Combining SIP mobility with MIP can, however, give excellent results in
some application scenarios by reducing both the delay and the handover time
[17], keeping, for example, the handover time at roughly 100 ms in 802.11 wire-
less networks independent of the delay between the mobile node and the cor-
responding node on the Internet.
Using MIP alone would have increased the handover time in a linear fash-
ion with the delay between the two nodes.
Multimodal Mobile Device Technology and Issues
As mentioned, multimodal mobile communication devices have the potential
to produce significant new business models, disruptions of existing mobile
services, and ensuing regulatory changes required for the benefit of users and
in the interest of public policy. Also as mentioned, quite a number of mobile
devices, besides the laptop computer have more than one network interface.
Figure 15.8 shows an example of a SIP UA for dual-mode PDAs and pocket
computers.
As more multimodal mobile devices are emerging in the market, SIP-based
mobile communications are becoming more widespread. The major technical,
interoperability, and standards approaches are still under development. We
will present in the following sections some of these technologies and issues.
SIP Application Level Mobility 265Mobile IP requires two addresses for the mobile host and has problems with
NAT transversal. NAT transversal problems are, however, not unique to Mobile
IP. SIP also needs firewall transversal support, as we show in Chapter 10, “NAT
and Firewall Traversal.”
SIP mobility does not have the drawback of long media paths and is,
therefore, better suited for real-time communications than Mobile IP.
We are not aware yet of any major deployments by Mobile IP by service
providers. This may probably be because the first service providers to
implement Mobile IP have little incentive to do so, since they would only serve
users belonging to other networks that just happen to visit their own network.
The complex payment settlement technologies between providers have also not
been deployed for MIP.