*3.1.2. Default mesh QoS mechanism*

In the mesh mode, a special MAC is defined in the IEEE 802.16, which provides two different types of scheduling mechanisms – centralized and distributed scheduling.

Centralized Scheduling (Mesh CS): the Mesh-BS is responsible for supplying resources for each link in response to resource requests. Mesh centralized scheduling messages transmitted in a scheduled control subframe are used for this purpose.

In centralized scheduling, when a node has packets to send to either other MSS or the MBS, it sends a request packet in the control subframe, using the Mesh Centralized Scheduling Message (MSH-CSCH message) to the MBS. The node sends one bandwidth request for each link it has and all requests belonging to that node are sent in one MSH-CSCH message. After receiving requests from all the MSSs in the network, the MBS applies its traffic scheduler to these requests, including its own traffic requests.

Based on the scheduler used in the MBS, these requests are granted, either wholly or partially. Then the MBS broadcasts these grants in a MSH-CSCH message. A grant packet describes the data subframe usage of a frame. This data subframe description belongs to a frame after the frame from which the grant is sent. Each MSS forwards this grant message to its children. However, these requests and grants only include the amount of data that a node can transmit [14]. Figure 7 illustrates how it works in mesh mode.

**Figure 7.** Overview of scheduling in the mesh mode

Distributed Scheduling (Mesh DS): The neighboring Mesh SS responds to a request with a corresponding grant for a link between two Mesh SSs. Mesh distributed scheduling messages are exchanged to perform this operation.

The scheduling policy for accessing data slots in coordinated distributed fashion, is not specified in the IEEE 802.16 standard. The standard only defines the Mesh Distributed Scheduling Message (MSH-DSCH message), and specifies the scheduling to avoid collisions between messages of different nodes. The MSH-DSCH message contains the scheduling information organized in Information Elements (IE): Request IE, Availability IE, Grant IE and Scheduling IE.

The scheduling procedure follows a three-way handshake to reserve the minislots. First, a node sends an MSH-DSCH message to one of its 1-hop neighbors, requesting a set of data slots. In the message, the node also includes the set of data slots that it has available for reservation. The 1-hop neighbor grants the request by replying with another MSH-DSCH message that specifies a set of data slots that confirms the availability of data slots at both nodes. Finally, the first node confirms the reservation of this set of data slots by repeating the grant in another MSH-DSCH message.

In contrast with point-to-multipoint WiMAX networks, the standard does not define sched‐ uling services for Mesh WiMAX networks [13].
