STN Roaming Network (POTS/PSTN Network Intra-PLMN Backbone Network Intra-PLMN Backbone Network Network 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. GPRS Introduction GPRS Introduction end user is able to access public servers such as the Web sites and private corporate intranet servers. GPRS can also receive voice services via the GSM PLMN. Voice services and GPRS services may be accessed alternately or simultaneously depending on the mobile station's capabilities. Several classes of mobile stations are possible, which vary in degree of complexity and capability. The actual end-user data terminal used can be a smart phone, a dedicated wireless data terminal, or a standard data terminal con- nected to a GPRS-capable phone. Circuit-Switched or Packet-Switched Traffic An On/Off model characterizes the typical Internet data. The user spends a certain amount of time downloading Web pages in quick succession, fol- lowed by an indeterminate long time of inactivity during which he or she may be reading the information, thinking, or maybe even have left the work space. In fact, the traffic is quite bursty (sporadic) and can be characterized as data packets averaging about 16 Kbps in size with average inter-arrival times of about seven seconds. If a circuit-switched connection is used to access the Internet, then the bandwidth that is dedicated for the entire duration of the session is underutilized. This inefficient use of the circuit- switched example shown in Figure 2-11 creates an undesirable scenario for the network operators. Instead, they would like to fill the channels (circuits) to the highest reasonable level and carry as much billable traffic as possible. GPRS involves overlaying a packet-based air interface on the existing circuit-switched GSM network shown in Figure 2-12. This gives the user an option to use a packet-based data service