Mobile IP (3G)

Mobile IP, whereas a packet-transport method, is quite different than Simple
IP in that it actually transports the data. Mobile IP utilizes a static IP
address that can be assigned by the PDSN. The establishment of a static IP
address facilitates ROAMing during the packet session, provided the static
IP address scheme is unique enough for the subscriber unit to be uniquely
identified.
With Mobile IP, the PDSN is the Foreign Agent (FA) and the Home Agent
(HA) is set up as a virtual HA. The mobile needs to register each time it
begins a packet data session, whether it is originating or terminating. Also,
the PDSN on the visited network terminates the packet session using an IPin-
IP tunnel. The HA delivers the IP traffic to the FA through an IP tunnel.
The mobile is responsible for notifying the system that it has moved to
another service area. Once the mobile has moved to another service area, it
need to register with another FA. The FA assigns the mobile a care of
address (COA).
The HA forwards the packets to the visited network for termination on
the mobile. The HA encapsulates the original IP packet destined for the mobile using the COA. The FA using IP-in-IP tunneling extracts the original
packet and routes it to the mobile.
The IP address assignment is a done via DHCP and is mapped to the
HA. However, PAP and CHAP are not used for Mobile IP as it is in
Simple IP.
In the reverse direction, the routing of IP packets occurs the same as if
on the home network and does not require an IP-in-IP tunnel unless the
wireless operator decides to implement reverse IP tunneling.
In summary:
■ The PDSN in the visited network always terminates the IP-in-IP
tunnel.
■ The HA delivers the IP traffic through the mobile IP tunnel to the FA.
■ The FA performs the routing to the mobile and assigns the IP address
using DHCP.
Figure 7-15 is a simplified depiction of Mobile IP.
Figure 7-16 is an example of a Mobile IP packet session flow.