Switching to VoIP

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Components of the PSTN

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65

Standard tones

Sounds that you hear when the switch needs to notify the caller (or receiver) the

progress of the call are called standard tones. These include the busy tone, the ring-

back tone (the ringing the caller hears while waiting for his call to be connected), and

the dial-tone. Standard tones are also defined by the ITU, in specs E.180 and Q.35.

City codes, area codes, and country codes

COs are grouped into logical, compact geographic areas that are large enough to

support somewhere around five or six million phone numbers apiece. These groups,

known commonly as area codes, are used to help the PSTN and SS7 route calls

between subscribers in remote areas of the network. In this way, a caller from Bos-

ton can reach a caller in Tampa and so forth. Area codes are prepended onto the

local CO portion of the phone number in order to place interarea phone calls. Coun-

try codes are similar prefixes, but for calls between different national PSTN designa-

tions, like the United States and Japan. City codes are a variation on area codes and

are mainly used outside of North America. More information about phone numbers

and international designations can be found in ITU recommendation E.164.

Out-of-Band Signaling and SS7

Common Signaling System 7, also called