## 3 Wireless Access-Metro Networks
### Scheduling and Resource Management
In the centralized scheduling mode, such as the PMP, each Mesh-SS (MSS) sends its request to the mesh-BS (MBS) that manages the network. In the distributed scheduling mode, each MSS distributes its scheduling information and one-hop neighbors to all its adjacent MSSs. A three-way handshake mechanism is deployed for bandwidth allocation. Coordinated (collision-free) and uncoordinated (non-collision-free) methods are used for distributed scheduling. The two different mesh scheduling methods can be applied together by subdividing the data part of the frame into two parts, one for centralized scheduling and another one for distributed scheduling [20]. Generally, WiMAX applies three different types of SFs: \((i)\) provisioned SFs, which are represented by an SFID; \((ii)\) admitted SFs, which show the admitted requests for the available resources and/or SFs; and \((iii)\) active SFs which exhibit the allocated resources and/or SFs. Each SS must send an UL-request to the BS, and the BS responds with an uplink bandwidth grant (UL-grant) subsequently. The UL-grant services assign bandwidth based on the QoS parameters of that connection. Although bandwidth requests are per connection, the BS does not allow to grant the entire uplink capacity to a single SS. When an UL-grant is sent by the BS, the receiver SS cannot change or share the size of the granted bandwidth [38]. In IEEE 802.16, there are two modes of transmitting the UL-requests: \((i)\) contention mode and \((ii)\) contention-free mode (polling). In the contention mode, SSs send their UL-requests during the contention period, whereby contention is resolved by means of back-off. In contention-free mode, the BS polls each SS and SSs reply by sending UL-requests. In WiFi, the so-called transmission opportunity (TXOP) is the main new MAC enhancement of HCF. In EDCA, a QoS-enhanced STA (QSTA) and, in HCCA, a QoS-enhanced access point (QAP) generate the TXOP to allow a QSTA to transmit a burst of data frames, separated only by a short inter-frame space (SIFS) from each other. In HCCA, QSTAs report their QoS requirements for each traffic class to the QAP by means of traffic specification (TSPEC) frames. A TSPEC describes the traffic characteristics and QoS requirements for a given traffic flow, also referred to as traffic stream (TS), e.g., minimum and maximum service interval, minimum data rate, and delay bound. A TID is applied by MAC clients to distinguish MAC service data units (MSDUs) for the MAC entity that supports QoS differentiation via different service classes. There are sixteen possible TID values, whereby eight identify traffic classes and eight identify parameterized TSs. The IEEE standard 802.11n does not specify any particular scheduling algorithm. Scheduling can be done in two ways: inter-QSTA scheduling and intra-QSTA scheduling. Inter-QSTA scheduling arbitrates the transmissions of different QSTAs, while intra-QSTA scheduling arbitrates the transmissions of different traffic classes or AC queues in each QSTA. There are two possible implementation methods. Either inter-QSTA scheduling is implemented at the QAP or TXOP holder, and each QSTA performs its own intra-QSTA scheduling, or both inter-QSTA and intra-QSTA scheduling are implemented at the QAP or TXOP holder. During a TXOP, a single block acknowledgment request (BAR) frame is transmitted at the end of data transmission by the TXOP holding QSTA. The receiving STA replies with a block acknowledgment (BA) frame to confirm the frames that were correctly received. Moreover, IEEE 802.11e is able to provide direct connections between QSTAs by using the so-called direct link protocol (DLP). In a direct connection, the QAP can monitor the involved QSTAs by sending beacons periodically. In IEEE 802.11n, HT STA and HT AP are able to provide QoS like the aforementioned QSTA and QAP in IEEE 802.11e.