TELEPHONE PBX Presented By M.Rajivgandhi M.E.,(Applied Electronics) 11/25/2020

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 11/25/2020

ROLM PBX The ROLM PBX was based on a general-purpose computer. Application Example Line card CPU Memory Line card 11/25/2020

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. 11/25/2020

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. 11/25/2020

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. 11/25/2020

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) 11/25/2020

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* 11/25/2020

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. 11/25/2020

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. 11/25/2020

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. 11/25/2020

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 11/25/2020

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. 11/25/2020

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 ………… 11/25/2020

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. 11/25/2020

Dual-tone Multifrequency (DTMF) 3 1 2 6 4 5 9 7 8 * # 11/25/2020

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. 11/25/2020

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. 11/25/2020

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. 11/25/2020

System Architecture Basic hardware architecture of PBX Telephone Line Card Line Card Line Card Call data ISA bus CPU 11/25/2020

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); } 11/25/2020

THANK YOU 11/25/2020