Bizfon 7000 User's Guide download pdf (Page 51)

System Administration Guide Remote IP Phone

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Revised: August 5, 2005 Page 47

12.3.2 Limitations

While connecting the Bizfon IP2 phone to a network behind a NAT firewall works well, there are a few

limitations.

1. While intercom works fine, paging functions usually do not. More specifically, sending pages from the

remote phone always works, but neither zoned nor overhead pages will be heard at the remote phone.

Solution:

Paging is implemented using IP multicasting. If all the routers between the Bizfon server and the remote

phone can be configured to pass multicast packets, then this limitation can be eliminated. Setting up a

VPN between the Bizfon server and the remote phone may be used to enable this functionality. A full

discussion of the general networking details and the configuration of various router brands is beyond the

scope of this document. However, here are the Bizfon server details necessary to accomplish this.

On the Servers / VOIP Server page of the Bizfon server, there are three parameters used to configure

where the server transmits zoned pages:

Paging Base IP Addr – This is the multicast base IP address used by the system. Each paging zone uses

the base address plus an offset. Zone 0 (the overhead zone), uses an offset of 0, zone 1 uses an offset of

1, etc. For example, if the base address was set to 239.255.10.0, then zone 2 would use multicast IP

address 239.255.10.2.

Paging Port – This is the UDP port number that the packets are sent to. All zones use the same port

number (but each have their own multicast IP address).

Paging Max Hop Count – This value controls the time-to-live (TTL) count in the IP header of all paging

UDP/RTP frames. Typically this value is set to 1 such that the packet won’t be sent beyond the local

subnet. However, if you have multiple subnets with phones on them, this value will need to be increased.

2. When there are multiple phones each at different remote locations and all behind their own NAT firewalls,

there may be problems passing audio between them when one calls another.

Solution:

Statically map (one to one) the remote phone’s RTP ports through the NAT firewall. This requires two

steps.

The first step is to identify and possibly change the RTP ports used by the phone. Normally, a large range

of ports are used. Because setting up a one to one mapping in the firewall may be a time consuming

manual operation, you will want to reduce the number of ports used.

Go to the Phone Options section of the Phone System / Handsets / View Configuration page for the

Bizfon IP2 remote phone. The RTP Media Port Range parameter is the range of ports to be used by

this phone.

This is configurable. Two ports are required for each active “conversation” on the phone. The phone’s

intercom capability counts as a “conversation”. A conference call uses two ports for each remote caller.

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Bizfon 7000 User's Guide Page 51