MSC. The MSC responds with an RP-ACK message (L4, GSM 04.11 Section 7.3.3). The network sends a CP-DATA message to the MS, carrying the RP-ACK payload in its RPDU. The MS responds with a CP-ACK message. The network releases the SDCCH with the RR Channel Release message. This implies a closure of the MM sublayer and triggers the release of L2 and L1. Mobile-Terminated SMS (MT-SMS) The transaction steps for MT-SMS are defined in GSM 04.11 Sections 5, 6 and Annex B. In the simplest case, error-free delivery outside of an established call, the transaction sequence is: The network pages the MS with the standard paging procedure. The MS establishes an SDCCH using the standard RR paging response procedure, which implies a CC sublayer connection. The network initiates multiframe mode in SAP3. The network sends the RP-DATA message as the RPDU in a CP-DATA message. The MS responds with the CP-ACK message. The MS processes the RPDU. The MS sends a CP-DATA message to the network containing an RP-ACK message in the RPDU. The network responds with a CP-ACK message. The network releases the SDCCH with the RR Channel Release message. This implies a closure of the MM sublayer and triggers the release of L2 and L1. Um security features GSM 02.09 defines the following security features on Um: authentication of subscribers by the network, encryption on the channel, anonymization of transactions (at least partially) Um also supports frequency hopping (GSM 05.01 Section 6), which is not specifically intended as a security feature but has the practical effect of adding significant complexity to passive interception of the Um link. Authentication and encryption both rely on a secret key, Ki, that is unique to the subscriber. Copies of Ki are held in the SIM and in the Authentication Center (AuC), a component of the HLR. Ki is never transmitted across Um. An important and well-known shortcoming of GSM security is that it does not provide a means for subscribers to authenticate the network. This oversight allows for false basestation attacks, such as those implemented in an IMSI catcher. Authentication of subscribers The Um authentication procedure is detailed in GSM 04.08 Section 4.3.2 and GSM 03.20 Section 3.3.1 and summarized here: The network generates a 128 bit random value, RAND. The network sends RAND to the MS in the MM Authentication Request message. The MS forms a 32-bit hash value called SRES