Unlike Wi-Fi which is used for small range communications, WiMAX is mainly applied to metropolitan area networks and therefore must master all data transmission decisions to/from SSs to avoid synchronization problems. In this section, we brief the WiMAX frame structure under TDD mode, describe the five service classes whose connections fill up the frame, and elaborate the detailed packet flow in the BS MAC.
According to the flow the bandwidth allocation module as well as its input and output is identified. Some related works investigating the allocation problem are discussed, leading to the statement of the research goals.
2.1 Overview of the MAC Protocol
2.1.1 TDD Subframe
Fig. 1 TDD subframe structure.
As shown in Fig. 1, the frame structure under TDD includes (1) UL-MAP and DL-MAP for control messages, and (2) downlink and uplink data bursts whose scheduled time is determined by the bandwidth allocation algorithm and is indicated
in the MAP messages. All UL-MAP/DL-MAP and bursts are composed of a number of OFDMA (Orthogonal Frequency Division Multiplexing Access) slots, in which a slot is one subchannel by two OFDMA symbols in uplink and one subchannel by three OFDMA symbols in downlink. This mode is named PUSC (Partial Usage of Subchannels), the mandatory mode in 802.16, and is considered throughout the work.
2.1.2 Uplink scheduling service classes
Table 1 summarizes the characteristics of the supported service classes, namely the Unsolicited Grant Service (UGS), Real-time Polling Service (rtPS), Non-real-time polling Service (nrtPS), Best Effort (BE), and the lately proposed Extended Real-time Polling Service (ertPS). Each service class defines different data handling mechanisms to fulfill service differentiation. The UGS has the highest priority and reserves a fixed number of transmission slots at each interval for bandwidth guarantee.
rtPS, nrtPS, and BE rely on the periodic polling to gain transmission opportunities from BS, while the ertPS reserves a fixed number of slots as UGS does, and notifies the BS in the contention period for possible request size changes. nrtPS and BE also contend for transmission opportunities if they fail to get enough bandwidth from polling.
Table 1 Characteristics of the scheduling service classes.
Feature UGS ertPS rtPS nrtPS BE
Priority 1 2 3 4 5
Request size Fixed Fixed/Variable Variable Variable Variable
Unsolicited grant Y Y Y Y Y
Unicast polling N N Y Y Y
Contention N Y N Y Y
Request duration Periodic Periodic Periodic Timely Sometime
Application VoIP without silence
2.1.3 Detailed Packet Flow in the MAC Layer
The complete packet flow in the uplink and downlink of a BS MAC is shown in Fig. 2. For the downlink processing flow, both IP and ATM packets from network layer are transformed from/to the MAC Convergence Sublayer (CS) by en/de-capsulating the MAC header. According to the addresses and ports, packets are classified to the corresponding connection ID of a service flow which further determines the QoS parameters. Fragmentation and packing are then performed to form a basic MAC Protocol Data Unit (PDU), whose size frequently adapts to the channel quality, followed by the dispatching of resulting packets into queues. Once the scheduler starts, the bandwidth management unit arranges the data burst transmissions to fill up the frame. The MAP builder then writes the arrangement, namely the allocation results, into the MAP message to notify the PHY interface when to send/receive the scheduled data in the time frame. Encryption, header checksum and frame CRC calculations are carried out to the packets before they are finally sent to the PHY. The uplink processing flow is similar to that of the downlink except the BS receives bandwidth requests which could be either standalone or piggybacked ones. Among the above operations, it is obvious that the bandwidth management, and thus the bandwidth allocation algorithm, are critical and need to be carefully designed in order to improve the performance of the system.
Fig. 2 Uplink/Downlink packet flow in the BS MAC.
2.2 Related Works
A number of works regarding the bandwidth allocation over 802.16 can be found.
Hawa and Petr [7] propose a QoS architecture applicable for both DOCSIS and 802.16 and use semi-preemptive priority for scheduling UGS traffic while priority-enhanced WFQ for others. Chu et al. [8] employ the Multi-class Priority Fair Queuing (MPFQ) for the SS scheduler and the Weighted Round Robin (WRR) for that
of the BS. Though innovative in the architectural design, both of them do not present experiment results validating the architecture. Wongthavarawat and Ganz [9]
introduce the Uplink Packet Scheduling (UPS) for service differentiation. It applies the Strict Priority to the selection among service classes, and each service class adopts a certain scheduling algorithm for queues within it. However, this scheme deals with only uplink channel so that overall bandwidth utilization suffers. The Deficient Fair
Priority Queue (DFPQ) [10], which uses the maximum sustained rate as the deficit
counter to specify the transmission quantum, dynamically adjusts the uplink and downlink proportion. Nonetheless, this method is suitable only for GPC rather than GPSS. Maheshwari et al. [11] support GPSS using proportion, though it is not alterable in run-time. Furthermore, the above schemes do not consider the PHY attribute when translating data bytes requested by SSs into OFDMA slots to practically determine the allocation of a time frame.
2.3 Research goals
To solve this problem which could lead to long latency and serious jittering, a well-designed bandwidth allocation algorithm shall possess three merits. First and obviously, the algorithm must implement GPSS to comply with the standard as well as to provide flexible packet scheduling in SSs. Second, service classes should adhere to the corresponding QoS requirements such as Maximum Sustained Traffic Rate (MSTR) and Minimum Reserved Traffic Rate (MRTR) for differentiated guarantees.
The former prevents a certain class from consuming too much bandwidth while the latter maintains a service class with least feeds. Third, in order to achieve high throughput, the proportion of the uplink and downlink subframes should be able to be dynamically adjusted. The separator was previously fixed and failed to adapt to situations in which uplink and downlink bandwidth needs vary.