Deferred Transmissions occur when you have too much traffic to transmit on an interface and the packet cannot be immediately transmitted (and has to be deferred and stored in a buffer). This typically means that you need more bandwidth on the link.

If the device with the Deferred Transmissions is a Cisco switch, and there are Deferred Transmissions along with Outbound Discards, it usually means that the interface is configured for VoIP QoS, and is deferring and dropping data packets. We have seen this many times with Cisco switches that have an incorrect QoS setting, and it drops data packets to make sure the VoIP packets get transmitted. If the QoS was configured properly, it would defer the data packets but not drop them.