Telecommunications Standards Institute (ETSI) standard for GPRS, GSM 03.60, recommends three positions for the PCU. In many Figure 4-1 Different places to locate the CCU and PCU. 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. Function of GPRS Elements Chapter 4 implementations, the PCU is located in the Packet Control Unit Support Node (PCUSN), which resides between the BSC and the Serving GPRS Support Node (SGSN). Thus, the PCUSN is aligned with the second and third options. The advantage of placing the PCU out at the CCUs is that the round-trip delay is shorter. The advantage of having the PCU located at the SGSN is that one PCUSN can provide service and control for as many as 12 BSCs. Each BSC can control up to 128 BTS systems (although installations typically are less than that). The Packet Control Unit Support Node (PCUSN) The PCUSN (Figure 4-2) is a new functional unit defined as part of the GPRS specifications. The PCUSN is a stand-alone node in the BSS. Its main function is to complement the BSCs (both 2G and 3G wireless stan- dards) with the PCU capability. The PCUSN is typically connected to the BSC with the proprietary Agprs interface and to the SGSN via the standard Gb interface. Its main function is to provide the interworking function between the two interfaces. The PCUSN can be connected to a single BSC or multiple BSCs. However, all Figure 4-2 The PCUSN is a new device. BTS + CCU Agprs CCU deals with PCUSN Network radio coding. PCUSN provides interworking function between GPRS and BSS. 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. Function of GPRS Elements Function of GPRS Elements signaling and traffic channels from the BSC must pass through the same