# On the Feasibility of Wireless Multimedia Sensor Networks over IEEE 802.15.5 Mesh Topologies
## 3 Technological Background
### IEEE 802.15.5 Low Rate  
IEEE 802.15.4 standard defines Physical (PHY) and Medium Access Control (MAC) specifications for Low-Rate Wireless Personal Networks (LR-WPAN). This standard does not define the network layer and, therefore, does not offer routing capabilities by itself. Consequently, with the aim of providing an efficient multi-hop scheme, the IEEE 802.15.5 standard emerged in 2009. This new standard has a low-rate part, which will be the focus of the paper, consisting of a set of recommendations, which, according to the standard, provide an architectural framework that enables low-power, low-rate WPAN devices to promote interoperable, stable, and scalable wireless mesh topologies [28].  
The objective of the standard is to support features such as unicast, multicast and reliable broadcast over multi-hop mesh links, synchronous and asynchronous communication for power saving, trace route functionality and portability of end devices [28]. In addition, being a mesh standard, it provides route redundancy, which enhances network reliability. Another key feature of the standard is that it fosters simplicity. There is no need for route discovery, reducing communication overhead, or for storing routing tables for all possible destinations. One of its main advantages over other mesh networking approaches is that it has a very similar set of service access points to the 802.15.4 MAC sublayer and, therefore, migration from non-mesh IEEE 802.15.4 networks to mesh networks is a straightforward process. In this regard, IEEE 802.15.5 aims at standardizing mesh networking over IEEE 802.15.4.  
Figure 2 shows a comparison of the topologies of an IEEE 802.15.5 network with a cluster-tree IEEE 802.15.4 network. The main difference lies in the mesh links (represented by dashed lines), which connect any node with any neighbor node within communication range. The types of device defined in an IEEE 802.15.5 network are also shown in the Figure. A _mesh coordinator_ is the root of the logical tree of the mesh network. It creates and manages the mesh network, and may also serve as the reference clock when network-wide synchronization is required. _Mesh devices_ are responsible for intelligently relaying sensor data toward their intended destination. Finally, _end devices_ are generally nodes with in-built sensors and do not include mesh routing capability.  
The establishment of the network topology is key to the operation of IEEE 802.15.5. During this process, logic addresses are assigned to nodes according to their position in the network. By binding logic addresses to the network topology, the Tunneled Direct Link Setup (TDLS) routing protocol [29] suggested by the IEEE 802.15.5 standard can forward packets without performing route discovery,  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**CRF** & **Bitrate (kbps)** & **Latency (s)** & **Mean PSNR (dB)** \\ \hline
10 (_medium-low compression level_) & 247 & 356.704 & 50.936 \\ \hline
24 (_medium-high compression level_) & 53 & 84.042 & 41.730 \\ \hline \hline \end{tabular}
\end{table}
Table 2: Comparison of bitrate, latency (Latency, see Section 5.1) and PSNR (Peak Signal-to-Noise Ratio (PSNR), objective video (or signal) quality metric, analyzed in Section 5.6) according to CRF.  
Figure 1: (a) Akiyo QCIF, CRF = 10; (b) Akiyo QCIF, CRF = 24.  
eliminating the associated latency and reducing communication overhead as well as avoiding the need for explicit route repair. This protocol, which measures route quality in terms of hop count, balances traffic among all the most suitable paths.