e owned by the LEC. Connection to an access tandem would permit an IXC access to all the subtending class 5 offices. Some carriers, most notably AT&T, would continue to connect directly to class 5 offices. Whichever access arrangement was chosen, the access point itself was referred to as the point of presence (POP). The class 4 offices owned by the IXCs now had the responsibility of tracking for billing purposes the calls that were routed to the IXC's network. For a long time after the AT&T breakup, this capability was not available, and carriers, particularly AT&T, paid the LEC to render its bills. Section 11.1.1 described how the telephone communicated the desired telephone num- ber to the local exchange office. If the local exchange office needed to send the call to an- other office, it also had to communicate the desired telephone number to that office. A method still in use today to accomplish this is to signal directly over the trunk selected for the purpose. A detailed description of the technique used will be left to Chap. 13. This method does not provide, however, the rich feature set available in the network today and largely has been replaced by an out-of-band signaling approach, commonly called common channel interoffice signaling (CCIS). CCIS uses a data network to communicate the desire to set up a call through the various switches in the call path. It is possible to reserve all the trunks needed to complete the call before switching occurs. This data network uses an internationally standardized protocol called Signaling System 7, commonly abbreviated SS7. This signaling system is essentially a datagram service designed with redundancy for high reliability. When the Bell System was bro- ken up, each operating company operated its own signaling system. Calls placed be- tween LEC and IXC traversed the POP using an access protocol (called Feature Group D) that was based on signaling over the interconnecting trunks. Today, SS7 is used com- monly to communicate between LECs and IXCs for a more seamless 