## 2 System Overview and Research Directions in WOBAN
### WOBAN Architecture and the Motivation behind WOBAN  
As stated earlier, a WOBAN consists of a multi-hop multi-radio wireless mesh network at the front end that connects to an optical access network providing connection to the Internet at the back end. At the back end, the dominant technology is the PON having OLTs located at the CO and optical network units (ONUs) that are connected to the wireless gateways routers. In the wireless infrastructure, standard WiFi and WiMAX technologies can be used for wireless mesh networks. The subscribers, i.e., the end-users (also known as mesh clients) send packets to their neighborhood mesh routers. The mesh routers inject packets to the wireless mesh of the WOBAN. The mesh routers can reach any of the gateways/ONUs through multi-hop routing. A WOBAN provides the flexibility of optimum utilization of the optical and wireless components to achieve scalability, cost-efficiency, and performance. In the upstream direction (mesh routers to ONUs), the routing is basically anycast and in the downstream direction (ONUs to mesh routers), the routing is unicasting as traffic is sent from an ONU to a particular mesh router only. The gateways/ONUs can be strategically placed over a geographic region to better serve the wireless users. The detailed WOBAN architecture is depicted in Figure 1c. Below, we summarize the advantages of WOBAN as opposed to fully optical and wireless access:  
1. A WOBAN can be more cost efficient as compared to a PON, since fibers in a WOBAN do not need to be penetrated to every subscriber's homes, premises or offices. One estimate [4] shows that the cost to wire 80% of the US households with broadband lies between $60-120 billion, whereas using wireless drastically reduces the cost to $2 billion. Even in developing regions, the cost of providing and maintaining wireline broadband connectivity is prohibitively expensive. WOBAN-like hybrid architecture is particularly appealing in these scenarios where fiber layout is only needed from central offices to some ONU points, beyond which the wireless frontend consists of mesh routers that can be flexibly deployed. 2. The wireless mesh architecture provides more flexible wireless access to users compared to optical access networks. It is often difficult to deploy optical fibers and equipments in highly populated areas as well as in rugged environments. In these environments, the wireless front-end can provide easy coverage and connectivity in a cost-effective manner. Hybrid optical-wireless networks, if properly designed, can achieve better deployment flexibility than traditional PON. 3. The self-healing nature of the wireless front end makes WOBAN more robust and fault tolerant than traditional PON. In traditional PON, a fiber cut between the splitter and an ONU or between the splitter and the OLT makes some or all of the ONUs disconnected from OLT. In WOBAN, the traffic disrupted by any failure or fiber cut can still be forwarded through the mesh routers using multi-hop routing to other ONUs and then to the OLT. 4. WOBAN enjoys the advantages of anycast routing [5,6]. If a gateway is congested, a wireless router can route its traffic through other gateways. This reduces load and congestion on that gateway and gives WOBAN a better load-balancing capability. Such schemes require information exchanges among the gateway nodes which is a key feature of a WOBAN, as they are connected through optical fibers but is absent in a pure wireless mesh network. 5. WOBAN has much higher bandwidth capacity compared to the low capacity wireless networks, which reduces the traffic congestion, packet loss rate as well as end-to-end packet delay. For example, in Figure 1b, multiple mesh routers in the mesh backbone result in traffic congestion, wireless interference, and channel contention, which can be improved using multi-channel communication. However, even in case of multi-channel communication, interference is a limiting factor, which can be alleviated by collaboratively using the optical and wireless technologies. 6.