15.2. Background
In this section, we give a brief discussion on hybrid wireless access networks, wireless ad hoc and sensor networks, and security and survivability for wireless networks in general.
Traditionally, wireless networks have been homogeneous with limited or no interoperability between various technologies. However, no single wireless technology is capable of supporting all the various application requirements such as coverage, data rates, error rates, mobility, and so on. The evolutionary trend is toward a mixture of various technologies and networks that must coexist and interoperate to provide required services [2]. Cellular service providers are currently deploying such hybrid architectures in the United States and Europe. As an example, a WLAN may be employed for local coverage, low mobility, and high data rates while an overlaid mobile data network (such as the General Packet Radio Service, GPRS, or Third-Generation Universal Mobile Telecommunications Service, 3G-UMTS) is used for wide-area coverage and high mobility, but lower data rates [7]. Figure 15.1 shows the architecture of a future hybrid wireless network with 3G-UMTS, IEEE 802.11 infrastructure WLANs, a backhaul mesh network (perhaps using 802.16 WiMAX), a single ad hoc network cluster, and a sensor network. MSs participating in such a hybrid network must have the capability to operate with multiple technologies and possess the intelligence to appropriately switch among technologies and networks. Entities in Figure 15.1 are grouped into different subsystems (such as the access subsystem), which are considered in more detail later in the chapter.
In Figure 15.1, the mobile switching center (MSC) in conjunction with several databases—the home and visitor location registers (HLR/VLR), the equipment identity register (EIR), and the authentication center (AuC)—manage access control (via authentication) and mobility for MSs. The base station/radio network controller (BSC/RNC) is involved in allocating and deallocating radio channels, handoff decisions, transmit powers, and so on. GPRS support nodes (GSNs) that support data traffic also play a role in mobility management. The serving GSN (SGSN) handles communications to and from MSs in its service area and is similar to the foreign agent (FA) in the mobile Internet Protocol (mobile IP). The gateway GSN (GGSN) is similar to the home agent (HA) in mobile IP. WLANs do not have the full range of mobility and radio resource management functions as the cellular network and rely on the AP (or mobile IP) to provide some of these services. Since LANs are broadcast in nature (there is no need to track MS locations) and based on carrier sensing, they do not require sophisticated management techniques. Authentication using 802.1X or RADIUS servers is also possible in WLANs and this is now a standard (802.11i). In backhaul mesh networks, mesh routers form a network through self-configuration and may also provide connectivity to isolated base stations or access points [8]. Mesh networking is possible using 802.11 WLAN technology or 802.16 WiMAX technology. Mobile stations in ad hoc clusters communicate in peer-to-peer fashion directly or in multihop fashion using mobile ad hoc networks (MANET) routing protocols.
Wide-area cellular systems are far more sophisticated than their local-area, metro-area, and ad hoc counterparts. There are little or no mechanisms or entities (like the HLR/VLR, BSC, or AuC) to handle mobility management, radio resources management, billing, or security in WLANs or ad hoc networks. For seamless operation between different technologies, some interworking functions are required [2, 7] that are not discussed here in great detail. Five different approaches for roaming between GPRS and WLANs were suggested in Pahlavan and Krishnamurthy [2]. The approaches can be broadly considered as using an emulator entity that makes the WLAN look like a location area or routing area of a UMTS network, using mobile IP or a proxy mobility gateway to reconcile the two networks. A network where the cellular operator owns the WLAN is considered in Salkintzis et al. [7]. A loose coupling approach where the WLAN is complementary to GPRS and only uses the GPRS databases (for authentication and subscriber information) but not the GPRS interfaces is proposed. The WLAN directly transports data to the Internet in this case. A tight coupling approach where data from a WLAN first goes through the GPRS core network is also proposed in Salkintzis et al. [7]. In this case, the WLAN looks like a GPRS radio access network with a different air interface. These two approaches are similar to the ones proposed in Pahlavan and Krishnamurthy [2] but include more operational details. Details of the messages and protocols used in these approaches can be found in Pahlavan and Krishnamurthy [2] and Salkintzis et al. [7]. Similarly, mesh routers with gateway bridges are used for interworking with base stations or MSs in mesh networks [8].
Recently, interconnecting MANETs and infrastructure wireless networks has been considered in Luo et al. [9] where the primary motivation is to allow MSs that have poor connectivity to a base station to get higher throughputs by using intermediate relays or proxies that have better connectivity. In this work, greedy and on-demand algorithms have been developed for discovering the proxies with the best connectivity and routing between the MSs. While the greedy algorithm generates lesser overhead on the cellular uplink, it results in larger energy consumption at the MSs compared to the on-demand algorithm. However, this work does not consider survivability or interworking aspects of the hybrid network.
Wireless sensor networks consist of hundreds of sensor nodes deployed over the environment to be sensed. Typically, a high-power, high-capability base station communicates with all the nodes through broadcast messages for network activities such as synchronization, querying, topology control, transmission schedules, and so on. [3]. Sensor nodes have low transmission powers, so they have to use multihop communications to report sensed data to sensor nodes closest to the base station. These closest nodes then relay the information (perhaps fused, aggregated, or processed cooperatively by all the sensor nodes) to the base station, which may be connected to the Internet. We consider a different kind of sensor network with heterogeneous sensor nodes and no base station in Section 15.6.3.