In stark contrast to the PMP mode, the QoS in MESH mode is provisioned on a packet-by-packet basis. Thus, the per-connection QoS provisioning using the DSx messages as introduced previously is not applicable. This design decision helps to reduce the complexity of implementing the MESH mode considerably. However, the MESH mode even with this simplification is quite complex.
The CID in the MESH mode is shown in Figure 1. The mesh CID is used to differentiate the forwarding service a PDU should get at each individual node. As can be seen from Figure 1 it is possible to assign a priority to each MAC PDU. Based on the priority the transmission scheduler at a node can decide if a particular PDU should be transmitted before another. The field reliability specifies the number of retransmissions for the particular MAC PDU (if needed). The drop precedence specifies the dropping likelihood for a PDU during congestion. Messages with a higher drop precedence are more likely to be dropped. In effect, QoS specification for the MESH mode is limited to specifying the priority of a MAC PDU, the reliability, and its drop precedence. Given the same reliability and drop precedence and MAC PDU type (see Figure 1), the MAC will attempt to provide a lower delay to PDUs with higher priority. This QoS mechanism, however, does not allow the node to estimate the optimal bandwidth requirement for transmissions on a particular link. This is because (just based on the previous interpretation as presented in the 802.16 standard), the node is not able to identify the expected arrival characteristics of the traffic and classify it into the different categories as traffic requiring UGS, rtPS, nrtPS, or BE service.
Figure 1: MESH connection identifier (CID).
To summarize, QoS mechanisms in the MESH mode are not consistent with those provided for the PMP mode. In addition, the per-packet QoS specification for the MESH mode does not allow a node to optimally estimate the amount of bandwidth required for transmission on a link, as no information about the data scheduling service required for the traffic is included explicitly in the QoS specification in the mesh CID.
We next give an overview of the existing bandwidth request and grant mechanisms specified for the MESH mode of 802.16. This is followed by a description of our proposed QoS architecture, which enables efficient bandwidth management in the MESH mode and allows support of the data scheduling services consistent with those outlined for the PMP mode.
