ansport backbone, or if there is no available room on the backbone, then we have to consider the mileage costs of backhaul. This leads us to placing the MSC near the geographic center of the service region SO that it is at the minimum to- tal distance from all its base stations. Of course, the availability of cheaper transport from some base stations rather than others will tend to bias MSC placement toward the more expensive transport base stations to minimize their backhaul costs. The transport costs to the public networks (PSTN and PPDN) are also an important factor. The backhaul links tend to be expensive because they are small pipes of a few DS-1 or E-1 links. The public network transport comes in larger quantities, DS-4 and E-4 levels, so we should get a better price per unit on PSTN and PPDN transport. How- ever, the voice traffic to the PSTN is uncompressed 64-kbps pulse code modulation (PCM), so the total volume of voice transport is higher from the MSC to the PSTN than it is from the MSC to the base stations. 2 If we have a very cheap transport backbone, then all the MSCs will be placed along that backbone, and the backhaul problem becomes the backbone access problem 2There is a possible exception to this. It may be possible to carry compressed signals all the way to the PSTN switching office and install our voice speech coders at the far end. If this option is available, we choose by comparing the reduced transport cost to the cost of leasing space for the speech coders and maintaining equipment at a separate site, as well as a risk calculation for the chance of equipment failure at the separate location. 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. Mobile Switching Center (MSC) Growth Increasing Capacity described in Sec. 35.3. Also, the transport costs between the new MSC and the public networks (PSTN and PPDN) are likely to 