## 2 System Overview and Research Directions in WOBAN
### Related Works  
We note that some of the key design issues of WOBAN, namely, quality based routing, anycasting, and channel assignment, have been extensively researched in the domain of (purely) wireless mesh network. In the following, we review some of the notable concepts from prior work on these topics. Later, we also summarize the research that addressed different architectural and routing challenges in WOBAN.  
**Anycasting Based Routing and Channel Assignment:** In [7], the authors propose a _multi-gateway wireless mesh network routing protocol_ (_MAMSR_) based on the _Dynamic source routing_ (_DSR_) protocol. The routing metric used in _MAMSR_ is hop count, where gateway and route selection is performed on the basis of the first RREP packet received by the source.A hybrid anycast routing is proposed in [8] where the network is divided into two regions: proactive and reactive. Nodes that are very close to any gateway are in the proactive region and send packets to this gateway only. Nodes that are not in the proactive region are part of the reactive region; these nodes choose gateways that carry minimum load. Another multi-gateway association scheme is proposed in [9], where the shortest paths from any node to each gateway and the available bandwidth in all the paths are computed. The path that has the largest available bandwidth is selected in a greedy manner. In [10], the authors propose a scheme where each source keeps track of its nearest gateway as the primary and other gateways as secondary gateways. All nodes generally send traffic to their primary gateways. If the primary gateway is congested, sources with high traffic are notified by the gateway to switch their traffic to some other gateways.  
The multi-channel assignment is also well-mined in wireless networks. The existing literature on channel assignment schemes can be broadly classified into three categories: static assignment, dynamic assignment and hybrid assignment. While static channel assignment strategies [11; 12; 13; 14; 15; 16] permanently assign channels to the network interfaces, the dynamic strategies [17; 18; 19; 20] switches their interfaces from one channel to the other in between successive data transmissions. On the other hand, the hybrid approaches [21; 22] adopt static assignment for few fixed interfaces, whereas use dynamic assignment on some switching interfaces.  
**Architecture and Routing in hybrid wireless-optical networks:** Hybrid wireless-optical networks have been manifested with different names such as WOBAN [4], FiWi [23], MARIN [24], GROWNet [25], ACCORDANCE [26]. Relevant surveys on wireless-optical networks are reported in [27; 28].  
Much literature has described the problem of optimal ONU placement based on the traffic demand in a geographic area. In [29], the authors propose a greedy algorithm where a number of predefined points are considered as initial candidates to place the ONUs. Then, each user identifies the primary ONU which is the closest and then the locations of the primary ONUs are calculated as the center of the users. An improvement of this greedy algorithm by using _simulated annealing_ and _hill climbing_ are described in [30]. In [31], the authors propose a _mixed-integer-programming_ approach to solve the problem of ONU placement.  
Several routing schemes are presented in the context of WOBAN. Delay and capacity aware routing is proposed in [32; 33]. A minimum-hop routing and shortest-path routing is proposed in [4]. An efficient radio and channel assignment scheme in a multi-radio WOBAN is discussed in [34]. Analytical modeling of the IEEE 802.11 Distributed Coordination Function (DCF) in radio-over-fiber (RoF) wireless LANs is proposed in [35].