The PMP topology supports both TDD and FDD duplexing modes, while Mesh topology supports only the TDD duplexing mode. In the case of a Mesh network, on the opposite side of the basic PMP mode, there can be no separate downlink and uplink subframes since all stations have the same hierarchy. An (optional) Mesh frame structure is defined in the 802.16 standard to facilitate Mesh networks. Figure 9.16 shows the global structure of this Mesh (TDD) frame. The contents of this Mesh frame are now described.
A Mesh frame consists of a control and a data subframe. This frame uses information contained in the MAC management message MSH-NCFG (Mesh Network Configuration) and, specifically, the Network Descriptor IE.
The control subframe serves two basic functions. The first function is defined as network control and realises the creation and maintenance of cohesion between the different systems. It is described in Section 9.6.1 below. The other function is defined as schedule control and realises the coordinated scheduling of data transfers between systems. It is described in Section 9.6.2. Frames with a network control subframe occur periodically, as indicated in the Network Descriptor, included in this subframe and detailed below. All other frames have a schedule control subframe. The length of the control subframe is fixed and of length MSH-CTRL-LEN x 7 OFDM symbols, where MSH-CTRL-LEN is a parameter indicated in the Network Descriptor IE of MSH-NCFG.
The Network Control subframe is made of two parts and is shown in Figure 9.17. The MAC PDUs of these two parts, the network entry and the network configuration, contain two Mesh messages: MSH-NENT and MSH-NCFG:
MSH-NENT (Mesh Network Entry) is a basic MAC management message that provides the means for a new node to gain synchronisation and initial network entry into a Mesh network.
MSH-NCFG (Mesh Network Configuration) is a broadcasted MAC management message that provides a basic level of communication between nodes in different nearby networks, whether from the same or different equipment vendors or wireless operators. Among others, the Network Descriptor is an embedded data of the MSH-NCFG message. The Network Descriptor contains many channel parameters (modulation and coding schemes, threshold values, etc.), which makes it similar to the UCD and DCD.
The Schedule Control subframe is made of three parts and is shown in Figure 9.18. The MAC PDUs of these three parts, the centralised configuration, the centralised scheduling and the distributed scheduling contain three Mesh messages: MSH-CSCF, MSH-CSCH and MSH-DSCH:
MSH-CSCF (Mesh Centralised Schedule Configuration) and MSH-CSCH (Mesh Centralised Schedule) are broadcasted MAC management messages that are broadcasted in the Mesh mode when using centralised scheduling. The Mesh BS broadcasts these messages to all its neighbours and all nodes forward (rebroadcast) them.
The Mesh BS may create a MSH-CSCH message and broadcast it to all its neighbours to grant bandwidth to a given node, and then all the nodes with a hop count lower than a given threshold forward the MSH-CSCH message to their neighbours that have a higher hop count. On the other hand, nodes can use MSH-CSCH messages to request bandwidths from the Mesh BS. Each node reports the individual traffic demand requests of each ‘child’ node in its subtree to the Mesh BS.
MSH-DSCH (Mesh Distributed Schedule) is a broadcasted MAC management message that is transmitted in the Mesh mode when using distributed scheduling. In coordinated distributed scheduling, all the nodes transmit a MSH-DSCH at regular intervals to inform all the neighbours of the schedule of the transmitting station. The coordination protocol is provided in the standard. Further, the MSH-DSCH messages are used to convey information about free resources, indicating where the neighbours can issue grants.