Snom 4S Manuale Utente

20 • Architecture

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S N O M

  4 S   N A T   F

I L T E R

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The snom 4S NAT Filter includes a STUN server that operates on 

the SIP UDP port. User agents should send their test packets to the SIP 

port.

Generally, there are two categories of user agents: The non NAT 

aware user agents and the STUN/ICE capable user agents.

Non-NAT aware user agents must have at least the following fea-

tures:
1. 

Must send SIP UDP packets from the port where they receive SIP 

traffic

2. 

Must start sending media immediately after receiving SDP

Requirement 1 is not fulfilled by the default configuration of the 

Cisco  7960;  however  there  is  a  setting  that  enables  this  feature.  Most 

known user agents support this feature, however.

Requirement  2  sometimes  creates  problems  with  user  agents 

who don’t send media during silent periods. In this case, users have to 

start speaking before two-way audio can be established.

In any case, customers are asked to contact their vendor in case 

of problems and explanations. In general, snom recommends using NAT-

aware user agents to reduce the network and support overhead.

STUN/ICE-Aware User Agents must implement the two IETF stan-

dards. It is ok if the user agents use the built-in STUN server for refresh-

ing the bindings and learning the public IP address.

snom phones starting with version 2.05a fall into this category.