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Sometimes the SIP channel is connected to a server that is removed from the RTP sessions entirely. This would likely be the
case when two SIP devices are registered to the same (or sometimes even different) providers. The SIP channel would instruct
the devices to create RTP sessions between them, rather than to the provider. This is known as the “SIP Triangle”.
But more commonly, a SIP device is interested in making and receiving calls to and from the “old fashioned” public switch
telephone (PSTN) or “plain old telephone” (POTS) network, whether wired or cellular. In this case both the SIP channel and
the RTP sessions are made to a server at the Internet Telephone Provider, and the provider acts as a gateway for the voice call
to the “legacy network”. The user would be delivered a “real” phone number (DID for Direct Inward Dial) and the provider
would handle all the necessary VoIP <-> PSTN conversions. We’ll focus on this scenario from here on.
SIP Details
The technical details of SIP are widely available on the web for further research. But essentially, commands and formats are
provided to invite users to a call, accept calls, end them, and reject them. SIP also provides a mechanism to register and
authenticate with a server.
Another useful function in SIP is encoder negotiation. The SIP protocol can inform users of which encoders are supported on
each end of a session and in which priority. In this way, it’s easy to make decisions about which encoder to choose that will
be in common with both ends, and to reject calls if no common encoder is found.
Like RTP sessions, the SIP channel utilizes the UDP protocol by default. There is a specific port defined, 5060, as the default
“well-known” port over which SIP operates, although it can usually be configured to be different.
A single SIP channel can manage multiple RTP sessions simultaneously. In this way, only a single account needs to be
registered with the Internet Telephone Provider and a single SIP channel maintained, but multiple VoIP calls can be run
simultaneously. Whenever a call is initiated or dropped, a pair of RTP sessions is created or destroyed on the fly for each call.
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