3.4.7 Ciphering mode setting procedure

3GPP44.018GSM/EDGE Radio Resource Control (RRC) protocolMobile radio interface Layer 3 specificationRelease 17TS

In dedicated mode, the ciphering mode setting procedure is used by the network to set the ciphering mode, i.e. whether or not the transmission is ciphered, and if so which algorithm to use. The procedure shall only be used to change from "not ciphered" mode to "ciphered" mode, or vice-versa, or to pass a CIPHERING MODE COMMAND message to the mobile station while remaining in the "not ciphered" mode. The ciphering mode setting procedure is always triggered by the network and it only applies to dedicated resources.

The cipher mode setting procedure shall not be applied in group transmit mode.

3.4.7.1 Ciphering mode setting initiation

The network initiates the ciphering mode setting procedure by sending a CIPHERING MODE COMMAND message to the mobile station on the main signalling link, indicating whether ciphering shall be used or not, and if yes which algorithm to use.

Additionally, the network may, by the use of the cipher response information element, request the mobile station to include its IMEISV in the CIPHERING MODE COMPLETE message.

The new mode is applied for reception on the network side after the message has been sent.

3.4.7.2 Ciphering mode setting completion

Whenever the mobile station receives a valid CIPHERING MODE COMMAND message, it shall, if a SIM is present and considered valid by the ME and the ciphering key sequence number stored on the SIM indicates that a ciphering key is available, load the ciphering key stored on the SIM into the ME. A valid CIPHERING MODE COMMAND message is defined to be one of the following:

– one that indicates "start ciphering" and is received by the mobile station in the "not ciphered" mode;

– one that indicates "no ciphering" and is received by the MS in the "not ciphered" mode; or

– one that indicates "no ciphering" and is received by the mobile station in the "ciphered" mode.

Other CIPHERING MODE COMMAND messages shall be regarded as erroneous, an RR STATUS message with cause "Protocol error unspecified" shall be returned, and no further action taken.

Upon receipt of the CIPHERING MODE COMMAND message indicating ciphering, the mobile station shall start transmission and reception in the indicated mode. A mobile station supporting Selective Ciphering of Downlink SACCH shall be able to decode both ciphered and not-ciphered SACCH blocks (see sub-clause 3.4.7a).

When the appropriate action on the CIPHERING MODE COMMAND has been taken, the mobile station sends back a CIPHERING MODE COMPLETE message. If the "cipher response" field of the cipher response information element in the CIPHERING MODE COMMAND message specified "IMEI must be included" the mobile station shall include its IMEISV in the CIPHERING MODE COMPLETE message.

Upon receipt of the CIPHERING MODE COMPLETE message or any other correct layer 2 frame which was sent in the new mode, the network starts transmission in the new mode.

Figure 3.4.7.2.1: Ciphering mode setting sequence

Upon starting ciphering with algorithm A5/1, the network may change the contents of the Neighbour Cell Description and Neighbour Cell Description 2 information elements and SI 6 Rest Octets spare padding (see sub-clauses 10.5.2.22, 10.5.2.22a and 10.5.2.35a, respectively) in the System Information messages to be sent to the mobile station, compared to those sent prior to ciphering and (for the Neighbour Cell Description and Neighbour Cell Description 2 information elements) those broadcast on the BCCH. In this case, such changes should be performed immediately after reception of the CIPHERING MODE COMPLETE message by the network.