Routing Code and Signaling System:
Routing Code and Signaling System – When dialing a subscriber in another part of the world, it is essential to identify the wanted telephone number uniquely, so that the international telephone network selects that number and no other. It simply would not do if a subscriber dialed the number 2345678 in New York from Boston and got the number 2345678 in Antwerp, Belgium, instead. Thus each country (or continent, in the case of North America) has a numbering scheme with unique area codes.
For example, the area code for New York is 212, that for Montreal, Canada, is 514, and so on. Again, countries must also have their unique codes, and these have been allocated in the CCITT World Plan. For example, North America has the country code 1, Australia 61 and Israel 972. An Australian subscriber must dial the digit sequence 0011 when making an international telephone call; different access digits are required in other countries, often consisting of fewer numbers. A subscriber in Sydney dialing a counterpart in New York would dial:
And, needless to say, the number is dialed smoothly and continuously, such as: 00111212921ABCD. The access digits are to tell the outgoing national network that this will be an international call, and the country code states where the call is going. The rest of the number is the same as would be dialed by a subscriber in North America residing outside the New York local zone.
In order for the wanted subscribers in the call described above to be interconnected, signaling systems must exist to send on the appropriate digits, ensuring that correct Routing Code and Signaling System is achieved. A number of signaling systems are in use around the world. The most common ones for national signaling are the decadic and Multifrequency Coding (MFC), while for international signaling CCITT No. 5 and No. 6 are internationally agreed. In the decadic system, which is on the way out in most countries, dc pulses are sent on the signaling circuit connected to the telephone, with a number of pulses equal to the digit dialed. In MFC, combinations of two tones out of 700, 900, 1100, 1300, 1500 and 1700 Hz are used to define each digit, and such supervisory signals as subscriber busy or no circuits available.
The signaling system is compelled, in that the receiving office acknowledges each digit sent. Such a system is not practicable for international dialing because of the propagation delays mentioned previously, which would tie up signaling and common equipment of telephone exchanges far too long. Thus the CCITT No. 5 system is used instead. This is also an MFC system, but here only the control signals are compelled, not the actual digits sent.
All the systems so far described use the actual telephone circuits for the signaling functions, before and after the telephone call. CCITT No. 6 is the first international signaling system which uses common-channel signaling. Here signaling circuits are established between the computers controlling each pair of inter-working telephone exchanges. These common channels are used exclusively for signaling, and telephone circuits themselves are used only for voice (or data). The international use of CCITT No. 6 was pioneered by the United States, Australia and Japan, during the late 1970s; CCITT is currently evolving a new common-channel signaling system, No. 7. Finally, it should be noted that the foregoing remarks generally apply also to telex, although the signaling systems themselves are somewhat different from those used for telephony.