A method performed by a connectivity-related network entity is provided. The method includes: receiving, from a Packet Data Network (PDN) gateway and a Serving Gateway (SGW), a Short Message Service (SMS) message to be sent in a downlink; determining to use a control plane only, out of the control plane and a user plane, for transferring the SMS message; transmitting a trigger message to a user equipment to initiate establishment of connectivity with the user equipment via the control plane only and not via the user plane, wherein the user equipment starts a timer when the establishment of the control plane connectivity is initiated; receiving a response message from the user equipment; encapsulating the SMS message in a security protected Non-Access Stratum (NAS) message; and transmitting the security protected NAS message to the user equipment, in receipt of which the user equipment stops the timer.