en. The HLR is con- nected to the MSC across the C interface. The HLR also plays a role in the mobile station attach function. When a mobile station attempts to attach to the GPRS network, the HLR provides subscription data to the SGSN. The HLR is updated to maintain information regarding the user's subscription services for GPRS. Figure 4-9 shows the HLR. 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 Figure 4-9 The HLR. IP/X.25 The Visitor Location Register (VLR) The VLR holds similar information as the HLR. However, the VLR is always distributed (that is, one is associated with each MSC). The informa- tion contained in this database is temporary and will only be available as long as the subscriber is in the area. The information relates to all sub- scribers in the MSC area only; anyone else in another MSC area is dropped from the database. The VLR also contains details of foreign mobile sub- scribers roaming in its area. In addition to the information contained in the HLR information, the VLR also contains additional information: Stored authentication parameters Location area identity Routing area identity Mobile Station Roaming Number (MSRN) Temporary Mobile Subscriber Identity (TMSI) and Packet TMSI (P_TMSI) The VLR is connected to the MSC using a B interface, as shown in Fig- ure 4-10. 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 Figure 4-10 "B" Interface The function of the VLR. Holds similar information as HLR One-to-one relationship per MSC Temporary storage Stored Authentication Information Mobile Station Roam # TMSI or P_TMSI Other Network Elements The intra-PLMN backb