quipment specifications play an important role not only in dimensioning but also in detailed planning. In a packet core network, SGSN and GGSN capabilities are the most important defining factors, SO it is important to understand the functioning of these two network elements. As shown in Figure 9.9, SGSN contains four main blocks: interface units (IF), router, GPLC/GTPU and management. An interface unit is required for handling the data Iu-ps Router Figure 9.9 Simplified block diagram of SGSN 3G CORE NETWORK PLANNING AND OPTIMISATION connections in the direction of the RNC on one side and the GSN (SGSN/GGSN) on the other side. A GPLC card is also present and is used for receiving, sending and managing the packet data. The router in the SGSN performs the functions of both a router and a pro- cessor. In fact, this is the unit that 'controls' the SGSN. All the units in SGSN are usually Ethernet or ATM interfaces and are connected to the router. The traffic movement is guided by IP addresses. GGSN is much simpler than SGSN as it can be termed as a simple router. SGSN is also responsible for the mobility management-related functions. The functionality of the GGSN router is only routing without much processing to be done, unlike in SGSN where the router performs processing functions as well. The main function of the GGSN is to generate the PDP contexts and to set up the GTP tunnel. Once this is done, all the GGSN has to do is to maintain them. Again, IP addresses play an important part in the process, as they are required for generation of the PDP contexts. In earlier chapters, we have already seen the planning aspects of the Iu-ps interface and SGSN. However, there are some extra interfaces (i.e. G and Ga) that need to be planned apart from planning IP addressing and routing. As other interfaces - such as G, between the GSN and external networks - in the core network involve external element characteristics, they are considered to be outside the scope of this book. GSN Inter-connection GSN inter-connections (Gn interf