■■ Mobility at the application layer (L5) provided by SIP—As we will show,
application-layer mobility can support the changing of L2 networks
and the changing of devices, and significantly enlarges the dimensions
of mobility.
It is possible to combine mobility at different layers, for example to combine
SIP mobility with MIP or with L2 mobility to improve handover performance.
Dimensions of Mobility
Henning Schulzrinne from Columbia University in New York and his research
group have introduced the concepts for SIP-based application-level mobility
[7] [8]. These concepts are very innovative and disruptive by nature, and have
already, in part, been proven in the market (such as the inherent mobility
offered by commercial VoIP networks). Table 15.1 examines SIP-based applica-
tion-level mobility. Some of the mobility modes discussed are, however, attrib-
utable to the intrinsic mobility on the Internet. These may apply equally well
for other VoIP protocols, other than SIP, such as is the case for Skype (though
Skype uses SIP gateways as well for PSTN termination).
Table 15.1 SIP-Based Application-Level Mobility
ROAMING USERS LOGGING AWAY FROM OFFICE: HOME AND
WHILE TRAVELING
Terminal mobility or The same endpoint moves between different
network level mobility attachment points to the same network. This is
familiar from all 2G mobile phone services, though
they are not IP-based.
Personal mobility User is reachable under the same Address of
Record on various networks and various devices
(for example, at work, in the office, on a PC/laptop,
office phone, office fax, mobile phone, and PDA).
Personal mobility that includes legacy phones, fax,
and 2G mobile services requires ENUM, as
discussed in Chapter 4.
Service mobility Users keep the same services when moving to
another location. VoIP users can travel across the
world and still call and be reached on the PSTN
home phone number. This is a common feature
found in almost all present VoIP services and is
rather an Internet property.
(continued)SIP Application Level Mobility 255