On the traditional PSTN, there was an operating assumption referred to as calling party pays. A party receiving a call was never charged for the call or any services related to the call unless he or she accepted a collect call. With the advent of wireless, all that changed. Wireless subscribers pay for air time when they receive calls. They also may pay roam- ing charges and other fees as well. From the perspective of call statistics, the switch must record all relevant data in the correct category, and all switches on the network must record the information in a con- sistent fashion. If this is done, the service provider's billing application can generate accurate telephone bills from the records. The subscriber uses his or her mobile tele- phone to make calls and receive calls just about anywhere, the switches record the data, and the bills come out straight. This topic is covered in more detail in Sec. 16.4. Billing errors not only frustrate the subscriber, they also increase customer service costs for the provider and may cause a loss of customers or a loss of revenue. 11.4.9 Speech coding The wireless MSC has to deal with speech coding of varying rates during a wireless telephone call. The normal digital landline telephone switches in PBXs and the PSTN have DS-0 or E-0 circuits coming into and going out of them, and each switch changes the paths of data flow. In wireless telephone systems, the MSC must communicate with the PSTN in some form of pulse code modulation (PCM) over standard DS or E con- nections, but the MSC also communicates with the base stations of the cellular network in more compressed speech-coded forms appropriate to the lower available bandwidth on the air interface. As a result, the cellular provider must add a translation function that translates the landline-to-wireless voice signal from PCM to the wireless data compression and also translates the compressed voice signal from the wireless network into PCM for delivery to the PSTN. This speech coding translation function sits be- tween t