 SIG converts Transaction Capabilities Application Part / Global Systems for Mobile (TCAP/GSM) MAP-encoded messages orig- inating from the GSM HLR in the SS7 network to User Datagram Proto- col/Internet Protocol (UDP/IP) messages containing the GSM MAP client interface for GSM messages destined for the GPRS SGSN nodes. It per- forms the reverse messaging that is originated by the SGSN and destined for the HLR. This interface is called the Gr. The SS7 Gateway resides on a high-availability duplex server and is scalable from one to four processors. Each processor is planned to support the traffic load for 250,000 to 300,000 GPRS subscribers. The normal SS7 network uses the bottom three layers in what is called the Message Transfer Part 1-3 (MTP1-3). These parts use a different layer of the OSI model to provide the routing and data-link layers across the physical link. Between Layer 3 and the applications layer is the Signaling 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 Function of GPRS Elements Performs the MAP/IP to MAP/TCAP conversion for Gr interface Figure 4-12 Interworks to the SGSN via IP The SS7 to IP Interwork to the HLR via SS7 GPRS HLR Gateway (SIG). SS7/IP Gateway Backbone Network GPRS HLR MAP Message MAP/IP MAP/SS7 Connection Control Part (SCCP) that is used when database queries are required and when providing both connection and connectionless access to the SS7 networks. The combination of the MTP1-3 and SCCP creates what is called the actual Message Transfer Part. When looking at the previous layers, the SS7 protocols support the use of the following: Telephony User Part (TUP) is used for a voice circuit-switched call across the PSTN. ISDN User Part (IUP) is a newer implementation and replaces the TUP. TCAP is an application layer that supports the features and functions of a network. MAP sits on to