 trunks may dominate the MSC facilities. The base station transport carries com- pressed voice and user data, whereas the PSTN voice trunks are not compressed. Com- pressed voice is 4000 to 13,000 bits per second, whereas full-rate pulse code modulated (PCM) voice is 64,000 bits per second. We also have to consider the data going to and from the speech coders that convert the compressed voice to full-rate PCM. A system that is primarily voice traffic is going to have a lot of speech coding and a lot of data going into and out of the speech coding equipment. We have to count the entire data service load as well. Wireless data service connects a subscriber's user terminal via IP to the landline data network. Subscriber data in- clude packets in and packets out, and those packets use telephone facilities in the MSC. We also have to count the signaling connections. The pipes from the MSC to the network should be a tiny fraction of the total data going through the MSC, but it has to be figured into the data flow equation. 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. Switching Capacity Capacity and Quality Principles There really is not any traffic engineering to be done here. Each component of the transport has to be designed to have enough capacity for adequate performance according to the service provider's quality requirements and the subscriber demand the provider wishes to serve. If each base station has to have 0.02 percent packet blocking, then this determines how many DS-1 or E-1 links are needed. If speech coders have to have 99.9 percent availability, then this determines how many packet and circuit links they need. all of these data-throughput requirements add up to more than the capacity of the switch hardware, then we have to buy another switch or to lower our expectations. 24.3 Circuit Switching Circuit-switching loads are pretty