1 Caller Identification Introduction to the signaling protocol for caller identification, single data message format (SDMF) and multiple data message format (MDMF). Appendix A List of Equipment Required Appendix B ASCII Conversion Table Bibliography We Value Your Opinion! X

Courseware Outline PRIVATE AUTOMATIC BRANCH EXCHANGE (PABX) Unit 1 Architecture and Basic Operation Introduction to the private automatic branch exchange (PABX). Role played by the PABX in the telephone network. Introduction to the architecture and the basic operation of a PABX implemented using digital technology. Ex. 1-1 Architecture of a Digital PABX Familiarization with the architecture of a digital PABX (the Lab-Volt PABX). Resemblances and differences between the architecture of a digital PABX and that of a central office. Description of how two digital telephone sets are interconnected in the Lab-Volt PABX. Ex. 1-2 Telephone Set Portability Introduction to telephone set portability in a PABX environment. Identification (ID) number, terminal (extension) number, and line interface address. Description of how telephone set portability is achieved in the Lab-Volt PABX. Ex. 1-3 Internal Call Establishment Procedure Comparison between subscriber signaling in the PSTN and subscriber signaling in a modern digital PABX. Description of how signaling is performed between digital telephone sets and the call processor in the Lab-Volt PABX. Description of the signaling procedure used in the Lab-Volt PABX to control a basic internal call. Ex. 1-4 Call Progress Indication Introduction to call progress tone generation in a PABX environment. Description of how call progress tones are generated and routed to digital telephone sets in the Lab-Volt PABX. Unit 2 Call Functions Introduction to the various call functions commonly available in today's digital PABXs: call holding, multiple call control, call transfer, conference calling, and intercom. Ex. 2-1 Call Holding and Multiple Call Control Familiarization with the call holding function. Description of the signaling procedure used in the Lab-Volt PABX to hold and retrieve a call. Description of how multiple call control is performed in the Lab-Volt PABX, using call reference values (CRVs). XI

Courseware Outline PRIVATE AUTOMATIC BRANCH EXCHANGE (PABX) Ex. 2-2 Call Transfer Familiarization with the call transfer function. Description of the signaling procedure used in the Lab-Volt PABX to transfer a call. Ex. 2-3 Conference Calling Familiarization with conference calling. Description of the function and basic operation of a digital conference bridge. Description of how conference calling is implemented in the Lab-Volt PABX. Description of the signaling procedure used in the Lab-Volt PABX to control conference calling. Ex. 2-4 Intercom Familiarization with the intercom function. Description of how the intercom function is implemented in the Lab-Volt PABX. Description of the signaling procedure used in the Lab-Volt PABX to control an intercom call. Appendix A List of Equipment Required Appendix B ISDN Overview Appendix C Setting Up and Operating the Digital Telephone Set Appendix D Digital (ISDN) Telephone Set Block Diagram Bibliography We Value Your Opinion! XII

Courseware Outline PABX ANALOG TRUNK Unit 1 PABX Analog Trunk Role of trunks in the public switched telephone network (PSTN). Analog versus digital trunks. Description of what an analog trunk is. Use of analog trunks to interconnect a PABX to the PSTN. Ex. 1-1 Familiarization with the Lab-Volt PABX Analog Trunk How to set up an analog trunk between a Lab-Volt PABX and a Lab-Volt central office. Making and receiving external calls using the digital telephone sets connected to the Lab-Volt PABX. Ex. 1-2 Analog Trunk Interface Role of the analog trunk interface in a PABX. The various functions of the analog trunk interface. Block diagram and operation of the analog trunk interface in the Lab-Volt PABX. Unit 2 Call Routing Over a PABX Analog Trunk Signaling over a PABX analog trunk. Conversion of the signaling information in the Lab-Volt PABX. Block diagram and operation of the analog trunk service circuit in the Lab-Volt PABX. Block diagram and operation of the trunk status demultiplexing and storage circuit in the Lab-Volt PABX. Ex. 2-1 External Call Answering and Termination Sequence of events that occurs in the Lab-Volt PABX when an external call is answered. Sequence of events that takes place in the Lab-Volt PABX when an external call is terminated. Ex. 2-2 External Call Establishment (Overlap Sending Method) Sequence of events that occurs in the Lab-Volt PABX when an external call is established using the overlap (conventional) sending method. Sequence of events that takes place in the Lab-Volt PABX when external call establishment fails because the PABX analog trunk is not available. Ex. 2-3 External Call Establishment (En-Bloc Sending Method) Sequence of events that occurs in the Lab-Volt PABX when an external call is established using the en-bloc sending method. XIII

Courseware Outline PABX ANALOG TRUNK Unit 3 PABX Configuration Familiarization with the configuration of various options found in most digital PABX's. Ex. 3-1 Configuring the Lab-Volt PABX How to configure the Lab-Volt PABX using the host computer running the Lab-Volt Telephony Training System (LVTTS) software. Parameters related to the operation of the Lab-Volt PABX that can be configured. Appendix A List of Equipment Required Bibliography We Value Your Opinion! XIV

Courseware Outline DIGITAL TRUNK Unit 1 Multiplexing Format and Basic Operation Description of what a trunk is. Role of trunks in the public switched telephone network (PSTN). The evolution of trunks from the simple nonmultiplexed analog trunk to today's digital trunks using the SONET/SDH technology. Multiplexing format and basic operation of digital trunks. Ex. 1-1 Familiarization with the Lab-Volt Digital Trunk Overview of the Lab-Volt digital trunk. How to set up a digital trunk between two CO's implemented with the Telephony Training System. What an inter-exchange call is. Making inter-exchange calls. Ex 1-2 Digital Trunk Interface Role of the digital trunk interface. TDM formats used in the Lab-Volt digital trunk. Simplified block diagram of the digital trunk interface used in Lab-Volt CO's. Operation of the transmitter and receiver in the digital trunk interface of Lab-Volt CO's. Ex 1-3 Alarm Indication Description of what alarm indication is. Role of alarm indication in digital trunks. Local alarm indication. Remote alarm indication. Illustration of common alarm situations that may occur between two Lab-Volt CO's interconnected through a digital trunk. Unit 2 Inter-Exchange Signaling Description of what common-channel signaling (CCS) is. Use of CCS in Lab-Volt CO's. Introduction to signaling system number 7 (SS7). Familiarization with the Integrated Services Digital Network (ISDN) signaling protocol used in Lab-Volt CO's to control inter-exchange calls established via the digital trunk. Ex. 2-1 Outgoing Inter-Exchange Call Routing Sequence Sequence of events that occurs in a Lab-Volt CO when an outgoing inter-exchange call is established. Sequences of events that can take place in a Lab-Volt CO when an inter-exchange call is terminated. Sequence of events that occurs in a Lab-Volt CO when establishment of an outgoing inter-exchange call fails. XV

Courseware Outline DIGITAL TRUNK Ex. 2-2 Incoming Inter-Exchange Call Routing Sequence Sequence of events that occurs in a Lab-Volt CO when an incoming inter-exchange call is established. Outgoing interexchange call establishment versus incoming inter-exchange call establishment. Ex. 2-3 Multiple Inter-Exchange Call Control Description of what a call reference value (CRV) is. Understanding the mechanism that enables Lab-Volt CO's to control several interexchange calls established via the digital trunk. Appendices A List of Equipment Required B ISDN Overview C Multiframe Structures of the DS1 and E1 TDM Formats Bibliography We Value Your Opinion! XVI

Sample Exercise Extracted from Analog Access to the Telephone Network 1

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Exercise 2-2 Hybrid Function EXERCISE OBJECTIVE When you have completed this exercise, you will be able to explain why two-wire to four-wire conversion (2W/4W conversion) is required to interface an analog telephone set to the local central office. You will be able to demonstrate the 2W/4W conversion performed by the subscriber loop interface circuit (SLIC). DISCUSSION Introduction To minimize the cost of the cables required to connect numerous subscribers to the telephone network, each analog telephone set is usually wired to the central office through a single pair of wires (the local loop). Since a telephone conversation is inherently bidirectional, the transmitted and received voice signals have to travel onto the local loop at the same time and in opposite directions, as shown in Figure 2-6. BALANCED TRANSMITTED VOICE SIGNAL BALANCED RECEIVED VOICE SIGNAL TRANSMITTED VOICE SIGNAL TRANSMITTER T RECEIVED VOICE SIGNAL RECEIVER ANALOG TELEPHONE SET LOCAL LOOP (TWO-WIRE CIRCUIT) TO AND FROM CENTRAL OFFICE R Figure 2-6. Balanced, transmitted and received signals traveling on the local loop (two-wire circuit). A local loop is known as a two-wire transmission circuit. The transmitted and received signals traveling on the local loop are balanced. This means that each of these signals travels on both the T and R wires of the local loop, the phase of the 3

Hybrid Function signal on one wire being opposite to that of the signal on the other wire. The use of balanced signals on local loops provides good immunity against noise and interference. In today's central offices, digital switching equipment is used to interconnect telephones. This type of equipment, however, uses a four-wire circuit to route the transmitted and received signals associated with a telephone conversation. Fourwire circuits are also used for links that interconnect central offices (trunks). In a four-wire circuit, one pair of wires is the transmit path and carries the transmitted voice signal, while a second pair of wires is the receive path and carries the received voice signal. The use of two separate paths for transmission and reception facilitates time-division multiplexing in the central office switching equipment as well as signal amplification in trunk circuits. To interface a subscriber's telephone line (two-wire circuit) to the digital switching equipment of the central office (four-wire circuit), a two-wire to four-wire conversion (2W/4W conversion) must take place somewhere in the system. This conversion is performed in the analog line interface by the subscriber loop interface circuit (SLIC), which is also referred to as the subscriber line interface circuit. Two-Wire to Four-Wire Conversion Figure 2-7 illustrates 2W/4W conversion performed by the SLIC of a line interface. The wire at the SLIC TXA output and a wire connected to the interface's common terminal form the transmit path of a four-wire circuit. Similarly, the wire at the SLIC RXA input and another wire connected to the interface's common terminal form the receive path of the four-wire circuit. The SLIC couples the balanced transmitted signal from the telephone line (two-wire circuit) to its TXA output (the transmit path of the four-wire circuit). It also couples the signal received at its RXA input (the receive path of the four-wire circuit) to the telephone line. Furthermore, the SLIC prevents the signal received at the RXA input and coupled to the telephone line from being sent to the TXA output. This prevents the received signal from being echoed in the transmit path of the four-wire circuit. The transmitted analog signal from the SLIC TXA output is converted into a digital signal by an encoder/decoder (CODEC) in the line interface, so that it can be processed by the digital switching circuit of the central office. Conversely, the received digital signal from the digital switching circuit is converted into an analog signal by the CODEC, so that it can be sent to the telephone set via the SLIC. Traditionally, the 2W/4W conversion is referred to as the hybrid function. This comes from the special multiple-winding transformer, called hybrid transformer, that performs 2W/4W conversion in older non-electronic analog line interfaces. 4

Hybrid Function CENTRAL OFFICE LINE INTERFACE TELEPHONE LINE (LOCAL LOOP) T T TXA TRANSMITTED ANALOG SIGNAL TRANSMITTED DIGITAL SIGNAL TX TRANSMIT PATH TRANSMIT PATH ANALOG TELEPHONE SET TRANSMITTED AND RECEIVED SIGNALS ARE ON EACH WIRE (BALANCED SIGNALS) SLIC RXA RECEIVED ANALOG SIGNAL CODEC RECEIVED DIGITAL SIGNAL RX TO AND FROM DIGITAL SWITCHING CIRCUIT OF CO R R RECEIVE PATH RECEIVE PATH 2-WIRE CIRCUIT 4-WIRE CIRCUIT Figure 2-7. 2W/4W conversion performed by the SLIC in the analog line interface. Implementing the Hybrid Function with Electronic Components Figure 2-8 is a simplified diagram that shows how the hybrid function can be implemented in a SLIC using electronic components. The single-ended signal received at the SLIC RXA input (triangle-wave signal in Figure 2-8) is passed through amplifiers A 3 and A 4, which are non-inverting and inverting amplifiers, respectively. This provides two signals of opposite phases that are sent to the T and R terminals of the SLIC to form the balanced received signal. The balanced, transmitted and received signals on the T and R terminals (sine-wave and trianglewave signals in Figure 2-8) are passed through amplifiers A 1 and A 2, which are noninverting and inverting amplifiers, respectively. This provides signals that are in phase at the inputs of summing point 1. Adding these signals together provides a single-ended signal that corresponds to the sum of the transmitted and received balanced signals on the T and R terminals. Note: A single-ended signal is available on a single wire. However, the voltage related to such a signal is measured (or sensed) by connecting an instrument (or any other electronic device) between this wire and a wire connected to the circuit's common terminal. 5

Hybrid Function SUBSCRIBER LOOP INTERFACE CIRCUIT (SLIC) T A 1 BALANCED TRANSMITTED AND RECEIVED SIGNALS œ 1 œ 2 TXA TRANSMITTED SIGNAL 2-WIRE CIRCUIT R A 2 INVERTING AMPLIFIER A 5 INVERTING AMPLIFIER ECHO CANCELLATION AMPLIFIER 4-WIRE CIRCUIT A 3 RXA RECEIVED SIGNAL A 4 INVERTING AMPLIFIER Figure 2-8. Hybrid function implemented using electronic components. Amplifier A 5 inverts the single-ended signal received at the SLIC RXA input. Summing point 2 adds this inverted signal to the output signal of summing point 1 (sum of the transmitted and received signals) to cancel the received signal, and thereby, prevent undesired echoes in the four-wire transmission circuit. The resulting signal at the output of summing point 2 (TXA output) is the single-ended transmitted signal (sine-wave signal in Figure 2-8). Procedure Summary In the first part of the exercise, you will set up a central office with the Telephony Training System (TTS). In the second part of the exercise, you will establish a connection between two telephone sets and apply sine-wave sound signals to the microphones of the 6

Hybrid Function handsets. You will observe the waveforms of the input and output signals of the SLIC to demonstrate the 2W/4W conversion. In the last part of the exercise, you will disable the echo cancellation function of the SLIC. You will observe the effect this has on the waveforms of the signals at the SLIC inputs and outputs. You will also hear the effect this has on a normal telephone conversation. EQUIPMENT REQUIRED Refer to Appendix A of this manual to obtain the list of equipment required to perform this exercise. PROCEDURE Setting Up the Central Office * 1. Make sure that the Reconfigurable Training Module, Model 9431, is connected to the TTS Power Supply, Model 9408. Make sure that there is a network connection between the Reconfigurable Training Module and the host computer. Install the Dual Analog Line Interface, Model 9475, into one of the analog/digital (A/D) slots of the Reconfigurable Training Module. Connect two analog telephone sets to the Dual Analog Line Interface. Make sure that the tone dialing mode is selected on the analog telephone sets. CAUTION! High voltages are present on the standard telephone connectors of the Dual Analog Line Interface. Do not connect or disconnect the analog telephone sets when the Reconfigurable Training Module is turned on. Connect the AC/DC power converter supplied with each analog telephone set to one of the AC power outlets on the TTS Power Supply. Connect the DC power output jack of each AC/DC power converter to the DC power input connector on either one of the analog telephone sets. Note: The analog telephone set requires an auxiliary DC power source for the digital display to be operative. 7

Hybrid Function * 2. Turn on the host computer. Turn on the TTS Power Supply then the Reconfigurable Training Module. * 3. On the host computer, start the Telephony Training System software, then download the CO program to the Reconfigurable Training Module. The CO program configures the Reconfigurable Training Module so that it operates as a central office. Note: If the host computer is unable to download the CO program to the Reconfigurable Training Module, it may not be using the proper IP address. Have your instructor or the LAN administrator check if the host computer uses the proper IP address to communicate with the Reconfigurable Training Module. Two-Wire to Four-Wire Conversion * 4. On the host computer, zoom in on ANALOG LINE INTERFACE A and connect Oscilloscope Probes 1, 2, and 3 to TP3 (balanced signal on the telephone line), TP4 (SLIC TXA output), and TP7 (SLIC RXA input), respectively. Note: Probes 1, 2, and 3 are associated with channels 1, 2, and 3 of the Oscilloscope, respectively. * 5. Start the Oscilloscope. Make the following settings on the Oscilloscope: Channel 1 Mode... Normal Sensitivity... 0.5 V/div Input Coupling...AC Channel 2 Mode... Normal Sensitivity... 0.2 V/div Input Coupling...AC Channel 3 Mode... Normal Sensitivity... 0.2 V/div Input Coupling...AC Time Base...1 ms/div Trigger Source... Ch 1 Level... 0 V Slope... Positive (+) Display Refresh... Continuous 8

Hybrid Function * 6. Lift off the handset of telephone set A and dial the number of telephone set B. Lift off the handset of telephone set B to answer the call and establish a communication. * 7. Using miniature jack leads, connect the two speakers provided with the Telephony Training System to the low-impedance auxiliary outputs (outputs C and D) of the Reconfigurable Training Module. Place the speakers connected to auxiliary outputs C and D beside telephone sets A and B, respectively. Note: The telephone sets should be positioned as far apart as possible. This will provide maximum acoustical isolation between the two speakers. Install the handset of telephone set A so that the microphone is located over the speaker connected to auxiliary output C. Install the handset of telephone set B so that the microphone is located over the speaker connected to auxiliary output D. * 8. On the host computer, set auxiliary outputs C and D of the Reconfigurable Training Module as follows: Auxiliary Output C Power... On Frequency... 400 Hz Amplitude... minimum Auxiliary Output D Power... Off Frequency... 800 Hz Amplitude... minimum * 9. Increase the amplitude of the signal at auxiliary output C while observing the Oscilloscope. Notice that a 400-Hz sine-wave signal appears at TP3 (telephone line). This signal represents the sound wave applied to the handset of telephone set A, that is, the signal to be transmitted. Set the amplitude of the signal at auxiliary output C so that the amplitude of the sine-wave signal at TP3 is about 0.5 V. Observe the signals displayed on the Oscilloscope screen. Describe how the SLIC routes the sine-wave signal present on the telephone line. 9

Hybrid Function * 10. Turn off auxiliary output C to remove the sound signal applied to the handset of telephone set A. Turn on auxiliary output D. * 11. Increase the amplitude of the signal at auxiliary output D while observing the Oscilloscope. Notice that an 800-Hz sine-wave signal appears at the SLIC RXA input (TP7). This signal represents the sound wave applied to the handset of telephone set B, which is received in the analog line interface of telephone set A via the central office switching circuitry. Set the amplitude of the signal at auxiliary output D so that the amplitude of the sine-wave signal at TP7 is about 0.2 V. Observe the signals displayed on the Oscilloscope screen. Describe how the SLIC routes the sine-wave signal received at the SLIC RXA input. Briefly explain why the received sine-wave signal is not routed to the SLIC TXA output (TP4), although it is present on the telephone line (TP3). * 12. Turn on auxiliary output C to reapply the 400-Hz sine-wave sound signal to the handset of telephone set A. Observe the signals displayed on the Oscilloscope screen. Describe what happens. 10

Hybrid Function Effect of Disabling the SLIC Echo Cancellation Function * 13. Turn off auxiliary output C to remove the sound signal applied to the handset of telephone set A. On the host computer, disable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE A while observing the signals on the Oscilloscope screen. Describe what happens. Briefly explain. * 14. On the host computer, enable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE A. Turn on auxiliary output C to reapply the 400-Hz sine-wave sound signal to the handset of telephone set A. The signals displayed on the Oscilloscope screen should show that normal two-wire to four-wire conversion is performed. * 15. On the host computer, disable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE A while observing the signals on the Oscilloscope screen. Describe what happens. Briefly explain. * 16. On the host computer, enable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE A. Disable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE B while observing the signals on the Oscilloscope screen. Describe what happens. Briefly explain. 11

Hybrid Function * 17. On the host computer, enable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE B. Turn off auxiliary outputs C and D of the Reconfigurable Training Module to remove the sine-wave sound signals applied to the handsets of telephone sets A and B. * 18. On the host computer, disable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE A while you are having a normal telephone conversation. Briefly explain what happens. * 19. On the host computer, enable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE A. Disable the echo cancellation function of the SLIC in ANALOG LINE INTERFACE B while you are having a normal telephone conversation. Briefly explain what happens. * 20. On the host computer, close the Telephony Training System software. Turn off the TTS Power Supply as well as the host computer (if it is no longer required). Disconnect the speakers from auxiliary outputs C and D of the Reconfigurable Training Module. Disconnect the AC/DC power converters from the TTS Power Supply and the analog telephone sets. Disconnect the analog telephone sets from the Dual Analog Line Interface. Remove the Dual Analog Line Interface from the Reconfigurable Training Module. 12

Hybrid Function CONCLUSION In this exercise, you saw that the SLIC in the analog line interface performs 2W/4W conversion, which is also called the hybrid function. You learned that 2W/4W conversion is required because the transmitted and received voice signals travel on a single pair of wires (the local loop) between the analog telephone set and the line interface, while they travel on two separate pairs of wires (4-wire circuit) in the digital switching circuit of the central office. You observed that the SLIC prevents the received voice signal from being transmitted back to its point of origin in order to have echo-free telephone conversations. REVIEW QUESTIONS 1. Why are the transmitted and received signals on the local loop referred to as balanced signals? 2. Briefly explain why 2W/4W conversion is required in the analog line interface. 3. Why do the transmitted and received voice signals travel on a single pair of wires between the telephone set and the analog line interface in the central office? 13

Hybrid Function 4. Why is 2W/4W conversion also referred to as the hybrid function? 5. Describe the 2W/4W conversion performed by the SLIC in the analog line interface. 14

Sample Exercise Extracted from Central Office Operation

Exercise 3-1 Call Processor Functions EXERCISE OBJECTIVE When you have completed this exercise, you will be familiar with the control functions performed by the call processor during the processing of a call. You will learn how to record and observe the control actions performed by the call processor of the Telephony Training System during the processing of a call. DISCUSSION Introduction As stated in the discussion of fundamentals of this unit, all interconnections made in the switching circuit of today's central offices are under stored program control (SPC), i.e., under the control of a central computer (call processor). Figure 3-1 shows a simplified diagram of a central office using stored program control. Each analog line interface (ALI), trunk interface, and service circuit (the service circuits are integrated to the SIGNALING CIRCUIT in Figure 3-1) is connected to both sides of the switching circuit (these connections are not shown to keep the diagram clear) to allow each of theses devices to transmit and receive digitized signals. The figure also shows that the call processor exchanges data with the analog line interfaces, the signaling and switching circuits, and the trunk interfaces to perform four control functions: system supervision, signaling, dialed telephone number reception and processing, and connection control (switching circuit control). & & & & System supervision is performed by reading circuit status information (telephone set hook status, trunk interface idle/busy status). Signaling mainly consists of transmitting commands to analog line interfaces to make telephone sets ring, and sending data to the signaling circuit to generate call progress tones which are routed to the proper telephone sets via the switching circuit. Dialed telephone number reception and processing is performed by reading dialed digits from the signaling circuit one by one to recover the complete number, and analyzing this number to determine the connections to be made. Note that when pulse dialing is used, dialed number reception is carried out by monitoring the circuit (hook status) status information. Connection control consists in sending the proper connection setup and release commands to the switching circuit. 17

Call Processor Functions TELEPHONE LINES TRUNKS ALI SWITCHING CIRCUIT TRUNK INTERFACE TO ALIs ALI ALI TELEPHONE RINGING CIRCUIT STATUS INFORMATION DIALED TELEPHONE NUMBERS SIGNALING CIRCUIT CALL PROGRESS TONES CALL PROCESSOR CONNECTION SETUP AND RELEASE COMMANDS TRUNK INTERFACE TRUNK INTERFACE TO TRUNK INTERFACE Figure 3-1. Simplified diagram of a central office using stored program control. Control Functions Performed by the CALL PROCESSOR in the CENTRAL OFFICE of the Telephony Training System Figure 3-2 is a simplified diagram of the CENTRAL OFFICE in the Lab-Volt Telephony Training System. This subsection explains how the CALL PROCESSOR in the CENTRAL OFFICE performs the control functions. System Supervision The CALL PROCESSOR supervises the system by cyclically reading the contents of the HOOK STATUS BUFFER MEMORY in the SIGNALING CIRCUIT. The hook status signals indicate the CALL PROCESSOR if a telephone set requests service, remains active or becomes inactive. Signaling The signaling function performed by the CALL PROCESSOR consists in sending commands to the TSAC of ANALOG LINE INTERFACEs to make telephone sets ring. The CALL PROCESSOR also sends data to TONE GENERATORs in the SERVICE CIRCUITs to control the generation of call progress tones. 18

Call Processor Functions SWITCHING CIRCUIT ANALOG LINE INTERFACE A TX0 TX1 SPACE-DIVISION SWITCH RX0 RX1 DC SOURCE T R RING RELAY 0 V TXA RXA SLIC HS0 CODEC TX0 RX0 SIGNALING CIRCUIT RING GENERATOR R / V TSAC DATA SERVICE CIRCUIT 1 FOR ANALOG LINE INTERFACES TX1 TONE GENERATOR CODEC RX1 DTMF DETECTOR TSAC SERVICE CIRCUIT 2 FOR ANALOG LINE INTERFACES TX1 RX1 CODEC TONE GENERATOR ANALOG LINE INTERFACE B DTMF DETECTOR TSAC DC SOURCE T R RING RELAY 0 V TXA RXA SLIC HS0 CODEC TX0 RX0 HOOK STATUS BUFFER MEMORY HOOK STATUS DEMULTIPLEXING AND STORAGE CIRCUIT HS0 DEMUX CONTROLLER DATA R / V TSAC PULSE DIALING DETECTOR CALL PROCESSOR DETECTED NUMBER DISPLAY Figure 3-2. Simplified diagram of the CENTRAL OFFICE in the Telephony Training System. Dialed Telephone Number Reception and Processing Depending on the type of dialing used (tone or pulse), telephone numbers are received by reading the dialed digits from DTMF DETECTORs in the SERVICE CIRCUITs or by scanning the contents of the HOOK STATUS BUFFER MEMORY. Once a complete telephone number is recovered, the CALL PROCESSOR analyzes this number to determine the connections required. 19

Call Processor Functions Connection Control Connection control is performed by writing data in the SPACE-DIVISION SWITCH Control Register, and by dynamically controlling the receive (RX) time slot assigned to each ANALOG LINE INTERFACE and each SERVICE CIRCUIT in the SIGNALING CIRCUIT according to the connections to be established. CALL PROCESSOR Log Function in the CENTRAL OFFICE of the Telephony Training System A special function of the CALL PROCESSOR in the CENTRAL OFFICE of the Telephony Training System allows the recording of the control actions performed by the call processor during the processing of a call. This function is referred to as the Call Processor Log function. The operation of the Call Processor Log function is similar to that of a recorder: the recording of the control actions can be started and stopped when required. A recorded sequence can be played back, or printed, to perform step-by-step observation. This function is included in the Telephony Training System for educational purposes only. Procedure summary In the first part of the exercise, you will set up a central office with the Telephony Training System (TTS). In the second part of the exercise, you will identify the paths through which the CALL PROCESSOR performs each of its control functions. In the third part of the exercise, you will use the log function of the CALL PROCESSOR to record the control actions performed when a call is initiated without being completed. You will then relate each action recorded in the log to a specific control function of the CALL PROCESSOR. You will also relate the actions performed when you attempt to make the call to the control actions recorded in the log. In the last part of the exercise, you will make a step-by-step observation of what happened in the Central Office for each of the actions recorded in the log. EQUIPMENT REQUIRED Refer to Appendix A of this manual to obtain the list of equipment required to perform this exercise. 20

Call Processor Functions PROCEDURE Setting Up the Central Office * 1. Make sure that the Reconfigurable Training Module, Model 9431, is connected to the TTS Power Supply, Model 9408. Make sure that there is a network connection between the Reconfigurable Training Module and the host computer. Install the Dual Analog Line Interface, Model 9475, into one of the analog/digital (A/D) slots of the Reconfigurable Training Module. Connect two analog telephone sets to the Dual Analog Line Interface. Make sure that the tone dialing mode is selected on each analog telephone set. CAUTION! High voltages are present on the standard telephone connectors of the Dual Analog Line Interface. Do not connect or disconnect the analog telephone sets when the Reconfigurable Training Module is turned on. Connect the AC/DC power converter supplied with each analog telephone set to one of the AC power outlets on the TTS Power Supply. Connect the DC power output jack of each AC/DC power converter to the DC power input connector on either of the analog telephone sets. Note: The analog telephone set requires an auxiliary DC power source for the digital display to be operative. * 2. Turn on the host computer. Turn on the TTS Power Supply, then the Reconfigurable Training Module. * 3. On the host computer, start the Telephony Training System software, then download the CO program to the Reconfigurable Training Module. The CO program configures the Reconfigurable Training Module so that it operates as a central office. Note: If the host computer is unable to download the CO program to the Reconfigurable Training Module, it may not be using the proper IP address. Have your instructor check if the computer is using the proper IP address to communicate with the Reconfigurable Training Module. 21

Call Processor Functions Paths Through Which the CALL PROCESSOR Performs the Control Functions * 4. Draw in Figure 3-3 the paths through which the CALL PROCESSOR performs each of the control functions listed below. Use the specified line symbols. System supervision (line symbol: ) Signaling (line symbol: ) Dialed telephone number reception (line symbol: + + + + ) Connection control (line symbol: ) The Log Function of the CALL PROCESSOR * 5. Make sure that the address of the TSAC in ANALOG LINE INTERFACE A is set to 01. On the host computer, display the Call Processor Log window. * 6. Start recording the control actions performed by the CALL PROCESSOR. Lift off the handset of telephone set A and dial two digits on the keypad, then hang up. While doing this, observe that control actions are recorded in the Call Processor Log window as they are being performed. Stop recording the control actions performed by the CALL PROCESSOR. * 7. Display the detailed information about each control action recorded in the Call Processor Log window. 22

Call Processor Functions SWITCHING CIRCUIT ANALOG LINE INTERFACE A TX0 TX1 SPACE-DIVISION SWITCH RX0 RX1 DC SOURCE T R RING RELAY 0 V TXA RXA SLIC HS0 CODEC TX0 RX0 SIGNALING CIRCUIT RING GENERATOR R / V TSAC DATA SERVICE CIRCUIT 1 FOR ANALOG LINE INTERFACES TX1 TONE GENERATOR CODEC RX1 DTMF DETECTOR TSAC SERVICE CIRCUIT 2 FOR ANALOG LINE INTERFACES TX1 RX1 CODEC TONE GENERATOR ANALOG LINE INTERFACE B DTMF DETECTOR TSAC DC SOURCE T R RING RELAY 0 V TXA RXA SLIC HS0 CODEC TX0 RX0 HOOK STATUS BUFFER MEMORY HOOK STATUS DEMULTIPLEXING AND STORAGE CIRCUIT HS0 DEMUX CONTROLLER DATA R / V TSAC PULSE DIALING DETECTOR CALL PROCESSOR DETECTED NUMBER DISPLAY Figure 3-3. Paths through which the CALL PROCESSOR performs the control functions. * 8. In Table 3-1, classify the control actions recorded in the Call Processor Log window according to the control function of the CALL PROCESSOR which they are related to. 23

Call Processor Functions CALL PROCESSOR CONTROL FUNCTION CALL PROCESSOR CONTROL ACTIONS System supervision Signaling Dialed telephone number reception and processing Connection control Table 3-1. Relating the control actions recorded in the Call Processor Log window to the control functions of the CALL PROCESSOR. * 9. In Table 3-2, relate the actions you performed when you attempted to make a call to the control actions recorded in the Call Processor Log window. EVENT CALL PROCESSOR CONTROL ACTIONS Handset of telephone set A is lifted Two digits are dialed on the keypad of telephone set A Handset of telephone set A is replaced on the cradle Table 3-2. Relating the actions performed when attempting to make a call to the control actions recorded in the Call Processor Log window. 24

Call Processor Functions * 10. In the control action named Service Request, what is the meaning of the following detailed information: received from HS0, 01? Explain. * 11. In the control action named Service Request, what is the meaning of the following detailed information: mark ALI HS0, 01 as busy? * 12. What is the purpose of the control action named Call Progress Tone Transmit Path Setup? * 13. Explain why the control action named Call Progress Tone Removal is not immediately followed by the control action named Call Progress Tone Transmit Path Release. Step-By-Step Observation * 14. In the LVTTS window, adjust the view so as to be able to see the circuitry of both ANALOG LINE INTERFACEs, the SWITCHING CIRCUIT, the SIGNALING CIRCUIT, and the CALL PROCESSOR. Using the playback function of the Call Processor Log, make a step-by-step observation of what happened in the Central Office for each of the control actions recorded in the log. Note: The Previous View function of LVTTS allows you to reobtain the initial general view after you have zoomed on a particular section. 25