NS2 based simulation framework to evaluate the ...

NS2 based simulation framework to evaluate the performance of Wireless Distribution Systems Amine Dhraief Abdelfattah Belghith Nicolas Montavont ENST-Bretagne, France ENSI, Tunisia ENST-Bretagne, France [email protected] [email protected] [email protected] Jean-Marie Bonnin Mohamed Kassab ENST-Bretagne, France ENST-Bretagne, France [email protected] [email protected] wireless distributed system invokes several questions to be investigated. Among these questions, we focus here rather on the gain provided by using more than one wireless communication channel and the way we may schedule different data frames from the different APs on the different channels for transmission within the WDS. In order to implement a WDS model and evaluate its performances, we use the NS2 simulator which is a de facto standard in simulation world. NS2 proposes a 802.11 model, however this model has many shortcomings and does not enable WDS simulation. In this paper, firstly we propose, test and validate an NS2 base simulation framework to investigate WDS environments and secondly we investigate the gain in throughput provided by using multiple interface cards and a scheduling scheme that maintains data frame sequenced delivery. This paper is organised as follow. In the first section, we present the WDS architecture, its applicability and some deployment issues. We highlight in the second section some of the shortcomings of the current implementation of the 802.11 infrastructure architecture in the NS2 simulator. In the third section, we propose a new design for IEEE 802.11 components in NS2 that allows WDS simulation. In the last section, we propose an analytical model of the throughput in an 802.11 wireless network and use it to test and validate our WDS design.

Keywords: 802.11, WDS, NS2, channel assignment problem

Abstract In this paper, we present an extensible and modular NS2 based simulation framework to simulate Wireless Distribution Systems (WDS). A WDS is the interconnection of access points (APs) via wireless links. We point out and clarify some of the shortcomings inherent to the current implementations of the IEEE 802.11 in NS2. We describe then a new design for different IEEE 802.11 models including the WDS component. Our design includes the use of several interfaces per AP and provides a framework for the integration and the study of WDS architectures. We show that using just two interfaces per AP for the WDS nearly doubles the throughput, maintains the sequencing of delivered data frames and improves the average data frame delay.

1 INTRODUCTION In the last few years, we are witnessing a tremendous gain in popularity of the IEEE 802.11b wireless LAN (WLAN). The cost of 802.11 wireless network cards for clients and AP dropped down significantly, making WLANs and Wi-Fi hotspots very attractive for public wireless Internet Access. 802.11 WLANs support various usage scenarios. They are often deployed in area such as companies, convention centres, conference locations, public hotspots, shopping malls, parking lots, airplanes, trains and campuses [13, 10, 6, 14, 12, 8]. The IEEE 802.11b MAC standard defined two different infrastructure architectures: a Basic Service Set (BSS) where stations are controlled by and via a unique AP, and an Extended Service Set (ESS) where many APs serving different sets of stations (different BSS) are interconnected through a given Distribution System (DS). The Ethernet technology is usually used as the distribution system. However, adopting Ethernet in certain situations where cabling is not already available can be very costly and time consuming especially for large scale WLAN networks or even impossible. The use of the wireless technology as a distribution system may then be considered as a rapid deployable and low-cost solution to form and extend existing WLANs. The use of a

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2 WIRELESS DISTRIBUTION SYSTEM The IEEE 802.11-999 [2] defines a Distribution System (DS) as a system that links IEEE 802.11 cells together. A single IEEE 802.11 cell is called a Basic Service Set (BSS), each one is driven by a single AP. A set of BSS linked together via a DS constitutes an Extended Service Set (ESS). The Ethernet bus is commonly used as a DS, however, in many cases, deploying wired link between AP is impossible. The IEEE 802.11 standard mentions the possibility to wirelessly interconnect APs together using a WDS. However, it does not specify the way to configure paths between APs. IEEE 802.11s task group is currently working on WDS standardisation. Its objective is to develop a protocol for auto-

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is mainly designed to simulate transport protocol and more specifically TCP and its different variants. In addition, NS2 implements two wireless technologies (on its core system): IEEE 802.11 and IEEE 802.15.4. The 802.11 model in NS2 was developed in 1998 by the CMU monarch group [1]. The main purpose of this model is the simulation of ad hoc networks. We expose here how the existing 802.11 implementation in NS2 can be enhanced to support a WDS. The mobile node (MN) model in NS2 is an extension of the classic node model (see Figure 1). It is mainly made of three levels. The upper level is in charge of the transport and application layers. The middle level is in charge of the LCC and ARP. The lower level models both Mac and physical layers.

configuring paths between APs [3]. It defines a new WLAN design, namely a WLAN Mesh in which APs are interconnected via a WDS.

2.1 WDS applicability Deploying Internet access in museums or archaeological sites can be very useful. It could provide several services to visitors and relevant information on paintings or the plan of the visited site. The most appropriate solution to have Internet access in such sites is to deploy wireless network for cabling is usually impossible. Thus, an appropriate solution to link the 802.11 BSS is to deploy a WDS.

2.2 WDS deployment issues In order to deploy a WDS, each AP should have at least two wireless interfaces: one interface to handle the BSS communication (intra BSS traffic), and the other one to handle communications within the WDS (inter BSS traffic). However, we can also use a single wireless interface per AP to support both intra and inter BSS traffic. This implies that all BSSs use the same channel within their corresponding cells yet this channel is also used within the WDS. Such a case, while it might be envisioned, leads to a dramatic increase in competition to access the unique used channel and creates a sure contention bottleneck [11]. In this paper and in the quest to investigate a wireless DS, we consider a dedicated channel for inter BSS communications that is different from any channel used within the different participating BSSs. Consequently, we have at least two wireless interfaces per AP. Furthermore, as the cost of 802.11 wireless network cards dropped down significantly, several interface cards within an AP could be dedicated to support the WDS. This will significantly increase the capacity of the WDS and will consequently avoid bottlenecks. However, this configuration requires an algorithm for interface selection which determines which interface to use for sending the current frame. We propose to integrate in the current NS2 simulator appropriate simulation components for the IEEE 802.11 BSS and ESS infrastructure architectures. These software components should be modular and extensible providing an adequate framework for any further addition of assignment policies, extensions to APs interaction and handling of any multi hop WDS set up. We then evaluate some WDS environments adopting a channel assignment that maintains sequenced delivery. In the following section, we present NS2 simulator and discuss issues related to the WDS implementation in this simulator.

Figure 1. Mobile Node structure in NS2.28 This design does not fulfil the requirement to simulate WDS as it can only assign a single wireless interface to each MN. We identify 4 issues for the WDS modelling.

3 BUILDING OVER THE EXISTING 802.11 IMPLEMENTATION

The channel management NS2 creates a linked list containing all the nodes configured on the same channel. This model is correct for MN equipped

NS2 is a discrete event simulator, created in 1989 and supported by DARPA. It is written in C++ and OTcl. NS2

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with a single wireless interface. If the MN has two wireless interfaces, each one configured on a different channel, the MN would be in two separate linked lists. This will cause a knotty problem whenever this MN is the head of the two lists. Thus, the MN has no mean to select an appropriate channel to send its data packets when it is equipped with several interfaces The Bridging functionality The bridging functionality within multiple wireless interfaces is not currently implemented in NS2. The MAC 802.11 model This model does not implement the management part of the IEEE 802.11 standard. The management frames such as association request/response, probe request/response and beacons are not implemented in the MAC 802.11 model. BSS management Even in infrastructure mode, if two MNs are in range of each other, they will exchange data frame directly. This is due to two reason : The first one is the absence of management functionalities in the Mac 802.11 sub layer. So, MNs are not associated with any AP. The second reason is the ad hoc routing agent. Indeed, NS2 integrates in each MN an ad hoc routing agent, and so the destination of the data frame depends only on the result of the route established by the ad hoc routing algorithm. Thus, a data frame sent by MN is not automatically forwarded by AP. Consequently, we propose a new design to represent an AP. The main purpose of this design is to enable the simulation of WDS architectures. Figure 2. Access Point with multiple interfaces

4 WDS DESIGN In order to simulate a WDS, we propose to extend the MN model in general and more particularly the AP model (see Figure 1). We propose a novel simulation model for an AP. This design allows to plug into our AP as many wireless interfaces as needed. We can see in figure 2 an AP having three wireless interfaces: one interface to handle communication within the cell (BSS Interface), and two interfaces to manage communications within the WDS (WDS Interfaces). Multiplying wireless interfaces per AP aims at resolving the bottleneck problem in the WDS. Our model takes into account not only WDS simulation but also bridging functionalists and IEEE 802.11 management aspects. Our AP contains three major components: LLC sublayer, bridge sublayer and wireless interfaces. While LLC sublayer and wireless interface were already defined by NS2, the bridge sublayer is a new component. Nonetheless, we

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introduced several modifications on the formers. We present here each of these components and we detail our contribution in the AP’s design. Mac 802.11 Simulating WDS scenarios implies that we can simulate the IEEE 802.11 infrastructure mode. The Mac 802.11 implemented in NS2 was initially intended solely to simulate ad hoc architectures. Therefore, we modified the MAC implementation in order to appropriately implement the infrastructure mode. Our modification has a great impact on the behaviour of MNs in both the WLAN and the WDS simulation contexts. For instance, even if we specify that we want to simulate infrastructure mode in NS2, the MNs still exchange data frames directly with each other as if APs were nonexistent. Whereas,

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with our Mac 802.11 design, data frames sent by MNs are transmitted to their destinations via their associated APs. In order to prove the impact of our modification on the infrastructure mode, we propose the following scenario (see figure 3 ). We simulate a 802.11b BSS having two MNs, a source and a sink. The source generates a CBR traffic. The data frame size is fixed to 512 bytes and we vary the input rate from 1Mbps to 11Mbps. We measure the throughput, using both NS2 802.11 infrastructure modelling and our design.

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Figure 4. Measured Throughput in a BSS interface equipped MNs. Indeed, NS2 creates a linked list containing all nodes configured on the same channel, while it is more accurate to have a linked list containing all the wireless interfaces configured on the same channel. Therefore, we propose a new structure of the wireless channel in order to simulate WDS scenarios. Let’s consider figure 5 to explain our wireless channel modelling. We have two APs: AP1 and AP2 managing two BSSs and interconnected with each other via a WDS. We assume that we have just one MN per BSS (MN1 and MN2 respectively). Each AP has two wireless interfaces I1 and I2. I2 serves the WDS and I1 is serves the BSSs. We assume that our WDS is configured on channel 6 (CH6), while the BSSs are respectively configured on channel 1 and 3 (CH1 and CH3). Our wireless channel structure is as follows: we create a list of wireless channel mentioned in the simulation script (CH1, CH6, CH3). To each channel we associate a list containing the wireless interfaces set to this channel. For example for the CH1 entry we have two interfaces: (AP1.I1) relative to the first wireless interface of the AP1 and (MN1) relative to the MN served by AP1.

Figure 3. Infrastructure Mode simulation Figure 4 shows the measured throughput versus CBR load. The throughput measured using NS2 infrastructure modelling is the double of the one measured with our modelling. Indeed, when stations communicate with each other in an infrastructure mode, all data frames are stored and forwarded by the AP, and thus they are sent twice into the wireless channel. Consequently, the throughput is divided by 2. While in ad hoc communications each data frame is sent only once into the wireless channel. For a CBR load less than 1.9Mbps, the two plots are superposed and increase linearly. This is due to the fact that the CBR load is lower than the BSS capacity. For a CBR load between 2Mbps and 2.7Mbps, the measured throughput decreases. This is due to the increase of the frequency of the collisions between data frames sent by the MN source and those forwarded by the AP. The manager The manager deals with the management aspects of the IEEE 802.11 standard, such as the layer two handoff. Even though management aspects are usually integrated within the Mac sublayer, we separate them in our design to enable to integrate the standard handoff scheme as well as fast handoff schemes. The Wireless Channel In the previous section, we mentioned that NS2 wireless channel modelling is unable to simulate WDS multiple

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Figure 5. Wireless Channel

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which is appended by the physical layer preamble. Thus, the duration of the ACK frame(Dack) is equal to 208µs.

The Bridging Level The bridging level is between the LLC level and the wireless interfaces of the AP. Its objective is to relay the incoming/outgoing data frames to the adequate interface. It has two components: the bridge sublayer and the MultiInterface Classifier. The MultiInterface Classifier is an array of pointers on the different Interfaces of the AP. The bridge is connected to the MultiInterface Classifier. It examines the Mac destination address in the data frame and then forwards it to the adequate interface.

We assume that we have always a frame to send and we assume that no collision occurs. The duration of the total emission cycle (D(n)) of the Mac frame is equal to: D(n) = DIFS + B + D f (n) + SIFS + Dack Thus the radio channel efficiency is given by:: ρ = D f (n)/D(n).

In order to evaluate the performance of our model, we conducted several simulations. We present these evaluations in the next section.

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Consequently the maximum throughput in an empty network in point to point communication is equal to: throughput = ρ × λ

5 VALIDATION

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The equation 3 will help us to validate our design. We plot both this equation and the measured throughput by simulation in order to see the accuracy of our model (see figure 6.

In this section, we first propose an analytical model of the throughput in an empty 802.11 network. We then compare this analytical model of the throughput and the results obtained from simulation. The aim of the two previous steps is to validate our proposed design of the WDS analytically. Finally, we compare our WDS model with the existing implementation of the DS in NS2.

5.2 Simulation scenario The purpose of the simulation that we conducted in this section is to compare the theoretical results presented above and the one obtained from the simulation. We define two BSSs connected via a WDS (see figure 5) The mobile station MN1 is the source of a CBR flow and MN2 is its destination. We configure the network load d (λ) to 11Mbps and we vary the data frame size (n) from 64bytes to 2312bytes (the maximum size of a 802.11 data frame).

5.1 Analytical model of the throughput In this section we propose to analytically evaluate the theoretical maximum throughput in an empty 802.11 network. We here assume a prerequisite knowledge of the CSMA/CA access method and the IEEE 802.11 distribution coordination function (DCF) [5]. The throughput is the product of the radio channel efficiency (ρ) by the network load (λ). Thus, let’s first determine ρ: the radio channel efficiency. We detail here the different duration involved in the cycle of the emission of a 802.11 Mac frame. We consider the following durations:

5.3 Results Figure 6 portrays the throughput as a function of the packet size for a submitted load of 11Mbps. This figure shows both the simulation and the theoretical curves which are nearly superposed. For a small data frame size, we have a small radio channel efficiency. For large frames, we have a greater radio channel efficiency and thus greater throughput for a fixed network load (11Mbps). Indeed, for frame having 1500bytes, the analytical throughput is equal to 6.70Mbps, and the measured throughput by simulation is equal to 6.66Mbps. As a conclusion, the throughput provided by the simulation is very similar to the one obtained theoretically.

• DIFS: Distributed Inter frame Space = 50µs. • B: Backoff average delay = 310µs. • SIFS: Short Inter Frame Space = 10µs. • Let’s consider a Mac frame having n bytes of payload and 34bytes of heading. The duration of the transmission of the payload (Dp(n)) is equal to n ∗ 8/ρ and the duration of the transmission of the payload and the headings (D f (n)) is equal to ((n + 34) × 8/ρ). The physical layer appends the Mac frames with a preamble which duration is equal to 192µs in the case of 802.11 DSSS and 96µs in the case of 802.11b. We consider the case of the 802.11b.

5.4 Average throughput in an ESS We maintain the simulation scenario presented in figure 5 in order to simulate an ESS scenario. In this section, we compare our WDS implementation and the DS implementation provided by NS2. We define two simulation scenarios: in the first one, we use a WDS to interconnect the APs while in the second one we use a classical distribution system (Ethernet

• The duration for the transmission of an acknowledgement frame (ACK) having 14bytes is equal to 112µs

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bus). We fix the frame length to 512byte and we vary the CBR load from 1 to 11Mbps. We measure the average throughput in Mbps. The result is given in figure 7. The upper curve corresponds to the result obtained with a wired distribution system, whereas the lower one corresponds to the results obtained with a WDS. The two curves take a similar shape: they grow linearly then they are stabilised. As long as the CBR load is lower than the network capacity, the curves grow linearly. As soon as the CBR load becomes greater than the network capacity, the curves are stabilised. The curve relative to the WDS scenario is stabilised at 3.66Mbps while the curve relative to the classical DS is stabilised at 4.06Mbps. This can be explained by the fact that in NS2 the Ethernet bus is a wired link without any access technique to the medium. Actually, there is no contention and only a delay is affected to each frame corresponding to a broadcast frame. Whereas in the WDS, we take into account the contention in the channel access method. Thus, it is more accurate to use our WDS model to simulate ESS scenario than the current DS proposed by NS2.

assignment policy that maintains the order of delivery of the data frames and hence no re-sequencing is needed at the destination node. To force this sequencing criterion, we require that an AP can only use just one of its interfaces at any given time even if it has several pending data frames (no data frame simultaneous transmission using different channels from the same AP). Transmissions from different APs could, however, take place at the same time. These transmissions succeed if they are using different channels. There are several assignment policies that maintain the order of delivery of data frames. Nonetheless, the objective of this paper is not to propose the best channel assignment policy. We target to validate our WDS design, and more specifically, the multi interfaced AP. Hence, we use an elementary assignment policy: the Round Robin . We recall that our design is open to integrate any other assignment policies with or without the sequencing criterion. In the scenario represented in figure 8, we measure both the delay and the throughput in a WDS context. We simulate 4 BSS, fully connected with a WDS. In each BSS, we configure an AP and a MN, so we have 4 MNs MN1, MN2, MN3, and MN4. Each AP has two types of interfaces: the first one is used for communication within its BSS, and the second one is used for communication within the WDS. Mobile nodes MN1 and MN2 are sources of two CBR flows. Their respective destinations are MN3 and MN4. The data frame size used in each flow is fixed to 512bytes and we vary the CBR rate from 1Mbps to 7Mbps. Consequently, we submit to the WDS two flows varying together from 2Mbps to 14Mbps. In 802.11b networks, for 512bytes data frames, transmitted at 7Mbps, we have an effective throughput equal to 3.82Mbps according to the equation 3. Consequently, for 2 flows having the same characteristics as mentioned before, we have an effective limiting throughput equal to 7.64Mbps.

6 CHANNEL ASSIGNMENT PROBLEM The WDS is an 802.11b network, so it has a limited capacity in comparison with a wired distributed system. As the number of inter BSS flows increases, the WDS becomes a real bottleneck. As a solution, we can increase the number of interfaces per AP connected to the WDS and hence, we increase its capacity. When a data frame has to be forwarded throughout the WDS, the question naturally arises on which interface (channel) the AP must transmit. We call this a channel assignment problem. Several recent works have dealt with this problem in ad hoc networks [4, 9, 7]. The objective here is rather to increase the WDS throughput as much as possible. We propose here one

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WDS is considered as an easy to deploy and a low cost solution to interconnect APs together. This is also a solution to deploy Internet access in sites where there is no existent infrastructure or where cabling is impossible. We proposed a new IEEE 802.11 modelling framework for NS2 that enables the study and evaluation of the WDS. Our proposal integrates the use of multiple interfaces to provide the WDS with a larger bandwidth. We first validated analytically our modelling. Then we compared our WDS design with the DS design proposed by NS2. We noticed in addition that the WDS might become a bottleneck with a growing number of flows. Thus, we proposed to multiply the number of the wireless card per AP connected to the WDS. This raised the problem of the channel assignment. Hence, we propose a simple channel assignment policy called round robin, that maintain the sequencing of data frames. The conducted simulation has shown that this assignment policy solved the bottleneck problem in the WDS, yet enhanced the average delay for data frame delivery.

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mission in round robin. The AP forces the sequencing criterion by not transmitting more than one data frame at the same time. Figure 9 shows the throughput versus the CBR load. With two interfaces, the maximum throughput is equal to 6.38Mb ps, which is less than 7.64Mb ps, the limit of the throughput of the two flows. In fact, with the round robin technique, we have synchronisation between the two flows if the AP starts at the same time sending their frame. This is the case when the AP uses at the same time the same interface to send their data frames into the WDS. Thats why, even when we use 20 interfaces, we never reach the limit 7.64Mb ps.

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REFERENCES Figure 10 shows the average delay versus the CBR load. For a load less than 2Mbps, the average delay is negligible. In fact the data frames are sent immediately as the queues are empty. We can clearly see that when we use 2 interfaces per AP linked to the WDS, the average delay is equal to 18.4s whereas, with only one interface the average delay is equal to 20s. Thus, having more than one interface connected to the WDS has an important impact on the average delay. We can clearly observe from this figure that using just two interfaces per AP increases the throughput to a value close to its theoretical limit, yet provides a lesser delay than just using a single interface. Moreover, operating with more than two interfaces per AP provides only very small increases in the throughput.

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[1] Cmu monarch extensions ns.http://www.monarch.cs.cmu.edu/.

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[2] Ieee802.11 : Part 11 : Wireless lan medium access control mac and physical layer phy specifications., 1999. [3] Terms and definitions for 802.11s. doc ieee 802.1104/1477r4, 2005. [4] A DYA , A., BAHL , P., PADHYE , J., W OLMAN , A., AND Z HOU , L. A multi radio unification protocol for ieee 802.11 wireless networks. In Proceedings Broadnets (2004).

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[5] B IANCHI , G. Performance analysis of the ieee 802.11 distributed coordination function. Selected Areas in Communications, IEEE Journal on 18 (2000). [6] B YERS , S., AND KORMANN , D. 802.11b access point mapping. Communications of the ACM 5, 46 (2003). [7] C HEN , J., AND S HEU , S. Distributed multi channel mac protocol for ieee 802.11 ad hoc wireless lans. Elsevier Computer Communication journal 8 (2005). [8] KOTZ , D., AND E SSIEN , K. Characterising usage of a campus wide wireless network. Proceedings of the 8th Annual Conference on Mobile Computing and Networking (2002). [9] K YASANUR , P., AND VAIDYA , N. Routing and interface assignment in multi-channel multi-interface wireless networks. WCNC (2005). [10] L IN , K., AND C HANG , J. Communications and entertainment onboard a high-speed public transport system. IEEE Wireless Communications 9, 1 (2002). [11] N G , P., L IEW, S. C., AND L IN , C. Voice over wireless lan via ieee 802.16 wireless man and ieee 802.11 wireless distribution system. In International Conference on Wireless Networks,Communications and Mobile Computing (2005). [12] OTT, J., AND K UTSCHER , D. Drive-thru internet: Ieee 802.11b for automobile users. IEEE INFOCOM 2004 (2004). [13] S CHWAB , D., AND B UNT, R. Characterising the use of a campus wireless network. IEEE INFOCOM04 (2004). [14] S INGH , J., BAMBOS , N., S RINIVASAN , B., AND C LAWIN , D. Wireless lan performance under varied stress conditions in vehicular traffic scenarios. In IEEE Vehicular Technology Conference (2002), vol. 2.

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