Short and simple about the complex - the service "8-800" (Freephone)

Service "8-800" or "free call" or Freephone (Toll-free) is known to everyone. We often choose the 800 series number to call, knowing that this call will be free for us.

But how does this work?
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Those who wish to read more about the history of the emergence and development of the service “8-800” I dare to send to Wikipedia or, even better, to the classic edition of B.S. Goldstein, etc. “Intellectual networks” .

And we will try to briefly and simply understand how this service works.

First of all, let me formulate the essence of the service "8-800" as a "forwarding service". This means that the numbers 8-800, in fact, do not exist - they are virtual. All calls to 8-800 numbers are redirected to real landline or mobile numbers. This is similar to the domain name system, when each text entry of the form, for example, “www.name.ru” corresponds to a specific IP address of the server.

In the recent past, this was implemented by simply substituting a pair of redirections on the MgMn switch, for example, as (simplified):

78001111111 → 74951234567

This means that all calls to number 8 (800) 111-11-11 will be unconditionally redirected to number 8 (495) 123-45-67. However, today it is rarely anyone who does this. Such a scheme is difficult to implement and support - after all, the MgMn switch for each operator is not one. This is a whole network and, not infrequently, of dissimilar equipment, each switch had to be manually configured.

Today, the so-called “intellectual redirection” reigns. “Intellectual” means redirect depending on conditions. As a condition for call forwarding, you can use the day of the week, the time of day, the called party’s belonging to a specific region, the load on the call forwarding numbers, and much more. In order to analyze these conditions and select the required redirect number, a separate telecommunication server is usually used - SCP ( Service Control Point in IN SS7 terminology). This server receives requests for routing calls to a specific number 8-800 from the Mgmd switch, and in response, after analyzing the conditions, it returns the forwarding target number for each case. Consider an example of a call to number 8-800 in a simplified scheme.



Subscriber A dials the number 8 (800) 111-11-11. The call through the PSTN goes to the MgMn operator switchboard (1). The switch queries the SCP 8800 server for the call routing number (2). The SCP 8800 server finds the required call forwarding number in the rules database (that same pair 78001111111-74951234567), additionally checking various conditions (if they were specified) and informs the RedWe switch to the forwarding number (3). The MgMn switch finds the MUS to establish the call (4). The ICC sets up a call to number 8 (495) 123-45-67. The voice channel is switched on (6). A MSC (mobile switch) can also be used as an ICC if the forwarding number is a mobile or SIP gateway, if the forwarding address is a SIP. Once again, the scheme is as simple as possible. Of course, there is also the network of the operator who owns the number A, its mobile, local and MgMn switches.

I propose to consider in detail the rules of intellectual redirection and practical examples of their use in the next article.