Answer
Universal Plug and Play support
Step-by-step explanation
A smooth wall is used as an open source firewall. A fire wall is a system which is specifically designed to provide network security. It does so by making sure that only access which are authorized are allowed to a network. For instance there is untrusted traffic on the network the firewall will stop it.
The key feature of smooth wall is Universal Plug and Play support which is a feature that allows devices on your network to discover each other access and certain services.