 landline telephone local offices, and their DS-1 link is actually one time slot on a DS-4. The physical layout might just as well look like Fig. 14.6. In this case, one machine performs the HLR and VLR functions for all three MSCs, as well as a digital cross- connect function to manage the MSC-to-MSC links. In case one breaks down, this net- work has two HLR/VLR machines for redundancy, and these machines share the load when both are running. Each MSC is connected to both HLR/VLR machines with an 2This distinction is completely separate from the distinction of layers in the OSI reference model. The OSI reference model, SS7, and ANSI-41 are all logical models with varying physical models used in implementation. 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 Signaling with SS7 Figure 14.4 Logical diagram of three MSCs, HLRs, and VLRs. VLR/MSC/HLR VLR/MSC/HLR VLR/MSC/HLR Figure 14.5 A physical diagram of three MSCs, HLRs, and VLRs. 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 VLR/HLR VLR/HLR Figure 14.6 Another physical diagram of three MSCs, HLRs, and VLRs. E-1 link, with 2 E-0 links dedicated to HLR and VLR traffic and the other 30 E-Os used for MSC-to-MSC data. The individual MSC-to-HLR and MSC-to-VLR links are four packet streams on the 2 E-0 links. The two HLR/VLR machines have a dedicated E-1 link between them to keep their databases synchronized. These are two very different physical models that implement the same logical model. 3 The ANSI-41 standard provides a logical model of some functions of a wireless telephone network that may be hard to recognize from the physical layout of the system. 14.5 Co