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Figure 3-10 DTMF tone pairings
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switch. Want to impress your friends Try pulse dialing on a DTMF telephone. Simply find a phone that has a real button or buttons that you can push down to hang up the phone. Pick up the handset, then dial the number you wish to call by rapidly pushing and releasing the buttons the appropriate number of times, leaving a second or two of delay between each dialed digit. It takes a little practice but it really does work. Try something simple like Information (411) first. DTMF has been around since the 1970s, but switches are still capable of being triggered by pulse dialing. Back to our example. Cristina finishes dialing Adam s number and a digits collector in the switch receives the digits. The switch then performs a rudimentary analysis of the number to determine whether it is served out of the same switch. It does this by looking at the area code (numbering plan area, or NPA) and prefix (NXX) of the dialed number. A telephone number is made up of three sections, shown here.
(802) 555-7837
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Telephony
3
The first three digits are the NPA, which identifies the called region. For example, I live in Vermont where the entire state is served by a single NPA 802. Other states, such as California, have a dozen or more NPAs to serve their much denser population bases. Each NPA identifies a calling area. The second three digits are the NXX, which identify the exchange, or office, that the number is served from, and by extension the switch to which the number is connected. Each NXX can serve 10,000 numbers (555-0000 through 555-9999). Remember the New York fire Twelve exchanges were lost, under which 104,000 subscribers were operating (out of a possible 120,000). A modern central office switch can typically handle as many as 15 to 20 exchanges of 10,000 lines each. In our example, Adam is served out of a different switch than Cristina, so the call must be routed to a different central office. Before that routing can take place, however, Cristina s local switch routes a query to the signaling network, known as Signaling System 7 (SS7). SS7 provides the network with intelligence. It is responsible for setting up, maintaining, and tearing down a call, while at the same time providing access to enhanced services such as Custom Local Area Signaling Services (CLASS), 800 Number Portability, Local Number Portability (LNP), Line Information Database (LIDB) lookups for credit card verification, and other enhanced features. In a sense, it makes the local switch s job easier by centralizing many of the functions that formerly had to be performed locally. The original concept behind SS7 was to separate the actual calls on the public telephone network from the process of setting up and tearing down those calls as a way to make the network more efficient. This had the effect of moving the intelligence out of the PSTN and into a separate network, where it could be somewhat centralized and therefore made available to a much broader population. The SS7 network, shown in Figure 3-11, consists of packet switches (signal transfer points, or STPs) and intelligent database engines (service control points, or SCPs) interconnected to each other and to the actual telephone company switches (service switching points, or SSPs) via digital links, typically operating at 56 to 64 Kbps. When a customer in an SS7 environment places a call, the following process takes place. The local switching infrastructure issues a software interrupt via the SSP so that the called and calling party information can be handed off to the SS7 network, specifically an STP. The STP, in turn, routes the information to an associated SCP, which performs a database lookup to determine whether any special call handling instruc-
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