The McGraw-Hill Companies. All rights reserved. Any use is subject to the Terms of Use as given at the website. System Architecture Chapter 3 be a separate hardware element associated with the BSC. Each BSC will require the installation of one or more PCUs and a software upgrade. The PCU provides a physical and logical data interface out of the Base Station System (BSS) for packet data traffic. The BTS may also require a software upgrade, but typically will not require hardware enhancements. When either voice or data traffic is originated at the subscriber terminal, it is transported over the air interface to the BTS, and from the BTS to the BSC in the same way as a standard GSM call. However, at the output of the BSC, the traffic is separated; voice is sent to the MSC per standard GSM, and data is sent to a new device called the SGSN, via the PCU over a Frame Relay interface. GPRS Network In the core network, the existing MSCs are based upon circuit-switched central office technology, and they cannot handle packet traffic. The deploy- ment of GPRS requires the installation of new core network elements called the Serving GPRS Support Node (SGSN) and Gateway GPRS Support Node (GGSN). Figure 3-2 shows some of the overlay elements. From a high level, GPRS can be thought of as an overlay network onto a second-generation GSM network. This data overlay network provides packet data transport at rates from 9.6 to 171 Kbps. Additionally, multiple Figure 3-2 The overlay network Intra-PLMN interworks between GPRS Backbone public and private networks. Operator-Specific IP Network External IP Network (for example, Internet) Router 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. System Architecture System Architecture users can share the same air-interface resources. GPRS attempts to reuse the existing GSM network elements as much as possible, but in orde