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Recently I started looking at SIP implementation for a future work. I was reading (Googling) about what SIP means and how to go about implementing a end-to-end SIP enabled VoIP network. What I did not get is what use does a SIP Gateway is for? How different is it with respect to SIP proxy servers or a SIP DNS/Locator like server? I understand probably QoS would be one primary factor - like dedicating a set bandwidth for SIP/VoIP specific I/O over a network. Anything else?

Can anyone help me with any other hints/pointers? I fully understand that is quite a basic question - but I really couldn't find any text which could clear my doubt about what 'Gateway' would mean in SIP context and what differentiates it from other SIP based network components (like Softphones, Proxies etc).

Thanks a lot.

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up vote 2 down vote accepted

In SIP terminology, proxy and gateway are the same thing. They route calls between peers. Your best source of information concerning SIP protocol would be the RFC3261.

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I am trying to read through the RFC 3261 but that explains SIP protocol only and not what Gateway would mean. Nevertheless, thanks for clarifying 'proxy' and 'gateway' terms. –  Shrey Oct 21 '10 at 9:30
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Both terms can be ambiguous depending on where they're being used and by whom, but ...

A proxy generally talks SIP only (i.e. on both sides), and acts as a router for inbound and outbound requests (just like a web proxy). This might be required for security, NAT traversal or other reasons.

A gateway generally talks something other than SIP as well, such as a PSTN gateway which allows calls to and from the public switched telephone network which is based on TDM protocols such as ISDN or SS7.

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