munications Ecosystem tracking of mobile IP subscribers as those move from one zone to another. It also ensures that the packets are forwarded to the correct devices. Foreign Agent: Foreign Agent (FA) is also a necessity for Mobile IP. FA is a router which stores information about mobile nodes visiting its network. Foreign agents also advertise care-of addresses which are used by Mobile IP. FA receives the tunneled packets, unpacks them and forwards them to the devices. 8.2.2.1 UMB Core Network Architecture A UMB system simplifies the CN and network interfaces by employing flat network architecture. In a UMB flat network architecture there is no need for centralized BSCs which is a requisite in 2G/3G networks. Figure 8.11 shows that eBSs of UMB radio access network are connected to a common access gateway (AGW) which provides a point of IP attachment for the AT (Access Terminal) to the packet data network [17]. The key UMB core network elements are AGW, AAA, HA (Home Agent), PCRF. The two new elements are AGW and PCRF while the rest of the entities have been reused from EV-DO systems: AGW: The AGW is the first hop router for the mobile terminal by which it gains access to packet data networks. The AGW performs layer 3 services and above, including hot-lining, policy enforcement and more. PCRF: The PCRF (Policy and Charging Rules Function) provides rules and regulations to the AGW. The purpose of these rules is: Detect a packet belonging to a service data flow. Provide policy control for a service data flow. Provide applicable charging parameters for a service data flow. The traditional hierarchical mobile-broadband radio access networks serve user traffic in multiple types of nodes. For example, in legacy cdma2000 and EV-DO networks, the BS, BSC, PDSN and mobile IP INTERNET Foreign Agent Home Agent Figure 8.11 UMB core network architecture Core Networks and Operations Support Systems Connectivity Service Network Foreign Agent ASN-GW Internet Agent IP backbone Servers Figure 8.12 WiMAX connectivity service node ho