The results in Figure 15.10 show a very smooth handover performance from
combining SIP with IEEE 802.21 media-independent handover. Without MIH,
the handover time for 802.11 test networks is about 4 ms. 3G mobile networks
may introduce up to 15 seconds handover delay without MIH because of the
lengthy authentication and signaling procedures in such networks. As we will
see in the next section, lengthy authentication and signaling can happen in
commercial 802 networks, as well to ensure payment for service, though prob-
ably not as lengthy as in 3G networks.
Network Selection Issues
Most present business models require secure (but complex) authentication
procedures to ensure the payment for Internet access service in wireless net-
works. This is usually the cause for quite complex network selection issues.
The reasons for the network selection issues include the following [20]:
■■ There may often be several wireless networks in the same location but
with different characteristics, different payment methods, and different
credentials required.
■■ Several service providers share the same wireless networks (for exam-
ple, a hot spot in an airport is accessible via subscription from various
wireless providers or aggregators).
■■ The path for the IP packets depends on the service provider connected
to the wireless access network.
■■ Complex roaming relationships may result in different cost structures
to the user.
■■ The user has to make network selection decisions and provide different
credentials for different providers.
■■ The user may have different credentials, such as one for company use
and another for personal use.Figure 15.11 shows one of the possible scenarios for network selection.SIP Application Level Mobility 269