Section: 6.5.4.1 COUNT-I
Content: The integrity sequence number COUNT-I is 32 bits long. For signalling radio bearers (RB 0-4) there is one COUNT-I value per up-link signalling radio bearer and one COUNT-I value per down-link signalling radio bearer. COUNT-I is composed of two parts: a "short" sequence number and a "long" sequence number. The "short" sequence number forms the least significant bits of COUNT-I while the "long" sequence number forms the most significant bits of COUNT-I. The "short" sequence number is the 4-bit RRC sequence number (RRC SN) that is available in each RRC PDU. The "long" sequence number is the 28-bit RRC hyper frame number (RRC HFN) which is incremented at each RRC SN cycle. Figure 16a: The structure of COUNT-I The RRC HFN is initialised by means of the parameter START, which is described in section 6.4.8. The ME and the RNC then initialise the 20 most significant bits of the RRC HFN to START; the remaining bits of the RRC HFN are initialised to 0.