be it file transfer, web browsing, or communications. For example, mobile IP is
a useful feature when moving with a wireless-connected laptop to another
office in the same building or campus.
Although there is no agreement yet in the IETF about application-level
mobility, many SIP developers feel that terminal mobility, personal mobility,
and service mobility (where users can change devices, networks, and the IP
address used for communications) are valid extensions of the more limited
notion of mobile IP. SIP mobility will allow users to communicate while on the
move, with a short handover, but an uninterrupted file transfer or web brows-
ing would not be possible with changing IP addresses.
Mobile IP and SIP mobility are, therefore, complementary capabilities with
different areas of application.
We believe SIP mobility is a wide-open field where many interesting devel-
opments are possible. SIP mobility is presented in Chapter 15, “SIP Application-
Level Mobility.”
Context-Aware Communications: Presence and IM
Presence and instant messaging (IM) are usually perceived as parts of a single
service, but, in fact, represent different communications service components.
We will discuss them here separately.
It is important to notice that presence and instant messaging based on SIP
use similar message flows to SIP and the same “infrastructure” that is used for
SIP-based voice and video communications: Endpoints, servers in the net-
work, message exchanges, software, and data. Presence and IM come virtually
at no (or little) extra infrastructure cost for SIP service providers, but enable
very innovative new services, such as the following:
■■ Push-to-talk for mobile networks
■■ Integration of applications with communicationsThese are the reasons not to deploy disparate voice systems (such as a PBX)
and separate IM systems.
SIP Presence
The presence information conveys the ability and willingness of a user to com-
municate [7]. Initially, presence has been limited to “online” and “offline” indi-
cators, but later work has added emoticons about the state and mood of the
remote party, as well as some other useful information, such as “is typing” [8].
Table 2.4 shows some examples of rich presence information.
Internet Communications Enabled by SIP 21