p of the TCAP as a means of supporting the different application service entities for mobile users. Base Station Systems Application Part (BSSAP) is a combination of the BSSMAP and the Direct Transfer Application Part (DTAP). Base Station Systems Mobile Application Part (BSSMAP) transmits messages that the BSC must process. This applies generally to all messages to and from the MSC where the MSC participates in radio resource (RR) management. 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. Function of GPRS Elements Chapter 4 DTAP transports messages between the mobile and the MSC, where the BSC is just a relay function, transparent for the messages. These messages deal with MM and connection management (CM). Domain Name System (DNS) A GPRS network will likely be connected to the Internet. Each registered user who wants to exchange data packets with the IP network gets an IP address. The IP address is taken from the address space of the GPRS oper- ator. In order to support a large number of mobile users, it is essential to use dynamic IP address allocation (in IPv4). Thus, a Dynamic Host Configura- tion Protocol (DHCP) server is installed. The address resolution between IP address and GSM address is performed by the GGSN, using the appropri- ate PDP context. The routing of IP packets are tunneled through the intra- PLMN backbone with the GTP. Moreover, a Domain Name Server (DNS) managed by the GPRS operator or the external IP network operator can be used to map between external IP addresses and hostnames. To protect the PLMN from unauthorized access, a firewall is installed between the private GPRS network and the external IP network. With this configuration, GPRS can be seen as a wireless extension of the Internet all the way to a mobile station or mobile computer. The mobile user has a direct connection to the Internet. To exchange data pac