POTS - pbx
Content
Design and Implementing a 1-to-4 PBX System Using a Microcontroller
Behdad Hosseini University of Isfahan, Iran May 2003
Preface
This is the presentation of a project in partial fulfillment of the requirement for the degree of Bachelor of Engineering in hardware engineering presented to Department of Computer Engineering, Engineering Faculty, University of Isfahan. The title of project is “Design and Implementing a 1-to-4 PBX System Using a Microcontroller”. The project started since September 2002 and lasted until May 2003. I’ve done lots of its work in my home without any laboratory facility. So I hope I’ve done something acceptable. There are some useful technical documents about my project like hardware schematics, a project report (but in Farsi) and its software in assembly language. All of them can be found in the project’s web site. I like to acknowledge everybody (especially my family) that helped my in my project.
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Abstract
Telephone centers or more precisely, branch exchange systems, are electrical and electronical systems that are responsible for establishing telephony communications. Their behavior can be explained by making conversations between connected phones. Private Branch eXchanges (PBXs) are some kind of such systems that are mainly used in companies, organizations and other private complexes. In this project, I’ve designed and implemented a PBX system that is capable of switching 4 phones (lines) and an external connection (trunk). A microcontroller from MCS-51TM family is used for handling and controlling the whole system. The switching unit is realized via 3 analog conversation circuits (one for each conversation connection). The hardware is implemented on 8 breadboards and the software is in the MCS-51TM assembly language.
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Introduction
Contents
Switching Theory Telephone Switching System About The Phonesets
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Switching Theory (1)
Switched Network A network consists of some entities (stations)
that can communicate with each other and some controlling systems. If we use some switching nodes instead of connecting every pair of stations, we’ve made a switched network.
Switching means controlling some connections to making favorite
paths. These selections (and its actuators) are under our control. A path can carry any signal from power to speech signals. About our task, assume we have some phones or equally some speech signals. We want to connect these phones to each other so we are concerned about switching these signals.
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Switching Theory (2)
Types of switching:
– Circuit the traditional form of the switching which makes the paths
physically by switches like relays and codecs. It is like an electrically close circuit that connects our two desired points. So we can have any kind of signaling if the frequency response of the path allows us and the current value is supported by the elements. It means we can have analog or digital data.
– Packet this kind of switching makes a virtual path not a physically one.
The data must be digital and encapsulated in packets. A packets is a collection of data bits plus control bits that constructs its frames. The switching can be performed by a simple PC or some specially designed digital processing systems. So we can only transmit digital data or digitized analog data.
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Telephone Switching System: Definition (1)
Definition An electrical and electronical circuit switching system is
called Telephone Switching System if the switched data is in the form of analog speech signals (i.e signals with frequency between 0 and 4kHz) and can understand the request signals like dialing and can also send meaningful informative signals like ringing. The electronic technology of the system doesn’t matter. The important aspect is the system behavior not the system structure. The switching section usually is implemented in circuit switching model and can be analog or can be digital as is done in new high performance systems.
Behavior From more practical point of view, Telephone Switching
Systems are capable of establishing conversation connections between some connected phones or some external links (e.g from another system).
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Telephone Switching System: Definition (2)
Types (Electronics) The Telephone Switching Systems can be
categorized by the electronic technology to:
– analog – digital
Types (Domain) There’s also another kind of categorization that is
related to the domain of use (will be discussed in the next slides):
– Global eXchange (GX) – Private Branch eXchange (PBX)
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Telephone Switching System: History
Point-to-point phones in 1876 (Bell) Manual Telephone Switching Center Crossbar (Matrix) Telephone Switching Center Automated Central Telephone Switching Analog Telephone Switching System Digital Telephone Switching System Exchange System
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Telephone Switching System: Some Technical Words (1)
Line A voice link is called a line if it is supposed to connect a phone
(or some systems alternatively) to the telephone network. Each lines contains just one conversation link.
Trunk A voice link is called a trunk if it connects two switching
systems. So a trunk may contain more than one conversation links (multiplexing).
Tip & Ring These are two wires that carries electrical voice signals
(in analog form). The positive one is called Ring and the negative is Tip.
Inbound Signaling If the request and respond signals (like dialing
and ringing) is carried over tip and ring, it is inbound signaling. It is more used in lines where one side is a phoneset.
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Telephone Switching System: Some Technical Words (2)
Outbound Signaling If we add some wires to the tip and ring just
for transmitting the control signals, it is called outbound signaling. In this form, conversation is isolated from other signals. The trunks usually uses outbound signaling because of its higher quality and ease of use.
Conversation Circuit In analog telephone switching system there
are some buses (separated from each other) that may hold conversations and is called conversation circuits (one conversation in one circuit). It means each line / trunk can be connected / disconnected from each bus (like a matrix).
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Telephone Switching System: GX
Global eXchange (GX) is a switching system that is implemented in
high scales to support a lot of lines and trunks. The formal telephone network is implemented by GXs. The local telephone wires that is came to our house is from a GX. These systems are connected hierarchically to build the whole telephone network in a country or in the world. The systems in leaf of that tree is connected to phonesets or alternatively to a PBX.
Connections As is obvious from next slide, a GX is connected to
phonesets (or PBXs) through lines and another GXs through trunks. A digital processing system can observe and control the policies of the system but can be removed later. Monitor panel is a panel to show some signals like current connections.
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Telephone Switching System: GX Connections
Monitor Panel
Phone 1 Phone 2
Line 1 Line 2 GX Switching System Line I … &Trunks Another GX Signals
Phone I
Power Data Processing System
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Telephone Switching System: PBX
Private Branch eXchange (PBX) is a switching system but
usually in much lower scales to support the lines and trunks just for an organization, company or another private complex. It designed for private use i.e to switch some phonesets internally and also for external connections through a few incoming lines (from the local GX). So a hospital, hotel and … can connect all of its phones to the public telephone network just through some few links.
Connections As is obvious from next slide, a PBX is connected to
phonesets (or PBXs) through lines and GXs through trunks. A digital processing system can observe and control the policies of the system but can be removed later. Monitor panel is a panel to show some signals like current connections. Operator is a phoneset-like system that has the ability to control connections specially from trunks to lines and some other permissions.
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Telephone Switching System: PBX Connections
Operator Monitor Panel
Phone 1 Phone 2
Line 1 Line 2 PBX Switching System Line I …
Trunk 1 Trunk 2 …
GX 1 GX 2
Phone I
Trunk J
GX J
Power Data Processing System
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About The Phonesets (1)
On-hook / Off-hook These are two traditional words to express the
physical mode of the phone. When the handset of a phoneset is in its place and can be ringed, it is in on-hook mode. When the handset is in hand (a conversation or dialing is in progress) the mode is off-hook.
Voice The most important aspect of a phoneset is its ability to send
and receive speech signals through two wires: tip and ring. In a handset there is a speaker and a microphone and the tip and ring pass through these components (serially). The switching system is responsible to put a dc power on these wires so the voice signals can be carried over this voltage. It is obvious that in the conversation mode the phoneset shows a dc resistance and lets dc current pass (a dcloop).
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About The Phonesets (2)
Dialing When a phone wishes to connect to another, there must be a
way of addressing the destination. In nowadays telephone networks, each phone (line) has a unique decimal number (with respect to its local). The process of repeatedly sending the digits of destination no is called dialing. There are two dialing modes: – Pulse the phoneset opens and closes the dc-loop in a predefined timing to
send the the dialed digit to switching system.
– Tone the phoneset sends some special wave with predefined timings and
frequencies to show the dialed no. It this mode the dc-loop always exists.
Ringing When a phone wishes to connect to another one, the
switching system sends an ac power to the destination. If the destination phone is in on-hook mode, it rings. In the on-hook mode there isn’t a dc-loop but an ac-loop establishes.
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About The Phonesets (3)
Notifying Tones There are some tones that switching systems send
to phones to inform the user of the result of its request. – Dial tone a continuous wave after the handset pickup. It means it is ready
to accept the dialing.
– Ring back tone a discrete wave synchronous with ring wave to inform
the requester that the requested phone is ringing.
– Busy tone a discrete wave which means the requested phone is busy. – Invalid tone a discrete one which means the dialed number is invalid.
Usually, its frequency is higher than the other tones.
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The Project
Contents
Title Behavior Design Considerations Hardware Software Sample Processes
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Title
“Design and Implementing a 1-to-4 PBX System Using a Microcontroller”.
PBX The project is a telephone switching system and is a Private
Branch eXchange.
1-to-4 It supports 4 lines (connection to phones) and 1 trunk
(connection to local GX).
Microcontroller The controller section is a microcontroller (a digital
processor + internal RAM + internal ROM + internal timers …).
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Behavior (1)
Connections The system is connected to 4 normal phones and 1
external line (from local GX). The monitor panel is the monitoring leds on the project’s boards. The input power is 220V, 50Hz ac voltage. The optional data processing system is not implemented in this version of project (see next slide for details).
Dialing mode The system supports only pulse dialing. It has no
reaction when encountering tone dialing.
Number assignments The system has 5 valid numbers: “0” for the
trunk, “1” for the 1st line, “2” for 2nd, “3” for 3rd and “4” for 4th line. Other numbers are invalid.
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Behavior (2)
Tones The system has 4 tones:
– – – – Dial tone system is ready for dialing. Ring back tone requested phone is ringing. Busy tone requested phone is busy. Invalid tone dialed no is invalid.
Services No special services (like diverting, auto answering, …) exist
in this version of implementation. Note that the design method guaranties adding new services and capabilities.
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Behavior: Connections
Monitor Panel
Phone 1 Phone 2 Phone 3 Phone 4
Line 1 Line 2 Line 3 Line 4 Project’s PBX Switching System
Trunk 1
GX 1
Power Data Processing System
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Design Considerations (1)
Modular design Everything in this project is designed with modular
method. In this method special & predefined tasks are assigned to each subsystem. These subsystems communicate with sending requests and getting answers to each other in order to perform a complex task. This method guaranties the following characteristics: – Readability, writablity, debugging A reader can understand the
mechanisms better. The modifications are easy and the bugs can be discovered quickly.
– Upgrade & development Hardware and software additions (like adding
lines or trunks, adding special services, …) are easy tasks. The project development is bright and straight.
Analog switching The system utilizes the circuit switching with
analog technology. So there are some conversation circuits (i.e 3) to switch connections.
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Design Considerations (2)
Nonblocking system Every phone can be connected to another
one at any desired time. It means a connection won’t be blocked because of lack of resources. There are 2 conversation circuits for internal switching (i.e 4 phones) and 1 for connecting the phones to trunk.
Digital controller A microcontroller from MCS-51TM family is selected
for controlling the whole system. It is observed that even though the switching is analog, the controller is digital making all internal signals (except voice signals) digital.
Trunk ring policy If a ring is sent to the trunk (by the local GX) the
system checks line 1 then 2 … to find first free phone and sends the ring to the found phone in order to connect it to the trunk. Therefore in this project, operator can be neglected.
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Hardware: Structure
Line 1 Line 2 Trunk 1 Line 3 Line 4
Line Unit
Monitoring Signals
Project’s PBX System
Switch Unit
Control Unit
Trunk Unit
Data Signals
Power Supply Unit
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Hardware: Units
Power Supply Unit This unit is responsible for supplying the necessary
voltages that other units need. This unit must supply the voice carrier voltage applied to the phoneset, so must be designed with care.
Line Unit This unit interfaces the system with phonesets. It detects the
dialing & phone mode and also send ring signal. In this unit, tip and ring of each phone (for use in switch unit) is constructed.
Trunk Unit It interfaces with trunk and has responsibilities similar to line
unit. The tip and ring of trunk (for use in switch unit) is constructed.
Switch Unit It has some conversation circuits and switches the phones
and trunk (under control of control unit) i.e their tip and ring wires.
Control Unit This unit controls the whole system. It gets detect signals
from line and trunk units and sends control signals to switch unit and (if necessary) to line and trunk units.
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Hardware: Some Components
Relays The main switch components that is used in this project are
relays. A relay is an electrical-mechanical component capable of attract a contact with an electrical command in order to connect / disconnect some connections. It is like a key with electrical control. Despite of its mechanical problems, it’s an idle component for passing a wide range of signals with different powers. Open collector not gates Just like normal logical not gates, open collector not gates, negates the input signals but with one major difference. The output layer transistors are open collectors, which means are not connected to power voltage. Opto-couplers A semiconductor device which consists of a normal led and a BJT transistor with optically sensitive base. If a current is passes through the led, it stimulated base just like a normal current. It’s main usage in this project, is detecting dc-loop in order to find phoneset mode.
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Hardware: Boards (1)
Power Supply board It produces the following voltages:
– +5Vdc: for the digital (gates and microcontroller) components. – +18Vdc: for carrying the voice signal and also for driving the relays. – 110Vac: This board also generates an ac voltage (around 50Hz) as the ring voltage for applying to phonesets.
Line boards Each line board contains 2 line interface circuits that
interfaces a phoneset to the system and do the following tasks:
– Detecting the phone mode (Off-hook or On-hook), also useful for detecting pulse dialing signals. – Applying the ring to phone, if necessary. – Carry tip and ring wires to switch unit for switching. – Showing the phone mode and phone ring signal by two leds.
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Hardware: Boards (2)
Trunk board The trunk board contains 1 trunk interface circuits that
interfaces a trunk to the system and do the following tasks:
– Detecting the trunk ring signal. – Carry tip and ring wires to switch unit for switching. – Showing the ring signal and connect mode by two leds.
Conversation Circuit boards Each conversation circuit board
contains 1 conversation circuit that is able to create one conversation between some lines or trunk. Control unit controls which relays are active and therefore switching of each circuit.
Control board This board has one microcontroller from MCS-51TM
(AT89C52) that controls the system by scanning the detect signals and retrieving the request information then sending controlling or monitoring signals and if necessary, switching the lines / trunk (via switch unit).
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Control Unit
Control board Line 0&1 board
Line Unit
L0_RNG L0_INTR L0_DTCT L1_RNG L1_INTR L1_DTCT L2_RNG
Line 2&3 board
L0_TIP L0_RING L1_TIP L1_RING
pbx.dsn (Line 0” sheet)“ pbx.dsn (Line 1” sheet)“
L2_INTR L2_DTCT L3_RNG L3_INTR L3_DTCT
L2_TIP L2_RING L3_TIP L3_RING
Hardware: Boards Diagram
VCC +18V Ring 1 Ring 2
pbx.dsn (Line 2” sheet)“ pbx.dsn (Line 3” sheet)“
Power Supply Unit
C0_S0 C0_S1 C0_S2 C0_S3 C0_TONE C1_S0 C1_S1 C1_S2 C1_S3 C1_TONE C2_S0 C2_S1 C2_S2 C2_S3 C2_TONE
Switch Unit
Conversation Circuit 0 board
Power Supply board
pbx.dsn )“Control” sheet(
pbx.dsn (Conversation Ckt 0” sheet)“
Conversation Circuit 1 board
pbx.dsn (Conversation Ckt 1” sheet)“
pbx.dsn )“Power Supply” sheet(
Conversation Circuit 2 board
pbx.dsn (Conversation Ckt 2” sheet)“
Trunk Unit
T0_CNCT T0_RDTCT
Trunk 0 board
T0_TIP
pbx.dsn (Trunk 0” sheet)“
T0_RING
Hardware: Photo
Software: Structure
Line Ports Trunk Ports
Line Hardware Interface Unit
Trunk Hardware Interface Unit
Line Control Unit
Trunk Control Unit
Central Control Unit
Project’s Software Architecture
Switch Control Unit
Ring & Tone Generator Unit
Switch Hardware Interface Unit
Switch Ports
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Software: Units (1)
Line Hardware Interface Unit This unit is responsible for
interfacing the line ports (hardware signals) to the software. Every access to line ports are through this unit. This unit also translate detect signals to some meaningful messages.
Trunk Hardware Interface Unit This unit is similar to Line one but
interfaces the trunk ports.
Switch Hardware Interface Unit This unit interfaces the switch
ports to software or in the other words, any access to switch ports must be through this unit.
Line Control Unit This unit controls each line by getting messages
from line interfaces and central control. In this unit the applied signals to the phone is constructed and it watches over any change concerning with the line (phone).
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Software: Units (2)
Trunk Control Unit Like the line one, this unit controls the trunk. It
receives messages from trunk interface and central control in order to build trunk control signals.
Switch Control Unit This unit has a collection of useful routines for
handling and monitoring the switching.
Central Control Unit This unit is responsible for making interroutine communication and higher policies are held in this unit. This unit applies number assignments and connects the three above control units to each other.
Ring & Tone Generator Unit This unit generates all of the tones
and waves that is needed in the software including: ring wave, dial tone, busy tone, ring back tone and invalid tone.
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Software: Considerations (1)
Concurrency Because of executing some routines (like control units)
in real time mode, we are encountering with a concurrency problem. This can be solved by use of hardware interrupts but this solution is not applicable in all situations where we have some software routines. The used method is time sharing and use of timer interrupt. For simplifying the context switching, all of the concurrent routines are state-based. Whenever a change has been detected in the system, an event is created and will be propagated in the related control units. So the switching between concurrent routines is done by themselves not by forcing and cutting their execution.
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Software: Considerations (2)
State-based routines State-based routines are routines that have
some finite states and are in one of those states in every specific time.When a messages or events is received by these routines, the state will be updated and if necessary some action will be done.
Event-driven routines Whenever a change has been occurred in
the system, an event is created usually by interface routines and will be reported to the other parts of the software. This mechanism guaranties that if and only if something is changed, processor will process it not always. Therefore the processor will have enough time to complete concurrent tasks.
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Software: Phone States
StIdle Normal state of the phone i.e on-hook. StDialTone The handset is picked up and the dial tone must be
applied (ready to dial). StDialing The user is dialing. No tone must be applied. StInvalidTone The requested no is invalid. Invalid tone must be applied. StBusyTone The requested no is busy. Busy tone must be applied. StTrunk The requested no is the trunk and the connection is established. StRingbackTone The requested phone is ringing. Ring back tone must be applied. StTalk A talk (conversation) is established and is in progress. StRing The phone is ringing (requested by another phone).
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EvPutdown
StRingback Tone
EvEOD+EvNoLineRB
EvTalk Accept
EvAllRel EvTalkCancel (EvToneReq(TnNo
(EvToneReq(TnRingback
EvPutdown
StTalk
EvPickup
EvAllRel
EvTalkAccept
Software: Phone State Diagram
EvEOR
StRing
EvConvReq (EvToneReq(TnDial
EvSOR
StIdle
EvPutdown
EvPickup
StDialTone
EvAllRel
EvSOD
StDialing
(EvToneReq(TnNo EvPutdown
EvDigitDialed
EvPutdown
StInvalid Tone
EvAllRel
EvEOD+EvNoInvalid
(EvToneReq(TnInvalid
EvPutdown
StBusy Tone
EvAllRel EvTrunkRel
EvEOD+EvNoLineBusy EvEOD+EvNoTrunkBusy (EvToneReq(TnBusy
EvPutdown
StTrunk
EvEOD+EvNoTrunkOK
Software: Routines (1)
Initialize routines These routines are executed once (usually after a
reset) and will initialize variables and set up the timer settings.
Timer ISR routine When an interrupt is received by the processor, it
will execute this routine that is responsible for invoking the timing routines repeatedly.
Timing routines These routines make the necessary timings like
1ms, 20ms, 1s … and will call the time sensitive routines (like interface routines) with special periods.
Memory Access routines Access to memory is through these
routines. Each routine that needs to restore / retrieve some value (like its state) will call the related memory access routine.
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Software: Routines (2)
Hardware Access routines These routines interface the software
and ports. Any routine that needs to change or read a port will do that via these routines.
Interface routines These routines translate the hardware signals to
events (if required) and will propagate it through the software.
Control routines These routines control the parts of the hardware
(lines, trunk and switches) by attending to events and messages from interface and other control routines.
Debugging routines There are some routines just for debugging
and development of the software. These routines can be removed in the final implementation of the software.
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Sample Processes (1)
Internal connection In this process a line connects to another line.
First, a user pickups the handset (off-hook). The hardware signal is detected by the interface routines and the appropriate event is generated. The state of its line control will be updated and the dial tone will be applied. Now, requester starts to dial, the software finds out the dialed no by watching over the hardware, then sends the no to central control and gets the meaning of the dialed no. Assume it is no of a free line. The destination starts to ring. When the handset is picked up, the connection is established (switched) and voice signals can be transmitted. When two sides put down the handsets (on-hook) the connection is ended and the resources are returned to the system.
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Sample Processes (2)
External connection In this process the trunk is ringed by the local
GX. First the ring is detected by hardware and the interface software makes an event to inform the central control of what has been happened. In this case, according to the trunk ring policy of the system, the first free phone with higher priority will be ringed. When the handset is picked up the connection starts and a dedicated conversation circuit will hold the conversation. When the destination put downs the handset (off-hook), the resources return to the system and the communication ends.
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Summary
Summary (1)
Development Suggestions:
– The switch unit can be implemented using digital technology. It needs major changes in hardware and software. – Tone dialing can be supported by adding some hardware component to line boards. Of course some software modification is needed. – Operator is another possible addition that can be implemented using a new hardware unit and its software drivers. – Lots of services like diverting, auto answering, conferencing … and monitoring and also logging can be supported by system just by adding required software routines.
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Summary (2)
Conclusion:
– This project is an electrical and electronical circuit switching system, a PBX i.e a Private Branch eXchange that is implemented in analog technology. – Mixture of electronics, communication, software and hardware theories and technologies are used to construct the project.
More Information:
– All of project documents are in electronic form and are stored as PC files, please refer to them for more technical information. – For information of the used microcontroller, please refer to the Intel® official site: www.intel.com.
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Behdad Hosseini University of Isfahan, Iran May 2003
Preface
This is the presentation of a project in partial fulfillment of the requirement for the degree of Bachelor of Engineering in hardware engineering presented to Department of Computer Engineering, Engineering Faculty, University of Isfahan. The title of project is “Design and Implementing a 1-to-4 PBX System Using a Microcontroller”. The project started since September 2002 and lasted until May 2003. I’ve done lots of its work in my home without any laboratory facility. So I hope I’ve done something acceptable. There are some useful technical documents about my project like hardware schematics, a project report (but in Farsi) and its software in assembly language. All of them can be found in the project’s web site. I like to acknowledge everybody (especially my family) that helped my in my project.
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Abstract
Telephone centers or more precisely, branch exchange systems, are electrical and electronical systems that are responsible for establishing telephony communications. Their behavior can be explained by making conversations between connected phones. Private Branch eXchanges (PBXs) are some kind of such systems that are mainly used in companies, organizations and other private complexes. In this project, I’ve designed and implemented a PBX system that is capable of switching 4 phones (lines) and an external connection (trunk). A microcontroller from MCS-51TM family is used for handling and controlling the whole system. The switching unit is realized via 3 analog conversation circuits (one for each conversation connection). The hardware is implemented on 8 breadboards and the software is in the MCS-51TM assembly language.
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Introduction
Contents
Switching Theory Telephone Switching System About The Phonesets
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Switching Theory (1)
Switched Network A network consists of some entities (stations)
that can communicate with each other and some controlling systems. If we use some switching nodes instead of connecting every pair of stations, we’ve made a switched network.
Switching means controlling some connections to making favorite
paths. These selections (and its actuators) are under our control. A path can carry any signal from power to speech signals. About our task, assume we have some phones or equally some speech signals. We want to connect these phones to each other so we are concerned about switching these signals.
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Switching Theory (2)
Types of switching:
– Circuit the traditional form of the switching which makes the paths
physically by switches like relays and codecs. It is like an electrically close circuit that connects our two desired points. So we can have any kind of signaling if the frequency response of the path allows us and the current value is supported by the elements. It means we can have analog or digital data.
– Packet this kind of switching makes a virtual path not a physically one.
The data must be digital and encapsulated in packets. A packets is a collection of data bits plus control bits that constructs its frames. The switching can be performed by a simple PC or some specially designed digital processing systems. So we can only transmit digital data or digitized analog data.
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Telephone Switching System: Definition (1)
Definition An electrical and electronical circuit switching system is
called Telephone Switching System if the switched data is in the form of analog speech signals (i.e signals with frequency between 0 and 4kHz) and can understand the request signals like dialing and can also send meaningful informative signals like ringing. The electronic technology of the system doesn’t matter. The important aspect is the system behavior not the system structure. The switching section usually is implemented in circuit switching model and can be analog or can be digital as is done in new high performance systems.
Behavior From more practical point of view, Telephone Switching
Systems are capable of establishing conversation connections between some connected phones or some external links (e.g from another system).
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Telephone Switching System: Definition (2)
Types (Electronics) The Telephone Switching Systems can be
categorized by the electronic technology to:
– analog – digital
Types (Domain) There’s also another kind of categorization that is
related to the domain of use (will be discussed in the next slides):
– Global eXchange (GX) – Private Branch eXchange (PBX)
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Telephone Switching System: History
Point-to-point phones in 1876 (Bell) Manual Telephone Switching Center Crossbar (Matrix) Telephone Switching Center Automated Central Telephone Switching Analog Telephone Switching System Digital Telephone Switching System Exchange System
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Telephone Switching System: Some Technical Words (1)
Line A voice link is called a line if it is supposed to connect a phone
(or some systems alternatively) to the telephone network. Each lines contains just one conversation link.
Trunk A voice link is called a trunk if it connects two switching
systems. So a trunk may contain more than one conversation links (multiplexing).
Tip & Ring These are two wires that carries electrical voice signals
(in analog form). The positive one is called Ring and the negative is Tip.
Inbound Signaling If the request and respond signals (like dialing
and ringing) is carried over tip and ring, it is inbound signaling. It is more used in lines where one side is a phoneset.
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Telephone Switching System: Some Technical Words (2)
Outbound Signaling If we add some wires to the tip and ring just
for transmitting the control signals, it is called outbound signaling. In this form, conversation is isolated from other signals. The trunks usually uses outbound signaling because of its higher quality and ease of use.
Conversation Circuit In analog telephone switching system there
are some buses (separated from each other) that may hold conversations and is called conversation circuits (one conversation in one circuit). It means each line / trunk can be connected / disconnected from each bus (like a matrix).
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Telephone Switching System: GX
Global eXchange (GX) is a switching system that is implemented in
high scales to support a lot of lines and trunks. The formal telephone network is implemented by GXs. The local telephone wires that is came to our house is from a GX. These systems are connected hierarchically to build the whole telephone network in a country or in the world. The systems in leaf of that tree is connected to phonesets or alternatively to a PBX.
Connections As is obvious from next slide, a GX is connected to
phonesets (or PBXs) through lines and another GXs through trunks. A digital processing system can observe and control the policies of the system but can be removed later. Monitor panel is a panel to show some signals like current connections.
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Telephone Switching System: GX Connections
Monitor Panel
Phone 1 Phone 2
Line 1 Line 2 GX Switching System Line I … &Trunks Another GX Signals
Phone I
Power Data Processing System
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Telephone Switching System: PBX
Private Branch eXchange (PBX) is a switching system but
usually in much lower scales to support the lines and trunks just for an organization, company or another private complex. It designed for private use i.e to switch some phonesets internally and also for external connections through a few incoming lines (from the local GX). So a hospital, hotel and … can connect all of its phones to the public telephone network just through some few links.
Connections As is obvious from next slide, a PBX is connected to
phonesets (or PBXs) through lines and GXs through trunks. A digital processing system can observe and control the policies of the system but can be removed later. Monitor panel is a panel to show some signals like current connections. Operator is a phoneset-like system that has the ability to control connections specially from trunks to lines and some other permissions.
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Telephone Switching System: PBX Connections
Operator Monitor Panel
Phone 1 Phone 2
Line 1 Line 2 PBX Switching System Line I …
Trunk 1 Trunk 2 …
GX 1 GX 2
Phone I
Trunk J
GX J
Power Data Processing System
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About The Phonesets (1)
On-hook / Off-hook These are two traditional words to express the
physical mode of the phone. When the handset of a phoneset is in its place and can be ringed, it is in on-hook mode. When the handset is in hand (a conversation or dialing is in progress) the mode is off-hook.
Voice The most important aspect of a phoneset is its ability to send
and receive speech signals through two wires: tip and ring. In a handset there is a speaker and a microphone and the tip and ring pass through these components (serially). The switching system is responsible to put a dc power on these wires so the voice signals can be carried over this voltage. It is obvious that in the conversation mode the phoneset shows a dc resistance and lets dc current pass (a dcloop).
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About The Phonesets (2)
Dialing When a phone wishes to connect to another, there must be a
way of addressing the destination. In nowadays telephone networks, each phone (line) has a unique decimal number (with respect to its local). The process of repeatedly sending the digits of destination no is called dialing. There are two dialing modes: – Pulse the phoneset opens and closes the dc-loop in a predefined timing to
send the the dialed digit to switching system.
– Tone the phoneset sends some special wave with predefined timings and
frequencies to show the dialed no. It this mode the dc-loop always exists.
Ringing When a phone wishes to connect to another one, the
switching system sends an ac power to the destination. If the destination phone is in on-hook mode, it rings. In the on-hook mode there isn’t a dc-loop but an ac-loop establishes.
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About The Phonesets (3)
Notifying Tones There are some tones that switching systems send
to phones to inform the user of the result of its request. – Dial tone a continuous wave after the handset pickup. It means it is ready
to accept the dialing.
– Ring back tone a discrete wave synchronous with ring wave to inform
the requester that the requested phone is ringing.
– Busy tone a discrete wave which means the requested phone is busy. – Invalid tone a discrete one which means the dialed number is invalid.
Usually, its frequency is higher than the other tones.
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The Project
Contents
Title Behavior Design Considerations Hardware Software Sample Processes
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Title
“Design and Implementing a 1-to-4 PBX System Using a Microcontroller”.
PBX The project is a telephone switching system and is a Private
Branch eXchange.
1-to-4 It supports 4 lines (connection to phones) and 1 trunk
(connection to local GX).
Microcontroller The controller section is a microcontroller (a digital
processor + internal RAM + internal ROM + internal timers …).
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Behavior (1)
Connections The system is connected to 4 normal phones and 1
external line (from local GX). The monitor panel is the monitoring leds on the project’s boards. The input power is 220V, 50Hz ac voltage. The optional data processing system is not implemented in this version of project (see next slide for details).
Dialing mode The system supports only pulse dialing. It has no
reaction when encountering tone dialing.
Number assignments The system has 5 valid numbers: “0” for the
trunk, “1” for the 1st line, “2” for 2nd, “3” for 3rd and “4” for 4th line. Other numbers are invalid.
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Behavior (2)
Tones The system has 4 tones:
– – – – Dial tone system is ready for dialing. Ring back tone requested phone is ringing. Busy tone requested phone is busy. Invalid tone dialed no is invalid.
Services No special services (like diverting, auto answering, …) exist
in this version of implementation. Note that the design method guaranties adding new services and capabilities.
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Behavior: Connections
Monitor Panel
Phone 1 Phone 2 Phone 3 Phone 4
Line 1 Line 2 Line 3 Line 4 Project’s PBX Switching System
Trunk 1
GX 1
Power Data Processing System
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Design Considerations (1)
Modular design Everything in this project is designed with modular
method. In this method special & predefined tasks are assigned to each subsystem. These subsystems communicate with sending requests and getting answers to each other in order to perform a complex task. This method guaranties the following characteristics: – Readability, writablity, debugging A reader can understand the
mechanisms better. The modifications are easy and the bugs can be discovered quickly.
– Upgrade & development Hardware and software additions (like adding
lines or trunks, adding special services, …) are easy tasks. The project development is bright and straight.
Analog switching The system utilizes the circuit switching with
analog technology. So there are some conversation circuits (i.e 3) to switch connections.
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Design Considerations (2)
Nonblocking system Every phone can be connected to another
one at any desired time. It means a connection won’t be blocked because of lack of resources. There are 2 conversation circuits for internal switching (i.e 4 phones) and 1 for connecting the phones to trunk.
Digital controller A microcontroller from MCS-51TM family is selected
for controlling the whole system. It is observed that even though the switching is analog, the controller is digital making all internal signals (except voice signals) digital.
Trunk ring policy If a ring is sent to the trunk (by the local GX) the
system checks line 1 then 2 … to find first free phone and sends the ring to the found phone in order to connect it to the trunk. Therefore in this project, operator can be neglected.
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Hardware: Structure
Line 1 Line 2 Trunk 1 Line 3 Line 4
Line Unit
Monitoring Signals
Project’s PBX System
Switch Unit
Control Unit
Trunk Unit
Data Signals
Power Supply Unit
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Hardware: Units
Power Supply Unit This unit is responsible for supplying the necessary
voltages that other units need. This unit must supply the voice carrier voltage applied to the phoneset, so must be designed with care.
Line Unit This unit interfaces the system with phonesets. It detects the
dialing & phone mode and also send ring signal. In this unit, tip and ring of each phone (for use in switch unit) is constructed.
Trunk Unit It interfaces with trunk and has responsibilities similar to line
unit. The tip and ring of trunk (for use in switch unit) is constructed.
Switch Unit It has some conversation circuits and switches the phones
and trunk (under control of control unit) i.e their tip and ring wires.
Control Unit This unit controls the whole system. It gets detect signals
from line and trunk units and sends control signals to switch unit and (if necessary) to line and trunk units.
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Hardware: Some Components
Relays The main switch components that is used in this project are
relays. A relay is an electrical-mechanical component capable of attract a contact with an electrical command in order to connect / disconnect some connections. It is like a key with electrical control. Despite of its mechanical problems, it’s an idle component for passing a wide range of signals with different powers. Open collector not gates Just like normal logical not gates, open collector not gates, negates the input signals but with one major difference. The output layer transistors are open collectors, which means are not connected to power voltage. Opto-couplers A semiconductor device which consists of a normal led and a BJT transistor with optically sensitive base. If a current is passes through the led, it stimulated base just like a normal current. It’s main usage in this project, is detecting dc-loop in order to find phoneset mode.
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Hardware: Boards (1)
Power Supply board It produces the following voltages:
– +5Vdc: for the digital (gates and microcontroller) components. – +18Vdc: for carrying the voice signal and also for driving the relays. – 110Vac: This board also generates an ac voltage (around 50Hz) as the ring voltage for applying to phonesets.
Line boards Each line board contains 2 line interface circuits that
interfaces a phoneset to the system and do the following tasks:
– Detecting the phone mode (Off-hook or On-hook), also useful for detecting pulse dialing signals. – Applying the ring to phone, if necessary. – Carry tip and ring wires to switch unit for switching. – Showing the phone mode and phone ring signal by two leds.
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Hardware: Boards (2)
Trunk board The trunk board contains 1 trunk interface circuits that
interfaces a trunk to the system and do the following tasks:
– Detecting the trunk ring signal. – Carry tip and ring wires to switch unit for switching. – Showing the ring signal and connect mode by two leds.
Conversation Circuit boards Each conversation circuit board
contains 1 conversation circuit that is able to create one conversation between some lines or trunk. Control unit controls which relays are active and therefore switching of each circuit.
Control board This board has one microcontroller from MCS-51TM
(AT89C52) that controls the system by scanning the detect signals and retrieving the request information then sending controlling or monitoring signals and if necessary, switching the lines / trunk (via switch unit).
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Control Unit
Control board Line 0&1 board
Line Unit
L0_RNG L0_INTR L0_DTCT L1_RNG L1_INTR L1_DTCT L2_RNG
Line 2&3 board
L0_TIP L0_RING L1_TIP L1_RING
pbx.dsn (Line 0” sheet)“ pbx.dsn (Line 1” sheet)“
L2_INTR L2_DTCT L3_RNG L3_INTR L3_DTCT
L2_TIP L2_RING L3_TIP L3_RING
Hardware: Boards Diagram
VCC +18V Ring 1 Ring 2
pbx.dsn (Line 2” sheet)“ pbx.dsn (Line 3” sheet)“
Power Supply Unit
C0_S0 C0_S1 C0_S2 C0_S3 C0_TONE C1_S0 C1_S1 C1_S2 C1_S3 C1_TONE C2_S0 C2_S1 C2_S2 C2_S3 C2_TONE
Switch Unit
Conversation Circuit 0 board
Power Supply board
pbx.dsn )“Control” sheet(
pbx.dsn (Conversation Ckt 0” sheet)“
Conversation Circuit 1 board
pbx.dsn (Conversation Ckt 1” sheet)“
pbx.dsn )“Power Supply” sheet(
Conversation Circuit 2 board
pbx.dsn (Conversation Ckt 2” sheet)“
Trunk Unit
T0_CNCT T0_RDTCT
Trunk 0 board
T0_TIP
pbx.dsn (Trunk 0” sheet)“
T0_RING
Hardware: Photo
Software: Structure
Line Ports Trunk Ports
Line Hardware Interface Unit
Trunk Hardware Interface Unit
Line Control Unit
Trunk Control Unit
Central Control Unit
Project’s Software Architecture
Switch Control Unit
Ring & Tone Generator Unit
Switch Hardware Interface Unit
Switch Ports
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Software: Units (1)
Line Hardware Interface Unit This unit is responsible for
interfacing the line ports (hardware signals) to the software. Every access to line ports are through this unit. This unit also translate detect signals to some meaningful messages.
Trunk Hardware Interface Unit This unit is similar to Line one but
interfaces the trunk ports.
Switch Hardware Interface Unit This unit interfaces the switch
ports to software or in the other words, any access to switch ports must be through this unit.
Line Control Unit This unit controls each line by getting messages
from line interfaces and central control. In this unit the applied signals to the phone is constructed and it watches over any change concerning with the line (phone).
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Software: Units (2)
Trunk Control Unit Like the line one, this unit controls the trunk. It
receives messages from trunk interface and central control in order to build trunk control signals.
Switch Control Unit This unit has a collection of useful routines for
handling and monitoring the switching.
Central Control Unit This unit is responsible for making interroutine communication and higher policies are held in this unit. This unit applies number assignments and connects the three above control units to each other.
Ring & Tone Generator Unit This unit generates all of the tones
and waves that is needed in the software including: ring wave, dial tone, busy tone, ring back tone and invalid tone.
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Software: Considerations (1)
Concurrency Because of executing some routines (like control units)
in real time mode, we are encountering with a concurrency problem. This can be solved by use of hardware interrupts but this solution is not applicable in all situations where we have some software routines. The used method is time sharing and use of timer interrupt. For simplifying the context switching, all of the concurrent routines are state-based. Whenever a change has been detected in the system, an event is created and will be propagated in the related control units. So the switching between concurrent routines is done by themselves not by forcing and cutting their execution.
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Software: Considerations (2)
State-based routines State-based routines are routines that have
some finite states and are in one of those states in every specific time.When a messages or events is received by these routines, the state will be updated and if necessary some action will be done.
Event-driven routines Whenever a change has been occurred in
the system, an event is created usually by interface routines and will be reported to the other parts of the software. This mechanism guaranties that if and only if something is changed, processor will process it not always. Therefore the processor will have enough time to complete concurrent tasks.
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Software: Phone States
StIdle Normal state of the phone i.e on-hook. StDialTone The handset is picked up and the dial tone must be
applied (ready to dial). StDialing The user is dialing. No tone must be applied. StInvalidTone The requested no is invalid. Invalid tone must be applied. StBusyTone The requested no is busy. Busy tone must be applied. StTrunk The requested no is the trunk and the connection is established. StRingbackTone The requested phone is ringing. Ring back tone must be applied. StTalk A talk (conversation) is established and is in progress. StRing The phone is ringing (requested by another phone).
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EvPutdown
StRingback Tone
EvEOD+EvNoLineRB
EvTalk Accept
EvAllRel EvTalkCancel (EvToneReq(TnNo
(EvToneReq(TnRingback
EvPutdown
StTalk
EvPickup
EvAllRel
EvTalkAccept
Software: Phone State Diagram
EvEOR
StRing
EvConvReq (EvToneReq(TnDial
EvSOR
StIdle
EvPutdown
EvPickup
StDialTone
EvAllRel
EvSOD
StDialing
(EvToneReq(TnNo EvPutdown
EvDigitDialed
EvPutdown
StInvalid Tone
EvAllRel
EvEOD+EvNoInvalid
(EvToneReq(TnInvalid
EvPutdown
StBusy Tone
EvAllRel EvTrunkRel
EvEOD+EvNoLineBusy EvEOD+EvNoTrunkBusy (EvToneReq(TnBusy
EvPutdown
StTrunk
EvEOD+EvNoTrunkOK
Software: Routines (1)
Initialize routines These routines are executed once (usually after a
reset) and will initialize variables and set up the timer settings.
Timer ISR routine When an interrupt is received by the processor, it
will execute this routine that is responsible for invoking the timing routines repeatedly.
Timing routines These routines make the necessary timings like
1ms, 20ms, 1s … and will call the time sensitive routines (like interface routines) with special periods.
Memory Access routines Access to memory is through these
routines. Each routine that needs to restore / retrieve some value (like its state) will call the related memory access routine.
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Software: Routines (2)
Hardware Access routines These routines interface the software
and ports. Any routine that needs to change or read a port will do that via these routines.
Interface routines These routines translate the hardware signals to
events (if required) and will propagate it through the software.
Control routines These routines control the parts of the hardware
(lines, trunk and switches) by attending to events and messages from interface and other control routines.
Debugging routines There are some routines just for debugging
and development of the software. These routines can be removed in the final implementation of the software.
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Sample Processes (1)
Internal connection In this process a line connects to another line.
First, a user pickups the handset (off-hook). The hardware signal is detected by the interface routines and the appropriate event is generated. The state of its line control will be updated and the dial tone will be applied. Now, requester starts to dial, the software finds out the dialed no by watching over the hardware, then sends the no to central control and gets the meaning of the dialed no. Assume it is no of a free line. The destination starts to ring. When the handset is picked up, the connection is established (switched) and voice signals can be transmitted. When two sides put down the handsets (on-hook) the connection is ended and the resources are returned to the system.
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Sample Processes (2)
External connection In this process the trunk is ringed by the local
GX. First the ring is detected by hardware and the interface software makes an event to inform the central control of what has been happened. In this case, according to the trunk ring policy of the system, the first free phone with higher priority will be ringed. When the handset is picked up the connection starts and a dedicated conversation circuit will hold the conversation. When the destination put downs the handset (off-hook), the resources return to the system and the communication ends.
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Summary
Summary (1)
Development Suggestions:
– The switch unit can be implemented using digital technology. It needs major changes in hardware and software. – Tone dialing can be supported by adding some hardware component to line boards. Of course some software modification is needed. – Operator is another possible addition that can be implemented using a new hardware unit and its software drivers. – Lots of services like diverting, auto answering, conferencing … and monitoring and also logging can be supported by system just by adding required software routines.
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Summary (2)
Conclusion:
– This project is an electrical and electronical circuit switching system, a PBX i.e a Private Branch eXchange that is implemented in analog technology. – Mixture of electronics, communication, software and hardware theories and technologies are used to construct the project.
More Information:
– All of project documents are in electronic form and are stored as PC files, please refer to them for more technical information. – For information of the used microcontroller, please refer to the Intel® official site: www.intel.com.
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