Mobility in the Wider Concept
This section examines mobility in the wider context of the Internet, the PSTN,
mobile networks, and the application-layer mobility based on SIP.
SIP can support application-level mobility across different networks where
the network and the device cannot be changed. Application-level mobility that
SIP can support is described in Chapter 15, “SIP Application Level Mobility.”Global Telephone Number Portability
Local number portability allows telephone subscribers to keep their phone
number when changing service providers, but only in the same local calling
area. Telephony number portability on a national scale poses an implementa-
tion challenge on the PSTN, and global telephone number portability seems
just not to be possible with the various national telephone networks of today.
Global number portability is, however, a trivial application for SIP services if
users have a domain name and a SIP URI in their own domain, such as the
following:sip:[email protected]A SIP URI is globally valid by contrast to telephone numbers that have a
local significance, except for the E.164 number format, though even 1-800-...
numbers have a local significance only.
Public VoIP providers with gateways in multiple cities and multiple coun-
tries are offering phone numbers from everywhere their SIP to PSTN gateway
service can reach. For example, a resident in Karachi, Pakistan can have a
phone number in Dallas, Texas, USA (this is a real service known to the
authors). The use of DNS is described in Chapter 4, “DNS and ENUM.”SIP Application-Level Mobility
You can make the distinction between the following:
■■ Terminal mobility—Terminal moves between subnets.
■■ Personal mobility—Different terminals, same address.
■■ Service mobility—Keep same services while mobile.SIP has been chosen for call control for the third generation (3G) wireless
networks by the Third-Generation [wireless mobile] Partnership Program
(3GPP and 3GPP2) initiatives.
Mobility for IP has been defined in the IETF by RFC 2002 with the basic con-
cept that a mobile host maintains its IP address while changing the point of
attachment to the IP network. Mobile IP is, therefore, valid for any application,20 Chapter 2