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Chapter 4: Trunks
115
know your public IP address. They will learn it through the registrations process (see
Appendix B for information on SIP registration).
The SIP registrations trunk is a popular type of SIP trunk and is easy to set up once the
ITSP account has been established. It requires the system to register to the provider, just
like an IP phone registers with the system. Like an extension that has been registered
with the system, SIP registration trunks require a user name or account (usually the
DID), a password, and the IP address or domain name of the SIP or proxy server.
The advantage of the SIP registrations trunk is that the IP address is dynamically bound
to the SIP registration, which allows the registration to be used from any IP address.
Although service providers often assume that a trunk registration involves an IP phone
or an ATA rather than a trunk, many phones can sit behind the snom ONE telephone
system and share the resources of the trunk (Figure 4-3). However, most service pro-
viders limit the number of calls that can be made over a SIP trunk and use a charging
method that simulates the charges accumulated by a physical legacy TDM trunk (i.e.,
the number of call paths).
Although SIP trunks are virtual trunks and can theoretically have an infinite number of
calls, SIP trunks are limited by the amount of bandwidth that is available to handle the
calls.
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PSTN
511
510
GSM
Network
Internet
ITSP
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Figure 4-3. SIP Trunks
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