Switches in Madras

All landline phones in Madras have a number with 7 digits. Along with the coutry code ( 91 ) and the Madras area code ( 44 ), the total number of digits is 11, thus satisfying the ITU specification for number of digits in an Internation Direct Dial ( IDD ) phone number.

I have listed the various exchange codes in Madras along with the exchange type. I explain how I found this out at the end. The following information is correct as on 28th April 2001.

220                             OCB 
221 and 223                     EWSD
224                             OCB
226 and 227                     EWSD
230                             AXE10
231 and 232                     OCB
233 - 237                       E10B
238                             OCB
239                             EWSD
240                             OCB
241                             EWSD
242 - 249                       OCB
251                             EWSD
254 and 255                     OCB
371, 372 and 374                EWSD
375                             OCB
376                             EWSD
430 - 434                       EWSD
435 and 436                     AXE10
440                             OCB
441                             E10B
442                             OCB
443                             E10B
445                             OCB     
4458/4459                       IIT Madras ( Tata Telecom IDX PABX )
446                             OCB
447 - 449                       EWSD
450                             OCB
461 and 462                     AXE10
466                             AXE10
467                             OCB
471 - 473                       OCB
474 - 476                       EWSD
477                             E10B
478 and 479                     EWSD
480, 481                        OCB
482 - 484                       E10B
485                             EWSD
486                             OCB
487 and 488                     E10B
489                             EWSD
490 - 494                       E10B
4909                            CorDECT WiLL Digital Interface Unit                             
495                             OCB
496                             EWSD
497                             E10B
498 and 499                     AXE10
501                             OCB - no known subscribers
521                             5ESS
522 and 523                     E10B
524 - 527                       OCB
528                             5ESS
530                             AXE10
531 - 533                       E10B
534 and 535                     OCB
536 - 538                       E10B
539 and 550                     5ESS
551 and 552                     OCB             
554 - 556                       5ESS
557                             E10B
558 and 559                     OCB
572 and 573                     5ESS
574                             OCB - no known subscribers
594                             5ESS
595                             AXE10
596 - 598                       OCB
619 and 620                     AXE10
621                             AXE10
622                             OCB
623                             EWSD
624 and 625                     AXE10
626 - 628                       E10B
629                             OCB
630 and 631                     E10B
632 - 634                       OCB
635                             E10B
637                             AXE10
638 and 639                     E10B
640                             5ESS
641 and 642                     E10B
643                             AXE10
644 and 645                     OCB
646 and 647                     5ESS
648                             OCB
649                             5ESS
650                             OCB
654 and 657                     AXE10
660 and 661                     5ESS
811                             AXE10
820 - 823                       OCB
824 and 825                     E10B
826 - 828                       FETEX Analog SPC
829                             OCB     
841 and 842                     AXE10
844 and 846                     OCB
851 and 852                     OCB
853                             E10B
854                             AXE10
855                             OCB
856                             unknown - PABX
857                             E10B
858 and 859                     OCB

These are all the landline phone exchange codes in Madras. I have tried my best to include all the exchanges. Here's how I did it.

My exchange is an E10B. For all landline phones, it starts analysing the dialled digits as soon as 3 digits are entered. If I were to dial 3 digits which was not the part of any subscriber number in the Madras area, I would get the hunt/call progress tone as soon as I finished dialling the first 3 digits of the number. This would tell me that there is no exchange with a code beginning with the 3 digits that I dialled. This feature of generating a hunt tone the moment there is enough data to decide is specific to my E10B switch, and I have noticed that this does not happen at a friend's place, who is served by an EWSD. This feature helped me to arrive at the conclusion that in the Madras area, there are no phone numbers beginning with the digit '7'. Similarly, if I have left out, say '463' and '464'. that's simply because they don't exist.
How did I identify the exchanges ? Consider this. When I dial a phone which is served by an E10B, the ring tone that I hear from the receiving end will coincide exactly with the ring tone that I will hear if I were to dial another number within my exchange itself. Here I'm not referring to the nature of the ring tone. That is always the same. It is a repeated burr-burr, as opposed to the US where it's a repeated long burrr. But the harmonics making up the ring tone generated by an E10B are different from the harmonics making up the ring tone that an AXE10 generates, and this helps me identify the exchange. So all ring tones that are similar to my exchange's ring tone clearly indicate that the exchange at the other end is an E10B. Logical enough, don't you think ? Fact is, it's true.
Another observation that helps me identify is the process of elimination. My exchange, the E10B, always generates a ring tone that starts perfectly. You never hear half-rings or long rings when you dial my phone number. And more importantly, the ring that you hear is exactly synchronised with the actual ringing of my phone. The OCB, on the other hand, does not generate perfect ring tones, meaning when you dial a phone number that is served by an OCB, you will hear the ringing start with a not-perfect burr-burr. But from the next cycle onwards, the ringing is OK. Only the start is non-perfect. And again, the ringing that you hear does not coincide with the ringing that is actually going on on the receipient's phone. When you are hearing the silence between two sets of burr-burr(s), the phone could actually be ringing at that instant. The design allows re-use of the ring tones, meaning more than 1 simultaneous caller could be hearing the same ring tone. So, conclusion ? If I hear non-perfect ring tones, it's not an E10B or an AXE10 - they never do that.
On a minor note, the E10B does generate a non-perfect ring tone on one condition - the called person is already busy with another call, and has the call waiting feature enabled. If this is the case, the subscriber line interface does not do anything because the called phone is already off-hook, and the T/R line voltage levels are not to be changed because the call is in progress. So, assuming we are calling this person when he is already busy with a call, we hear a ring tone that will not be synchronised with the actual ringing of the phone, simply because the phone will not ring. So, the E10B looks for some other phone legitimately ringing the usual way, and assigns that ring tone generator to you too. So you hear a ring anyway. This is an excellent way of using available resources ( ring tone generators ) as efficiently as possible. This ring tone that you hear will either start imperfectly or will start after a discernable delay. This has helped me conclude many a time that the person whom I'm calling is already on another call, and I usually surprise the other person by my knowledge of the fact that he/she was talking to someone else at that time. This is possible only when the phone you are calling is served by an E10B. An OCB would never generate perfect ring-tones so you wouldn't be able to do this. I have tried to look for patterns when I dial an OCB phone when I go in second, but none have emerged.
I am sure about my identification of the E10B, AXE10, EWSD and the OCB. I had some doubts about the 5ESS, because I did not have conclusive proof that any one of the 5ESS installations was a 5ESS in the first place. I had assumed it to be so because I knew there were only 5 different switches in use in Madras, and the fifth was the 5ESS. So, if I could get proof that one of them was a 5ESS, then the rest would all be surely 5ESSs, because they had identical ring tone characteristics. I got the proof by getting in touch with the DoT, and the people were nice enough to confirm my findings.

The entire exercise of identifying these switches was absolute fun. Countless people in Madras would have cursed when their phones rang and stopped immediately. That was me, listening to the ring tone and identifying the exchange.

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