 of the time, packets are carried over single direct link from point to point. These links are called packet pipes. Packet pipes are more flexible than the fixed pipes of traditional PSTN telephony and are potentially more efficient, as discussed briefly in Sec. 13.2.1 and in more detail in Chap. 35 and Sec. 44.1. While packet switching using packet pipes of flexible size may be the future of land- line telephony, the present world of landline telephony is steady-state circuit switching using PCM links of fixed size. Current CDMA technology adds speech coding from MSC to base station and from base station to user terminal over the air link, achieving greater compression, but for the most part, it still uses fixed pipes. Packet transport is a new concept in the telephony world, and it will require new equipment and new en- gineering methods. The equipment is being developed and deployed, but it will be some time before cellular systems move to the voice over IP (VoIP) packetized technology de- scribed in Sec. 19.4 and realize the benefits of flexible pipes (and other benefits of VoIP as well). This conversion will be accelerated where there is demand for greater data ca- pacity and providers deploy 3G cellular solutions such as cdma2000 to meet that de- mand. However, it will be a fair number of years before the legacy systems of the PSTN are converted to VoIP. 11.5 Conclusion Cellular networks make new demands of switching systems beyond the requirements of the traditional PSTN. These come from specific technical requirements of mobile technology, including the need to support roaming, the need to track many new system performance parameters, and the need to bill customers for use of specific facilities. Each of these functions makes demands for particular types of capacity within switches. In this chapter we have discussed the functions and the switches. In the next chapter we will take a closer look at the telephony engineering principles that describe how the switches and other system components function to su