Figure 15.8 SIP user agent for GSM- and Wi-Fi-enabled PDA (courtesy Cicero Networks).
(a) HP PDA, (b) Qtek mobile phoneNetwork Control versus User Control of Mobility
Network control of mobility as shown in Figure 15.6 assumes the existence of
one single network-based service and this does not apply to most users.
Mobile IP makes no assumptions about network ownership, and this is not
satisfactory either.
The emergence of multimodal mobile devices (as shown in Figure 15.8)
places the control of mobility in the hands of the user who can now chose
between several types of networks, not all necessarily owned by the same
organization or service provider. We will show in the following discussion
how the handover between different network types (referred to here as
“media”) can be accomplished, and what the issues are when selecting net-
works from different owners.266 Chapter 15