e MS may transition to Sleep Mode after negotiating the sleep intervals with the serving BS, and it may exit Sleep Mode on receiving a traffic indication message or availability of uplink traffic. 4.1.1 Initialization State Figure 4-3 provides a decomposition of the Initialization State. It is shown that the Initialization State consists of two steps: (1) cell search; and (2) cell selection. In the first step, the MS scans for the candidate base stations to which the MS can attach and detect the Cell Identification (Cell_ID). The Cell_ID is a unique code or a synchronization sequence that is assigned to each cell (i.e., a BS or a BS sector) in order to identify the cell exclusively.' ii The scanning is performed by measuring appro- priate metric corresponding to the received RF signal strength. In the second step, the MS acquires the system configuration parameters and concludes cell selection. Upon power ON/OFF or after any loss of RF signal, the MS acquires a downlink RF channel. The MS is required to use nonvolatile storage in which the last operational parameters are stored and therefore it may first try to acquire the previous downlink RF channel. If the MS fails to acquire the last it must be clarified that Cell_ID or physical layer identifier is not unique throughout the operator's network. In practice, the number of Cell_ID sequences are limited, thus they are reused as long as the cells that are assigned the same Cell_ID are sufficiently far apart (geographically) such that there is no confusion in the signals received from those cells. CHAPTER 4 IEEE 802. 16m System Operation and State Diagrams Power ON/OFF Power ON/OFF Power Down Power Down Scanning and DL Synchronization Scanning and DL (Preamble and Synchronization Cell ID Detection) From Access, (Primary and From Access, Connected, Secondary Advanced Connected, or Idle State Preamble and Cell_ID or Idle State Detection) Acquire Frame Control Header and DL MAP Primary and Secondary Superframe Detect DL/UL Header Channel Acquisition Descriptors Cell S