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Design and ImplementingDesign and Implementinga 1-to-4 PBX Systema 1-to-4 PBX System

Using a MicrocontrollerUsing a Microcontroller

Behdad Hosseini

University of Isfahan, Iran

May 2003

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May 20031-to-4 PBX Project2

PrefacePreface

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.

Ive done lots of its work in my home without any laboratory facility. So I

hope Ive done something acceptable.

There are some useful technical documents about my project likehardware schematics, a project report (but in Farsi) and its software in

assembly language. All of them can be found in the projects web site.

I like to acknowledge everybody (especially my family) that helped my

in my project.

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AbstractAbstract

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 bymaking 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, Ive 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 handlingand 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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IntroductionIntroduction

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ContentsContents

Switching TheorySwitching Theory

Telephone Switching SystemTelephone Switching System

About The PhonesetsAbout The Phonesets

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Switching Theory (1)Switching Theory (1)

Switched NetworkSwitched 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 ofstations, weve made a switched network.

SwitchingSwitching means controlling some connections to making favoritepaths. 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 concernedabout switchingthese signals.

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7/48May 20031-to-4 PBX Project7

Switching Theory (2)Switching Theory (2)

Types of switching:Types of switching:

CircuitCircuit the traditional form of the switching which makes the paths

physically by switches like relays and codecs. It is like an electrically closecircuit 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.

PacketPacket 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:Telephone Switching System:

Definition (1)Definition (1)

DefinitionDefinition An electrical and electronical circuit switching system iscalled Telephone Switching System if the switched data is in the form

of analog speech signals (i.e signals with frequency between 0 and4kHz) and can understand the request signals like dialing and can also

send meaningful informative signals like ringing. The electronic

technology of the system doesnt 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.BehaviorBehaviorFrom more practical point of view, Telephone SwitchingSystems are capable of establishing conversation connections

between some connected phones or some external links (e.g from

another system).

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Definition (2)Definition (2)

Types (Electronics)Types (Electronics) The Telephone Switching Systems can becategorized by the electronic technology to:

analog digital

Types (Domain)Types (Domain) Theres also another kind of categorization that isrelated to the domain of use (will be discussed in the next slides):

Global eXchange (GX)

Private Branch eXchange (PBX)

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HistoryHistory

Point-to-point phones in 1876 (Bell)Point-to-point phones in 1876 (Bell)

Manual Telephone Switching CenterManual Telephone Switching Center

Crossbar (Matrix) Telephone Switching CenterCrossbar (Matrix) Telephone Switching Center

Automated Central Telephone SwitchingAutomated Central Telephone Switching

Analog Telephone Switching SystemAnalog Telephone Switching System

Digital Telephone Switching SystemDigital Telephone Switching System

Exchange SystemExchange System

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Some Technical Words (1)Some Technical Words (1)

LineLine 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.TrunkTrunk A voice link is called a trunk if it connects two switchingsystems. So a trunk may contain more than one conversation links

(multiplexing).

Tip & RingTip & 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 SignalingInbound Signaling If the request and respond signals (like dialingand 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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Some Technical Words (2)Some Technical Words (2)

Outbound SignalingOutbound Signaling If we add some wires to the tip and ring justfor transmitting the control signals, it is called outbound signaling. In

this form, conversation is isolated from other signals. The trunksusually uses outbound signaling because of its higher quality and ease

of use.

Conversation CircuitConversation Circuit In analog telephone switching system thereare 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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GXGX

Global eXchange (GX)Global eXchange (GX) is a switching system that is implemented inhigh scales to support a lot of lines and trunks. The formal telephone

network is implemented by GXs. The local telephone wires that iscame 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.

ConnectionsConnections As is obvious from next slide, a GX is connected to

phonesets (or PBXs) through lines and another GXs through trunks. Adigital 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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GX ConnectionsGX Connections

GX Switching System

Data Processing

System

Monitor Panel

Power

Trunks&

Another GX

Phone 1Line 1

Phone 2Line 2

Phone ILine I

Signals

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PBXPBX

Private Branch eXchange (PBX)Private Branch eXchange (PBX) is a switching system butusually in much lower scales to support the lines and trunks just for an

organization, company or another private complex. It designed forprivate 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.

ConnectionsConnections As is obvious from next slide, a PBX is connected to

phonesets (or PBXs) through lines and GXs through trunks. A digitalprocessing 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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PBX ConnectionsPBX Connections

PBX Switching System

Trunk 1GX 1

Data Processing

System

Operator Monitor

Panel

Power

Trunk 2GX 2

Trunk JGX J

Phone 1Line 1

Phone 2Line 2

Phone ILine I

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About The Phonesets (1)About The Phonesets (1)

On-hook / Off-hookOn-hook / Off-hook These are two traditional words to express thephysical 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 inhand (a conversation or dialing is in progress) the mode is off-hook.

VoiceVoice The most important aspect of a phoneset is its ability to sendand 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 canbe carried over this voltage. It is obvious that in the conversation mode

the phoneset shows a dc resistance and lets dc current pass (a dc-

loop).

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DialingDialing When a phone wishes to connect to another, there must be away of addressing the destination. In nowadays telephone networks,

each phone (line) has a unique decimal number (with respect to itslocal). The process of repeatedly sending the digits of destination no is

called dialing. There are two dialing modes:

PulsePulse the phoneset opens and closes the dc-loop in a predefined timing tosend the the dialed digit to switching system.

ToneTone the phoneset sends some special wave with predefined timings and

frequencies to show the dialed no. It this mode the dc-loop always exists.

RingingRinging When a phone wishes to connect to another one, theswitching 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 isnt a dc-loop but an ac-loop establishes.

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Notifying TonesNotifying Tones There are some tones that switching systems sendto phones to inform the user of the result of its request.

Dial toneDial tone a continuous wave after the handset pickup. It means it is readyto accept the dialing.

Ring back toneRing back tone a discrete wave synchronous with ring wave to informthe requester that the requested phone is ringing.

Busy toneBusy tone a discrete wave which means the requested phone is busy.

Invalid toneInvalid 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 ProjectThe Project

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ContentsContents

TitleTitle

BehaviorBehavior

Design ConsiderationsDesign Considerations

HardwareHardware

SoftwareSoftware

Sample ProcessesSample Processes

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TitleTitle

Design and Implementing a 1-to-4 PBX System Using a

Microcontroller.

PBXPBX The project is a telephone switching system and is a PrivateBranch eXchange.

1-to-41-to-4 It supports 4 lines (connection to phones) and 1 trunk(connection to local GX).

MicrocontrollerMicrocontrollerThe controller section is a microcontroller (a digital

processor + internal RAM + internal ROM + internal timers ).

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Behavior (1)Behavior (1)

ConnectionsConnections The system is connected to 4 normal phones and 1external line (from local GX). The monitor panel is the monitoring leds

on the projects 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 modeDialing mode The system supports only pulse dialing. It has noreaction when encountering tone dialing.

Number assignmentsNumber assignments The system has 5 valid numbers: 0 for thetrunk, 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)Behavior (2)

TonesTones The system has 4 tones:

Dial toneDial tone system is ready for dialing.

Ring back toneRing back tone requested phone is ringing. Busy toneBusy tone requested phone is busy.

Invalid toneInvalid tone dialed no is invalid.

ServicesServices No special services (like diverting, auto answering, ) existin this version of implementation. Note that the design method

guaranties adding new services and capabilities.

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Behavior:Behavior:

ConnectionsConnections

Projects PBX Switching

System

Trunk 1GX 1

Data Processing

System

Monitor Panel

Power

Phone 1Line 1

Phone 2Line 2

Phone 4

Line 4

Phone 3Line 3

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Design Considerations (1)Design Considerations (1)

Modular designModular design Everything in this project is designed with modularmethod. In this method special & predefined tasks are assigned to

each subsystem. These subsystems communicate with sendingrequests and getting answers to each other in order to perform a

complex task. This method guaranties the following characteristics:

Readability, writablity, debuggingReadability, writablity, debugging A reader can understand themechanisms better. The modifications are easy and the bugs can be

discovered quickly.

Upgrade & developmentUpgrade & development Hardware and software additions (like addinglines or trunks, adding special services, ) are easy tasks. The project

development is bright and straight.

Analog switchingAnalog switching The system utilizes the circuit switching withanalog technology. So there are some conversation circuits (i.e 3) to

switch connections.

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Design Considerations (2)Design Considerations (2)

Nonblocking systemNonblocking system Every phone can be connected to anotherone at any desired time. It means a connection wont be blocked

because of lack of resources. There are 2 conversation circuits forinternal switching (i.e 4 phones) and 1 for connecting the phones to

trunk.

Digital controllerDigital controllerA microcontroller from MCS-51TM family is selectedfor 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 policyTrunk ring policy If a ring is sent to the trunk (by the local GX) thesystem 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:Hardware:

StructureStructure

Control Unit

Line Unit

Trunk Unit

Power

Supply

Unit

Switch Unit

ProjectsPBX

System

Monitoring

Signals

Data

Signals

Line1

Line2

Line4

Line3

Trunk

1

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Hardware:Hardware:

UnitsUnits

Power Supply UnitPower Supply Unit This unit is responsible for supplying the necessaryvoltages that other units need. This unit must supply the voice carrier

voltage applied to the phoneset, so must be designed with care.

Line UnitLine Unit This unit interfaces the system with phonesets. It detects thedialing & phone mode and also send ring signal. In this unit, tip and ring of

each phone (for use in switch unit) is constructed.

Trunk UnitTrunk Unit It interfaces with trunk and has responsibilities similar to lineunit. The tip and ring of trunk (for use in switch unit) is constructed.

Switch UnitSwitch Unit It has some conversation circuits and switches the phonesand trunk (under control of control unit) i.e their tip and ring wires.

Control UnitControl Unit This unit controls the whole system. It gets detect signalsfrom line and trunk units and sends control signals to switch unit and (if

necessary) to line and trunk units.

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Hardware:Hardware:

Some ComponentsSome Components

RelaysRelays The main switch components that is used in this project arerelays. A relay is an electrical-mechanical component capable ofattract 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, its an idle component for passinga wide range of signals with different powers.

Open collector not gatesOpen collector not gates Just like normal logical not gates, opencollector not gates, negates the input signals but with one majordifference. The output layer transistors are open collectors, which

means are not connected to power voltage.Opto-couplersOpto-couplers A semiconductor device which consists of a normalled and a BJT transistor with optically sensitive base. If a current ispasses through the led, it stimulated base just like a normal current.Its main usage in this project, is detecting dc-loop in order to findphoneset mode.

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Hardware:Hardware:

Boards (1)Boards (1)

Power Supply boardPower 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 boardsLine boards Each line board contains 2 line interface circuits thatinterfaces 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:Hardware:

Boards (2)Boards (2)

Trunk boardTrunk board The trunk board contains 1 trunk interface circuits thatinterfaces 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 boardsConversation Circuit boards Each conversation circuit boardcontains 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 boardControl 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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Software:Software:

StructureStructure

Trunk Hardware Interface Unit

Trunk Control Unit

Trunk Ports

Ring & Tone Generator

Unit

Central Control Unit

Projects

Software

Architecture

Switch Hardware Interface Unit

Switch Control Unit

Switch Ports

Line Hardware Interface Unit

Line Control Unit

Line Ports

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Software:Software:

Units (1)Units (1)

Line Hardware Interface UnitLine Hardware Interface Unit This unit is responsible forinterfacing 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 UnitTrunk Hardware Interface Unit This unit is similar to Line one butinterfaces the trunk ports.

Switch Hardware Interface UnitSwitch Hardware Interface Unit This unit interfaces the switchports to software or in the other words, any access to switch ports

must be through this unit.

Line Control UnitLine Control Unit This unit controls each line by getting messagesfrom 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:Software:

Units (2)Units (2)

Trunk Control UnitTrunk Control Unit Like the line one, this unit controls the trunk. Itreceives messages from trunk interface and central control in order to

build trunk control signals.

Switch Control UnitSwitch Control Unit This unit has a collection of useful routines forhandling and monitoring the switching.

Central Control UnitCentral Control Unit This unit is responsible for making inter-routine 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 UnitRing & Tone Generator Unit This unit generates all of the tonesand 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:Software:

Considerations (1)Considerations (1)

ConcurrencyConcurrency 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 notby forcing and cutting their execution.

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Software:Software:

Considerations (2)Considerations (2)

State-based routinesState-based routines State-based routines are routines that havesome 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 routinesEvent-driven routines Whenever a change has been occurred inthe 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 timeto complete concurrent tasks.

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Software:Software:

Phone StatesPhone States

StIdleStIdle Normal state of the phone i.e on-hook.

StDialToneStDialTone The handset is picked up and the dial tone must beapplied (ready to dial).

StDialingStDialing The user is dialing. No tone must be applied.

StInvalidToneStInvalidTone The requested no is invalid. Invalid tone must beapplied.

StBusyToneStBusyTone The requested no is busy. Busy tone must be applied.

StTrunkStTrunk The requested no is the trunk and the connection isestablished.

StRingbackToneStRingbackTone The requested phone is ringing. Ring back tonemust be applied.

StTalkStTalk A talk (conversation) is established and is in progress.

StRingStRing The phone is ringing (requested by another phone).

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Software: Phone State DiagramSoftware: Phone State Diagram

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Software:Software:

Routines (1)Routines (1)

Initialize routinesInitialize routines These routines are executed once (usually after areset) and will initialize variables and set up the timer settings.

Timer ISR routineTimer ISR routine When an interrupt is received by the processor, itwill execute this routine that is responsible for invoking the timingroutines repeatedly.

Timing routinesTiming routines These routines make the necessary timings like1ms, 20ms, 1s and will call the time sensitive routines (like interface

routines) with special periods.

Memory Access routinesMemory Access routines Access to memory is through theseroutines. Each routine that needs to restore / retrieve some value (like

its state) will call the related memory access routine.

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Software:Software:

Routines (2)Routines (2)

Hardware Access routinesHardware Access routines These routines interface the softwareand ports. Any routine that needs to change or read a port will do that

via these routines.

Interface routinesInterface routines These routines translate the hardware signals toevents (if required) and will propagate it through the software.

Control routinesControl 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 routinesDebugging routines There are some routines just for debuggingand development of the software. These routines can be removed in

the final implementation of the software.

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Sample Processes (1)Sample Processes (1)

Internal connectionInternal 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 betransmitted. 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)Sample Processes (2)

External connectionExternal connection In this process the trunk is ringed by the localGX. 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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SummarySummary

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Summary (1)Summary (1)

Development Suggestions: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)Summary (2)

Conclusion: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: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 Intelofficial site: www.intel.com.
http://www.intel.com/http://www.intel.com/

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