Chapter 14. RECENT ADVANCES IN WIRELESS NETWORKS

14.1 INTRODUCTION

It is expected that by the end of this decade, the predominant mode of Internet access will be over wireless networks. In order to support high data rates on short ranges, new promising technologies such as the ultra-wide band (UWB) transmission scheme and optical wireless wavelength division multiplexing (WDM) networks are currently under research. In addition to the demands on increasing the performance of the lower layers of the system, pressure is on to efficiently reuse the existing frequencies, which also demands allocation of newer bands for commercial wireless communications. The additional areas of interest in next-generation wireless networks include freedom in selection of a particular network from a set of heterogeneous wireless networks, seamless handoff across such networks, support for multimedia traffic, and all the above services at an affordable cost. The first step in this is the development of wireless fidelity (Wi-Fi) systems that integrate the high-speed wireless LANs with the wide-area packet cellular infrastructure. The demand for high bandwidth for high-quality multimedia traffic has also resulted in utilizing the WDM technology in wireless communication. This chapter discusses the recent advances in the area of wireless networking such as UWB technology, wireless fidelity, optical wireless communication, and IEEE 802.11a/b/g.

14.2 ULTRA-WIDE-BAND RADIO COMMUNICATION

The major differences between the ultra-wide band (UWB) technology and the existing narrow-band and wide-band technologies are the following: (i) The bandwidth of UWB systems, as defined by the Federal Communications Commission (FCC), is more than 25% of the center frequency or a bandwidth greater than 500 MHz. (ii) The narrow-band and wide-band technologies make use of a radio frequency (RF) carrier to shift the base band signal to the center of the carrier frequency, whereas the UWB systems are implemented in a carrier-less fashion in which the modulation scheme can directly modulate base band signals into an impulse with very sharp rise and fall time, thus resulting in a waveform ranging several GHz of bandwidth [1]. These impulses have a very low duty cycle.¹ A simple example of UWB transmission is any radio frequency (RF) transmission with a bandwidth of 500 MHz at a center frequency of 2 GHz. The IEEE 802.11b is a narrow-band system with a 22 MHz bandwidth with center frequencies ranging from 2.412 GHz to 2.462 GHz. One of the major approaches for generating a UWB waveform is the impulse-based approach which is explored later in this section. The principles behind the operation of UWB technology had been applied in radar applications since the 1980s. With proper emission restrictions (restrictions on the upper limit on the transmission power) in place, the UWB spectrum can overlay the existing narrow-band spectrum, resulting in much more efficient use of the existing radio spectrum.

¹ The duty cycle of a pulse is defined as the ratio of the duration of the pulse to the sum of the pulse duration and the period between two successive pulses. Lower values of duty cycle refer to shorter pulse durations compared to the period.

Figure 14.1 illustrates the upper limits of transmission power permitted for the UWB system in comparison with the IEEE 802.11a. The 802.11a standard has three bands, each with 100 MHz bandwidth with an upper limit of effective isotropically radiated power (EIRP),² as follows: 16 dBm for the band 5.15-5.25 GHz, 24 dBm for the band 5.25-5.35 GHz, and 30 dBm for the upper band with frequency range 5.725-5.825 GHz. This results in a power spectral density of -3 dBm/MHz, 4 dBm/MHz, and 10 dBm/MHz, respectively. In contrast to these, FCC has restricted the maximum power emitted by the UWB systems to be a power spectral density of -41.3 dBm/MHz. As depicted in Figure 14.1, the UWB signals appear as noise to the legacy systems that utilize the same spectrum.

² EIRP of a transmission system in a given direction is the transmission power that would be needed with an isotropic (otherwise called omnidirectional) radiator to produce the same power density in the given direction.

Figure 14.1. Spectrum of UWB systems compared with IEEE 802.11b and 802.11a.

14.2.1 Operation of UWB Systems

The operation of UWB systems is based on transmission of ultra-short pulses (that yields a wide-band bandwidth signal [1]) which are also called monocycles. Each monocycle is similar to a single cycle of an ultra-high-frequency sine wave and is a single ultra-short pulse. The pulse widths of monocycles range from 0.10 to 1.6 ns with pulse-to-pulse intervals of between 25 and 1,000 ns. A single monocycle of width 0.5 ns is shown in Figure 14.2 (a). Figure 14.2 (b) shows a pulse train involving a sequence of monocycles generated at regular intervals. Monocycles are inherently wide bandwidth signals. The time domain function of a Gaussian monocycle (any sharp pulse whose time domain behavior can be represented as the first derivative of a Gaussian distribution) can be represented as , where t is time and τ is the time-decaying factor that decides the duration of the monocycle. It can be represented as in the frequency domain, where the center frequency (f_c) is proportional to . This means that the shorter the monopulse duration, the higher the center frequency. For a monopulse similar to the one shown in Figure 14.2 with a pulse width of 0.2 ns, the center frequency is around 5 GHz. The frequency domain function of an ultra-short pulse shows that the frequency components spread across (bandwidth) a wide range greater than 110% that of the center frequency. For a monopulse with 0.2 ns duration, the bandwidth is approximately around 5 GHz, as illustrated in Figure 14.3.

Figure 14.2. An illustration of a monocycle and pulse train.

Figure 14.3. Bandwidth of a monocycle.

In the pulse train of monocycles, the information is modulated either by using pulse position modulation (PPM) or similar modulation techniques [2], [3]. The effect of modulation with PPM on a train of monocycles is illustrated in Figure 14.4 (a), in which the precise timing of the monocycle varies with respect to its nominal position. For example, in a 100 Mpps (million pulses per second) system, monocycles are transmitted every 10 ns (refer to Figure 14.4). In this case, a PPM modulation may advance the monocycle for 10 picoseconds for representing a digital bit "1" and delay the monocycle for 10 picoseconds representing a digital "0". An encoding can be performed over the unmodulated pulse train to provide distinct time-hopping codes for channelizing the pulse train (channelizing refers to provisioning of multiple channels in the same spectrum). In such a multiple access system, each user would have a unique pseudo-random noise (PN) codes for encoding the pulse trains so that multiple simultaneous transmissions can coexist [4]. Figure 14.4 (b) shows the PN coded pulse train without data content. The nominal pulse position in the train is decided by the PN code as shown in Figure 14.4, where the pulses appear with different time differences from the reference points Tn+1, Tn+2, and Tn+3. This encoding process on the modulated pulse train makes the signal appear like white noise (noise containing all spectral components). A receiver cannot detect the transmission and the corresponding data without having the unique pseudo-random (time-hopping) code with which it was coded, even when the receiver is located at very close proximity to the transmitter.

Figure 14.4. The PPM coded pulse train.

14.2.2 A Comparison of UWB with Other Technologies

The trends that drive the short-range wireless technologies, including UWB technology, are the following: (i) The growing demand for data and multimedia capability in portable devices at high data rate but at low cost and power consumption. (ii) Increasing pressure on the wireless spectrum demanding higher reuse. (iii) Decreasing semiconductor cost and availability of low-power devices. Some of the competing technologies for such wireless access scenario are IEEE 802.11b, IEEE 802.11a, and Bluetooth.

The bandwidth, transmission range, and capacity comparison in bps/m² among 802.11b, Bluetooth, 802.11a, and UWB is illustrated in Figures 14.5, 14.6, and 14.7, respectively. In North America, the 802.11b spectrum ranges from 2,400 MHz to 2,483 MHz and is divided up into 11 channels from 2,412 MHz to 2,462 MHz, spaced 5 MHz apart, as illustrated in Figure 14.8. However, each channel is 22 MHz wide, resulting in great overlap. For example, the spectrum of channel 1 centered at 2,412 MHz overlaps with neighboring channels 2, 3, 4, and 5. This leads to a situation where, at any given point, only three channels (1, 6, and 11) can be simultaneously used. In Europe, of the 13 permitted channels, four channels can be used simultaneously.

Figure 14.5. Bandwidth comparison of UWB with other competing technologies.

Figure 14.6. Range comparison of UWB with other competing technologies.

Figure 14.7. Capacity comparison of UWB with other competing technologies.

Figure 14.8. An illustration of 802.11b channels.

Hence in North America, three channels among the 11 channels can operate simultaneously, providing a total bandwidth of 33 Mbps. The capacity of the system with respect to the area of coverage can be 33 Mbps over an area of a circle with a 100 m radius. Hence, 802.11b has a capacity of 1 Kbps/m².

Bluetooth has a transmission range of 10 m in its low-power mode and a peak bandwidth of 1 Mbps. There can be around 10 simultaneous piconets operating in the 10 m radius circle, making the total bandwidth 10 Mbps. Hence, Bluetooth has a capacity of 31 Kbps/m². The 802.11a has an operating transmission range of around 50 m with a maximum bandwidth of 54 Mbps, and 12 such systems can simultaneously operate within a 50 m radius circle. Thus the capacity of 802.11a is approximately around 82 Kbps/m². UWB systems can vary widely in their projected capacities, but systems with 50 Mbps with a transmission range of 10 meters have been demonstrated. With minimal interference, more than six such systems can operate simultaneously. Hence, UWB transmission systems have a capacity of 955 Kbps/m². The capacity of each technology is shown in Figure 14.7. Thus, UWB systems have great potential for support of future high-capacity short-range wireless systems.

14.2.3 Major Issues in UWB

With the evolution of wireless networks with higher data rates, similar advancement in all the layers of protocol stack is required to make use of the high data rates provided by the physical layers [5]. The increasing application space for multimedia communications puts additional requirements on latency and delay performances. The error-prone and time-varying nature of wireless link results in large delay, delay variations, and misordering of packets. In this section, the major issues at the physical and MAC layers are discussed.

Physical Layer

The wide-band receiver used for the UWB system is susceptible to being jammed by the traditional narrow band that operates within the UWB pass band. Issues such as the wide bandwidth required for the filters and antenna subsystems and filter-matching accuracy are major physical-layer issues difficult to solve without adding to the cost of the physical-layer interface. At the receiver, accurate timing is required for detecting the modulated narrow pulses accurately (refer to Figure 14.4). In addition to all the above, noise from the on-board micro-controller can also result in interference that cannot be easily solved using traditional mechanisms such as band-pass filters, due to the wide bandwidth of the system.

MAC Layer

The most important issues in the design of MAC-layer protocol for UWB systems are the following: (i) controlling channel access, (ii) maintaining QoS, and (iii) providing security. In addition to these, the design of MAC protocols for UWB systems is dictated by other properties of UWB. For example, UWB systems have unique features such as precise timing or position information. The position information can be obtained by the following feature. A 10 GHz UWB system can distinguish signal echos (signal echos refer to the multipath signals that arrive at the receiver through different paths with corresponding delays) that differ by 100 picoseconds. By analyzing the different signal echos that take different delays, a nearly exact position and time information can be obtained. For a 10 GHz system, the delay differences could be of the order of 100 ps and hence an accuracy of 3 cm in distance estimation can be achieved. Utilization of this feature at the MAC layer can improve performance of multimedia unicast and multicast communication. MAC protocols can also benefit from the flexibility of trading off the throughput with respect to the transmission range. The peak amplitude of pulses and the pulse repetition frequency (PRF) can be varied to obtain constant average power. Using this mechanism, MAC protocol can provide different data rates and transmission ranges on a per-link or per-packet basis. UWB systems can be implemented using spread spectrum technology in order to provide better coexistence with the existing systems. Above all, another major aspect of the design of MAC protocol for UWB systems is the compatibility and coexistence of UWB systems with existing WPANs and WLANs.

14.2.4 Advantages and Disadvantages of UWB

Promising applications of UWB in communication include cable-free audio/video devices, broadband WPANs, and high-speed wireless links. UWB systems are potential candidates for high data rate, low power, and short- to medium-range communication applications. UWB systems have a wide range of applications other than communications, some of which are automobile collision-detection devices, medical imaging similar to x-rays and ultrasound scans, through-wall imaging for detecting people and objects in law-enforcement applications, and ground-penetrating radars in construction applications. The major advantages of UWB systems include simplicity of implementation, high data rate, inherent robustness to multipath fading, flexibility of operation, low power consumption, and low cost of implementation. The disadvantages of UWB systems include stringent design requirements of communication subsystems and chances of interference from the existing technologies.

14.3 WIRELESS FIDELITY SYSTEMS

Wireless fidelity (Wi-Fi) system is the high-speed wireless LAN that was originally intended to extend the wired Ethernet in offices to wireless clients. The coverage area and ability to support high bit rates are the two major reasons behind the name Wi-Fi. Though the popular wireless LAN standards IEEE 802.11b and 802.11a are considered as the standard Wi-Fi candidates, conceptually any high-speed wireless LAN protocol such as HiperLAN can be used. The integration of Wi-Fi hotspots (wireless LAN access points) with wide area wireless networking technologies such as GSM and GPRS provides an added advantage for the mobile nodes. Such an integrated system provides secure, reliable, and high-speed wireless connectivity. A Wi-Fi network can be used to connect computers to each other, to the Internet, and to wired networks. Wi-Fi networks operate in the unlicensed 2.4 GHz and 5 GHz radio bands, with an 802.11b or 802.11a, or with products that contain both bands (dual band), so that they can provide an enriched network experience. Wi-Fi systems are potential candidates for provisioning high-speed multimedia content delivery in areas such as indoor offices, airport lounges, and shopping malls. The Wi-Fi Alliance [6] is a non-profit international association formed in 1999 to certify interoperability of IEEE 802.11-based products in order to make the objectives of Wi-Fi a reality. The advantages of Wi-Fi systems are ease-of-use, high-speed Internet access, low cost of operation, and flexibility of reconfiguration.

14.3.1 The Service Provider Models for Wi-Fi Systems

• The Wi-Fi micro carrier model: In this model, small business operators can set up their own access points (APs) and maintain customer relations and billing with subscribers. An example of this category is a restaurant operating a small Wi-Fi system with a set of APs on its premises.

• The franchisee-franchisor model: This model for Wi-Fi systems is that a franchisor company making an agreement with a franchisee (e.g., a restaurant which has an inbuilt Wi-Fi system for its internal purposes) for providing Wi-Fi connectivity on a revenue-sharing basis. The external communication, access network costs, and back-office softwares may be supplied and maintained by the franchisor. Hence, the franchisor company can extend its services to the public.

• The Wi-Fi carrier model: In this model, a particular company referred to as Wi-Fi carrier can own, deploy, and operate a number of Wi-Fi system-enabled APs at public places. The subscribers can utilize the designated carrier's network services in their coverage area based on acceptable billing models.

• The Wi-Fi aggregator model: This model refers to an abstract service provider which strikes wholesale partnerships with Wi-Fi operators. Such aggregators mainly focus on two major things: (i) reselling of the services provided by the Wi-Fi operators and (ii) giving their subscribers access to a large number of networks. The advantages of this model are easy scale-up of network coverage as the aggregator does not own the infrastructure and hence, by having more partnerships, a service provider can increase the coverage area and the customer base.

• The extended service provider model: The synergistic operation of Wi-Fi systems with existing cellular systems, especially with the 3G systems, can increase profits for cellular operators. This can even lead to reduction in the deployment cost of 3G systems. The widespread deployment of Wi-Fi systems can be considered as complementing the 3G systems. Also, the availability of wireless devices equipped with Wi-Fi and cellular interfaces encourages the possibility of switching to the Wi-Fi systems whenever an AP is detected. Vertical handoff (the handoff performed across two networks which are operating at widely varying coverage regions, for example, the handoff performed between a wireless LAN and wireless WAN) can be used to switch back to the wide area cellular networks as and when necessary in such cases. Thus, the extended service provider model envisions the provisioning of Wi-Fi services as an extension to the existing service provided by the cellular network operators. An illustration of such a model is shown in Figure 14.9, where the users can access information and data through either the Wi-Fi AP or through the 3G BS. Referring to Figure 14.9, the node C can communicate directly with the BS in order to make a call to the destination node E, and node K can communicate with the BS through the nearest Wi-Fi AP. This choice of selection can be based on network availability, cost of access, bandwidth availability, and specific user requirements. Such a system requires placement of APs at places such as crowded traffic junctions, public places, and railway stations. The presence of multiple service providers trying to use this model would result in managed utilization of spectrum in the bands assigned for 802.11.

Figure 14.9. A typical scenario of a Wi-Fi system.

14.3.2 Issues in Wi-Fi Systems

The major issues in Wi-Fi systems are the following:

Security

Security in Wi-Fi systems is important and requires viable solutions. The IEEE has proposed a long-term security architecture for 802.11, which is referred to as robust security network (RSN). The major objectives of RSN, which is based on IEEE 802.1x, are access control, authentication, and key management. Even though the research work done by Mishra and Arbaugh in [7] indicates the vulnerability of the initial versions of the IEEE 802.1x, IEEE 802.1x remains the only viable option as of this writing. In this section, the building blocks of security, major elements of threats and major types of attacks that a Wi-Fi system could be subjected to, are discussed. In the infrastructure mode of operation of 802.11 standard, a client has to establish a relation with an AP called an association. There are three main states in the transition process of an association. These are (i) unauthenticated and unassociated, (ii) authenticated and unassociated, and (iii) authenticated and associated.

The major elements in a Wi-Fi system that can contribute to its vulnerability are discussed below.

• The eavesdropper: Wireless medium is inherently broadcast, making it trivial for the adversary with a good receiver to eavesdrop on other stations' traffic. The use of a single key for encryption proved inadequate for this purpose. Dynamic changing of the encryption key is a must for making the eavesdropping a difficult task. Since the majority of the Wi-Fi systems are not WEP (wired equivalent privacy – any privacy solution in which the effort required to break the cipher is expected to be roughly equivalent to the effort required by an intruder to tap into a wired Ethernet)-enabled, it is easier for an inexperienced hacker to snoop on valuable informations such as credit card number, PIN numbers, or social security numbers from a public hot spot Wi-Fi system.

• The rogue client: The authenticated client that has intentions of acting against the organization is another problem aggravated by the wireless environment. A Wi-Fi client running a routing protocol and an additional wired interface can easily use its authentication information on its corporate wireless network infrastructure to transfer valuable information. This problem is extremely difficult to trace and prevent as the loss incurred cannot easily be detected.

• The rogue AP: The rogue AP is the unauthorized AP attached to an organization's network, either on its premises or in its authorized employee's home. An AP without the necessary security provisions, or in situations where the security is disabled temporarily, can act as a rogue AP. Even with security features such as WEP, organizations that use virtual private network (VPN) to secure their wireless LANs may be compromising valuable data. A VPN is a communication network between two or more machines or networks, built for the private use of an organization over a shared public infrastructure such as the Internet. The mechanisms to identify rogue APs include sniffing, pinging, and wide spectrum channel monitoring around the premises of the organization.

The two major types that make a Wi-Fi network particularly vulnerable are session hijacking and man-in-the-middle attack. Both exploit the authentication process.

• Session hijack attack: In this case, a hijacker node waits until the successful completion of a node's authentication process with an AP. On completion of the authentication, the hijacker sends a forged disassociate message that appears to be originated by the AP. On reception of the message, the original node gets disassociated. The AP continues to consider the node as part of the network. Now the hijacker, using the originally authenticated node's address, can utilize network services until the session expires. The initial versions of IEEE 802.1x security framework for 802.11 protocols are vulnerable to this attack, as indicated in [7].

• Man-in-the-middle attack: When an adversary spoofs messages to represent itself as AP to a node under attack, it can obtain the victim's authentication information. Using this information, the adversary authenticates with another AP masquerading as the victim node. This attack is known as man-in-the-middle attack. This attack is particularly effective when the victim node is not present within the transmission range of the AP.

Authentication

The IEEE 802.1x standard is proposed for providing authentication and controlling traffic to a protected network, as well as for dynamically changing encryption keys. The use of the extendible authentication protocol (EAP) as a framework for wired and wireless network provides a mechanism for multiple authentication methods such as certificate-based authentication, Kerberos³ authentication method, token cards, one-time passwords, and public key authentication. The interactions between specific entities in 802.1x is discussed below. The beginning of a series of messages that mark the authentication process is when a client (referred to as the supplicant in 802.1x terminology) sends an EAP-start message to the AP (authenticator). The authenticator responds with an EAP-request identity message. The client replies with an EAP-response packet that contains the identity of authentication server. This authentication server uses a specific authentication algorithm to verify the credentials of the client by performing verification based on any of the multiple authentication mechanisms mentioned above. Upon verification of the client's information, the AP originates an EAP-success or EAP-failure packet to the client. Once the authentication is successful, the AP permits data traffic between the client and the network.

³ Kerberos is a network authentication protocol, designed to provide strong authentication for client/server applications using secret key cryptography.

Quality of Service (QoS)

Provisioning of QoS is important in supporting time-sensitive traffic such as voice and video. The IEEE 802.11e standard which is under consideration is aiming at providing enhanced QoS for Wi-Fi systems.

Economics of Wi-Fi

• Billing schemes: Billing for Wi-Fi systems assumes importance as the commercial viability is a major factor for the existence of the Wi-Fi systems. The possible billing schemes include the following. The entity that has the responsibility of customer care and billing can be different from the one which actually provides network access service. Such an agency is referred to as a billing agency. The billing agency can employ different methods to accept payments from the customers. The major billing approaches that can be employed in the case of Wi-Fi systems are flat-rate schemes and volume-based pricing. In flat-rate schemes, the user is permitted to utilize network services for a specified amount of time without restricting the bandwidth. The volume-based approach charges the customer based on the amount of data transacted over the network. In addition to both these schemes, business establishments can provide Wi-Fi services as a value addition to the customers visiting the premises for the core business activity. In such cases, the billing is not considered as the bandwidth provided belongs to the organization's unutilized bandwidth; it can even be considered without additional charging.

• Revenue sharing model: In the franchisee-franchisor model and the aggregator model, the sharing of revenue is important as there exist multiple business entities in the process of customer relationship, billing, and providing service. Different revenue-sharing models that include a fixed fraction-sharing model (in which the amount shared among the parties involved is prefixed) and a variable-fraction volume-based sharing model (in which the percentage of revenue that goes to different parties involved varies with the volume of data transferred) can be employed. Such systems can consider a constant-fraction for the user per bit of data transferred, and in the high-traffic-density environments, a variable rate per bit of data transferred can be used. In the variable-fraction per bit of data transferred, application level mechanisms which communicate the cost of communication at any particular location is essential.

Spectrum Issues

The issues related to spectrum management are important as Wi-Fi becomes popular with its increasing use as a critical business communication infrastructure. The current allocation of free ISM band in the 2.4 GHz band for 802.11b raises several questions of interference. The source of interferences can be either naturally generated by other devices that are designated to operate at the same band or artificially generated by a rogue interference generator node. Since ISM band is unlicensed, any user with an 802.11 interface can disrupt communication at a specific location without inviting prosecution. The major interference sources for Wi-Fi systems are the following: (i) interference from cordless phones, microwave ovens, and Bluetooth-enabled devices and (ii) interference from jammers. The first issue can be reduced to some extent by the following ways: (i) use of different frequency-hopping patterns for communication-technology-related interference as used in Bluetooth standard and (ii) proper electromagnetic shielding in devices using microwave band for noncommunication-related purposes such as microwave ovens.

14.3.3 Interoperability of Wi-Fi Systems and WWANs

The wireless wide area networks (WWANs) use base stations (BSs) that cover a few tens of kilometers in radius. Traditionally, the mobile nodes (MNs) communicated only through these BSs. However, WWANs provide only low bandwidth (in terms of tens of Kbps) compared to the broadband LAN services (in terms of tens of Mbps) offered by the Wi-Fi networks. There are many situations where a WWAN cannot serve the requirements well, some of which are the following: (a) interiors of buildings, basements of buildings, subways, and where the signal-to-noise ratio (SNR) may not be sufficient to provide a high-quality service (b) heavily loaded BSs (cells), where the call blocking ratio is high due to the high offered traffic and the limited spectrum of WWAN. The Wi-Fi operators can leave the responsibilities such as billing, brand-building, and advertising to the WAN SP and hence, the Wi-Fi SPs are likely to derive a sustained revenue at a low cost. Hence, the interoperability between heterogeneous networks, especially between the existing WWANs and Wi-Fi networks, is mutually beneficial. The growing deployment of Wi-Fi hotspots⁴ also underlines the importance of using them as a complementary access system to the existing WWANs. The concentration of hotspots is especially high in places such as commercial complexes, business districts, airports, and educational institutions. In such scenarios, an MN currently registered with a WWAN BS may enter the coverage regions of Wi-Fi APs very frequently. The MN may then choose to relinquish its connection to the WWAN BS and instead use the Wi-Fi AP for communication. This leads to a reduction in the load on the cellular network, thus enabling more MNs to be supported. The interoperability of Wi-Fi systems and cellular networks is advantageous to both the network users and the network service providers. This interoperability is the basis of the extended service provider model described in Section 14.3.1.

⁴ Wi-Fi hotspots are implemented using WLAN access points (APs) and hence, hotspots and APs are used interchangeably.

Network Selection and Wi-Fi Interoperability

The current-generation MNs such as laptop and palmtop computers can support a wide variety of functions such as voice calls, streaming multimedia, Web browsing, and e-mail, in addition to other computing functions. Many of them have multiple network interfaces that enable them to communicate with different networks. Active research is underway to develop a commercially viable unified network interface that can operate across several heterogeneous networks (multimode 802.11 interface, discussed in Section 14.5, is a beginning in this direction). These varied functions place different requirements on the communication networks supporting them. For example, an MN involved in streaming multimedia traffic requires a much higher bandwidth than an MN involved in downloading a Web page. A user-profile defines the abstract behavior of the network user or an MN derived from the resource requirements and the user preferences. All nodes that follow a particular user-profile can be considered to belong to a particular class. The user-profile [8] of an MN determines the overall network access behavior of the MN, including whether it should connect to the Wi-Fi AP or to the WWAN BS when it is in the coverage regions of both. Switching between Wi-Fi APs and WWAN BSs on the basis of user-profiles also balances the different network resources among the different MNs with varied requirements. Three distinct user-profiles for MNs are given below:

• Bandwidth-conscious user-profile: This is the profile of a user or a class of users in which the user always chooses to connect to a network with the highest free bandwidth. An estimate of the free bandwidth available at an AP or a BS is sent along with beacon messages periodically transmitted by the respective AP or BS. A bandwidth-conscious MN, on receiving such a beacon of sufficient signal strength, will switch to the advertising BS or AP if the advertised bandwidth is greater than the free bandwidth estimate at the BS or the AP it is currently registered to. In order to avoid frequent switching between two different networks, a bandwidth threshold can be used. In such a bandwidth-threshold-based network-switching process, a handoff decision is made only when the new network has a bandwidth difference that exceeds the bandwidth-threshold. The MNs with high bandwidth requirements (like those engaged in multimedia data transfers) can possess this type of user profile.

• Cost-conscious user-profile: This user-profile represents the profile of a user or a class of users in which the user always prefers to be connected to the network that offers the lowest per-byte transmission cost among the available choices of networks. Each BS or AP advertises its associated per-byte transmission cost in the periodic beacons sent by it. A cost-conscious MN will switch to a different AP or BS only if the advertised transmission cost is less than that of the AP or BS with which it is currently registered. APs or BSs belonging to the same network service provider may advertise different transmission costs depending on their current load or their geographic location. An MN engaged in non-real-time file transfer can possess a cost-conscious user-profile.

• Glitch-conscious user-profile: A glitch is defined as an interruption in the transmission or connectivity which occurs when an MN switches to a new AP or BS. Thus an MN with a glitch-conscious user-profile tries to minimize the number of both vertical⁵ and horizontal⁶ handoffs it undergoes in order to achieve the smoothest possible transmission. One strategy to achieve this goal is to remain connected with the BSs of the cellular network as long as possible. The larger coverage regions of the cellular BSs result in fewer horizontal handoffs than in the case of the Wi-Fi APs.

⁵ A handoff that takes place across different networks, for example, across a WAN and a LAN, is called a vertical handoff.

⁶ The handoff that takes place between two network access entities of the same type, for example, between two APs or between two BSs, is called a horizontal handoff.

In all the three different user-profiles, maintaining connectivity is of utmost importance to the MN. This may result in an MN registering with an AP or a BS whose parameters go against the MN's user-profile. For example, when a cost-conscious MN finds the signal strength from its registered AP falling below a specified threshold value, it will switch to a different AP or BS in its coverage region, even if the per-byte transmission cost associated with the new AP or BS is higher than that of its current AP. The user-profile of an MN does not remain constant with time. For example, an MN involved in low-bandwidth non-real-time file transfers may switch to a bandwidth-intensive multimedia application. Moreover, an MN can possess multiple user-profiles at the same time – for example, MNs can be both bandwidth- and glitch-conscious. In such cases, the decision to switch between the APs and the BSs is more complex.

The user-profile of an MN determines its behavior and resource consumption as it moves across the terrain, encountering different APs and BSs on the way. The distinct behavior of nodes belonging to different user-profiles is described through Figure 14.10. An MN moves from point A to point E along the dotted line shown in the figure. The maximum bandwidth available at BS1 is 100 Kbps, while those available at the two APs, AP1 and AP2, are 11 Mbps each. In the scenario depicted here, it is assumed that the free bandwidth available at AP1 is much less than that at either BS1 or AP2 due to a very large number of MNs currently registered with AP1. It is also assumed that the free bandwidth available at AP2 is greater than that at BS1. The per-byte transmission cost associated with the BS is higher than that associated with the APs. The behavior of each class of MNs that holds the various user-profiles is described below:

Figure 14.10. Behavior of MNs with different user-profiles as they move across the terrain.

Bandwidth-Conscious MNs

A bandwidth-conscious MN always tries to register with the BS or AP offering the maximum free bandwidth. It can be seen from Figure 14.10 that the MN registers with the sole AP (AP1) accessible to it at the beginning of its journey (point A). At point B, it comes under the transmission range of BS1 also. Since BS1 has more free bandwidth than AP1, the MN will switch over to BS1 and will remain registered with it until it reaches the point D. On entering the range of AP2 at point D, the MN switches over to AP2, although it is still in the range of the BS. This is because AP2 advertises a higher amount of free bandwidth than BS1. The MN remains with AP2 until the end of its journey (point E).

Cost-Conscious MNs

A cost-conscious MN tries to register with the least-cost AP or BS at all times. After starting its journey from point A, the MN remains registered with AP1 until point C is reached. It must be noted here that the MN does not switch over to BS1 after it enters BS1's transmission range at point B. This is because the transmission cost associated with BS1 is higher than that associated with AP1. At point C, however, the MN goes out of the range of AP1 and is thus forced to register with the higher cost BS1 in order to maintain connectivity. On reaching point D, the MN registers with the lower cost AP2 and remains with it until the end of its journey (point E).

Glitch-Conscious MNs

A glitch-conscious MN tries to minimize the number of glitches in its connection by registering with the BS that has a much larger coverage. The MN remains registered with AP1 between points A and B. However, once it enters the range of BS1 at point B, it switches over to the BS and remains with it until the end of the journey at point E.

Table 14.1 shows the points at which network switching takes place as the MN at point A moves across the terrain shown in Figure 14.10.

Table 14.1. Handoff behavior of the MNs that have different user-profiles

14.3.4 Pricing/Billing Issues in Wi-Fi Systems

Pricing/billing schemes for a Wi-Fi systems assume significance as the commercial viability is very important for the survival of Wi-Fi systems. In the case of Wi-Fi systems, unlike the traditional wired or wireless networks, the entity that has the responsibility of customer care and billing can be different from the one which actually provides network access service. Henceforth, the agency that is responsible for providing customer care, billing, and revenue-sharing is referred to as the billing agency (BA). The BA can employ different methods to accept payments from the network users. The major billing approaches that can be employed in the case of Wi-Fi systems are flat-rate pricing and volume-based pricing. In the case of flat-rate pricing, a user is permitted to utilize the network services for a specified period of time without any restrictions on the bandwidth consumed. The volume-based pricing approach charges a user based on the amount of data which he/she transacted over the network. In addition to both these schemes, business establishments can provide Wi-Fi network services as a value addition to customers visiting their premises for core business activities. In such cases, the billing is not an issue as the bandwidth provided belongs to the organization's (unutilized) bandwidth, hence it can be provided without additional charging.

In the franchisee-franchisor and the aggregator models discussed in Section 14.3.1, the sharing of revenue is important because there exist multiple business entities in the process of customer relationship, billing, and providing the actual network access service. Two major revenue-sharing models are: the fixed-fraction sharing model (in which the revenue shared among the service providers involved is fixed a priori) and the volume-based variable-fraction sharing model (in which the percentage of revenue that goes to the different parties involved varies with the volume of data transferred). Service providers can consider a constant rate for the user per-bit of data transferred when the traffic load is light, and in the high-traffic-density environments, a variable rate per-bit of data transferred can be used. In the variable rate scheme, application layer mechanisms which periodically communicate the cost of communication are essential.

Billing Agency Models

The two main aspects in any network access are the actual network service provisioning and billing. Traditionally, these two were carried out by the same agency. However, in Wi-Fi systems, there can be different models for the BA and the network service providers (SPs). As illustrated in Figure 14.11, the basic BA models are the following: (a) billing-free service model, (b) associated BA model, (c) revenue-sharing-based BA model, (d) integrated BA model, and (e) independent BA model.

• Billing-free service model: As shown in Figure 14.11 (a), this is the simplest model and is in existence today in most places. Here, the WAN BA charges MNs for their network access whereas the Wi-Fi access is provided as a free service to the users, or as a value addition to the Wi-Fi SP's existing core business service.

• Associated BA model: In this case, Wi-Fi systems and WANs have independent BAs, each charging the MNs for their respective network services provided. Figure 14.11 (b) illustrates this model.

• Revenue-sharing-based BA model: This model provides a single-point billing for the MNs, and at the same time enables the MNs to avail network services from WAN or Wi-Fi networks. A revenue-sharing agreement must be in place between the WAN BA and the Wi-Fi SPs. Since the WAN SPs cover a larger geographical area, it is more appropriate to have the BA closely associated with the WAN SP as shown in Figure 14.11 (c).

• Integrated BA model: This model is the one that is associated with the extended service providers who operate a range of different networks including WANs and Wi-Fi APs. This is illustrated in Figure 14.11 (d).

• Independent BA model: In this case, the BA is an independent entity that has billing tie-ups with several WAN SPs and Wi-Fi SPs. This is illustrated in Figure 14.11 (e).

Figure 14.11. Billing agency models.

14.3.5 Pricing/Billing Schemes for Wi-Fi Systems

Volume-based pricing schemes and directions for pricing in the various Wi-Fi service models are discussed in this section. The two key aspects of the pricing schemes are the revenue-sharing approach and a reimbursement scheme for link-level successful delivery (similar to the pricing model discussed in [9]). Since the traffic considered in a Wi-Fi system includes at least a single AP, the AP can act as an efficient and reliable accounting station with the responsibility of gathering and delivering the pricing information to the BA. Also, due to the nature of the traffic, it can be observed that there is no need to aggregate any packet information at the MNs.

Revenue-Sharing Models

Revenue-sharing-based models describe the way in which the money paid by the user is split among the multiple SPs involved in providing the network access service. Generally, in the micro-carrier model, the Wi-Fi SP may lease a fraction of the bandwidth from an existing wired network service provider and resell it among its customers either on a flat-rate-based or a volume-based pricing scheme.

The number of bytes that an MN receives from a Web site or any Internet content provider would be significantly larger than the request packets it originates, leading to an asymmetry in the traffic flow. In order to take the traffic asymmetry into account, the costs for receiving and originating a packet are different. Here R_cost (O_cost) is the per-byte cost that the MN has to pay the BA for every byte of data received (originated) by the MN.

We can envisage the simplest billing scheme (referred to as the simple revenue-sharing scheme) in which the WAN SP and the BA treat the Wi-Fi AP as an intermediate node that participates in the forwarding process and reimburses the Wi-Fi AP through the Wi-Fi SP with the reimbursement amount β for every byte of data it has forwarded. The value of β can be either an absolute monetary unit or a fraction of the revenue the BA charges the customer for the same traffic unit. The simple revenue-sharing scheme is a volume-based fixed-fraction revenue-sharing model and is illustrated in Figure 14.12, where the Wi-Fi AP receives an amount of β from the WAN BA for every packet successfully forwarded. The value of β has to be decided in such a way that the WAN SP's revenue does not fall below a minimum threshold, and at the same time, the Wi-Fi SPs also get a fair share of the generated income. Since this model pays the APs on the basis of the traffic that they have transmitted instead of equal sharing among the APs, this model can also be used when each AP is operated by a different Wi-Fi SP. The MNs that connect directly to the WAN are required to pay O_cost and R_cost per unit of traffic or per byte of data originated and received, respectively.

Figure 14.12. Illustration of simple revenue-sharing scheme.

The net revenue of the Wi-Fi SP is given by , where the refers to the total number of bytes that a Wi-Fi SP forwarded successfully. The WAN SP's net revenue will then become

(14.3.1)

where Paid_i is the total amount paid by an MN i to the BA. The SP's revenue is a significant entity, as the SPs should be able to generate a minimum revenue in order to sustain the network services. This minimum revenue can be achieved by altering the values of β, R_cost, and O_cost depending on the node density and the traffic distribution in the network.

The volume-based variable-fraction revenue-sharing model uses different values of β_i depending on the volume of traffic transacted through a particular Wi-Fi SP. For example, a three-level volume-based variable-fraction revenue-sharing model may reimburse β₁ for 0 < t < T₁, β₂ for T₁ < t < T₂, and β₃ for T₂ < t, where t is the traffic-volume transacted through a particular Wi-Fi SP, and T₁, T₂, and T₃, are the different traffic-volume thresholds.

Another scenario in which a Wi-Fi system can operate is the multi-hop Wi-Fi system⁷ where the MNs can also act as the forwarders on behalf of other nodes which are unable to communicate directly to the Wi-Fi AP. Examples of such systems are the single interface MCN (SMCN) architecture [9] and multi-hop WLANs (MWLANs) [10]. In an area where the Wi-Fi APs are sparsely distributed with a broadband Wi-Fi system, enabling multi-hop relaying at the MNs can actually extend the coverage of the APs as well as raise the revenue-generation potential of the Wi-Fi SPs. The catch here is to enable the MNs to forward others' packets. Figure 14.13 illustrates a multi-hop Wi-Fi scenario where MN B accesses the services of the Wi-Fi AP through the intermediate relay MN K. The following two pricing models discuss the revenue-sharing in such multi-hop Wi-Fi systems.

⁷ Multi-hop Wi-Fi systems are also called ad hoc Wi-Fi systems as they use ad hoc radio relaying in the Wi-Fi environment.

Figure 14.13. Illustration of multi-hop Wi-Fi revenue-sharing scheme.

The payment scenarios in a multi-hop Wi-Fi network with a fixed-fraction revenue-sharing model (referred to as multi-hop Wi-Fi revenue-sharing scheme) is depicted schematically in Figure 14.13. In this case, for every successfully delivered packet, the Wi-Fi AP receives an amount of β per packet and an intermediate MN that forwards the packet on behalf of the original sender receives an amount of α × C_p, where α is the reimbursement factor and C_p is the cost incurred at the intermediate node per byte of data forwarded by MNs.

The net revenue of Wi-Fi SP is given by . Equation 14.3.2 represents the total reimbursement (Repay_i) that an MN i receives from the BA, where is the total number of bytes that a node i has successfully forwarded for node j.

(14.3.2)

The reimbursement factor α is applicable only for those MNs that are acting as forwarders for other MNs. The reimbursement becomes an incentive when α > 1.

The WAN SP's net revenue then becomes

(14.3.3)

Another possible revenue-sharing scenario is where the billing parameter R_cost is split into separate billing parameters R_costAP and R_costBS for the Wi-Fi SP and WAN SP, respectively. Similarly, O_cost is split into O_costAP and O_costBS. As per this scheme, those MNs which access the WAN SP through a Wi-Fi SP need to pay a differentiated billing rate O_costBS and R_costBS for the unit traffic originated or received by them, respectively. Such MNs are required to pay the Wi-Fi SPs O_costAP and R_costAP for their packet forwarding services. Lowering the O_costBS and R_costBS compared to O_cost and R_cost can help the WAN SPs to reduce the traffic load by encouraging more MNs to use the Wi-Fi services. Due to the differentiation in the billing rates, this scheme is called differentiated multi-hop revenue-sharing scheme. This scheme requires the MNs to individually pay the Wi-Fi SP and the WAN SP as shown in Figure 14.14. In this case, the amount to be paid by a given MN (Paid_i) is obtained as follows:

(14.3.4)

Figure 14.14. Illustration of differentiated multi-hop revenue-sharing scheme.

where is the total number of bytes originated by a node and is the total number of bytes received by a node. The Wi-Fi SP's revenue is simply the amount that is paid by the MNs that have been in its domain, and no further sharing needs to be done with the WAN SP. O_costBS and R_costBS provide bandwidth or additional service provided by the WAN. In this scenario, one can think of a number of Wi-Fi SPs each offering the service to a customer who can choose the SP (if available) which provides the most economical service. The registration mechanism by which an MN joins a particular network may also have to take into account the cost parameters such as O_costAP and R_costAP-

Similar to the previous scheme, the following equation represents the total reimbursement that an MN receives from the BA.

(14.3.5)

The reimbursement comes into the picture only for the MNs that act as intermediate forwarding nodes for other MNs.

Equation 14.3.6 gives the total expenditure that an MN incurs from forwarding on behalf of other MNs.

(14.3.6)

where is the total number of retransmitted bytes that a node i has forwarded on behalf of node j. The following equation shows the total amount that an MN has to pay the BA:

(14.3.7)

In this case, the WAN SP may distinguish between those MNs connected directly to them from those MNs that connect through a Wi-Fi AP. In such a differentiated charging mechanism, in order to encourage people to spare the low bandwidth WAN resources, the O_cost and R_cost for those MNs which access the WAN SP directly will be greater than O_costBS and R_costBS which are applied for those MNs which access the WAN through Wi-Fi APs.

14.4 OPTICAL WIRELESS NETWORKS

The discussion of wireless networks so far was restricted to the communication based on radio waves. Optical wireless communication enables communication using infrared rays and light waves operating at frequencies well beyond the visible spectrum for high data rate local communication. Optical wireless communication technology exhibits a number of properties that makes it a suitable alternative to indoor RF communication. The advantages of optical wireless communication include significantly less interference due to its lack of penetration through walls, positioning of spectrum at a completely unregulated and unlicensed band, increased security, and high data rate. Optical wireless technology promises broadband data delivery at short ranges in point-to-multipoint LANs and point-to-point medium-distance optical links. Optical wireless transmission can be classified into short-range communication and long-range communication systems. A comparison of these two types of optical wireless transmission schemes is given in Table 14.2. Long-range communication systems are mainly used for outdoor point-to-point optical links and short-range systems are used in indoor and outdoor applications. Unlike the long-haul networks [12] in fiber-based optical networks, the long-range optical wireless systems can operate over a distance of hundreds of meters only. The short-range systems operate over a distance of few meters. With the ever-growing demand for broadband wireless connectivity, the utilization of RF spectrum is a bottleneck due to the spectrum congestion, licensing requirements, and unsuitability of certain bands for broadband applications.

Table 14.2. A comparison of optical wireless technologies

14.4.1 Short-Range Infrared Communication

The use of infrared radiation for wireless communication was first proposed in the late 1970s. In 1993, the Infrared Data Association (IrDA), a non-profit organization, was founded by major hardware, software, and communications equipment manufactures for establishing and promoting an infrared standard that provides convenient cordless connectivity and fosters application interoperability over a broad range of platforms and devices. As a result of the activities of IrDA, short-range infrared communication was in widespread use in the last decade, with IrDA interfaces built into several hundred million electronic devices including desktop, notebook, palm PCs, printers, digital cameras, public phones/kiosks, cellular phones, pagers, PDAs, electronic books, electronic wallets, toys, watches, and other mobile devices. IrDA has developed standards that work on widely ranging data transfer rates (9.6 Kbps − 4 Mbps).

The indoor short-range communication can be classified into (i) directed transmission and (ii) diffusion-based transmission. In the directed transmission, the transmitter and receiver are required to be pointed to each other and there should exist a line of sight (LoS) transmission link between them. In diffusion-based transmission, the transmitter and receiver need not have a LoS for communication. Table 14.3 summarizes the differences between directed and diffused transmission systems.

Table 14.3. A comparison of two short-range transmission schemes

14.4.2 Optical Wireless WDM

The use of wavelength division multiplexing (WDM) [12], [13], which is a method of sending many light beams of different wavelengths simultaneously down the core of an optical fiber, has been successful in utilizing the tremendous bandwidth offered by the optical fibers. Operating on the basis of the WDM technology, an optical wireless network utilizes different wavelengths between a point-to-point wireless link. This enables carrying large number of simultaneous sessions across a sender-receiver pair. The system operates at a spectrum centered around 1,330 nm or 1,550 nm in order to be compatible with the wavelengths used for traditional fiber-based WDM systems.

First-generation point-to-point wireless WDM systems with four wavelengths, each carrying a data rate of 2.5 Gbps, make a total transfer rate of 10 Gbps. An illustration of such a system is shown in Figure 14.15. Optical wireless WDM technology is expected to increase the bandwidth to a greater extent over the wireless technologies. However, it has several disadvantages, some of which are (i) LoS link break can lead to the loss of a tremendous amount of data and (ii) dense smoke, rain, birds, kites, and other atmospheric changes can lead to link breaks. Reliability is one of the major design objectives for designing wireless optical WDM networks. Figure 14.16 illustrates an optical wireless metro area ring network formed by several optical wireless point-to-point links. In cities where laying of fibers is difficult or expensive, optical wireless rings are suitable alternatives.

Figure 14.15. An illustration of the wireless optical WDM point-to-point link.

Figure 14.16. An illustration of the wireless optical WDM ring network.

14.4.3 Optical Wireless WDM LAN

The infrared wireless LANs fail to exploit the bandwidth available in the 1,330 nm and 1,550 nm optical spectrum windows to the fullest. The multi-party communication in a wireless LAN environment using the radio spectrum can be replaced with the optical wireless WDM (OWWDM) system that can provide a much higher bandwidth with reduced interference and high wavelength reusability. Table 14.4 compares the RF wireless LANs and OWWDM wireless LANs. An illustration of the operation of OWWDM LAN is shown in Figure 14.17. Here the stations can connect to the OWWDM AP (access point) using the control wavelength (λ_C) and obtain any data wavelength (λ_i where i = 1, 2,..., N) for data transfer. This necessitates wavelength tuning capabilities at the stations and the ability to operate over multiple wavelengths at the APs.

Figure 14.17. A conceptual illustration of OWWDM LAN.

Table 14.4. RF wireless LANs versus OWWDM wireless LANs

Issues in OWWDM LANs

The major issues in a practical OWWDM LAN include the choice of the protocols and subsystems. MAC protocols for optical fiber LANs that belong to the category of broadcast-and-select⁸ can be extended to work in OWWDMs also. The choice of receivers includes PIN diodes or Avalanche diodes with the former less expensive and sensitive and the latter more sensitive. The choices for a transmitter include Laser diodes or inexpensive LEDs, with the former able to provide higher data rate with more wavelengths at a higher cost. Additional capabilities such as wavelength conversion and tuning are required for the operation of OWWDM. The factors affecting the performance of such a system are path loss, interference, receiver sensitivity, and protocol efficiency. In addition to all other technical issues, safety issue assumes significance in the design due to the fact that high optical energy in the specified optical bands can damage human eyes. This necessitates an upper limit on the maximum power of the transmitter used in the system.

⁸ A broadcast-and-select network consists of a passive star coupler connecting the nodes in the network. Each node is equipped with one or more fixed-tuned or tunable optical transmitters and one or more fixed-tuned or tunable optical receivers. Different nodes transmit messages on different wavelengths simultaneously. The star coupler combines all these messages and then broadcasts the combined message to all the nodes. A node selects a desired wavelength to receive the desired message by tuning its receiver to that wavelength.

14.5 THE MULTIMODE 802.11 — IEEE 802.11a/b/g

The different physical layer specifications for IEEE 802.11 operating at different frequency bands led to another dimension of the operation of WLANs, that of interoperability of different IEEE 802.11 [14] variants such as 802.11b, 802.11a, and 802.11g. In addition to these physical layer differences, enhancements to the basic medium access mechanism such as 802.11e for providing QoS support add to this interoperability issue. Thus was born the multimode 802.11 (i.e., IEEE 802.11a/b/g) which can operate in all of the three major variants of IEEE 802.11 standards to date. Multimode 802.11 is the WLAN client interface implementation that can seamlessly work with APs operating according to any of the three IEEE 802.11 standards. At any given time, an IEEE 802.11a/b/g interface works with only one AP. The major advantage of multimode 802.11 is the backward compatibility of the newer 802.11a and 802.11g with the millions of existing 802.11b installations. In addition to the interoperability issue, the use of 802.11a and 802.11b simultaneously at a given service area can actually increase the available bandwidth. Table 14.5 provides the total bandwidth available at any given service area by the combination of 802.11a, 802.11b, and 802.11g. The entire bandwidth may not be available for a single station as it requires several network interface cards and APs.

Table 14.5. Comparison of bandwidth in multimode 802.11 systems

The multimode WLAN clients that are 802.11a/b/g compatible can connect to any 802.11 AP transparently, leading to enhanced roaming services across different networks. An 802.11a/b/g client that detects carriers from different APs using 802.11b, 802.11a, and 802.11g is illustrated in Figure 14.18 (a). The 802.11a/b/g client searches for carrier in all the bands of operation and selects the most appropriate one. Hence, the new generation of wireless LAN clients are expected to be capable of operating in all these different modes, requiring a wide frequency tuning range (2 GHz to 6 GHz), multiple MAC implementations, and multiple baseband processors. Early solutions for multimode 802.11 were designed to have an integrated chipset that combined the 2.4 GHz chipset, 5 GHz chipset, and the baseband chipset for PCI,⁹ mini-PCI, or CardBus interfaces. A multimode LAN client is designed to automatically select the strongest detected carrier. A schematic diagram of the 802.11a/b/g client interface is shown in Figure 14.18 (b). The major difference between this interface card and the traditional single-mode client interface is the presence of multiple chipsets that can process the physical layer information belonging to different 802.11 variants. It is necessary to have at least two such physical layer chipsets — one for the 2.4 GHz frequency range and the other for the 5 GHz frequency range to handle 802.11a/b/g physical layers. The control signals from the baseband controller are used to switch to one particular physical layer chipset depending on the availability and signal strength from the APs. In addition to the switching decisions, the baseband module includes the MAC implementations and the interfaces to the PC bus standards such as PCI, mini-PCI, and CardBus.

⁹ PCI stands for peripheral component interconnect — a widely accepted local bus standard developed by Intel Corporation in 1993. mini-PCI is a PCI-based industry-wide standard for modems. CardBus is a 32-bit extension of PCMCIA (Personal Computer Memory Card International Association), PC card standard and has operation speeds up to 33 MHz.

Figure 14.18. Multimode 802.11. (a) Operation of the multimode client. (b) Schematic diagram of the multimode 802.11 client interface.

Figure 14.19 shows a time line of multimode 802.11 in wireless LANs. One can expect a k-mode operation which can cover almost every existing standard ranging from wireless PANs and LANs to satellite networks with a single handset by using programmable interfaces by 2010. It is extremely difficult to predict the evolution of wireless networks beyond a certain point, considering the revolutionary changes that wireless communication technology has undergone every decade. The existing solutions for 802.11a/b/g based on embedding different chipsets handling different physical layer standards are not scalable for a flexible k-mode operation where interoperability among many networks would be required. Also, the cost of such multimode interfaces increases linearly with the number of modes of operation. Programmable radio interfaces (popularly known as software-defined radio or software radio in short) point the way toward achieving a low cost k-mode wireless communication interface for future wireless communication networks.

Figure 14.19. A time line of multimode systems.

14.5.1 Software Radio-Based Multimode Systems

Software-defined radio (popularly known as software radio) [15] is a proven technology in military wireless communication systems. In a software-defined radio system, the different modules of a radio interface such as IF (intermediate frequency), base band, and bit stream processors are implemented through general-purpose programmable digital signal processors (DSPs). Hence such programmable, multimode, and multiband radio interfaces can provide low-cost terminals that can operate across several different networks. Different radio interfaces have different RF bands, waveform modulation types, voice-encoding algorithms, and encryption schemes. The waveform software in a software-defined radio includes software for all these operations. A software-defined transmitter can characterize the available transmission channels, probe the propagation path, construct an appropriate channel modulation scheme, electronically steer the transmission beam in the desired direction, and transmit at an appropriate power. Some of these capabilities may be applicable only in a military communication paradigm and may not be required for commercial civilian communication systems. Similarly, a software radio reception process can characterize the energy distribution in the adjacent channels, recognize the mode of the incoming transmission, adaptively nullify interferences, combine the multipath signals, decode the channel coding, correct the errors using FEC, and decode the received signal with minimum BER. All these activities are carried out in DSPs controlled by software. By changing appropriate software modules, the same system can be used with any other system. The Speakeasy [16] military radio system was designed to emulate more than 15 military radio systems. The waveform software required for a particular network can be preloaded, taken from a predefined standard set, or downloaded through over-the-air (OTA) data interfaces. Figure 14.20 shows an illustration of a software-defined multimode and multiband wireless network interface operating across a GSM network and a W-CDMA network, by downloading network specific waveform software. Every network is assumed to be using a control protocol or a control channel with the necessary information about the waveform software. In the presence of a new network, the multimode interface detects the network over the control channel and downloads the waveform software and the necessary protocol software. Once the new waveform and associated protocol software are loaded onto the radio interface, the mobile terminal becomes ready to communicate with the network similar to any other single-mode interface designed for that network.

Figure 14.20. An illustration of operation of software-defined multimode wireless client.

Even though software radio is currently used at the BSs only, recent developments are aimed at using it at the terminal side also. The major constraints for achieving this are processing power and power consumption. Experimental software radio-based terminals that operate in dual mode between GSM and W-CDMA exist today. They consume higher amounts of power for operation compared to the existing single-mode terminals. With the development of high-speed processors and low-power VLSI devices, multimode terminals based on software radio may soon become a reality.

14.6 THE MEGHADOOT ARCHITECTURE¹⁰

¹⁰ This is a prototype development project [17] currently being carried out at the High Performance Computing and Networking Laboratory, Department of Computer Science and Engineering, Indian Institute of Technology, Madras, India.

The Meghadoot¹¹ architecture is a packet-based wireless network architecture for low-cost rural community networks. Traditional wireless networks for rural telephony, such as wireless in local loop (WLL), require extensive infrastructure for service deployment. The high investments required for such networks, and the low revenue prospects in rural regions, discourage commercial service providers from providing communication services in the rural regions. Packet-based radio networks are considered as an ideal alternative for low-cost community networks, both in the urban developed environments and also in the rural regions.

¹¹ This is a Sanskrit word meaning Cloud Messenger, derived from the epic love story written by legendary Indian poet Kalidasa. The theme of this story is a message sent by an exiled Yaksha in Central India to his beloved wife in the Himalayas through a cloud.

The major objectives of the Meghadoot project are (i) to develop a fully distributed packet-based hybrid wireless network that can carry voice and data traffic, (ii) to provide a low-cost communication system in the rural regions, and (iii) to provide an alternate low-cost communication network for urban environments.

Meghadoot uses a routing protocol called infrastructure-based ad hoc routing protocol (IBAR). An illustration of routing process in the Meghadoot architecture is shown in Figure 14.21. The infrastructure node (IN) controls the routing process in its k-hop neighborhood (also referred to as k-hop control zone) and aids in routing for the calls originated beyond k-hops and destined to a node within the k-hop region. Any node registered to the IN assumes that the routing and other control activities would be taken care of by the IN, and hence it stays away from initiating its own path-finding process. Nodes that are not under the control of the IN operate in the ad hoc mode, and hence are required to perform self-organization, path-finding, and path reconfiguration by themselves. The region beyond the k-hop neighborhood from an IN is called the ad hoc routing zone. Meghadoot requires the gateway nodes (GNs) to hold the additional responsibility of interfacing the nodes in the ad hoc routing zone (operating in the ad hoc mode) to the IN in order to enable such nodes to find routes efficiently to the nodes inside the control zone of the IN.

Figure 14.21. An illustration of the Meghadoot architecture. (a) Control zones. (b) Gateway nodes.

Nodes in the ad hoc routing zone broadcast RouteRequest packets in order to find a path to the destination. Every intermediate node that receives the packet forwards it further until the packet reaches the destination. When the destination node receives the packet, it responds by sending back a RouteReply packet. This mechanism is similar to that used in the dynamic source routing (DSR) [18] protocol. The disadvantage of using DSR protocol is the high control overhead generated by the broadcast packets used for path-finding. In Meghadoot, this routing overhead is reduced whenever INs are present. Whenever a GN receives a RouteRequest packet, instead of flooding, it forwards the packet to the IN. If the destination node lies in its control zone, the IN returns the path information to the GN using a RouteReply packet. The GN receives the RouteReply packet and forwards to the original sender node. Using the IBAR protocol, the IN maintains the approximate topology of the nodes within its zone. Whenever a source node (say, node S) in the k-hop needs to send a packet to a destination node (say, node D), it sends a RouteRequest packet over multiple hops to the IN. The IN runs a shortest path algorithm to find a path to node D and returns the path found to node S. Node S can now start using this path provided by IN. When a path break is detected by the sender node, it sends a new RouteRequest packet to the IN for reconfiguring the broken path. If an intermediate node detects a path break, it sends a RouteError packet to the IN, upon reception of which the IN obtains a new path and informs the sender.

14.6.1 The 802.11phone

The end user equipment in Meghadoot is an IEEE 802.11 enabled device, either a laptop computer with an 802.11 adapter, or a small handheld device with an 802.11 interface. Meghadoot is aimed at deployment in rural areas, where other communication infrastructure is not available, using an 802.11phone (an inexpensive handheld device with an 802.11b card). The 802.11phone could either be a general-purpose palmtop device or a dedicated processor-based device similar to a GSM handset. The actual usage of Meghadoot not only aims at voice communication, but also aids the rural community to utilize other applications, such as data gathering, accounting, limited data processing, and for using local language-based applications. Therefore, Meghadoot utilized the Picopeta Simputer (Version 3) [19] as one of the devices for implementation of the 802.11phone. Figure 14.22 shows the software components used in the 802.11phone, implemented with Meghadoot. For voice communication, a Linux-based voice-over IP (VoIP) protocol stack that works with TCP/IP is used.

Figure 14.22. The software architecture of 802.11phone used in the Meghadoot system.

Meghadoot aims at providing community networking for voice or data over multi-hop wireless networks, with or without the use of infrastructure nodes. The deployment scenarios in the remote rural communities add to the additional pressure on the choice of power source for the 802.11phone. Meghadoot envisions the usage of a simple bicycle dynamo for powering the 802.11phone. The users in rural regions can charge their 802.11phones by connecting their devices to the bicycle dynamo through a cycle-based 802.11phone (CB8) charger.

Meghadoot provides an affordable communication system for rural communities where service providers may not be willing to establish infrastructure-based communication networks because of their return-on-investment constraints. This is typically the case in many developing nations. Meghadoot provides an ideal alternative in such situations.

14.7 SUMMARY

The evolution of wireless networking has had tremendous influence on human life and the ways of communication, from the first-generation (1G) cellular networks to the current third-generation (3G) wireless networks. The recent popularity of wireless networks has given rise to many additional requirements of the system, especially the requirement of greater bandwidth and demand for high-quality multimedia services. The ultra-wide band technology is providing a new direction to utilize the radio spectrum for the broadband short-range radio communication and hence offering a new direction to the development of wireless networks. The multi-mode IEEE 802.11 systems enable the end users to roam seamlessly across different WLANs — IEEE 802.11a, IEEE 802.11b, and IEEE 802.11g. Wireless fidelity (Wi-Fi) systems are aimed at integrating the high-speed wireless LANs and the wide-area packet cellular networks. Another recent trend in wireless networks is the use of optical wireless WDM networks for high bandwidth support. The Meghadoot wireless network architecture described in this chapter is a low-cost solution for rural community networks.

14.8 PROBLEMS

1. Compare and contrast UWB communication with conventional wide-band communication techniques based on spread spectrum techniques.
2. What is the single most important feature of the UWB system that makes it suitable for RADAR (radio detection and ranging) applications?
3. What are the important features that make the UWB systems applicable for high-speed mobile communications?
4. Calculate the duty cycle and average power to peak power ratio of a UWB system in which the monocycle duration is 0.5 ns, pulse repetition interval is 25 ns, and pulse amplitude is 0.1 mW.
5. Which of the service provider models require revenue-sharing schemes for pricing/billing schemes? Explain why.
6. Discuss the adaptability of the PICR-PDBR scheme (proposed for hybrid wireless networks; discussed in the previous chapter) as a Wi-Fi pricing/billing scheme.
7. A micro service provider installs two APs in his premises. What is the maximum data rate he could provide in his service area for the following cases: (a) Both 802.11b APs, (b) An 802.11b AP and an 802.11a AP, (c) One 802.11b AP and an 802.11g AP, and (d) Both 802.11a APs?
8. A coffee shop owner decided to provide Internet access to his customers. From the Internet Web site www.buy-here-or-get-lost.com, he found the best deals for the 802.11b, 802.11a, and 802.11g APs as $20, $80, and $40, respectively. Advise him on the best possible bandwidth in his shop with a budget limit of $100, given the fact that his neighbor, an ice-cream shop owner, is already using channel 6 among the available 802.11b channels.
9. Consider the simple revenue-sharing scheme. Calculate the revenue generated by a particular Wi-Fi SP for a per-month traffic of (i) 85 traffic units and (ii) 400 traffic units for the following cases: (a) if a fixed β value of 0.02 cents per traffic unit is reimbursed and (b) if a variable β with β₁ = 0.03 cents, β₂ = 0.01 cents with a traffic threshold of 100 traffic units is used.
10. Consider the simple revenue-sharing scheme with a volume-based variable-fraction revenue-sharing model where the reimbursement factor β has two levels, that is, β₁ for traffic up to 100 units, β₂ for traffic above 100 units. Assume that the operating cost for a Wi-Fi service provider with a single AP is $10 per-month. Calculate the corresponding β values for achieving break-even for (a) 85 traffic units per month and (b) 500 traffic units per month.
11. Calculate the revenue generated by a Wi-Fi SP if β is a fraction of the billing rate charged by the BA for the Wi-Fi traffic rate of (a) 85 units per month and (b) 400 units per month. What are your comments on the profitability? Assume that the BA charges a billing rate of 0.1 cents per traffic unit and β is 0.5.
12. Wireless optical WDM rings provide high data rate networks in metropolitan areas. Discuss possible solutions and factors to be considered for providing reliability for a wireless optical WDM ring network.
13. What are the major factors to be considered while selecting a light source for designing a mobile network using optical wireless technology?
14. What is the preferred light source for point-to-point metro area optical wireless links? Give reasons.
15. Give two advantages and disadvantages in using laser diodes as light sources for optical wireless networks.
16. Discuss the advantages and disadvantages of a hardware-based multimode 802.11 terminal.
17. Discuss the advantages and disadvantages of software radio-based multimode terminals.
18. Discuss the effects of increasing the control zone radius in Meghadoot architecture in terms of the resource requirements at the IN and the routing efficiency.

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