llows about 160 seven-bit ASCII char- acters without compression. If a simple compression scheme is used, this can be in- creased to about 200 characters. 5 However, a 16-bit character set is limited to only 70 characters unless compression is used. SMS messages can be concatenated, allowing an SME to send a message that is longer than what can fit in a single message. The specification allows concatenation of up to 255 messages. However, it is strongly recommended that SMS users and appli- cations concatenate no more than three or four SMS messages. The service rides along as an extra on the cellular network's signaling channel, and that channel is not in- tended to carry large quantities of data. There are a variety of other elements to the SMS message format. Many of them allow interaction with personal digital assistants (PDAs), fax devices, specialized wireless devices, or computers. Software application developers can make use of such For the most part, complex compression schemes have not been implemented because they ex- ceed the processing capacity of most user terminals. 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) Short Message Service (SMS) features to develop custom applications that integrate SMS with other tools. However, technical limitations and some vagueness in the data transmission standards for SMS from cellular devices to other devices have made development of these services some- what challenging. 20.3.2 Logical components and message delivery The first logical component of the SMS system is the originating SME. This can be user terminal or an ESME. In configuring an originating SME, a serving SMSC is de- fined. There are a wide range of possibilities for the birth of an SMS message. A cellu- lar telephone user can type in a message to a friend and add cute "emoticons." Or at the other ex