A method of providing a CS service circuit switching to a UE, (27) through an SAE core network (11) and an LTE radio access network (12), comprising method the steps of: extending an SGs interface to provide a SGs + interface (63) between a mobility management entity, MME, (13) in the SAE core network and a central mobile switching server, MSC- S, (26) wherein the SGs + interface supports the transmission of upper layer packet data units; transfer (8, 9, 12, 14) of CS service messages into SGs + messages sent between the MSC-S (26) and the MME (13) through the use of the SGs + interface, where the transfer step of the CS service messages includes sending from the MSC-S (26) to the MME (13), including a downlink data transfer message SGs + the upper layer packet data units destined for a terminating UE or sending from the MME (13) to the MSC-S (26), including an uplink data transfer message SGs + the upper layer packet data units originated by a source UE; and transfer (2, 15) of CS service message contents between the MME (13) and the UE through the LTE radio access network into non-access layer messages, NAS, where the CS service is the short message service, SMS, and the CS service message transfer step includes sending from the MSC-S (26) to the MME (13), a downlink data transfer message SGs +, including downlink data transfer message SGs + SMS control plane packet data units and SMS message content destined for the terminating UE.