econd-generation (2G) cellular service pro- viders came up with a system designed to compete with, and perhaps supplant, pagers. 1 For the paging services industry, the result is that pagers are now a niche market rather than a growth market. Signals to pagers do a better job of reaching their destination than signals on the cellular network, particularly inside buildings. Customers who want to be sure to receive their pages have kept pagers, even when they also have gotten cellular phones. 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. Short Message Service (SMS) Key Data Concepts This was the initial purpose of the short message service (SMS). SMS was a detail speci- fication of the Global System for Mobility (GSM) time division multiple access (TDMA) cellular telephone standard. As we explained in Chap. 7, the GSM standard defined more than the air interface and, as a result, contained many detail specifications. The goal of GSM was to standardize enough interfaces to allow multiple companies to com- pete in the development of equipment and services while all being compatible across Europe. SMS was included in the package, and the design of the standard took this ap- proach. Interfaces essential to compatibility were specified in detail. Interfaces not es- sential to compatibility, such as the user interface, were left open, allowing for com- petitive design of different services. Component design was left open, as long as the components delivered the results for the interface standard so that they would com- municate successfully on the network. With the exception of a few small glitches, this worked very well. SMS has been adopted worldwide, operating on almost all 2G and 3G cellular net- works. SMS operates at Open Systems Interconnect (OSI) layer 7 (the application layer), riding on top of GSM in the signaling channel. Due to layer i