mine whether the called party is locally hosted. If the number is in the same switch, the call is established. If it resides in another switch, the task is a bit more complex. First, the local switch must pass the call on to the local tandem switch, which provides access to the points of presence (POPs) of the various long-distance carriers that serve the area. The tandem switch typically does not connect directly to subscribers; it connects to other switches only. It also performs a database query through SS7 to determine who the subscriber's long-distance carrier of choice is so that it can route the call to the appropriate carrier. The tan- dem switch then hands the call off to the long-distance carrier, which transports it over the long-distance network to the carrier's switch in the remote (receiving) office. The long-distance switch passes the call through the remote tandem, which in turn hands the call off to the local switch that the called party is attached to. SS7's influence once again becomes obvious here. One of the problems that occurred in earlier telephone system designs was the following. When a subscriber placed a call, the local switch handed the call off to the tandem, which in turn handed the call off to the long-distance provider. The long-distance provider seized a trunk, over which it trans- ported the dialed digits to the receiving central office. The signaling information, therefore, traveled over the path designed to produce rev- enue for the telephone company, a process known as in-band signaling. As long as the called party was home, and wasn't on the phone, the call 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 Chapter 3 would go through as planned and revenue would flow. If they weren't home, however, or if they were on the phone, then no revenue was gen- erated. Furthermore, the network resources t