## 2 System Overview and Research Directions in WOBAN  
### Current Trends in Access Networks  
Current access networks can be broadly divided into two classes: those using high-bandwidth optical access networks, and those that use wireless access networks. The dominant broadband optical access technology is PON (shown in Figure 1a). A PON connects the telecom central office (CO) to businesses and residential users by using a single-wavelength channel in the downstream direction (i.e., from Optical Line Terminal (OLT) at CO to Optical Network Units (ONU)), and a channel on a different wavelength in the upstream direction (i.e., from ONUs to OLT). The key function of OLT is to control the information flow across the PON, while residing in a central office. On the other hand, ONU resides on the customer's premises to provide last-mile connectivity. The PON interior elements are basically passive combiners, couplers and splitters. Since no active elements exist between the OLTs and the ONUs, PONs are robust networks that are cost and power efficient as well. To ensure multiple access capability, several Time-division multiple access (TDMA) PONs such as Ethernet PON (EPON), Gigabit PON (GPON), Broadband PON (BPON) are deployed by several carriers. Eying for higher bandwidth requirements and better quality of service of future access networks, Wavelength Division Multiplexing-PON (WDM-PON) architecture is proposed in the literature which employs WDM multiplexing in both upstream and downstream directions of the access network. Broadband optical networks extend broadband communication links close to the user and leaves the user with the option of using short range mobile access technologies such as WiFi.  
On the other hand, the popular approach for extending the range of untethered or mobile access is to use wireless mesh networking, for instance, using WiFi and WiMAX. WiFi (using the IEEE 802.11 standard) technology is mainly used in local-area networks and offers low bit rate (max 54/11/54 Mbps for 802.11a/b/g, respectively) and limited range (typically 100 m) communications. WiMAX (standard: IEEE 802.16) is particularly suitable for wireless metropolitan-area networks (WMAN) because of its offers high bit rate and long range communications. WiMAX supports data rates up to 75 Mbps in a range of 3-5 km, and typically 20-30 Mbps over longer ranges. Wireless mesh networks (shown in Figure 1b) use wireless mesh routers to forward traffic to a gateway using multi-hop communications. The growing demands on broadband connectivity require development of novel approaches for the access network architecture. A potential solution may be achieved by integrating the advantages of the both worlds, where \"wireless needs optical\" for greater bandwidth availability, especially at the back-end, and the \"optical needs wireless\" for cost effective and flexible network deployment over large geographic areas. The WOBAN architecture proposes achieving this compromise between high-bandwidth optical access and untethered wireless access networks to extend the reach of the Internet in a cost-effective and efficient manner.  
Figure 1: Access network architectures: (**a**) a PON for optical access; (**b**) a typical wireless mesh network for wireless access; (**c**) a WOBAN, which is a combination of PON and wireless mesh.