es with variable packet sizes as opposed to fixed packet sizes when silence suppression or discontinuous transmission is used. Non-Real-Time Polling Service (nrtPS): The nrtPS, like its name is designed to support non-real-time service flows like FTP (File Transfer Protocol). This service supports applications that require variable size data grants on a regular basis but not periodically. The SS using nrtPS is allowed to use contention and piggyback requests. This is the only mechanism that uses unicast and multicast polling processes. Best Effort (BE): Today's most common method for data communications is based on BE like Internet Browsing. Due to its non sensitive nature to delay and jitter, strict QoS restrictions are not applicable but SLAs can be signed on some minimum QoS parameters. 4.2.1.5 MAC Scheduling The scheduling of resources is a tedious task that requires good traffic engineering. The scheduler is located in each base station for a rapid response to traffic requirements including QoS and channel conditions. The scheduler is vendor dependant since the standard does not address the specifics of it. The scheduling service is provided for both DL and UL traffic. In the uplink the CQICH (Channel Quality Indicator Channel) provides rapid feedback to enable the scheduler to choose the appropriate coding and modulation for each allocation. The scheduler can operate on different types of sub-channels including frequency-selective and frequency-diversity. The frequency-selective selecting can allocate mobile users to their corresponding strongest sub-channels; whereas, the frequency-diversity scheduling helps to establish QoS with fine granularity especially where sub-channels are of similar quality. 4.2.2 Physical Layers The standard defines a MAC layer that supports multiple physical layer specifications that are customized for specific frequency bands, LOS/NLOS conditions, and access techniques (Figure 4.4). These physical layers are designed to support functions like data randomization, forward error co