Wimax standards and Security The Wimax

Power-Saving Mechanism

The power-saving mechanism of IEEE 802.16e enables MSs to operate in one of the two operational modes: wake mode and sleep mode [7]. In wake mode, MSs are always powered up to communicate with their serving BS, but in sleep mode they can power down to conserve energy during prenegotiated inter- vals. In sleep mode, there are two operational windows (i.e., time intervals): sleep window and listening window. MSs in sleep mode basically switch between the two windows. During a sleep window, they turn off most of their circuits to minimize energy consumption and so cannot receive or transmit any data. During a listening window, they synchronize with their serving BS and receive small amounts of data or a traffic indication message.
The IEEE 802.16e standard provides three kinds of power-saving class (PSC), which operate according to the characteristics of the traffic for vari- ous types of service. Each PSC uses a different operational mechanism and parameter set appropriate to the traffic characteristics. If an MS has multiple concurrent service connections, each with different traffic characteristics, it can utilize several PSCs at the same time, each of which is appropriate for a different service connection. Figure 6.1 depicts an example of sleep mode operation with two PSCs. Class A contains several connections of best effort (BE) and nonreal-time variable rate (NRT-VR) type, and Class B contains a single connection of unsolicited grant service (UGS) type. To accommodate the use by an MS of multiple PSCs, an unavailability interval is defined as a time interval that does not overlap with any listening window of any active PSC. During the unavailability interval, a BS does not transmit to the MS and buffers or drops downlink packets addressed to the MS, so the MS can power down components for physical operation. By contrast, an availability

Power-saving clas connections

s A: BE and NRT-VR UGS data transfer

UGS data transfer

UGS data UGS data transfer transfer

Power-saving clas connection

s B: UG

S

State of MS as a whole


Listening windows Intervals of availability

Sleep windows Intervals of unavailability