These are also exchanged during the registration process so that the network can verify
a subscription when the device accesses the network. These are lessons carried over
from GSM implementations over the years.
This is where VoIP implementations failed for many operators, because the protocols
were simply not robust enough to ensure that every user was authenticated prior to
being able to access services. Security is discussed in more detail in Chapter 6.
Also part of the identity and registration process is the IP address. The subscriber
device will provide the assigned IP address along with its identity. This then allows the
network to ???follow??? wherever the user is connected, mapping the URI to the assigned
(and dynamic) IP address.
Domain Names and URIs
In the SIP domain, subscription addresses use the form of a Universal Resource
Identifier (URI). This is analogous with the Universal Resource Locators (URLs) we
use to reach Web sites on the Internet, but they are assigned to subscriptions for reaching
individual subscribers.
It is the concept of the URI that makes communications models in the IMS unique.
The ability to reach a subscriber based on these very personal identities rather than
numbers, and to apply these addresses to all forms of communications, is the purpose
of the SIP protocol within the IMS.
A URI can take two forms.
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