 con- nectionless and connection-oriented services to these applications. 1 SCCP can be thought of as filling in the OSI layer 3 services that are missing from MTP-3 and adding some layer 4 services. 14.3.4 ISDN user part (ISUP) A peer of the SCCP is the ISDN user part (ISUP). ISUP is used to signal the voice and data calls carried over the PSTN. ISUP does not use SCCP, so ISUP messages are ad- dressed directly to the switch itself instead of to other applications. ISUP has functions that straddle layers 4 through 7 of the OSI model. 14.3.5 Transaction capabilities application part (TCAP) TCAP defines the messages and protocol used to communicate between applications. It is necessary to get the TCAP messages out of the MTP envelope and to direct them to the correct application. TCAP uses SCCP for this routing function. Among the services supported by TCAP are database lookup services such as needed to provide 1-800 ser- vice and calling-card service. TCAP can be thought of as providing a subset of OSI lay- ers 5 and 6 services. 1A connectionless service is a datagram service such as provided by User Datagram Protocol (UDP) over IP and a connection-oriented service is a virtual circuit such as provided by the Trans- mission Control Protocol (TCP) over IP. 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. Signaling with SS7 Key Telephony Concepts 14.3.6 Mobile application part (MAP) MAP provides services to mobile switching centers (MSCs). MAP messages are carried within TCAP messages. MAP is used to communicate between the databases in a bile telephone network and facilitates the authentication of mobile subscribers and the support of roaming. MAP can be thought of as an OSI layer 7 service. MAP is used to support ANSI-41, which is the topic of Chap. 15. 14.3.7 Base station system application part (BSSAP) BSSAP provides similar functionality to