7 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 Fig- ure 3-11, consists of packet switches (signal transfer points, or STPs) and intelligent database engines (service control points, or SCPs) intercon- nected to each other and to the actual telephone company switches (ser- vice 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- Downloaded from Digital Engineering Library @ McGraw-Hill (www.digitalengineeringlibrary.com) Copyright © 2004 The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. Telephony Telephony Figure 3-11 The SS7 network Switch: Switch: tions apply. For example, if the calling party has chosen to block the delivery of caller ID information, the SCP query will return that fact. Once the SCP has performed its task, the call information is returned to the STP packet switch, which consults routing tables and then selects a route for the call. Upon receipt of the call, the destination switch will cooperate with SS7 to determine whether the called party's phone is available, and if it is, it will ring the phone. If the customer's number is not available due to a busy condition or some other event, a packet will be returned to the source indicating the fact, and SS7 