In the GSM/GPRS world, how is SMS transmitted? In CDMA, SMS is encapsulated within a data burst signaling message (received/sent during idle mode) or if it's too long then a traffic channel is setup (with specific service option 6 or 14) and the SMS is carried in traffic. Is GSM/GPRS the same?
In GSM/GPRS, SMS is part of the non-access stratum (NAS) signalling protocol, and is always (as far as I know) sent over a control (signalling) channel. UMTS is the same. See 3GPP TS 24.007 for more details.
LTE is slightly different, because an IP connection is formed at network registration. See Myth 2: SMS isn't supported over LTE (half way down the page) for an explanation of the standard way to send SMS.
The principle of being able to send an SMS without any connection other than being registered on the network holds.