ACITY AND QUALITY OPTIMIZATION Chapter Short Message Service (SMS) As you may have noticed, pagers are almost a thing of the past. This is only partly due to the fact that people prefer cellular telephones to pagers. The other major reason is that cellular telephones have become pagers. Cellular telephones act as pagers due to the short message service (SMS), which carries short text messages over the signaling chan- nel, delivering messages to users without setting up a cellular voice or data channel. On the air interface, signaling is carried over the paging and access channel in the forward direction and over the access channel in the reverse direction. Within the code division multiple access (CDMA) network, SMS messages are carried on the Signaling System 7 (SS7) network as specified by the ANSI-41 protocol. From the business perspective, SMS is a vertical service that provides paging-type message service to subscribers while using very little air-interface capacity and using network signaling capacity, which is readily available. There is often no charge for SMS services, but there is value. Offering SMS makes it easier to sell subscribers the switch from paging services to more expensive cellular service because subscribers do not have to give up anything when they upgrade. The latest versions of SMS-type services add the ability to transmit small data at- tachments, including alternate ring tones and small images. As the cellular market be- comes increasingly competitive, these services draw some consumers away from one brand of cellular phone to another. We say consumers rather than subscribers because so far these services are not offering much of value to business customers. This chapter will provide the history, benefits, and specifications of SMS. We also introduce the successors to SMS, enhanced message service (EMS) and multimedia message service (MMS). 20.1 History In 1980, pagers were all the rage, and they had significant value to business customers. It is not surprising, therefore, that early s