 the network and its context is deleted. The following entities may start MS deregistration process [4,5]: MS, when initiates shutdown; ASN, as result of shutdown or failure triggers in the network; Home AAA server located in CSN also is able to trigger MS deregistration. The MS deregistration procedure covers different scenarios as follows: MS deregistration as a result of MS shutdown; MS deregistration from the serving BS (and probably re-initialization in other BS/ network); Enforcing MS to halt any transmissions (including MAC management messaging); Enforcing MS to halt traffic transmissions; Erasing MS context in the ASN entities when radio link with the MS has been lost. Deregistration signaling over the air interface is performed using IEEE 802. 16m [2] (or alternatively corresponding procedures in reference [1] for the legacy systems) MAC control/management messages as follows: Deregistration command used by the BS to signal deregistration command to MS. It may be unsolicited or in response to an MS-initiated deregistration request. Deregistration request that the MS sends to the BS to request deregistration. CHAPTER 4 IEEE 802. 16m System Operation and State Diagrams In order to send or receive short message service (SMS) during the Idle Mode without performing network re-entry, the SMS data may be included in a ranging request/response message. The maximum size of the short message payload is 140 bytes [2] 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 negoti- ation, authorization, authentication, key exchange, registration with the network, IP connectivity establishment, and transfer of operational parameters [1,2]. It must be noted that, depend