Yealink SIP-T4X Administrator's Manual page 199

3. Enter the desired value in the SIP Qos (0~63) field.
4. Click Confirm to accept the change.
A dialog box pops up to prompt that the settings will take effect after reboot.
5. Click OK to reboot the IP phone.
Network Address Translation (NAT) is essentially a translation table that maps public IP
address and port combinations to private ones. This reduces the need for a large
number of public IP addresses. The NAT feature ensures security since each outgoing or
incoming request must first go through a translation process. But in the VoIP environment,
NAT breaks end-to-end connectivity.
NAT Traversal
NAT traversal is a general term for techniques that establish and maintain IP
connections traversing NAT gateways, typically required for client-to-client networking
applications, especially for VoIP deployments. STUN is one of the NAT traversal
techniques supported by IP phones.
STUN (Simple Traversal of UDP over NATs)
STUN is a network protocol, used in NAT traversal for applications of real-time voice,
video, messaging, and other interactive IP communications. The STUN protocol allows
applications to operate behind a NAT to discover the presence of the network address
translator, and to obtain the mapped (public) IP address and port number that the NAT
Configuring Advanced Features
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