1.1 What�s the difference between GPRS and UMTS? This is quite confusing for many people. According the 3GPP standard (TS 23.060) GPRS defines the packet data services provided by a common core network to two main types of access networks - GSM based and WCDMA based (UMTS). 1.2 What�s the relation between GPRS and IP? GPRS uses IP based signaling/control protocols between its components. Beyond that it transparently carries IP packets between the mobiles and the Internet. This is done by method known as encapsulation and tunneling. 1.3Can you do voice calls with GPRS? GPRS provides an IP data pipe. Therefore by definition you can do anything on top of it including voice. QoS is a major issue though for voice over GPRS, and IMS (IP Multimedia Subsystem) tries to resolve it. 1.4What is the relation between GPRS and SIP? SIP rides transparently over GPRS just like any other IP based protocol. IMS and QoS muddy this picture a bit. You can read all about it in 3G IMS Illu...