Wimax standards and Security The Wimax

Tree-Based Scheduling Broadcast Channels

Centralized scheduling MSH-CSCH and MSH-CSCF messages are transmitted in a tree-based scheduling broadcast channel. Scheduling of transmissions

in this channel is performed by following a breadth-first traversal of a glob- ally known tree topology. The global tree topology is first distributed with MSH-CSCF messages, which carry the entire routing tree the messages are multicast on. If the topology changes, further MSH-CSCF messages notify the nodes of the changes. As nodes receive MSH-CSCF messages, they learn the multicast routing tree, as well as which node in the topology is currently broadcasting the MSH-CSCF message, so they can calculate the transmission opportunity in which they should transmit. In the case of MSH-CSCH mes- sages, the nodes know the topology prior to any transmission of messages, so they can also calculate their next transmission opportunity the same way they would for MSH-CSCF messages.

4. 
   Best Effort Broadcast Channel
   

The best effort broadcast channel is used for transmission of distributed scheduling MSH-DSCH messages. This channel consists of unused transmis- sion opportunities in the data channel. The 802.16 standard does not set any rules on how this channel should be accessed.

5. 
   Transmission Scheduling in the Data Channels
   

In 802.16 mesh protocol, there are two types of data channels: the central- ized scheduling data channel and the distributed scheduling data channel. The difference between the two data channels is in how their transmission opportunities are assigned. In the centralized scheduling data channel, the transmission opportunities are assigned with the centralized scheduling pro- tocol, which relies on the base station to assign connection bandwidths and distribute them to all nodes. The nodes use the knowledge of the bandwidth assignments to independently calculate the global transmission schedule. In the distributed scheduling channel, the transmission opportunities are dis- tributed with the decentralized scheduling protocol, which uses pairwise negotiation of connection bandwidths to achieve conflict-free schedules using only local information.
The centralized scheduling data channel uses the first MSH-CSCH-
DATA-FRACTION transmission opportunities of the data subframe. These transmission opportunities are assigned with the centralized scheduling pro- tocol. In the centralized scheduling protocol, the nodes request bandwidth from the base station by sending MSH-CSCH messages to their parents in the scheduling tree (Figure 8.6). Once all the requests reach the base station, the base station uses them to calculate the bandwidth for each connection in the network and multicasts the connection bandwidth assignments using a new wave of MSH-CSCH messages. The connections whose centralized scheduling bandwidth is positive, form a tree coinciding with the scheduling tree for MSH-CSCH and MSH-CSCF messages. If by changing the connection bandwidths, the base station also changes the routing tree for the network, it multicasts routing changes with MSH-CSCF messages before it multicasts MSH-CSCH messages. MSH-CSCH messages coming from the base station

BS:1
^BS:1 MSH-CSCH,MSH-CSCF

SS:2 SS:2
(a) (b)