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100 WIRELESS LAN EVOLUTION communication medium. In contrast to the CSMA/CD protocol used in Ethernet, where collision detection can be easily realized, the CSMA/CA protocol (developed for an 802.11 wireless network) makes an effort to avoid collisions, because the wireless receiver has difficulty with collision detection. The receiver uses the following features and functions: • Adaptive collision window (CW) based random backoff time to reduce the probability of collisions • Different interframe space (IFS) to prioritize different types of transmissions • Acknowledgement frame to realize the stop and wait ARQ • Request to send (RTS) and clear to send (CTS) handshaking to solve the hidden terminal problem • Network Allocation Vectors (NAV) to realize virtual carrier sense As in other random access protocols, the random backoff time in CSMA/CA works to avoid collisions between transmissions from different STAs. The random backoff time can be calculated from this equation: Backoff time = Random() ∗ Slot time (4.1) In this equation, Random() = [0, CW], (CW min ≤ CW ≤ CW max ),andSlot time is the value of the corresponding PHY characteristic. Suggested values are CW min = 31 and CW max = 255. If it is the current packet’s first transmission, CW is set to CW min .After each collision of this packet, the collision avoidance mechanism doubles CW until it reaches CW max . CW new = (CW old + 1) ∗ PF − 1 (4.2) In this equation, PF is equal to 2. This is referred to as the exponential backoff algorithm. The offered load to the channel is high when experiencing a collision, so increasing the CW to increase the backoff time of each colliding STA helps decrease the collision probability. The IFS is a time interval after a busy state of the channel. This interval plays an important role in CSMA/CA for collision avoidance and prioritized transmissions. The IFS requires an STA to wait for a period of time after it senses the idle state of the channel. Then, the STA waits for a random backoff time before transmitting its frame. There are four basic types of IFS: • Short IFS (SIFS) • Point IFS (PIFS) • Distributed IFS (DIFS) • Extended IFS (EIFS) Each type has a distinct interval time. The four types are designed for transmitting different types of frames. SIFS is used to transmit frames with the highest priority, such as acknowledgment (ACK), CTS, and poll response. PIFS is used in the point coordinate WIRELESS LAN EVOLUTION 101 function when an AP issues poll frames. DIFS is used by ordinary asynchronous traffic. EIFS is used when a MAC frame is received with an error. Some examples of IFS relationships are shown in Figure 4.8. A stop-and-wait ARQ is combined with CSMA/CA. An ACK frame is sent by the STA that successfully receives a data frame. An SIFS is used for sending an ACK frame to guarantee the highest transmission priority. There is a well-known hidden terminal problem in CSMA-type protocols. RTS-CTS handshaking is used to solve this problem. Accordingly, the concept of network allocation vector (NAV) is introduced. Figure 4.9 shows the time chart of the CSMA/CA with RTS- CTS handshaking. The source STA sends an RTS to the nearby STAs to make a reservation and start a NAV period. The destination STA sends a CTS to respond to the reservation and start a SIFS PIFS DIFS DIFS Busy Medium Defer Access Contention Window Backoff Slots Slot Time Select Slot and Decrement Backoff as long as medium is idle Next Frame Immediate access when Medium is free >=DIFS Figure 4.8 Some IFS relationships RTS DIFS CTS SIFS SIFS Data NAV (RTS) ACK SIFS DIFS NAV (CTS) Defer Access Backoff Contention Window Sender STA Destination STA Other STA NAV Set Figure 4.9 IEEE 802.11 MAC RTS-CTS handshaking 102 WIRELESS LAN EVOLUTION NAV period for neighboring STAs. NAV protects the current transmission, thus solving the hidden terminal problem. 4.2.2 PHY Technologies The four IEEE 802.11 PHY standards are listed in Table 4.2. The fifth is being developed in IEEE 802.11 TGn, targeting a new PHY to support a throughput of more than 100 Mbps. This section briefly introduces the OFDM-based PHY technologies in 802.11, known as 802.11a and 802.11g. As described in Chapter 3, the multicarrier transmission is an efficient scheme for solv- ing the problem of severe frequency-selective fading in broadband wireless access systems. Figure 4.10 depicts such a mechanism. After experiencing a multipath propagation, an impulse waveform at the transmitter becomes widely spread in the time domain at the receiver. This results in intersymbol interference (ISI) in digital communications. When the symbol rate is low, the problem of ISI can be solved by using an equalizer or canceler at the receiver. The higher the symbol rate, the more complex the equalizer/canceler. This is one of the fundamental problems of broadband wireless access. One solution is a multicarrier transmission that can reduce the symbol rate at each subcarrier, so narrowband solutions can be used in this situation. OFDM is one of the most spectrum-efficient multicarrier transmission methods. Figure 4.11 shows a block diagram of OFDM transceiver. A channel-encoded data stream is input for the transmitter. The serial data stream is first transformed into paral- lel and then modulated separately. An Inverse Fast Fourier Transform (IFFT) is used as Direct path Path 1 Path 2 time Transmitting Waveform (impulse shape) time Combination of direct wave and delayed waves Receiving Waveform Multipath propagation in time domain multipath time Transmitting waveform frequency time Avoid ISI Receiving waveform frequency Multi-carrier Solution (OFDM) Figure 4.10 Multicarrier transmission in a multipath propagation environment WIRELESS LAN EVOLUTION 103 Coded Data S/P Trans- form … … modulation IFFT P/S Insert GI … OFDM amplitude frequency … TX Decoded Data P/S Trans- form Symbol Sync Carrier Frequency Compen- sation S/P Delete GI RX FFT … … subcarrier detection Figure 4.11 OFDM transceiver block diagram the processing algorithm to create OFDM symbols. To keep the subcarriers orthogonal in a multipath propagation environment, a guard interval (GI) is inserted in each OFDM symbol. After a parallel-to-serial transform, the OFDM symbols are transmitted. The figure shows that the subcarriers overlap each other. These overlapped carriers do not interfere with each other, improving the spectrum utilization efficiency. At the receiver, the GI is deleted and the FFT is used as the algorithm to transform OFDM symbols from a frequency domain into a time domain. Figure 4.12 shows an important mechanism that uses a GI to reduce multipath effect in OFDM communications. After multipath propagation, the received waveform may involve the direct wave as well as delayed wave components. If there are no means of protection, these components will exist in the results of FFT, that is, each parallel signal stream. The GI is designed to reduce the effect caused by delay spreads. As shown in Figure 4.11, the GI is generated by copying the bottom parts of OFDM symbol and inserting them into the top parts. The multipath effect of the GI is shown in Figure 4.12. The ISI effect can be reduced if the delayed waves arrive at the receiver within the window of GI. Table 4.2 shows the parameters related to OFDM in 802.11 standards. A 52-subcarrier OFDM symbol consists of 48 subcarriers for information and 4 subcarriers for pilot. Pilot is a known signal sequence to detect and compensate for frequency synchronization errors. The transmission rate varies from 6 to 54 Mbps, according to different modulation schemes and coding rates used. The GI is 800 ns, enabling the WLAN to work in a multipath environment with a root mean square (RMS) delay spread of 100 to 200 ns. Each of the subcarriers is spaced 312.5 kHz apart and the GI is added to each symbol to make the total symbol duration 4 s. 104 WIRELESS LAN EVOLUTION Cyclic extension Subcarrier f1 f2 f3 f4 Direct path signal Multi path delayed signals GI DataGI Data GI Data Timing ISI Optimum Early Late Small ISI Large ISI Optimum FFT window Early FFT window Late FFT window Figure 4.12 Reduction of multipath effect using OFDM Table 4.2 IEEE 802.11a parameters Parameter Value Data rate 6, 9, 12, 18, 24, 36, 48, 54 Mbps Modulation OFDM with BPSK, QPSK, 16-QAM, 64-QAM Number of subcarriers 52 subcarriers including 4 for pilot 64 point FFT FEC Convolution coding with K=7, R=1/2, 2/3, 3/4 Viterbi decoding Interleaving within an OFDM symbol OFDM symbol duration 4 s Guard interval 800 ns Subcarrier spacing 312.5 kHz -3 dB bandwidth 16.56 MHz Channel spacing 20 MHz Figure 4.13 shows the PHY frame format of 802.11g. For backward compatibility, the Physical Layer Convergence Protocol (PLCP) preamble and header are the same in both 802.11 and 802.11b. The Physical Service Data Unit (PSDU) uses OFDM and has the same structure as in 802.11a. Depending on the 48-subcarrier BPSK, QPSK, 16-QAM, and 64-QAM, the raw data rate can reach to 12–72 Mbps. In order to reduce the effect of fading, WIRELESS LAN EVOLUTION 105 SYNC (128bits- Scrambled Ones) SFD (16 bits) Signal (8 bits) Service (8 bits) Length (16 bits) CRC (16 bits) OFDM Sync (Long Sync – 8 us) OFDM Signal Field (4 us) OFDM Data Symbols OFDM Signal Extension (6us) PLCP Preamble (144 bits) PLCP Header (48 bits) PSDU (Data Modulation) PPDU DBPSK Modulation DBPSK Modulation OFDM Modulation Figure 4.13 IEEE 802.11g PHY frame format the convolutionary channel coding with a rate of 1/2 and soft-decision Viterbi decoding is specified. Although 802.11g takes advantage of both 2.4 GHz and OFDM technologies, its per- formance is not as high as expected. Figure 4.14 shows the upper limits of throughput for 802.11a/b/g (Morikura and Matsue 2001). Note that the throughput of CCK-OFDM does not increase significantly as the PHY layer transmission rate increases. The main reason for this is the relatively long PCLP preamble and header. 4.3 Evolution of WLAN WLAN has become increasingly popular over the past few years, and customers are demand- ing additional functionality. To provide high-speed Internet access in a public-access sce- nario, a WLAN must make an optimal trade-off between bit rates and range. In the home environment, significant challenges include the simultaneous distribution of high-definition video, high -speed Internet, and telephony. Such applications demand efficiency, robustness, and QoS from the WLAN. The forthcoming WLAN system is expected to provide a variety of services not currently available, such as: • Higher data rates (more than 100 Mbps) and low power consumption • Extended coverage areas and scalability using the multihop/mesh network • Coexistence of heterogeneous access devices in the same environment 106 WIRELESS LAN EVOLUTION 0 5 10 15 20 25 30 35 0 6 12 18 24 30 36 42 48 54 PHY Rate [Mbit/sec.] IP Throughput [Mbit/sec.] 802.11a CCK-OFDM (short) CCK-OFDM (long) ă802.11b (short) ă802.11b (long) Figure 4.14 The maximum IP throughput. Reproduced by permission of Dr. Morikura • Seamless mobility support: – Handoff mechanism and seamless AAA during handoff – Interworking with other systems, seamless mobility between various access tech- nologies, allowing continuity of existing sessions • Differentiated service support for differing reliability needs • Indoor location estimation • Quality of service assurance, including support of real-time applications • Enhanced security features, including authentication/authorization and data cipher A number of the issues that limit current WLAN services can be addressed through new technologies. This chapter focuses on the WLAN issues that will be most urgently needed to create solutions complementary to XG mobile networks. The following sections discuss in more detail the technologies related to mobility support, QoS, and enhanced security. WIRELESS LAN EVOLUTION 107 4.3.1 Higher Data Rates and Low Power Consumption Typical office applications, such as the downloading of large e-mail attachments, are data intensive. In a public hotspot, such as a hotel or airport, the time available for download is likely to be limited. A public wireless access solution should ideally be able to offer very fast transmission capacity. Both simulation and experience have shown that the throughput in an 802.11a network is actually limited to a point significantly below the 54 Mbps theoretically achievable by the PHY layer. There is also a theoretical maximum throughput for 802.11 MAC (Xia and Rosdahl 2002; Xiao and Rosdahl 2002). However, a WLAN that uses the CSMA/CA mechanism employs four different interframe spaces (IFSs) to control access to the wireless medium. These IFSs act as overhead, which limits the improvement of throughput perfor- mance. To reduce this MAC overhead, new systems may use multiple antennas solutions, bandwidth increment, turbo codes, and higher-order constellations, all of which can help to increase the theoretically achievable capacity (Simoens et al. 2003). The TGn of IEEE 802.11WG is now working on improving the current MAC and PHY throughput. The next generation of WLAN should be able to improve throughput performance significantly, with data rates of more than 100 Mbps. However, much of the research that targets maximum throughput does not consider increased power consumption. Energy efficiency is becoming crucial to the design of next- generation wireless systems, especially for WLAN that is used by mobile devices with limited battery life. Although WLAN does include a power-management scheme, further power efficiency from both PHY and MAC solutions will be needed. 4.3.2 Extended Coverage Areas and Scalability Multihop mesh network communication is gaining popularity, both for pure ad hoc commu- nication networks and for coverage extension in wireless networks. A mesh network differs from an ad hoc network in that each WLAN node operates not only as a host but also as a router. User packets are forwarded to and from an Internet-connected gateway in mul- tihop fashion. The network is dynamically self-organizing and self-configuring; the nodes in the network automatically establish and maintain routes among themselves. This makes the meshed topology reliable and it provides good area coverage. Systems are scalable and initial investment can be minimal because the technology can be installed incrementally, one node at a time, as needed. As more nodes are installed, both reliability and network coverage increase (Fitzek et al. 2003; Jun and Sichitiu 2003). This option would decrease installation costs for WLAN hotspots of the next generation. A mesh network’s traffic pattern is different from that of an ad hoc network. In the mesh network, most traffic is either to or from a gateway, while in ad hoc networks, the traffic flows between arbitrary pairs of nodes. Because of poor support for multihop opera- tions in the current IEEE 802.11 standard, current WLAN systems show poor performance for such multihop/mesh networks. To improve this, we need to find more-efficient MAC schemes that make it possible to operate these devices in multihop mode without exces- sive performance degradation. In the IEEE 802.11 WG, a Mesh Network Study Group was approved to be a TG in March 2004 to create a new standard for mesh networks over WLAN. 108 WIRELESS LAN EVOLUTION 4.3.3 Coexistence of Access Devices The WLAN operates in the 2.4-GHz industrial, scientific, and medical (ISM) unlicensed band. In the unlicensed ISM band, frequencies must be shared and potential interference tolerated as defined in Federal Regulations Part 15 of Federal Communications Commis- sion (FCC). Spread spectrum and power rules are fairly effective in dealing with multiple users in the band as long as the radios are physically separated, but not when the radios are in close proximity. This would be a problem for IEEE 802.11 WLAN and Bluetooth that, for example, come together in a laptop or desktop. To operate in the 5-GHz range, WLAN must share with other systems, such as military, aeronautical, naval RADARs, and satellite systems. In Europe, for example, WLAN oper- ating on the 5 GHz band is required to implement dynamic frequency selection (DFS) and transmit power control (TPC) in order to share with radar systems. Current research is focused on the coexistence of wireless devices in the 2.4-GHz band and other bands. • The IEEE 802.15.2 standard specifically addresses coexistence between WLAN and Bluetooth systems. This standard has adopted an adaptive frequency hopping (AFH) mechanism, which modifies the Bluetooth frequency hopping sequence in the presence of WLAN direct sequence spectrum devices (Golmie 2003; Golmie et al. 2003). • The TGh standard in the IEEE 802.11 WG met the European regulatory requirement for coexistence with radar systems. • The IEEE 802.19 Coexistence Technical Advisory Group (WG19) is working on policies that define the responsibilities of 802 standards developers to address issues of coexistence with existing standards and other standards under development. 4.3.4 Seamless Mobility Support Smooth on-line access to corporate data services in hot spots should allow users to move freely from a private, microcell network to a wide-area cellular (3G) network. In the next generation, various complementary RANs, including WLAN, will be used in combination with 4G RANs to provide full coverage services. Seamless communications over these heterogeneous environments will require effective vertical handoff support. Current applications primarily move data through the WLAN. In future, users expect to use VoIP over WLAN through the corridor or public space. With VoIP, a user requires handoff support to keep voice connection when moving from one AP to another. In other applications, such as video streaming, users want a seamless connection while roaming through different rooms and corridors. Mobility support and security are not currently sufficient to support a seamless con- nection over WLAN. Currently, WLAN does not have any coordination when the station (STA) moves from one AP to another, which causes connections to break during the hand- off. Fast-scanning and fast-authentication technologies will be key factors in reducing the handoff blackout time. To create solutions for these needs, the research community is studying authentica- tion, authorization, and accounting (AAA) and QoS mapping between different access WIRELESS LAN EVOLUTION 109 networks (Koin and Haslestad 2003). Standards work in this area is being done by the 3rd Generation Partnership Project (3GPP). WGs are currently developing technical require- ments for UMTS-WLAN interworking systems, reference architecture models, network interfaces, and AAA. The IEEE 802.11 WG has also formed a Study Group on Wireless Interworking with External Network, which will soon become a TG, working to standardize an interworking interface between WLAN and other wireless networks. There are two interworking solutions, tight coupling and loose coupling, based on the type of integration formation. The two solutions have different pros and cons: • Tight coupling uses the WLAN as a part of 3G RAN in which all necessary func- tions are located in the core network. This solution has the advantage of fully integrated mobility management (handover) and possible QoS mapping by the 3G core network. The 3G core network also provides sufficient AAA functionality. However, deployment is time consuming, and significant standards work will be needed. • Loose coupling considers WLAN as equivalent to the 3G networks. It adapts the IP protocol architecture and requires few changes to the WLAN standard. It has a low deployment cost and fast time to market. However, it is not easy to achieve QoS mapping or mobility support, and there is a possible risk of AAA compromise to 3G mobile networks. 4.3.5 Location Estimation by WLAN The recent growth of interest in pervasive computing and location-aware systems and ser- vices provides a strong motivation to develop techniques for estimating the location of devices in both outdoor and indoor environments. Indoor location estimation is particularly challenging because of the poor coverage of global positioning systems (GPS). There are several approaches that use existing wireless LAN infrastructures. Early work in this area included the RADAR system (Bahl and Padmannabhan 2000), which showed that accurate indoor location estimation could be achieved without deploy- ing separate sensor network infrastructures. Their idea is to infer the location of a IEEE 802.11b wireless LAN user by leveraging received signal strength information available from multiple WLAN beacons. In following work (Bahl et al. 2000), RADAR was enhanced by a Viterbi-like algo- rithm that specifically addresses issues, such as continuous tracking and signal aliasing. The Nibble system (Castro 2001) took a probabilistic approach in a similar WLAN environment. The MultiLoc system (Pandya et al. 2003), which utilizes information from multiple wireless (or wired) technologies, was proposed. The MultiLoc system employs two simple sensor fusion techniques to illustrate the benefit of combining heterogeneous information sources in location estimation. DoCoMo USA Labs proposes two location-estimation algorithms (Gwon et al. 2004), Selective Fusion Location Estimation (SELFLOC) and Region of Confidence (RoC), which can perform estimation and tracking of the location of stationary and mobile users. More research is still needed for practical deployment. For details of the research, see (Gwon et al. 2004). [...]... is terminated is different in Mobile IPv4 and Mobile IPv6 The next two sections describe tunnel termination and last hop routing in Mobile IPv4 and Mobile IPv6 Home-agent Tunnel Termination in Mobile IPv4 In Mobile IPv4, the last hop router for the mobile host’s current subnet typically manages the assignment of a care-of address for a newly arriving mobile host and takes care of terminating the tunnel... separate versions of Mobile IP for IPv4 (Perkins 2002b) and IPv6 (Johnson et al 20 04) Mobile IP effectively separates the routing identifier and endpoint identifier for the mobile host The endpoint identifier is the original IP address, assigned to the mobile host when it is in its home network This address is called the mobile host’s home address The home address is propagated into the DNS and is used by other... (IETF) standards, are still being developed as standards, or are still experimental and are being discussed in the Internet Research Task Force (IRTF) prior to standardization Discussion of standards is appropriate because, at this time, most of the standards have not yet been deployed or, in some cases, have not even been implemented as products The large cellular standards organizations (3GPP and 3GPP2)... the IEEE 802.11 WG in March 20 04 and is investigating further improvement of the fast-handoff capability 4. 4.1 Fast Channel Scanning The scanning process – when mobile stations scan for available networks to determine which network to join – is one of the most time-consuming processes in the handoff (Mishra et al 2002b) 802.11 Wireless LAN has two ways of scanning: passive and active Passive scanning... authentication mechanism for supporting mobile users moving from one AP to another within the coverage area of a WLAN system Mobile communication systems, such as 2G and 3G do not require authentication during handoff because their security and encryption features guarantee that the user is valid WLAN currently defines three mobility types that do not include seamless handoff (IEEE 1999a): No-transition:... the purposes of charging, QoS, and service provisioning While these functions can be performed at the Mobile IP home agent, they are not part of the Mobile IP protocol, and not part of the basic Internet wireless /mobile network architecture In addition, the 3G gateways also contain an interface between legacy, SS7-based cellular control protocols, and the IP network Since Mobile IP was originally developed... interface The mobile host is responsible for maintaining the binding between the home address and the care-of address at the home agent When the mobile host moves to a new subnet and receives a new care-of address, it sends a message to its home agent containing the binding between the new care-of address and the home address In Mobile IPv4, this process is called home-agent registration In Mobile IPv6,... the AS and the STA The AS can be located in the DS or in the AP IEEE 802.11i provides three cryptographic algorithms to protect data traffic: WEP, TKIP, and Counter mode with the CBC-MAC protocol (CCMP) WEP and TKIP are based on the RC4 algorithm and CCMP (Counter with CBC-MAC (CCM) 2003) is based on the advanced encryption standard (AES) (FIPS 2001) In this new security standard, CCMP is a mandatory... WLAN and GSM into a single account using GSM and WLAN Another benefit is easy roaming Unlike most Internet service providers, mobile operators have the infrastructure and support roaming between different operator networks So these solutions focus on single bill and roaming rather than supporting authentication method during handoff The main design challenge for these solutions was transporting standard... used to prevent replay protection 4 A new cryptographic mixing function creates a temporal key This key mixing function is designed to defeat weak key attacks Details of the TKIP and CCMP algorithms can be found in the IEEE 802.11i standard, which is expected to be published in mid 20 04 5 IP Mobility James Kempf 5.1 Introduction To those familiar with 3G mobile standards, the Internet looks like a . of wireless devices in the 2 .4- GHz band and other bands. • The IEEE 802.15.2 standard specifically addresses coexistence between WLAN and Bluetooth systems. This standard has adopted an adaptive. support, and there is a possible risk of AAA compromise to 3G mobile networks. 4. 3.5 Location Estimation by WLAN The recent growth of interest in pervasive computing and location-aware systems and. define the responsibilities of 802 standards developers to address issues of coexistence with existing standards and other standards under development. 4. 3 .4 Seamless Mobility Support Smooth on-line