In the example of the highrise building with 100 appartments, each with a phone connected to central box, you are going to have to configure the circuit with a cross point switch with 100 lines coming into the ROWs side if the cross point switch. If you then configure the cross point switch with N columns then there is a limit of N concurrent room to room or room to security guard calls. Actually the security phone needs to connect to the ROWS side to so you will need 101 rows in the matrix.

There are ways to configure a matrix that is smaller but then you limit the number of possible concurrent calls to an even lesser number. The way the smaller matrix is done is to connect say each floor of 10 rooms into a 10xM matrix. If support 4 calls per floor at once then this local floor matrix is 10x4. The 4 columns of each floor then go to the central box as the ROW inputs to the central matrix. In case of 100 phones then there are 10 seperate floor matrix modules each sending 4 columns to the central box. The matrix in the main box can now be 40xN (plus 1 for security) 41xN. N needs to be at least twice the size of M in the floor matrix so thus this matrix can be at least 41x8. The columns of the main matrix are used as cross bars to interconnect any two row coming in together so that you can perform floor to floor routing.

It is also common to supply additional columns of the main matrix that bring the tone signals trom the main matrix to/from the floor matrix modules and thus to each phone. These tone signals can consist of dial tone, busy signal, ring back tone and DTMF from the phone to the DTMF detector(s) in the main box. Additional columns can be allocated based upon how many simultaneous DTMF detectors you may wish to be active at once. It may not be necessary that the number of DTMF deectors be as many as the number of active calls you support at once.

Michael Karas