has out-of-band signaling on the twenty-fourth channel. This is different from T1 circuits in which signaling is carried in-band along with voice or data on each channel. On data communications, the signaling channel leaves each of the bearer channels "clear" capacity for all 64,000 bits. PRI does not require any bearer channel capacity for signaling such as call setup or teardown of signals. Using PRI Trunks for Caller Identification Many corporations use PRI ISDN for incoming voice traffic. The local telephone com- pany sends the caller's name and phone number over the signaling channel. The tele- phone system captures the information and sends it to the display-equipped telephone. Figure 5-10 illustrates a PRI line for transporting caller ID. Employees who receive heavy volumes of calls from vendors or who prefer to only take calls from certain callers use ISDN to screen calls. Unanswered calls are automatically forwarded into voice mail. Large call centers use PRI ISDN to receive the telephone number of the person calling. With ISDN, the telephone number is sent at the same time as the call. However, it is sent on the separate D, or signaling channel. This is significant because it enables the telephone system to treat the telephone number information differently than the call. It can send the telephone number to a database that matches the telephone number to the customer account number. The data network sends the account number to the agent's terminal to which the call is sent, which saves agents time by eliminating the need to key in account numbers. ISDN-compatible phone system Central office ISDN circuit One signaling channel "Hello, Susan. How are you?" ISDN = Integrated Services Digital Network Figure 5-10 PRI ISDN is typically used in an organization to identify callers' names and phone numbers. Broadband and Wide Area Network Services Sharing Signaling Channels to Enable Circuit- Switched Video Companies with multiple PRI trunks can share the twenty-fourth signaling channel among a group of PRI trunk