Comrex ACCESS MultiRack, Product Manual

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same as the server/proxy address, and if this field is not populated, that is the default. If, for some reason,
the domain is different than the server/proxy address, then this field is used.
sIp troubleshootIng
In a nutshell, SIP establishes a communication channel from the calling device to the called device (or server) on
port 5060 (instance #1). All handshaking takes place over this channel, and a separate pair of channels is opened
between the devices: one to handle the audio, and the other to handle call control. The original communication
channel is terminated once the handshaking is complete. Note that firewalls must have all three ports open for calls
to be established correctly.
The primary area where SIP complicates matters is how an audio channel is 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. This
is the default method Comrex devices use to create and maintain a connection.
But SIP includes a separate “forward address” or “return address” field, and requires that a codec negotiating a call
send to that address only. This is important in the case of having an intermediate server, and works fine as long as
each codec knows its public IP address.
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 does not know its public address. A codec will populate its private IP address (e.g.
192.168.x.x
style) into that “return address” field. The called codec will attempt to connect to that address and fail,
as its private IP Address can’t be reached from the public Internet.
IncomIng cAll Issues
Incoming calls to codecs behind routers are complicated by the need to forward ports on the router to the codec. In
the case of SIP, this must be three discrete ports for each instance. As the “forward address” is negotiated in SIP, the
incoming unit is likely to populate the “forward address” field with its private address as well.
On MultiRack, the following ports are used for incoming SIP traffic (all ports are UDP):
Instance Number SIP RTP
1 5060 6014, 6015
2 5062 6024, 6025
3 5063 6034, 6035
4 5064 6044, 6045
5 5065 6054, 6055