A QoS-Based Vertical Handoff Scheme for ... - Semantic Scholar

A QoS-Based Vertical Handoff Scheme for Interworking of WLAN and WiMAX Dong Ma and Maode Ma School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798 E-mail: [email protected]; [email protected] Abstract—In this paper, we investigate the issue of vertical handoffs in heterogeneous wireless networks. A QoS-based vertical handoff scheme for WLAN and WiMAX interworking networks is proposed with aim to provide always best service to users. We also present a simple, yet efficient method to estimate the available bandwidth in WLAN and WiMAX networks to evaluate the real-time status of the overlay networks and make a handoff decision based on the information. By our proposal, a handoff process will not only be triggered by unaccepted signal strength but also by unsatisfied QoS parameters. Simulation results show that the proposed scheme can keep stations always be best connected with their QoS requirements met.

I. INTRODUCTION

T

HE fourth-generation (4G) wireless networks tend to integrate different wireless access technologies for the purpose of offering users the best possible service. It is expected that devices will increasingly have multiple heterogeneous interfaces and obtain service from different networks simultaneously. Recently, the family of IEEE802.11 standards has become the most popular technology to support Wireless LANs (WLAN), which have been widely deployed for the broadband wireless access due to their low cost and high capacity. Meanwhile, another family of standards - IEEE 802.16 provides detailed specifications on the emerging broadband wireless access technique, Worldwide Interoperability for Microwave Access (WiMAX), which has attracted much more attention recently due to its promised high bandwidth over long-range transmission with QoS supports. The integration of WLAN and WiMAX is a promising approach to implement always best connected (ABC) [1] service by broadband wireless access techniques. In a heterogeneous wireless network, a handoff process will be different from that in a unique network. There could be two types of handoff processes: horizontal handoff (HHO) and vertical handoff (VHO) [2]. A HHO happens between different sectors controlled by different base stations (BS) or access points (AP) with the same wireless technology. A VHO is a handoff between different sectors with different wireless technologies. In conventional HHOs, only signal strength is considered for making a handoff decision. But in VHOs, some other metrics could be considered for the handoff decisions, such as QoS parameters. The design of seamless and efficient VHOs is an essential and challenging issue in the development of the 4G wireless networks. Recently, there are some proposals on the topic of VHOs in the WLAN and 3G interworking networks. However, very few

solutions for the WLAN and WiMAX interworking networks have been reported in the literatures. In [3], the authors have proposed a movement-aware VHO algorithm which exploits movement pattern to avoid unnecessary handoffs. QoS parameters have been taken into account in making the handoff decision. But the paper has not provided any technical details to obtain those parameters. In [4], the authors have proposed a mechanism that supports ABC QoS for IEEE 802.11e WLAN and IEEE 802.16d WiMAX networks. The handoff decision is made by comparing the QoS parameters provided by AP and BS. The stations cannot detect and estimate the conditions of each network proactively and the solution cannot be extended to other IEEE 802.11 standards due to the need of QoS support in AP. In [5], a VHO scheme for the interworking of the IEEE 802.11n WLAN and the IEEE 802.16a WiMAX networks has been presented, by which bandwidth is taken as the major QoS metric for the handoff decision. An algorithm to estimate the available bandwidth in WLANs has also been introduced. However, by this proposal, IEEE 802.11n WLANs have been taken to be the preferred networks and mobiles are assumed to try to stay in WLANs as long as possible. The solution is not applicable to the normal scenarios where the data rates of WLANs may change to lower than that in WiMAX networks due to heavy traffic load or wireless fading channel. Some parameters used in the WLAN bandwidth estimation scheme are not feasible to obtain. In this paper, we aim to design an effective QoS-based VHO scheme for a wireless overlay network consisting of WLANs and WiMAX heterogeneous wireless networks. The major contributions of this paper can be summarized as follows. 1) A general VHO management (VHOM) scheme to make VHO decisions in WLANs and WiMAX interworking networks has been designed for each station to intelligently monitor the quality of connections and proactively detect network conditions. VHOM always selects the network which can provide better service to serve the station rather than a preferred network. 2). In order to obtain QoS parameters to make a handoff decision, two bandwidth estimation algorithms have been proposed to predict the available bandwidth in WLAN and WiMAX networks. To the best of our knowledge, this is the first piece of work to estimate the available bandwidth in WiMAX networks. Our proposed solutions are practical to obtain the information required for the estimation and feasible to implement.

978-1-4244-4148-8/09/$25.00 ©2009 This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.

The rest of the paper is organized as follows. In Section II, the system architecture and the framework of the VHO scheme of our design is described. In Section III, two bandwidth estimation algorithms are proposed. In Section IV, the performance evaluation is presented to show the effectiveness and the feasibility of our solution. And in Section V, a summary is presented to conclude the paper.

II. SYSTEM MODEL AND PROPOSED VHOM SCHEME A. Interworking Architecture In this work, we study the interworking architecture of WLAN and WiMAX networks as shown in Fig. 1. The BS and overlaid APs can access to the same gateway. They belong to the same subnetwork at the IP layer. Each station is equipped with only one IP address but two medium access control (MAC) addresses for each of two interfaces. Any station will be in the connection with either a WLAN AP or a WiMAX BS at a time. The interface that is not serving the station will be in sleep mode. In the interworking system, the WiMAX network can be supported by either IEEE 802.16d or IEEE 802.16e standard without restriction on the physical and MAC layer of the networks. The WLAN can be supported by any one type of the IEEE 802.11 standards, where the network allocation vector (NAV) mechanism has been employed at the MAC layer. When mobility is supported in the interworking system, a VHO can be triggered by either a station moving out of the coverage of one cell of WLAN or the lack of available bandwidth to meet the QoS requirement in the current network. Otherwise, when mobility is not supported by the interworking system, a station can perform VHO when bandwidth is insufficient. B. VHO Management Scheme Based on the interworking architecture, in order to enable stations to intelligently monitor the quality of current connections and make an accurate handoff decision, we propose and design a VHOM scheme, which works on the MAC layer protocols of IEEE 802.11 and IEEE 802.16 standards. The major functions of the proposed VHOM include traffic measurement, network status detection, handoff decision, and connection transition. The functions are performed by four modules as illustrated in Fig. 2: • Traffic measurement module (TMM) which periodically

Fig. 1. Interworking architecture of WLAN and WiMAX.

VHO Manager (VHOM)

Traffic measurement module (TMM) Uplink traffic measurement

Control

Downlink traffic measurement

Context

Network availability detection Available bandwidth estimation

Network condition detection module (NCDM)

Handoff decision module (HDM) Connection transition module (CTM)

Fig. 2. Overview of VHOM.

measures the throughput of the network and provides reports to the Handoff decision module (HDM). • Network condition detection module (NCDM) which detects the availability of possible networks and estimates the performance of the available network. • Handoff decision module (HDM) which is the core component of VHOM. It gathers information from other modules and manipulates their operation. It decides the need to launch a handoff decision process and selects the target network based on the available handoff policies. • Connection transition module (CTM) which transfers all the connections of the station from the serving network to the selected target network. The operation of the proposed VHOM scheme can be described as following. When HDM receives a given number of successive reports on the performance of the current network from TMM, which indicate the throughput of the uplink or downlink connections cannot satisfy the QoS requirement of the traffic at the station, it will initiate the NCDM to detect the performance of the other network. The availability of the network will be detected first. If the other network cannot be found, HDM will terminate this handoff attempt and restart after a period if the measured traffic still cannot meet QoS requirement. Otherwise, if the network can be detected, NCDM will awaken the corresponding MAC layer from the sleep mode to estimate the bandwidth of the detected network domain using the algorithms presented in section III. Subsequently, the estimated reports will be submitted to HDM to make a decision on handoff. If the estimated available bandwidth can satisfy the QoS requirement of the traffic at the station, HDM will decide the new network domain as the target network and initiate CTM to perform a handoff operation. Otherwise, NCDM will be started to evaluate the bandwidth again after a period of time. Once CTM receives a command to launch a handoff, it will issue a gratuitous Address Resolution Protocol (ARP) message to notify the binding of the IP address and the MAC address corresponding to the target network. Hereafter, the data destined to the station will be transmitted by access gateway via the target network. Since the IP address is not changed, the

978-1-4244-4148-8/09/$25.00 ©2009 This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.

VHO process is transparent to upper layers. When making a handoff decision, it should be noticed that the resource allocation approach in WiMAX systems is different from that in WLANs. In WLANs, the medium is shared by multiple stations in a contention manner. And only the total available bandwidth can be estimated. However, the wireless resource in WiMAX networks is scheduled by BS and is partitioned into downlink and uplink sections. Then the available bandwidth should be estimated for downlink and uplink directions, respectively. Therefore, the handoff direction should be considered when making a VHO decision. For the scenario of the handoff from the WiMAX system to the WLAN, the condition to trigger a handoff is Bt ≥ ThrU + Thr D + h1

(1)

where Bt is the estimated total available bandwidth of the WLAN domain, ThrD and ThrU are the expected downlink and uplink throughput, h1 is the hysteresis which is used to guarantee the available bandwidth of the target network sufficiently stronger to serve the desired station. For the scenario of the handoff from the WLAN to the WiMAX system, both the downlink and uplink available bandwidth should satisfy the given conditions B D ≥ Thr D + h2

BU ≥ ThrU + h3 .

(2) (3)

Where BD and BU denote the downlink and uplink available bandwidth of WiMAX network, h2 and h3 are hystereses for the downlink and uplink connections respectively.

III. PROPOSED BANDWIDTH ESTIMATION ALGORITHMS In [6], a general expression for available bandwidth has been presented, which is the difference between the total capacity and aggregated utilized bandwidth. The following presented solutions evolve from this definition and take into account the characteristics of WLANs and WiMAX networks. A. Bandwidth Estimation in WLANs The fundamental access method of the IEEE 802.11 MAC is Distributed Coordination Function (DCF) known as carriersense multiple access with collision avoidance (CSMA/CA) [7]. The carrier sense at MAC layer can be achieved by advertising reservation information announcing the impending use of the medium. The exchange of request to send/clear to send (RTS/CTS) frames is one means of this medium reservation. Prior to the data transmission, the source station sends RTS and the destination station replies with CTS. Other stations that hear RTS/CTS will update their NAV to be busy state for a period of time given in the RTS/CTS frames. During this period, the station will defer its own data transmission as shown in Fig.3. A station determines the medium to be busy in two cases. One case is that NAV is set, which means the medium is occupied by another station. The

Fig. 3. IEEE 802.11 frame exchange sequence.

other case is that the medium is occupied by this station itself as a sender or receiver. Consequently, by aggregating these busy intervals, the station can estimate the utilization of the medium and the available bandwidth in succession. We assume the estimation period is τ. The period that the medium is occupied by data transmission during τ can be expressed as Tdata ( t − τ , t ) =

∑

t t −τ

Tdata _ NAV + ∑ t −τ Tdata _ trans t

(4)

where Tdata_NAV denote the data transmission time in a NAV interval, Tdata_trans denote the data transmission time in a transmission opportunity obtained by the desired station. Then the average utilization of the medium can be written as u (t − τ , t ) =

Tdata ( t − τ , t )

τ

.

(5)

However, as shown in Fig. 3, the reserved medium is not only used for data transmission in every busy interval. Time intervals between frames such as interframe space (IFS) are mandated by CSMA/CA algorithm and every transmitted data frame should be acknowledged by acknowledgement (ACK) frame. The proportion of the reserved resource that can be used purely for data transmission is

γ (t ) = μ data

(6)

μ data + μ R / C / A + ( 3T SIFS + T DIFS + Tbackoff ( t )) C (t ) / 8 where μdata is the mean data packet size in bytes of the desired station, μR/C/A is the sum of RTS/CTS/ACK frames size, TSIFS and TDIFS are time durations of SIFS and DIFS. Tbackoff(t) denote the average random backoff time, and C(t) is the data rate at time t. Based on the above information, we can calculate the available bandwidth through the expression BW available ( t ) = (γ (t ) − u (t − τ , t )) C (t ).

(7)

In (6), Tbackoff(t) and C(t) are time varying parameters. Both of them are determined in a way by the traffic in the network. In order to obtain accurate values for estimation, the scheme requires the desired station sends several Probe-Request messages at a speed of expected throughput before entering estimation process. But during the estimation period τ, we assume the data rate remains constant.

978-1-4244-4148-8/09/$25.00 ©2009 This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.

Note that if RTS/CTS mechanism is disabled, the proposed scheme can still work well after making minimal modification in the formulations. The methodology is similar. B. Bandwidth Estimation in WiMAX Systems In IEEE 802.16 protocols, the frame consists of a downlink subframe and an uplink subframe as illustrated in Fig. 4. The bandwidth resources will be allocated in the form of data bursts. And each burst constitutes an integer number of physical slots (PSs). Based on the resource demands and QoS parameters, BS determines the bandwidth that a station can obtain in one frame for receiving downlink data or transmitting uplink data, and then broadcasts this allocation information through information element (IE) in the DL-MAP and UL-MAP messages at the beginning of each frame [8]. For different PHY specifications, the bandwidth allocation information in DL-MAP/UL-MAP messages is expressed in different granularities, such as minislot in WirelessMAN-SC PHY specification and subchannel-symbol pair in WirelessMAN-OFDMA PHY specification. But all these granularities can be specified as a function of PS. For simplicity and explicitness, we use PS as a uniform unit. In the proposed scheme, desired stations are expected to check all the IEs in the DL-MAP/UL-MAP messages and aggregate the number of allocated downlink and uplink PSs, respectively. We use Tframe to denote the time duration of a frame. SDL-total and SUL-total denote the number of PSs in a downlink/uplink subframe, respectively. Assume that there are an integer number of frames during the probing period τ, which can be given by n=

τ

(8)

T frame

Then the number of unused PSs of this period is equal to S DL − free ( t − τ , t ) = nS DL − total − ∑ i =1 S DL − used ( i )

(9)

S DL − free ( t − τ , t ) = nS DL − total − ∑ i =1 S DL − used ( i )

(10)

n n

where SDL-used(i) and SUL-used(i) denote the utilized PSs in the ith downlink and uplink subframe. Based on the parameters introduced above, the available bandwidth can be estimated by the following expressions BW DL − available ( t ) =

1

τ

(11)

S DL − free ( t − τ , t ) C DL − slot (t )

Fig. 4. TDD frame structure of IEEE 802.16.

1

BW UL − available ( t ) =

τ

S UL − free ( t − τ , t )C UL − slot ( t )

(12)

where CDL-slot(t) and CUL-slot(t) are the number of bits that can be transmitted in one downlink or uplink PS. The values depend on the modulation and coding schemes (MCS) used at the physical layer. We further define the utilization of the resource in WiMAX as u DL ( t − τ , t ) = u UL

∑

∑ (t − τ , t ) =

n i =1

S DL − used (i )

(13)

nS DL − total n i =1

S UL − used ( i )

nS UL − total

.

(14)

IV. PERFORMANCE EVALUATION To demonstrate the effectiveness and feasibility of our proposed approach, we have built a simulation model based on the architecture illustrated in Fig.1 by the simulation software QualNet 4.0. Both the proposed VHOM scheme and the two bandwidth estimation algorithms have been tested. In simulation, we have taken IEEE802.11a as a representative example of WLAN standards. Meanwhile, IEEE 802.16d and IEEE 802.16e have been tested in different scenarios as examples of WiMAX standards. A. Evaluation of Bandwidth Estimation Algorithms The available bandwidth estimated by the proposed algorithms has been compared with the corresponding simulation results in Fig. 5 and Fig. 6 for a WLAN and a WiMAX system, respectively. The important parameters used in the simulation have been shown in Table I. TABLE I SIMULATION PARAMETERS FOR AVAILABLE BANDWIDTH ESTIMATION Parameter Value Data rates tested in WLAN 54/36/24/18/12/6 (Mbps) Mean packet size in WLAN 2000 (bytes) Time duration of SIFS/DIFS in WLAN 16/34 (μs) PHY specification in WiMAX OFDMA Channel bandwidth in WiMAX 20 (MHz) FFT size in WiMAX 2048 Frame duration in WiMAX 20 (ms) Downlink subframe duration in WiMAX 10 (ms)

The relationship between the available bandwidth and the network utilization in WLAN networks has been shown in Fig. 5. It is shown that the maximum bandwidth available purely for data transmission is about 60%-90% of the data rate. The higher the data rate, the lower the maximum utilization, which is implied by (6). The maximum utilization corresponds to γ(t). If data rate C(t) is increased, γ(t) will be decreased. It is also shown that there are very small differences between the estimated and simulated results. But it is obvious that the estimated results under higher utilization conditions are more crucial for making an accurate VHO decision. At this part, the proposed algorithm works very well. Moreover, some compensation to the estimation difference

978-1-4244-4148-8/09/$25.00 ©2009 This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.

B. Evaluation of Proposed VHOM Scheme We evaluate the performance of the proposed VHOM scheme in the following two scenarios. For systems with mobility support, the proposed scheme is compared with existing WLAN preferred solutions in the first scenario. For systems without mobility support, the proposed scheme is compared with conventional cases where handoff cannot be supported in the second scenario. The integrated system built in the simulator consists of one WiMAX network and two WLANs, just as shown in Fig. 1. The simulation parameters used are shown in Table II.

Available bandwidth (Mbps)

40 35

estimated simulated

30 data rate: 54 Mbps

25

36 Mbps

20 15

24 Mbps 18 Mbps

10

12 Mbps 5 0 0

6 Mbps 0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Utilization Fig. 5. Available bandwidth in WLAN.

Availabe bandwidth (Mbps)

35

estimated simulated

30 data rate: 31.752Mbps

25 20

TABLE II SIMULATION PARAMETERS FOR VHO Parameter Value IEEE 802.11a/ 802.16e (scenario 1) Standards IEEE 802.11a/ 802.16d (scenario 2) Data rates supported by WLAN 6/9/12/18/24/36/48/54 (Mbps) Max. Radius in WLAN 130 (m) Max. Radius in WiMAX 1000 (m) Frame duration in WiMAX 5 (ms) Transport layer protocol TCP(uplink) UDP(downlink) Number of stations in WiMAX20/20 only area/each overlay area

21.168Mbps 15 14.112Mbps

10

7.056Mbps 5 0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Utilization (a)

In the first scenario, a mobile station moves from the WiMAX domain into WiMAX/WLAN overlay region at a speed of 1m/s. There are two connections at the application layer. One is uplink VBR/TCP application with an expected throughput of 4Mbps. The other is downlink CBR/UDP application with an expected throughput of 3.2Mbps. Initially, the traffic measurement module of VHOM found that the average throughput of uplink traffic fell below the accepted

Available bandwidth (Mbps)

35

estimated simulated

30 data rate: 32.076Mbps

25 20

21.384Mbps 15 14.256Mbps 10

(a)

7.128Mbps

4.5

5

4

0 0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

3.5

(b) Fig. 6. Available bandwidth in WiMAX: (a) downlink; (b) uplink.

can be made by setting proper hysteresis h1 in (1). Fig. 6 shows the relationship between the available bandwidth and the network utilization in WiMAX networks. The simulation results and the estimated results are very close to each other for both the downlink and uplink transmissions. Slightly differences are caused by partially use of some PSs. In the uplink, the utilization of the medium cannot reach 0. The reason is that even no data transmission, some inherent PSs must be set apart for initial ranging or bandwidth request purpose.

Throughput (Mbps)

Utilization

3 2.5 2 1.5 DL-WLAN prefer UL-WLAN prefer DL-VHOM UL-VHOM

1 0.5 0 0

10

T1

20

30

40

50

T2

60

70

80

90

T3

100 T4

Simulation time (second)

(b) Fig. 7. Throughput comparison under the movement pattern.

978-1-4244-4148-8/09/$25.00 ©2009 This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.

obtained at the station by the proposed VHOM scheme has been compared with the similar system without VHO support in Fig. 8 (a). The results show that larger throughput can be obtained by the proposed solution. The sequence number of the uplink TCP connection around the handoff time has been compared in Fig. 8 (b). It is shown that for a given packet (identified by the sequence number), lower delay can be obtained by our solution.

3

Throughput (Mbps)

2.5 2 1.5 1

DL-no VHO UL-no VHO DL-VHOM UL-VHOM

0.5 0 0

10

20

V. CONCLUSION

30

40

50

60

70

80

90

100

Simulation time (second) (a)

TCP sequence (bytes)

1.78e+07 1.76e+07

VHOM

1.74e+07

no VHO

1.72e+07 1.7e+07 1.68e+07 1.66e+07 1.64e+07 1.62e+07 1.6e+07 50

51

52

53

54

In this paper, we have proposed a novel VHOM scheme to make VHO decisions in the interworking architecture of WLANs and WiMAX networks. The scheme aims to provide always the best QoS in terms of throughput for both mobile users and fixed users. The available bandwidth has been taken into account in making the vertical handoff decisions. We have also developed two algorithms to estimate the available bandwidth in WLANs and WiMAX networks respectively, which are based on the inherent features of the bandwidth allocation schemes and the message transmission approaches in both networks. The parameters required for the bandwidth evaluation are easy to obtain in the real networks and the complexity of the evaluation is computationally low. Those make the proposed scheme to be feasible to implement. By the simulation experiments, we have proven the feasibility and effectiveness of our proposed VHO scheme.

55

Simulation time (second) (b) Fig. 8. VHOM supported case versus no VHO support case for a fixed station: (a) throughput; (b) sequence number of the uplink TCP connection.

3Mbps. But there was no other network available at that time. At the moment of T2= 25 second, a WLAN domain has been found. VHOM initiated the available bandwidth estimation process immediately. However, the estimated available bandwidth could not satisfy the QoS requirement until T3= 65 second. The moving pattern and simulation results are shown in Fig. 7. It is shown that the handoff process started at 65 second by VHOM while at 25 second by WLAN preferred scheme. The simulation results show that better service can be achieved by the proposed solution. In the second scenario, the observed station located in the WLAN/WiMAX overlay region without movements. The station worked with an uplink CBR/TCP application and a downlink CBR/UDP application. Initially, the station was served by WLAN service domain. At around 50 second, VHOM detected that the network traffic tended to be overloaded and the downlink throughput could not fulfill the accepted 1.5Mbps for a period of 10 seconds. The NCDM module was initiated to estimate the available bandwidth of WiMAX. The estimated results showed that both the downlink and uplink available bandwidth of WiMAX could satisfy the requirements. Then the connections were transferred to the WiMAX domain immediately at the station. The throughput

REFERENCES [1] E. Gustafsson and A. Jonsson, "Always best connected," IEEE Wirel. Commun., vol. 10, no. 1, pp. 49-55, Feb 2003. [2] M. Liu, Z. Li, X. Guo, and E. Dutkiewicz, "Performance analysis and optimization of handoff algorithms in heterogeneous wireless networks," IEEE Trans. Mobile Comput., vol. 7, no. 7, pp. 846-857, July 2008. [3] L. Wonjun, K. Eunkyo, K. Joongheon, L. Inkyu, and L. Choonhwa, "Movement-aware vertical handoff of WLAN and mobile WiMAX for seamless ubiquitous access," IEEE Trans.Consum.Electr., vol. 53, pp. 1268-1275, 2007. [4] J. J. J. Roy, V. Vaidehi, and S. Srikanth, "Always best-connected QoS integration model for the WLAN, WiMAX heterogeneous network," IEEE Int. Conf. on Industrial and Information Systems (ICIIS), pp. 361366, 2006. [5] J. Nie, J. C. Wen, D. Qi, and Z. Zhou, "A seamless handoff in IEEE 802.16a and IEEE 802.11n hybrid networks," IEEE Int. Conf. on Commun. Circuits and Systems, vol. 1, pp. 383-387, 2005. [6] R. Prasad, C. Dovrolis, M. Murray, and K. Claffy, "Bandwidth estimation: metrics, measurement techniques, and tools," IEEE Network, vol. 17, no. 6, pp. 27-35, NOV.- DEC. 2003. [7] IEEE 802.11 WG, Part 11: Wireless LAN medium access control (MAC) and physical layer (PHY) specifications, IEEE standard, 1999. [8] IEEE 802.16 WG, Part 16: Air Interface for fixed broadband wireless access systems, IEEE Standard, 2004.

978-1-4244-4148-8/09/$25.00 ©2009 This full text paper was peer reviewed at the direction of IEEE Communications Society subject matter experts for publication in the IEEE "GLOBECOM" 2009 proceedings.