1. TELEPHONE PBX Presented By M.Rajivgandhi M.E.,(Applied Electronics)
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2. WHY PBX?
A telephone PBX(Private Branch Exchange) presents an interesting array of problems to the embedded system designer such as
High data rates
Sophisticated functionality
Low cost
High reliability
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3. ROLM PBX The ROLM PBX was based on a general-purpose computer.
Application Example
Line card
CPU
Memory
Line card
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4. ROLM PBX(Cont...) ROLM uses two buses:
1.The CPU has one bus for data and instruction fetching.
2. The telephone interfaces( called line cards) were attached to a separate bus.
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5. Telephone Answering Machine
The answering machine will be connected to a telephone subscriber line.
At the other end of the subscriber lines the central office.
All information is carried on the phone line in analog form over a pair of wires.
In addition to ADC and DAC to send and receive voice data, we need to sense two other characteristics of the line.
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6. Telephone Answering Machine(Cont…)
Ringing: The central office sends a ringing signal to the telephone when a call is waiting.
The ringing signal is in fact a 90V RMS sinusoid, but we can use analog circuitry to produce 0 for no ringing and 1 for ringing.
Off-hook: The telephone industry term for answering a call is going off-hook.
On-hook: The technical term for hanging up is going on-hook.
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7. Telephone Answering Machine(Cont…)
Off-hook= telephone is active
On-hook=telephone is not in use(but the terminology starts to make sense after a few uses)
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8. Class Diagram for Answering Machine:1
Microphone* 1
Outgoing-message
Record * *
Controls 1 1
Line-in* *
Playback 1 1 1
Line-out* 1
Incoming-message * 1
Buttons* 1
Speaker* 1
Lights*
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9. Operational Classes The major operational classes- Controls Records
Playback
The Controls class provides an operate() behavior.
The record and Playback classes provide behaviors that handle wiring and reading sample sequence.
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10. Theory of Operation of PBX
The first problem is to understand the procedure for completing calls.
Each distinct telephone connection is called a line, so a call is completed by connecting two lines.
A telephone can be on-hook or off-hook and that telephone voice data are sampled at an 8khz rate.
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11. Theory of Operation(Cont…)
When making a call , actions are required on each of the two lines to be connected
One must initiate the call by dialing a number, while the other side must pick up the phone.
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12. State Transition Diagram for Placing and Receiving Calls
Take off-hook
Incoming call
Receive call
Rcv ringing
Call
On-hook
Return on-hook
Other phone off-hook
Take off-hook
Off-hook
Dial decoder ready
Dial tone
Place ringing
Dialing
Number complete
Place call
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13. Contd…
When the PBX recognizes that it has received a complete call, it goes into its own ringing state and waits for an answer.
The rec ringing and place ringing states are distinct even though they both denote a line ringing.
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14. The PBX as a set of coordinated lines
Each line is represented by a state machine.
An additional state machine takes care of the coordination and interaction between lines.
Call coordination
Line 1
Line 2
Line n
…………
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15. Contd…
One of the important steps in placing a call is routing the call based on the telephone number.
If the telephone call is to another line within the PBX, the call is sent there directly.
If the call is outside the PBX, then it must be routed through an external line(called a trunk line) to another telephone switching system.
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16. Dual-tone Multifrequency (DTMF)3 1 2 6 4 5 9 7 8 * #
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17. Contd… We use DTMF dialing rather than pulse dialing.
Each key on a DTMF dialing pad generates two tones when depressed.
One to designate its row and another for its column.
The PBX senses digits dialed by looking for pairs of frequencies.
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18. Contd… The PBX must also collect billing information for all calls.
This function requires keeping track of the destination number and duration of each call.
The length of a call must be determined accurately to create accurate billing statements.
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19. Contd…
Finally, the PBX must support standard maintenance operations, such as
Adding new telephone lines and corresponding phone numbers,
Updating call routing information
Testing the PBX.
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20. System Architecture Basic hardware architecture of PBX Telephone
Line Card
Line Card
Line Card
Call data
ISA bus
CPU
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21. Software Coding
for(i=0;i<n_calls; i++){ /*transfer data from line 1 to line 2*/ data=read_line(call[i].line1); write_line(call[i].line2,data); /* transfer data from line 2 to line 1 */ data=read_line(call[i].line2); write_line(call[i].line1,data); }
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22. THANK YOU
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