What is the result of enabling "SSL" in the hub under Settings->Advanced->SSL ?
When you enable "SSL" in the hub under Advanced->SSL, all internal communication (probe-to-probe, probe-to-robot, and robot-to-hub communication) will be encrypted in SSL via native SSL support that exists in the nimsoft SDK's. (To encrypt communications between hubs, a tunnel is needed.)
In some cases, "Compatibility Mode" will be preferred especially if a customer is using older probes or custom probes that may not support SSL. Many older probes did not support SSL encryption, (but used a native Nimsoft encryption algorithm). As probes were compiled with newer SDKs, SSL support was embedded in the probe. So, a good rule of thumb is, any Java probe compiled after October 2011 will support SSL, and any C probe compiled after August 2011 will also support SSL.
Compatibility mode allows all probes that support SSL to use it, while allowing those custom/older probes which do not support SSL to use the standard Nimsoft communication protocol.