## 2 Background
### Hybrid Wireless Mesh Protocol (HWMP)  
In addition to the frequency bands used, the routing protocol is another important factor that dramatically affects the network performance. The Hybrid Wireless Mesh Protocol (HWMP) is used as the mandatory routing protocol to adhere to the IEEE 802.11s draft standard. In this protocol, the network is allowed to operate in both proactive and reactive modes.  
In the proactive mode, the MPs communicate with each other with the help of the root and the tree topology will be built with either the Route Request (RREQ) or the Root Announcement (RANN) mechanism. The RREQ message will be broadcasted by the root MP or the root mesh portal to create the paths between the root MP and all the other MPs in the WMN, while the RANN message distributes the routing information to reach the root MP [4]. The tree will then be created and maintained for all the MPs in the WMN to reach the root MP because it contains the information for the tree topology [21].  
The reactive mode allows the MPs to communicate with each other through the path created on-demand by the network and no root MP is required [4]. If there is no root MP available in the WMN, the WMN will be able to operate solely on the reactive mode. The RREQ message will be broadcasted by the MP as the route discovery process begins if it needs a path to its destination MP and the destination MP is not in its routing table [21]. The received RREQ message will be used to create a path to the source or update the path if the RREQ consists of the same or greater sequence number with an improved metric [4]. The destination MP or any intermediate node that is connected to the destination MP will respond to the RREQ with a unicast RREP message [21]. However, the initial latency for the path formation that lasts until the first data packet is received by the MP that initiated the route discovery process [21].  
A root portal is required for the system to operate in the hybrid mode [21]. Because the root portal has the information for all MPs in the WMN when the registration mode is used, it can accelerate the route discovery process by forwarding the data frames to the MP that wants to form a path with the other MP that is in the same WMN [21]. This information will be sent to the requesting MP along with the signal that these two MPs are in the same WMN, and thus, a route discovery will be initiated and the optimal path between these two MPs will be formed [21]. This hybrid mode will improve the conventional reactive mode with the information obtained from the proactive mode [21]. The initial latency in the path settings is then eliminated [21].  
To find the best path for route formation, the link cost for each link in the WMN had to be computed. The link with the lowest metric value will be the chosen path for the transmission of the data [4]. This metric value can be found in the metric field of the RREQ, Route Reply (RREP) and RANN messages, and will be propagated to build or update the path [4]. In IEEE 802.11s, the airtime link metric was defined as the default link metric computation method for the path selection [4]. Thisairtime link metric is a radio-aware routing metric. By transmitting a frame over the particular wireless link, the metric can measure the amount of channel resources consumed [4].