ACCESS Product Manual
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handshaking. But we believe most codec calls will be handled directly, and
SIP supports both techniques, so we'll ignore the topic of a SIP server here.
The main area where SIP complicates matters is in how an audio channel
gets established once the handshake channel is defined. In the common
sense world, the call would be initiated to the destination IP address, then
the called codec would extract the source IP address from the incoming
data and return a channel to that address. In fact, that's how the default
mode of Comrex codecs work, and it works well.
But SIP includes a separate "forward address" or "return address" field, and
requires that a codec negotiating a call send to that address only. And this
works fine as long as each codec knows what it's public IP address is.
Outgoing Call Issues
A unit making an outgoing call must populate the "return address" field.
But any codec sitting behind a router has a private IP address, and has no
idea what the public address is. So, naturally, it will put its private (e.g.
192.168.x.x style) address into that "return address" field. The called codec
will dutifully attempt to connect to that address and undoubtedly fail,
since that can't be reached from the Internet at large.
Incoming Call Issues
Incoming calls to codecs behind routers are complicated by the fact that
ports on the router must be forwarded to the codec. In the case of SIP,
this must be three discrete ports (For Comrex codecs these are UDP 5060,
5014 and 5015)<6014 and 6015 with 3.0 firmware>. And since even the
"forward address" is negotiated in SIP, the incoming unit is likely to popu-
late the "forward address" field with it' private address as well.
So to clarify, because SIP has this requirement, it makes things more dif-
ficult and complex when connecting from or to behind a router than the
default Comrex connection mode.
Work-Arounds
All is not lost here, since we can find some hacks to make this work. The
first place to look is your router, since many modern routers are aware of