Idle Mobility

Cellular networks are operated by different service providers, and each service provider manages the network by dividing the network into manageable network areas in a hierarchical fashion, all the way down to the cell level. Mobile nodes are identified by location based on which cell the user is presently in. Cellular networks perform location management by continuously tracking the location of mobile nodes with the help information received from the mobile nodes. The location information determines the cell (or a larger network area) where the mobile node is currently located. The location information is broadcast to all the mobile nodes in the network or cell area.

When someone calls the user, the network infrastructure, and the mobile switching center in particular, retrieves the latest location information of the mobile node and delivers the call. Mobile nodes periodically update their location information to the network. These location updates are otherwise referred to as idle mobility because they are performed when the mobile nodes are not engaged in any active call or other services. These updates are mainly timer based or event based, in case the mobile nodes may cross the network management areas or even network borders into an area operated by a different service provider .

The IETF Mobile-IP Group has defined two different mobility mechanisms for IP, one for IPv4 and another one for IPv6. The fundamental principles are similar, but the protocol capabilities and definitions are quite different. The basic principle is to provide a local care of address (CoA) to the mobile node while it is away from the home network. The mobile node may have a permanent or home address assigned in the home network. The local CoA is provided by the visiting or foreign network where the mobile node is currently present. In IPv6, the mobile node can form its own local CoA through stateless address autoconfiguration by listening to the router advertisement messages from the routers serving that subnet.

The mobile node performs a registration or a binding to the home agent (HA) to indicate the local CoA. The HA creates a binding cache between the permanent address and the CoA to tunnel all the packets addressed to the permanent address to the CoA. The same binding is also established at the correspondent node (CN) if there are any active sessions with any nodes. Whenever the mobile node's network point of attachment changes, it obtains a new CoA and performs updates on the binding to the HA and CN. If the mobile is far away from the home or CN, it may take a more latency to update the binding. To reduce the frequency of these updates, micro or localized mobility management (LMM) mechanisms are proposed. Cellular-IP, Hawaii, Regional Registration, and heirarchical mobile IP (HMIP) are some of the techniques.