[Part 1 begins with an introduction and a look at IEEE 802.16M mobile station (MS) state diagrams. Part 2 continues with a look at the mobile station Access State, Connected State, and Idle State, and their respective operating modes.]
4.2 NETWORK ENTRY
This section describes the network entry/re-entry procedures from the perspective of the mobile and base stations. Network
entry is a set of procedures that an MS must follow in order to enter the operator's network and to obtain network services. The network entry procedures that follow the cell search and cell selection include initial ranging and uplink synchronization, basic capability negotiation, authorization, authentication, key exchange, registration with the network, IP connectivity establishment, and transfer of operational parameters [1,2]. It must be noted that, depending on the current operational state of the mobile station, the network entry procedures can be different.
A mobile station which is just powered on must perform an initial network entry which, as described earlier, includes successful completion of the Initialization and Access State procedures to establish data-and control-planes and to enable any data transfer in the downlink or uplink. The initial network entry procedures in the mobile station and base station are illustrated in Figures 4-12 and 4-13. There are other access-and core-network entities such as ASN-GW/Authenticator and Home AAA Server involved in the initial network entry process which are not shown in the figures. Those network entities are involved in authentication, authorization, and key exchange, as well as in the registration procedures with the BS .
The MAC management messages in IEEE 802.16m are distinguished from their legacy counterparts by an "AAI" prefix denoting "Advanced Air Interface" messaging. There are some timers associated with execution of each procedure that are denoted by Ti. These timers are used to control the amount of permissible time to enact or execute a specific procedure that once expired, forces the restart of the same or a different procedure. The MAC management messages that are shown in Figures 4-12 and 4-13 will be introduced and discussed in Chapter 6. However, the procedures such as initial ranging (AAI_RNGREQ/RSP), basic capability negotiation (AAI_SBC-REQ/RSP), and registration with the BS (AAI_REG-REQ/RSP) have already been discussed in the description of IEEE 802.16m states.
FIGURE 4-12 Initial network entry procedures in the mobile station [1,2]
FIGURE 4-13 Initial network entry procedures in the base station [1,2]
Network re-entry is a sequence of procedures similar to network entry, where the MS attempts to transition from the Idle State to the Connected State or during handover and prior to establishment of data-plane with the target BS. The MS may exit the Idle State and re-enter the network either in response to a page by the BS (i.e., network-initiated), or to start communication with the BS (i.e., MS-initiated).
The network re-entry steps may differ depending on whether the serving ASN has the MS context. If the serving ASN does not have the MS context when the MS is trying to re-enter the network from Idle State, the entire context has to be retrieved from the Anchor Paging Controller. In other words, the MS tries to re-enter the network when the management resource holding timervii has expired in the network, and consequently the MS must perform the normal network re-entry procedures.
If the serving ASN has the MS context or the context in the serving BS is not revoked before the management resource holding timer expires, the procedures for the MS to exit Idle State can be further simplified. The serving BS releases the MS context and the data paths for this MS only upon expiration of this timer. In this case, if the MS has maintained downlink synchronization with the BS during Idle State and if the system information at the MS is up-to-date, the MS may skip Initialization State procedures and directly transition to Access State and perform a light ranging, re-authentication, and re-registration with the serving BS. This form of network re-entry is referred to as fast network re-entry.
Note that in the IEEE 802.16 standard, an MS is deregistered with the serving BS prior to entering the Idle State. In addition, during handover, the MS must search and select a target BS and perform network re-entry process in order to establish a data-plane with the target BS and detach from the serving BS. Since the MS context may remain in the network when the MS transitions to the Idle State for a limited time, there is no need for a full authentication and registration on transition from the Idle State.
Cell reselection is the process where an MS scans and associates with one or more base stations in order to determine their suitability, along with other performance considerations, as a handover target. The MS may use neighbor advertisement messages from the serving BS or may negotiate scanning intervals with the BS to search and select candidate base stations for handover. The cell reselection is performed in the Scanning Mode and as part of the Connected State.
vii When the MS enters Idle State, the BS in the serving ASN starts a timer called "Management Resource Holding Timer" to keep the time that the BS maintains connection information of the MS after the BS sends the deregistration command to the MS. The BS retains the R1 context as well as the R4 and R6 data paths for the MS until the timer expires or until the context is revoked by the Anchor PC. When located in the same ASN, the Anchor PC sends a control message to the serving BS to revoke the MS context if the MS has entered the network at a different BS before the management resource holding timer at the serving BS expires [4,5].