SMS system are short message entities (SMEs) and short message service centers (SMSCs). 3 SMEs are devices that can send or receive short messages via SMS. Cellu- lar user terminals with SMS capability are SMEs. SMEs that access the cellular network via modem, Internet, or other landline connection are called external SMEs (ESMEs). An ESME could be a corporate mainframe carrying a roster of on-duty tech- nicians that pages them when they are needed, or it could be an Internet server that allows someone who is browsing the World Wide Web to send a quick note to a friend. The SME components need to interact with key cellular network components. The in- terface between the SMSC and the cellular network is the signal transfer point (STP), the access point for Signaling System 7 (SS7) networks, described in Chap. 14. Through the STP, the SMSC interacts with the mobile switching center (MSC) via the ANSI-41 Q interface and with the home location register (HLR) via the N interface. 4 The mes- sage itself is carried across the cellular backhaul network to a base station, where it is transmitted across the air interface to a user terminal. The cellular user terminal is the SME receiving the message. Let us take a closer look at SMS message format, SMS logical components, and the process of sending an SMS message. The advanced details of the SMS specification are 2OTAPA is described in Sec. 15.5 SThat would be short message service centRE in Europe. So much for worldwide compatibility. 4The diagram specifying these interfaces may be found in Fig. 15.2. In the ANSI-41 specifica- tion, the SMSC is sometimes referred to as the message center (MC). 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 of interest only to SMS equipment designers and programmers creating custom SME functions. As such, they are beyond 