WLANâUMTS Integration: Architecture, Seamless Handoff, and ...

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WLAN-UMTS Integration: Architecture, Seamless Handoff, and Simulation Analysis Faouzi Zarai, Noureddine Boudriga and Mohammad S. Obaidat SIMULATION 2006; 82; 413 DOI: 10.1177/0037549706070275 The online version of this article can be found at: http://sim.sagepub.com/cgi/content/abstract/82/6/413

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WLAN–UMTS INTEGRATION

WLAN–UMTS Integration: Architecture, Seamless Handoff, and Simulation Analysis Faouzi Zarai Noureddine Boudriga University of Carthage, Tunisia NJ, USA [email protected] Mohammad S. Obaidat Monmouth University NJ, USA Providing Quality of Service (QoS) for sophisticated and QoS-constraining applications (e.g. videobased applications, multimedia services, and high speed data services) and ensuring continuous connectivity for universal mobile stations (UMS) in heterogeneous networks are important issues in the design of fourth generation (4G) networks. The latter will utilize multiple radio access technologies including cellular technologies (such as GSM, GPRS, and UMTS), satellite-based communications, and wireless local area networks, which are seamlessly integrated to form a heterogeneous wireless network. In the paper, first, an architecture for WLAN-UMTS integration is defined that makes the composing networks transparent to users and provides seamless services. Second, a novel protocol is proposed to reserve resources for handoff. Third, modeling and simulation is used to evaluate the QoS performance. Keywords: Heterogeneous wireless network, WLAN, UMTS, seamless handoff, security, resources reservation, performance evaluation, modeling and simulation

1. Introduction Currently, there is a large number of developed and deployed wireless networking technologies (Cellular networks (such as 2G, and 3G networks), Wireless Local Area Network (WLANs with access points or ad hoc WLANs), and Personal Area Networks (PANs) such as Bluetooth and Home RF). In fact, the Universal Mobile Telecommunication System (UMTS), which represents a major 3G wireless technology, offers wireless Internet services at a world-wide scale, extending the scope of second generation wireless networks from simple voice telephony to complex data applications including voice over IP, video conferencing over IP, web browsing, multimedia services, and high-speed data transfer. The major disadvantage of UMTS is that it is less suitable for small, indoor, and densely populated areas. Therefore, researchers and standardization bodies [1–4] have considered other types of technologies that could be utilized to extend 3G/UMTS network services in such dense areas. A WLAN network provides high-speed data communication in restricted coverage areas at a relatively SIMULATION, Vol. 82, Issue 6, June 2006 413-424 © 2006 The Society for Modeling and Simulation International DOI: 10.1177/0037549706070275

low cost. It allows users to move around in a confined area while they are still connected to the network. The distinctive advantages of the UMTS and WLAN networks can be combined to provide seamless connectivity when a user moves across heterogeneous access networks. Also, due to the convergence of core networking infrastructure on the Internet Protocol (IP), there is a significant need to interconnect the different access networks with a core network. In fact, the integration of heterogeneous wireless technologies can offer wireless Internet services at a world-wide scale and improve the network performance (in terms of data rate, blocking probability, and capacity). But the integration of two different technologies, such as UMTS and WLAN network, introduces a number of technical issues that must be resolved in order to maximize the benefits acquired from such integration [2]. In fact, UMTS and WLAN networks are heterogeneous technologies that have different attributes, requirements, and mechanisms as pointed out through the following parameters: signal quality, data rate, handoff decision, coverage discovery, security services, and authentication models. This will need more research on architectures and protocols to interconnect those heterogeneous networks in a seamless fashion. In this context, many technical challenges need to be faced such as those associated with heterogeneous architectures, seamless handoff, and secure

Volume 82, Number 6 SIMULATION 413


Zarai, Boudriga, and Obaidat

mobility. The major challenges can be summarized as follows. 1. QOS guarantees during handoff: Handoff introduces packet loss and latency, which can severely damage the data communications provided for a constraining service. Therefore, the handoff mechanisms must be managed to minimize these issues and maintain good network performance (e.g. no disruption to user traffic). These include: • an extremely low latency; • a signaling messages overhead and processing time; • a near-zero handoff blocking probability; • a near-zero call blocking probability. 2. Security: The heterogeneous network should maintain the same level of security services to the users when they roam across different access networks. Most of the recent work on heterogeneous networks has been dedicated to improving the coverage and capacity [3, 5] of networks. The work developed in [3], for instance, analyzes the cellular architecture from the specific perspective of coverage extension, capacity performance and QoS tradeoffs in a multi-cellular environment, while in [5], the authors presented an experimental study of internetworking mobility between GPRS cellular and 802.11bbased WLAN hot-spots, and analyzed its impact on active transport TCP flows. Other authors have focused on internetworking Bluetooth Piconet in indoor environments [4]. Seamless mobility has been the subject of a large number of research projects [6] and is now being studied to accommodate mobility across heterogeneous networks. Our contribution in this paper is three-fold. First, we define an architecture for WLAN-UMTS integration that makes the composing networks transparent to users and provides seamless and secure services. Second, we propose a novel protocol to reserve resources for handoff. Third, we show how quality of service is achieved and the level of efficiency of the approach by investigating its performance using simulation modeling. The remainder of the paper is organized as follows. In Section 2, we give an overview of the architecture of wireless networks. In Section 3, we present the proposed hierarchical architecture for the integration of WLAN and WPAN networks with UMTS. Section 4 describes our handoff procedure with a resource reservation scheme that we have developed based on the notion of signal strength. In Section 6, we show the simulation environment and present some results illustrating the efficiency of our method. Finally, Section 7 concludes the paper. 2. Wireless Networks Overview 2.1 UMTS UMTS wireless technology is a proposed standard as a part of the 3GPP solutions to satisfy the International

Mobile Telecommunications – 2000 (IMT-2000) requirements. It is based on Wideband Code Division Multiple Access (W-CDMA) technology. The latter means that mobiles and networks transmit over a single, widely-spread, frequency band and different transmitters are distinguished by differences in the code used to spread the transmission. The UMTS Terrestrial Radio Access (UTRA) provides two modes: 1. W-CDMA/FDD (Frequency Division Duplexing) mode: this is utilized to make efficient use of the paired and unpaired band of the allocation spectrum. 2. W-CDMA/TDD (Time Division Duplexing) mode: this is intended for applications in macrocell and microcell environments with medium data rates and high mobility. The W-CDMA/TDD mode is particularly well suited to environments with high traffic density and indoor coverage, where client applications require high data rates. 2.2 WLAN We assume in this paper that a WLAN operates in two modes: ad hoc and wireless fixed infrastructure with access point. WLAN with access point: In the wireless infrastructure mode, an access point (AP) coordinates the transmission among nodes within its radio coverage area, called the Basic Service Set (BSS). A mobile node (MN) can only associate with one AP at a time. All the MNs associated with an AP communicate with each other either through the AP or directly coordinated by the AP. Roaming across APs is supported in layer-2 through Inter-AP Protocol (IAPP). The APs generate beacons periodically that contain the network-id (or Extended Service Set Identifier, ESSID) and cell-id (which is the AP’s MAC address) in addition to other information. Ad hoc networks: A mobile ad hoc network represents an evolution of wireless networks in which no fixed infrastructure is needed. It consists of a group of mobile nodes that may communicate with each other without a fixed wireless infrastructure. It can be characterized by a flat topology or a hierarchical topology. In the flat topology, all the nodes are on the same level of communication. On the other hand, defining a hierarchy on an ad hoc network sets distinction between nodes. This approach organizes all nodes of the ad hoc network into clusters. Each cluster is controlled by a node called the cluster head and contains nodes other than the cluster head, these nodes are called mobile nodes and they use the cluster head to describe their cluster. The idea of clustering ad hoc networks has introduced some structure into the dynamic nature of the network. Clustering is critical for building and maintaining cluster-based virtual network architectures [8]. A WLAN may be extended by using various radio technologies to achieve a greater coverage range [4].

414 SIMULATION Volume 82, Number 6



2.3 Bluetooth In Bluetooth, the small number of users is arranged in a piconet. Each piconet has one master device and one or more slaves [10]. A master is the only one that may initiate a Bluetooth communication link. However, once a link is established, the slave may request a master/slave switch to become the master. Slaves are not allowed to talk to each other directly. All the communication is done between the slave and the master. Slaves within a piconet must also synchronize their internal clocks and frequency hops with that of the master. Each piconet uses a different frequency hopping sequence. When two or more piconets in the same area are interconnected in Bluetooth technology, the resulting wireless network is called Scatternet. The Bluetooth standard defines the concept of gateway node to take care of communication inter-piconet. The Bluetooth network can also be extended beyond a scatternet by using UMTS technology.

Figure 1. Cell without WLAN integration

3. WLAN-Cellular Networks Integration Architecture 3.1 Integration Benefits The integration of WLAN-cellular networks offers considerable benefits for users as for cellular networks and WLAN networks. The mobile user should be able to switch seamlessly from one access network to another. For the network, it will improve the quality of service and increase capacity by reducing interference. Also, new services can be made available in WLAN networks. The expected benefits for the UMTS cellular network include the following: • Cellular coverage may be extended by means of WLAN relaying as depicted in Figure 1 and Figure 2. While Figure 1 represents a cell without WLAN integration, Figure 2 shows a cell with multiple radio areas covered by different WLANs on the border. • Intra-cell and extra-cell interference may be reduced by using relaying technologies. This may result in an increase of capacity. • Load balancing between different cells can be provided by means of WLAN relaying. WLANs, operating in the unlicensed ISM spectrum, provide high speed wireless Internet access. However, in the WLAN-UMTS integration system, mobile users can achieve extended coverage area as shown in Figures 1 and 2. The mobile user can choose the access methods: direct cellular link (from the mobile user to the base station) or indirect cellular link (from the mobile user to base station via access points). 3.2 WLAN-UMTS Integration Architecture The design of a network architecture that efficiently integrates WLAN and UMTS networks is a challenging task.

Figure 2. Cell with WLAN integration

IP backbone IP access system

Cellular networks WLAN-UMTS Hybrid units

WLAN WPAN-WLAN Hybrid units

WPAN Universal Mobile Station Figure 3. WLAN-UMTS integration architecture

The proposed network architecture for WLAN-UMTS Integration is illustrated in Figure 3. In this architecture, the hierarchical cell structure of UMTS offers a global radio coverage and each of the wireless access networks has its own ability to handle IP networking within its own operating domain; some have a master (Bluetooth),Access Point (WLAN), CellularAccess Point (GSM, GPRS, and UMTS), and Cluster Head Volume 82, Number 6 SIMULATION 415



(ad hoc network). The proposed network architecture for the wireless 4G network is illustrated in Figure 1. Lower levels are comprised of high bandwidth wireless cells that cover a relatively small area connection over a larger geographic area. In our configuration, we have three layers: • The lowest layer is comprised of collection wireless local area networks (WLAN) and Wireless Personal Area Networks (WPAN), which offer mobile users high bandwidth wireless internet connectivity. • The second layer plays the role of the operation, providing protocol interoperability. • The final layer is a wide area network, cellular network, which provides a much lower bandwidth and covers a larger area. It provides connection with the external network (IP backbone, etc.). The major elements of the proposed configuration of WLAN-Cellular networks integration can be summarized as follows. IP access system: An IP access system contains a set of wireless routers that provide the necessary functionality for interconnecting the cellular systems to the IP backbone. As far as UMTS and GPRS is concerned, these routers are typically called Gateway GPRS Service Nodes (GGSNs), and they are able to cache locally the information needed to support mobility, quality of service and security elements. The Home Agents for macro-mobility of IP is implemented in wireless routes. It also acts as the root router of the domain for micro-mobility of IP. Hybrid Unit (HU): The hybrid unit is a node serving as a bridge between two different access networks. It enables integration of various heterogeneous networks. It plays the role of the operation providing protocol interoperability and provides abstracted services to pairs of different access technologies. The HU has the ability to operate as a network AP for a WLAN with infrastructure network, a master for a Bluetooth network and a cluster head in an ad hoc network. Each WLAN subnet (i.e., ad hoc network, WLAN with infrastructure, or Bluetooth) may contain some WLAN-UMTS hybrid units and the UMTS base station can be used to interconnect these hybrid units in order to allow inter-subnet communication. Such a consideration leads to a hierarchical architecture (as depicted in Figure 3), in which WLAN provides the basic wireless connectivity to the final users, while UMTS-TDD serves as a backbone, interconnecting the ad hoc sub-networks. Moreover, the UMTS base station can allow network access to users equipped with a cell phone as well as private cordless communication among cell phones. The election of the hybrid unit is based on radio frequency measurements, the status of the neighboring access point network, the availability of particular procedures at the candidate nodes, and the mobility scheme of the node. The hybrid units are selected from the set of cluster heads in

an ad hoc network, the set of access points in WLANs with infrastructure, and from the set of masters in a Bluetooth setup. Universal Mobile Station (UMS): A UMS is a multimodal terminal equipped with a network interface for each technology involved in the network. It is also capable of maintaining active UMTS/UTRAN and WLAN connections simultaneously. Other components can be added to a UMS in order to provide better management of resources and secure the communication architecture. Finally, note that when a mobile switches between access points supporting the same technology, an intratechnology handoff (also referred to as a horizontal handoff) takes place. On the other hand, an inter-technology handoff (also referred to as vertical handoff) takes place when the mobile switches between access points supporting different technologies [5]. Reasons for a vertical handoff may include better quality of service provision, coverage, cost, etc. 4. Interference Analysis for QoS provision A promising air interface technology for cellular networks is the Code Division Multiple Access with Time Division Duplex (TD-CDMA), which has been shown to be interference-limited and a good solution for supporting unbalanced traffics between uplink and downlink. In this subsection, we present the interference analysis for our architecture. 4.1 Signal Quality for UMTS This subsection develops bit energy to total noise power spectral density, say ( NEb0 ), as an estimation technique for a TD-CDMA system in the case where each cell has its own slot allocation according to the level of traffic asymmetry. To express ( NEb0 ), we assume the following: • Nj,n : number of active mobiles in cell j during slot n; • Vj,d,n : set of neighboring cells of cell j with the nth slot as downlink; • Vj,u,n : set of neighboring cells of cell j with the nth slot as uplink; • β: multi-user detection factor; • Pi,j,n : power level transmitted by mobile i and received at base station j within slot n; • Tj,n : total transmission power at base station j within slot n; • νi : activity factor of the service used by mobile i;




• αi,n : power portion dedicated to mobile i during the time slot n;

• V wi,n : the set of active neighbouring nodes of node i with the nth slot;

• PN : thermal noise power;

• PN : the thermal noise power for WLAN (N is taken for NW LAN );

• W : transmission bandwidth;

• WW LAN : the transmission bandwidth for WLAN.

• Ri : information data rate of mobile i; • N0 : thermal noise spectral density; • Lk,j : path loss probability between terminals k and j .

The ratio of bit energy to total noise power spectral density received at the access point (or cluster head) j coming from mobile i, denoted by ( NEb0 )i,j , can be expressed as

On the uplink: The ratio of bit energy to total noise power spectral density received at the base station j , coming from mobile i in the uplink during slot n and denoted by ( NEb0 )ui,j,n , is affected first by the set, say Ihci,j,n , of interfering signals coming from active mobiles occurring inside the home cell of i; and second by the set, say Ioci,j,n , of interfering signals transmitted by or received at mobiles at all cells in Vj,d,n ∪ Vj,u,n [15]. Values of Ihci,j,n and Ioci,j,n are easily shown to be given by I hci,j,n =

Nj,n

νt Pt,j,n

(1)

t=1

I oci,j,n =

l∈Vj,d,n

Lk,j νk Pk,l,n Lk,l l∈Vj,u,n k=1 Nl,n

Lj,l Tl,n +

(2)

The value of ( NEb0 )ui,j,n can be expressed by:

Eb N0

= i,j

WW LAN Ri

Pi

Li,j PN +

NW LAN k∈V wj,n

(5)

Pk Lk,j

4.3 Load Factor for UMTS We define the load factor of mobile i in unit j (base station, access point or cluster head), denoted by ηi,j , as given by: ηui,j = 1 − IPi,jN , where Ii,j is the total interference of mobile i in unit j . The uplink load factor during slot n, denoted by ηui,j,n , is given in [5] by the following equation:

=

η

u i,j,n

i,j,n

= (1 − β)(1 + fi,j,n )

Nj,n i=1

1

(3)

where fi,j,n is the intercell interference factor of mobile i connected to cell j in slot n [5]. On the downlink: Using a similar approach to the uplink, the value of ( NEb0 )di,j,n can be expressed as Eb N0

u

W Pi,j,n . Ri (1 + fi,j,n )(1 − β)I hci,j,n + PN − νi Pi,j,n

Eb N0

d =

With the assumption of using only one service with rate R and activity factor νι , the downlink load factor on the slot n, denoted by ηdi,j,n , is also given in [5] as

i,j,n

ηdi,j,n =

W αi,n Li,j Tj,n Ri (1 + fi,j,n )(1 − α)I hci,j,n + PN − νi αi,n Li,j Tj,n (4)

4.2 Signal Quality for WLAN Network Let us now consider a WLAN network consisting of a set of MW LAN access points and NW LAN nodes connected to the network. Let us define:

(6)

W u 1+ Eb νi Ri N0 i,j,n

(1 − α)Fj,n + (1 − α)Nj,n     νi + 

Ri

W Eb N0

d

(7)

   

i,j,n

where Fj,n is a Gaussian random variable with mean mFj,n and standard deviation σFj,n . Fj,n can be written as: Fj,n = Nj,n fi,j,n . i=1




4.4 Load Factor for WLAN In a similar approach to that for UMTS, we define the load factor of node i connected to access point j ; denote it by ηi,j, , as given by: ηi,j = 1 − IPi,jN , where Ii,j is the total interference of node i. For a WLAN network, the total interference Ii,j can be expressed as

NW LAN

Ii,j =

k∈V wj,n

Pk Lk,j

A straightforward computation of the load factor ηi,j for a WLAN is given by η=i,j 1 −

1−

PN

NW LAN k∈V wj,n

(8) Pk Lk,j

5. Handoff Reservation Resources Supporting seamless handoff across heterogeneous access networks requires several functionalities to be taken into account, such as continuity, quality of service and security service provision. In this section, we present a method for continuity and QoS services during a handoff. Handoff between a hybrid unit and UMTS base station has long been an important issue within the wireless telecommunication field. A higher level of handoff complexity, and thus its related issues, is introduced due to the differences between inter-networked heterogeneous wireless networks. This added complexity causes additional delays to the handoff process and requires more resources from the mobile and the network. Seamless mobility has been the subject of much research effort and is now being studied to accommodate mobility across heterogeneous networks. This section develops a solution that would predict the occurrence of handoffs and reserve resources for these events. 5.1 Handoff Decision In most technologies, the conventional criteria used to reflect the condition of the current network connection are the Received Signal Strength (RSS), Signal to Interference Ratio (SIR), coverage area, and the bit error rate. UMTS takes into consideration other factors such as the status of the neighboring base stations, in addition to the radio link measurements. However, WLANs do not have such capabilities, and thus rely primarily on physical layer measurements and the MAC layer evaluation of such measurements to determine the appropriate time for triggering a handoff operation [11]. The current non-optimized method for detecting different types of handoffs in areas that have heterogeneous wireless networks requires simultaneously active

network adapters on dual-mode. The following define the taxonomy of handoff classes based on the nature of the technology. 1. Handoff Intra-technology: This means handoff between base stations supporting the same technology. The handoff decision is based on the radio link measurements (e.g. the values of RSS and SIR). 2. Handoff Inter-technology: This means handoff between base stations supporting different technologies. Because the priority of handoff is to the highdata rate (we assume here that the handoff priority is given for WLAN) and the signal powers received from the base stations of the different networks are incomparable, we prefer to use two thresholds, TADD−W LAN and TDROP −W LAN . TADD−W LAN is greater than TDROP −W LAN . A node currently attached to the WLAN network evaluates the Received Signal Strength value to detect when a handoff is needed. Upon noticing degradation in the RSS value, the WLAN adapter starts scanning to detect the presence of another access point (such as 802.11 AP, CH, and Bluetooth AP). After a certain scanning period, if no other CH is detected, the handoff to UMTS-TDD is triggered. • If the Received Signal Strength is greater than or equal to TADD−W LAN , the mobile is in the coverage area of WLAN. • If the Received Signal Strength is between TADD−W LAN and TDROP −W LAN , then the method for detecting the presence of an access point is executed. If successful it follows the Handoff Intra-technology procedure. • If the Received Signal Strength is less than TDROP −W LAN , then the mobile looks for the UMTS-TDD too by signaling to the other interface to search for an available access network. 3. Handoff from UMTS-TDD to WLAN: This handoff is not done for coverage but rather for performance optimization purposes related to the density of users connected locally to UMTS or the need for local connection. As call rejection during call establishment is preferred to call dropping, the priority should be given to handoff requests in admission decision by defining, at each decision node, two load factor thresholds, denoted by η_a and η_h, satisfying the inequality: η_a η_h. The first factor, η_a, is the new call admission threshold, while the second, η_h, is the handoff admission threshold. As resources are passively reserved for mobiles that are maintained connected to their old cells, it is possible to over-allocate the soft capacity by stating that handoff reservation could be




Mobile currently connected to UMTS

Mobile currently connected to a WLAN access point network

Mobile searches an Access Point

Evaluation of RSS

no no

RSSTDROP-WLAN

Mobile triggers the UMTS-WLAN handoff

Figure 4. Handoff decision when the mobile moves from UMTS to WLAN

performed until a threshold η_p is reached. Threshold η_p satisfies:

yes Mobile triggers the WLAN-UMTS

Mobile search another AP network

Detection of another AP network yes

η_a η_h η_p 5.2 Resource Reservation for Handoffs WLAN-UMTS As mentioned earlier, the handoff from UMTS to WLAN is done to improve the transmission rate. Conversely, the WLAN-UMTS handoff resources reservation can be performed. This subsection presents a protocol for this. Reservation decision: To maintain a low handoff blocking probability, we choose to reserve a maximum bandwidth in all neighboring access point networks of the serving access point network for handoff. To maintain a low new call blocking probability, this reservation should be optimal. Our solution is based on RSS. For this, we define a threshold TRESV whose value belongs to [TADD−W LAN , TDROP −W LAN ]. Resource Reservation for WLAN-UMTS Handoffs. A node currently attached to the WLAN network evaluates the RSS value. If RSS is smaller than TRESV for the exceeding time, it is necessary to reserve resources at the related UMTS base station. The exceeding time is used to minimize the probability that a reservation request, once established, will operate without drop. Threshold Management: The reservation is to be optimized in the sense that a large reservation could increase the new call blocking probability and also it could increase the process complexity, due to the large number of mobiles

no

Mobile triggers the WLANWLAN handoff

End Figure 5. Handoff decision when the mobile is moving to UMTS

that can be involved. Conversely, a small reservation does provide a low handoff blocking probability. To optimize the bandwidth utilization, each cell should manage dynamically using the following thresholds: • η_aj,k : the new call admission threshold of cell j for mobile comes from a cell k; • η_pj,k : the passive reservation admission threshold of cell j for mobile comes from a cell k. The value η_aj,k is determined by the handoff blocking probability that the mobile operator plans to offer on the wireless network. The value of η_pj,k is chosen according to the number of passive reservations that will not be active. We have to count, for each cell, the number of handoffs performed during a period of time in order to measure the required mobility through a cell. This is done if the mobile Volume 82, Number 6 SIMULATION 419



sends a handoff request to its new cell. Each cell j counts the number of accepted passive resource reservations for each neighboring cell k, say RP −OK,j,k, and RP −NOK,j,k . The active handoff number RA−OK,j,k /RA−NOK,j,k is counted, as well. If the blocking passive handoff probability is larger, then η_pj,k should increase. If the blocking active handoff probability is larger, then η_aj,k should decrease. To count the number of active handoffs, we adopt the following approach: a cell j counts the number of active accepted (respectively, rejected) reservation requests, say RA−OK,j,k (respectively RA−NOK,j,k ) for each neighboring cell k, requested by nodes for each neighboring cell k when its signal strength exceeds the threshold of the reservation request, TADD−W LAN . As a consequence, each cell j knows the amount of reservation coming from every neighboring cell k that will be in active state. This amount, denoted by Aj,k , is used to calculate the η_pj,k value. Aj,k is given by: Aj,k =

RA−OK,j,k + RA−NOK,j,k RP −OK,j,k

A nominal value for ηp,j,k is determined from the total number of active and passive handoffs. This value corresponds to the amount of reservation, say Bj,k that will be active. If the mobile comes from a cell for which we do not have relative statistics, we use the newest value. Otherwise, the admission threshold is fixed at η_pj,k + (Bj,k − Aj,k )(η_pj,k − η_h) 6. Performances Analysis The goal of this section is to present the simulation model and the related numerical results to show the contributions of our scheme. 6.1 Simulation Environment 6.1.1 Simulation Models and Functions In the following, we present the main aspects of the simulation models. For the simulation, a scenario of 7 cells is considered (as depicted in Figure 6). The cells are arranged as a pattern of 7 cells, with a hexagonal pattern used for the cell shapes. Each cell is served by one base station. The nominal cell radius is defined as 500 m. Only the situation in the central cell is considered for simulation. The simulation assumes several models for mobility, path loss, traffic, admission, scheduling, power control, and handoff. These models are defined as follows: • Mobility model For UMTS, we consider a realistic model, in which the mobile initially chooses a speed that is uniformly distributed over interval [0, 10 km/h]. It also

Figure 6. Simulation architecture

chooses a direction for motion, which is uniformly distributed over [0, 360◦ ]. In the considered WLAN, nodes move according to the random waypoint mobility (RWM) model [14]. The pause time is 20 seconds with a uniformly random speed over the interval [0, 15m/s]. • Path Loss model The assumed propagation model that we use in this simulation is the “cost 231 indoor office model” without floor losses. It has been considered in [5] and is given by

Li,j (dB) = 37 + 30 log di,j where di,j is the distance between nodes i and j . On the other hand, the path loss in WLAN and Bluetooth is given by the following expression:

Li,j (dB) = 40 + 35 log di,j • Traffic Model We assume that the arrival rate of new calls follows a Poisson process with parameter λ1 for the voice service and λ2 for the data service. The duration of a call is assumed to be exponentially distributed. • Admission Control The radio link admission control performs the signal quality testing process. This testing makes use of the load factor η (or interference) and the estimate of the load increase that the establishment of the new call would cause in the radio network. The load factor is the ratio of the total interference to the background noise PN . The decision for admission is based on the following inequality: η∗i,j,n + ∆ηnew−user η∗N




Table 1. Simulation parameters

where – ∆ηnew−user is the increased load of the new user, – η∗i,j,n is the current load of mobile i at base station j on the nth slot in uplink (*=u) or downlink (*=d), – η∗N is the new call admission threshold for uplink or downlink. • Scheduling The scheduling algorithm assigns dynamically the priority according to the local information and instantaneous wireless channel quality [15]. • Power control In the performed simulation, the interference is realistically modeled. At any simulation time step, the received power from all cells, except from the best server, is counted as interference, and hence the corresponding coupling effect is fully taken into account. • Handoff Decision The handoff decision is assumed to be based on the signal to interference ratio measurements.

Simulation Parameters Simulation time UMTS Parameters Cell radius Max BS transmit power Min BS transmit power MUD Orthogonality factor Thermal noise power Tadd Tdrop TRESV E Req. ( Nb ) in up (down), voice

Values 20 min Values 500 m 43 dBm 33 dBm 0.7 0.5 –105 dBm –14 –18 dB –16 dB 6 (9) dB

Req. ( Nb ) in up (down), data 0 Voice rate in up (down) data rate in up (down) Call rate, voice Call rate, data Average call length, voice Average call length, data WLAN Parameters WLAN range Number of Nodes RF band Data rate AP Transmit power AP sensitivity Thermal noise power

2,5 (3,5) dB 16 (16) kbs/s 144 (384) kbs/s 2 calls/h 1,3 calls/h 90 s 600 s Values 30 m 50 ISM 2.4 GHz 2 Mbits/s 32 mW –83 dBm –126 dB

E

0

6.1.2 Simulation Parameters Two different wireless technologies were considered in the simulation environment: WLAN and UMTS. We assumed two simulation scenarios by varying the center cell coverage. In the first scenario, we defined the center cell radius to be 500 m and assumed that no WLAN access point is used. Then we computed the utilization, load factor, call blocking probability and handoff blocking probability. In the second scenario, we reduced the radius to 400 m and assumed that WLAN access points are placed in order to extend this cell to cover an area with radius 500 m. Table 1 represents the general system parameters chosen for these simulations (similar to what is considered in [13]).

3. each active mobile requests the network to allocate resources 4. if (new call or handoff request) – if (the increased load of the new user does not affect the established connections) – then accepts the call and assigns bandwidth – Else reject the call 5. The network estimate the increased load of the new user and accept the request if it does not affect the established connections

6.2 Pseudo-code for the Simulation

6. Schedule the arrived packet of each mobile with the highest priorit,

We consider the following algorithm in our simulation to process a call:

7. Determine if the packet has missed its end-toend deadline, such packets are then dropped.

Begin • Initialization (uniform distribution of mobiles, speeds, generation of population) • For each simulation step, 1. we monitor the power and distance of each mobile 2. we determine the set of active mobiles (the current time is equal to their arrival call time)

End. 6.3 Simulation Results and Discussion This sub-section is dedicated to describing the results in terms of performance and benefits of the WLAN-UMTS integration and reservation schemes. Figure 7 displays the capacity (utilization) of the wireless link. The y-axis presents the capacity rate (%) while Volume 82, Number 6 SIMULATION 421



10

With WLAN-UMTS integration Without WLAN-UMTS integration

80 70

Call blocking probability (%)

60 50 40 30 20 10 0

Without WLAN-UMTS integration

9 8 7

With WLAN-UMTS integration

6 5 4 3 2 1

Figure 7. Resource utilization

240

220

200

180

160

140

120

100

80

60

220

40

100 140 180 Number of users per cell

Number of users per cell Figure 8. Call blocking probability

• Call blocking probability and handoff blocking probability increase when the number of users increases. This can be explained by the increase of the number of admitted users, while the resources for handoff are kept unchanged. • Call blocking probability and handoff blocking probability in the case where access points are used


22 0

18 0

14 0


10 0

2 1.8 1.6 1.4 1.2 1 0.8 0.6 0.4 0.2 0 20

the x-axis represents the number of users. The figure shows that WLAN-UMTS integration can be considered as a system-level diversity technique to increase the UMTS capacity by using access points as relays. UMSs utilize the good channels to increase the capacity. In fact, each UMS periodically tries to find the good way to connect to the base station. The first way is by using access points as relays (indirect connection, cellular link through WLAN access point). The second is a direct cellular link. Also Figure 7 shows the good potential for UMTS capacity increase based on WLAN extensions. For instance, in the case of using WLAN access points, the maximum utilization bandwidth percentage could be 80%, whereas this percentage falls to around 78% in the case without WLANUMTS integration. Based on the results from worked experiments, it is shown that these values are reached if the number of users per cell exceeds 180. This gain (of about 2.5%) is also observed for simulations involving more than 350 mobiles. We justify this increase in utilization by the decrease of interference level. We believe that the gain may be higher than observed here if the number of access points is increased and if more than one intermediate access point is used to interconnect the mobiles to base stations. Figures 8 and 9 show the variation of the values obtained for the call and handoff blocking probability, assuming that the number of UMS users varies from 0 to 240. From these figures we observe the following:

Handoff blocking pobability (%)

60

20

0 20

60

Utilization (%)

100 90

Number of users per cell Figure 9. Handoff blocking probability

as a relay is lower than the value obtained in the case with no integration of WLAN-UMTS. This is mainly explained by the increase of the available capacity from one case to the other. • Call and Handoff blocking probabilities are near zero if the user number does not exceed approximately 100 users. When the number of users does not exceed this threshold, the values of the probabilities are omitted from Figures 8 and 9 for the sake of comparison simplicity.




Load factor

1


0.8


0.6 0.4 0.2 0 20

60

100

140

180 220

Number of users per cell

Figure 10. Load factor

• Handoff blocking probability is smaller than the call blocking probability. This is due to the priority given to handoff requests with respect to call admissions. • In the presence of integration, the call blocking probability gain can reach approximately 1.5% and the handoff blocking probability gain can reach about 0.4%. We observe also from Figures 8 and 9 a large variation in call and handoff blocking probabilities in the range 180 to 200 users. This is due to the rapid increase in the interference level. As shown in Figure 10, the increase in capacity and the reduction in call blocking probability and handoff blocking probability can be explained by the growth of interference in the case without WLAN-UMTS integration. In fact, without integration, the UMSs that are far away from the base station need to use an overly large amount of transmission power. But with integration, that is in the relay communication case (when using access points as relays), the transmission power required for the cellular link can be reduced. This fact has a direct consequence of reducing interference. 7. Conclusions We have considered in this paper the design of heterogeneous wireless networks using a UMTS core and integrating a variety of WLANs. We also developed a seamless handoff and showed that integration improves bandwidth utilization, which is necessary to provide good quality of service in heterogeneous networks. We also highlighted the influence of WLAN-UMTS handoff admission priority on handoff blocking probability and new call blocking probability. Results of a simulation analysis show the efficiency of the proposed schemes. Future work will address handoff authentication in order to provide seamless and secure handoff between different access networks.

8. References [1] 3GPP TS 22.234 (2004) Requirements on 3GPP system to Wireless Local Area Network (WLAN) internetworking, version 6.1.0, 06-2004. (Available at http://www.3gpp.org/ftp/Specs/htmlinfo/22234. htm) [2] Gerla, M., Y. Lee, R. Kapoor, T. Kwon, and A. Zanella. 2002. UMTSTDD: A solution for internetworking Bluetooth piconets in indoor environments. Proceedings of IEEE Symposium on Computers and Communications (ISCC’02), Taormina, Italy, 1–4 July. [3] Lungaro, P. and E. Wallin. 2003, Coverage, capacity and QoS tradeoffs in hybrid multi-hop ad hoc cellular access systems. Proceedings of the 3rd Swedish Workshop on Wireless ad hoc Networks, Stockholm, 6–7 May. [4] 3GPP TR 22.934, 2003. Feasibility study on 3GPP system to WLAN interworking, version 6.2.0, September. [5] Chakravorty, R., P.Vidales, K. Subramanian, I. Pratt, and J. Crowcroft. 2004. Performance issues with vertical handovers: experiences from GPRS cellular and WLAN hot-spots integration. Proceedings of the IEEE Pervasive Computing and Communications (PerCom’04), Orlando, Florida, 14–17 March. [6] Zarai, F., N. Boudriga, and M.S. Obaidat. 2004. Resources reservation for handoff in UTRA TDD systems. Proceedings of International Symposium on Performance Evaluation of Computers and Telecommunication Systems (SPECTS’04), California, USA, July, pp. 274–281. [7] Ma, X., Y. Liu, and K.S. Trivedi. 2001. A soft handoff scheme for improving utilization efficiency of traffic channels. Proceedings of the 5th WSES International Conference on Circuits, Systems, Communications and Computers, Greece, June. [8] Basagni, S. 1999. Distributed clustering for ad hoc networks. Proceedings of I-SPAN 99 International Symposium on Parallel Architectures, Algorithms, and Networks, June, pp. 310–15. [9] Aust, S., D. Proetel, N. A. Fikouras, C. Pampu, and C. Gorg. 2003. Policy based mobile IP handoff decision (POLIMAND) using generic link layer information, Proceedings of the 5th IEEE International Conference on Mobile and Wireless Communication Networks (MWCN 2003), Singapore, October. [10] Haarsten, J.C. 2000. The Bluetooth radio system. IEEE Personal Communication Magazine 17(1):28–36. [11] ANSI/IEEE Standard 802.11, 1999. IEEE Standard for Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications. (Available at http://standards.ieee.org) [12] Harri, H., L. Sanna, Otto-Aleksanteri, and T. Anti. 2000. Interference considerations for the time division duplex mode of the UMTS terrestrial radio access, IEEE Journal on Selected Areas in Communications 18:1386–93. [13] Sook Jeon, W. and D. Geun Jeong. 2002. Call admission control for CDMA mobile communications systems supporting multimedia services. IEEE Transactions on Wireless Communication 1(4): 649–59. [14] Tacconi, D., C. U. Saraydar, and S. Teknay. 2003. Improving UMTS throughput by utilizing ad hoc routing. ACM MSWiM, San Diego, CA, September. [15] Zarai, F. and N. Boudriga 2005. Provision of Quality of Service in open wireless architecture. Proceedings of 12th IEEE International Conference on Electronic Circuits and Systems (ICECS), Tunisia, 11–14 December.

Faouzi Zarai received the engineer and master degrees in telecommunication from School of Telecommunications engineering (Univ. of Carthage, Tunisia) in 2002 and 2003, respectively, where he is currently a PhD student. Since joining the Communication Networks and Security Research Lab. (CN&S), in 2002, he has been involved in research projects on 4G mobile communications systems. His research interests are




network architectures, access protocols, admission control, radio resource management, and security. Noureddine Boudriga received his PhD in Algebraic topology from University Paris XI (France) and his PhD in Computer science from University of Tunis (Tunisia). He is currently a Professor of Telecommunications at University of Carthage, Tunisia. He is the recipient of the Tunisian Presidential award in Science and Research (2004). He has served as the General Director and founder of the Tunisian National Digital Certification Agency. He was involved in very active research and authored or coauthored many chapters and books. He published over 200 refereed journal and conference papers.

Mohammad S. Obaidat is an internationally known scientist/engineer/academic. He received his PhD in Computer Engineering from Ohio State University. He is currently a Professor of Computer Science at Monmouth University, NJ, USA. He has received extensive research funding and authored or coauthored 5 books and over 340 refereed journal and conference papers. He served as IEEE Computer Society Distinguished Visitor/Speaker and currently is a Distinguished Lecturer for ACM and SCS. He is the Chief Editor of the International Journal of Communication Systems, and editor of many other journals including two IEEE Transactions. He is the Senior Vice President of SCS. Prof. Obaidat is a Fellow of IEEE and SCS.



WLANâUMTS Integration: Architecture, Seamless Handoff, and ...

WLANâUMTS Integration: Architecture, Seamless Handoff, and ...

WLANâUMTS Integration: Architecture, Seamless Handoff, and ...

WLANâUMTS Integration: Architecture, Seamless Handoff, and ...