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This is a robust, flexible, extensible approach compared to all its predecessors. In the past exchanges communicated by a wide variety of means, including with the use of special tones on the (analog) voice lines themselves. This precluded signaling between exchanges on those lines when the call was in progress. Other methods included using tones or digital data on inter-exchange lines that were separate from those which carried the calls.

All the exchanges of a particular carrier, including potentially many models of exchange from several manufacturers (although carriers generally prefer to use exchanges from a single manufacturer), are connected to form that carrier's SS7 network. The exchanges connect via multiple 2 Mb/s permanent links, which are carried by fiber, and most of the 64 kb/s 'pipes' within these links are reserved for carrying telephone calls. Some of them are reserved to carry the SS7 traffic. Each 2 Mb/s link is carried as part of a much larger datastream on a pair of fibers - for instance an SDH fiber pair or ring which carries 622 Mb/s.

SS7 is a packet-switched protocol, comparable to TCP/IP which forms the basis of the Internet, but it is less flexible and does not support packets arriving out of order. Each exchange can send a message to any other exchange in the network, and if the destination exchange is not directly connected to the originator, then intermediate exchanges automatically forward the message. Each exchange has two or more links to other exchanges, so there is no single point of failure - in terms of exchanges themselves or inter-exchange links - which could cause the network to fail.

Placing a Call with SS7
SS7 is an efficient way to perform ordinary call setup and termination functions. It also enables the exchanges to work together to perform many complex functions, and to interrogate databases of computers which are connected to the network.

An example of an 'ordinary' function is when a customer in one city dials a number of a customer on the other side of the country. The originating exchange monitors the incoming tone-dial tones and determines when a complete number has been entered. By looking up a database stored in its memory or hard-disk, it determines which exchange the call is destined for. It sends an SS7 message to that exchange, and then the two exchanges send further packets to intermediate exchanges in order to verify that there are spare 64 kb/s 'pipes' and sufficient exchange switching capacity to route the call. This can be a complex matter, since there may several possible paths between two exchanges, and some may be unusable due to congestion at that point in time.

Once the links and switching capacity has been reserved, they are all activated and the terminating exchange sends a ringing signal to the destination phone. At the same time it sends a ringing tone (not necessarily in synchronization with the ringing pulses it sends to the telephone) to the originating telephone via the 64 kb/s 'pipe' which has just been connected via intermediate exchanges.

The ability for the above process to occur reliably on a countrywide basis, often within a second of the customer dialing the last digit of the number, is quite remarkable.

Higher Level SS7 Applications
Examples of a more sophisticated SS7 applications include:

• Routing calls to a national free-call number to the nearest call center, depending on the location of the caller, the time of day, and other factors such as how busy each call center is.
• Forwarding a call from one number to another, potentially dependent on whether the first number is busy or not.
• Routing calls to the current destination of a person's 'personal phone number ' - one on which they can always be reached, but for which they notify the network of any particular number to which such calls should be directed to.
• Providing number portability between carriers, so that a call to a number which is not currently served by the caller's carrier can be re-routed to the exchange of the carrier which currently does provide that service.
• Enabling relatively free interworking between the networks of several carriers in the same country, and with carriers in other countries, including the transfer of call data such as the caller's number, and a Calling Number Display (CND - Caller ID) flag which indicates whether this may be forwarded to the destination telephone service.