Packet Data elements. Network Intra-PLMN Intra-PLMN Backbone Backbone GPRS PLMN GPRS PLMN Inter-PLMN Backbone 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 SS7/IP Gateway Functions The MSC is the central-switching function of the GSM network. The MSC is connected to a SS7 network for the purpose of signaling and performing database queries. The SS7 network uses a network node called the Signal Transfer Point (STP), which is a packet-switching node (can be SS7, IP, or X.25). Using a 64-Kbps channel connection between STPs, the network can process its signaling information. Next in a SS7 network is the use of the Signal Control Point (SCP) that houses the databases congruent to the network. In many cases, these databases interact with the HLR, VLR, Equipment Identity Register (EIR), Authentication Center (AuC), and Public-Switched Telephone Network (PSTN) nodes. The SCP is used when- ever a Global Title Translation is required, which converts numbers ([800] 322-2202 equates to [480] 706-0912) and whenever the Mobile Application Part (MAP) is used. These services link across an SS7 interface. In order to communicate to the legacy-based, circuit-switched nodes (MSC, HLR, SMSC, and SO on), according to GPRS standards, SGSNs are supposed to use MAP/BSSAP+ on SS7. Because of the emphasis on the Internet and IP protocols, several providers have chosen to develop MAP/BSSAP+ over IP in the SGSN using a separate SS7 Gateway server to perform the conversion between MAP on IP to MAP on SS7 Message Transfer Part (MTP). The SS7/IP Gateway server, or (SIG), as shown in Fig- ure 4-12, provides interworking between GPRS nodes in an IP network and GSM nodes in an SS7 network. Because multiple SGSNs exist in the GPRS network, the SIG is responsible for routing messages from the GSM HLR to the correct SGSN. Additionally, the