A Survey on Enterprise WLAN Techniques

Transcription

1 A Survey on Enterprise WLAN Techniques Jaegook Lee, Young-Joo Suh and Chansu Yu POSTECH WCU Division of ITCE, Pohang, S. Korea, {ljk8918, Cleveland State University Department of ECE, Cleveland, OH, Abstract WLANs are getting more and more popular. Even though an independent WLAN works in an optimal manner, many problems occur when they are deployed densely particularly with the advancement of new standards (e.g., n and ac) and new devices supporting dual- or non-standard features (e.g., dual-band devices such as iphone 5 and adaptive power save mode operation found in most smartphones). Research community has been exploring an integrated system to monitor or control the APs and clients so that the network performance can be maximized. Enterprise WLAN (EWLAN) uses a central controller to address problems arising from a collection of APs and their associated clients due to their behavior with the lack of knowledge of other APs and clients in the proximity. This article presents EWLAN s architecture and characteristics. Then it presents a survey on several solutions in terms of interference mitigation, traffic balance, and mobility support. I. INTRODUCTION While data service through cellular networks such as 3G and LTE has been a dominant technology, based WLANs would continue to exist as they can offload enormous demand on Telco networks and offer a higher throughput than cellularbased data networks. For example, the current n standard supports up to 150Mbps and the anticipated ac supports as high as 1.73Gbps while 3G and LTE supports 14.4Mbps and 150Mbps, respectively. On the other hand, WLAN deployments has been observed chaotic even in seemingly controlled environment such as office buildings, manufacturing sites, shopping malls, and campuses [1], [2] due to the random additions of APs for convenience, fast advancement of mobile devices as well as standards such as a/b/g/n/ac/ad and no or limited control over WLANs. This becomes serious when AP density is high. For example, it is observed that about 60% of retransmissions is the frame type of probe response because APs keep retransmit the frame even if a mobile station transmitted a probe request frame but left the channel to scan the next [2]. This becomes even more serious in large-scale venues contributing to a large volume of non-data traffic. Unmanaged WLANs are destined to suffer from unnecessary overheads as well as significant performance degradation, which can be addressed by having a central control entity with prudent monitor and control mechanisms in the context of enterprise WLAN (EWLAN). This article discusses the EWLAN architecture, interference and mitigation, traffic monitor and control, and mobility support. II. ARCHITECTURE OF EWLAN Control mechanism in an EWLAN can be either centralized or distributed. However, centralized mechanism in most cases makes more sense because the communication with the controller uses wired connections and thus, it does not disrupt wireless communications. Most popularly, they target interference mitigation or traffic balance. Figure 1 shows a general architecture of EWLAN. An EWLAN system consists of APs, a central controller, client devices, and LAN switches. The controller can control various parameters such as operating channel and transmission power for all APs considering locations of APs, interference among APs, and traffic loads on each cell. Furthermore, when a client device attempts to associate with one of several candidate APs, it dynamically decides the target AP based on monitored information. The controller also performs MAC operations such as packet scheduling for associated clients to mitigate inter-cell interference and load balancing. EWLAN also can conduct security related operations as opposed to most of open wireless networks that do not maintain logs or mechanisms for forensic investigations. Fig. 1. Architecture of EWLAN

2 To realize those control objectives, it needs information such as RSSI of client/ap, traffic, channel, and so on. Several standards specify how to collect information k is for radio resource management and explains the information collection scheme on a single BSS. According to this standard, an AP can query statistics such as channel status, retries, packets transmitted and packets received. Based on this statistics, the AP can manage their radio resource f (Inter Access Point Protocol or IAPP) supports AP-to-AP communication via distribution system (DS) for the purpose to assist client s handoff. III. INTERFERENCE MONITORING AND MITIGATION A. Monitoring With drastically increasing number of APs in wireless networks, interference problem becomes critical as it degrades performance [3], [4]. However, in order to mitigate the interference, it is important to understand the status of interference between APs, necessitating the proper method to monitor interference and build up the interference map, which can be done in a passive or active manner. Passive interference monitor: One advantage of passive is that it doesn t need an additional traffic while a disadvantage is that the mechanism typically takes more time to analyze the interference. For example, in [5], the key elements to monitor the interference are timestamp and reception status of packets. When two packets timestamps are overlapped but only one of them succeeds, it can be concluded that one link is interfering the other but not vice versa. Fig. 2. scenario Active interference monitor: In contrast, active mechanisms are generally more accurate than passive at the cost of the additional test traffic to construct a conflict graph. For instance, a prior work on conflict graph construction uses bandwidth tests [6]. This mechanism tries to take all of cases between all APs and clients in the network into account so that the result is very accurate but takes a long time to run and incurs a large control traffic. One possible approach to avoid the intolerable control traffic is found in [7]. This mechanism does not try all possible cases enumerating all APs and all potential client positions. Instead, it divides all links into several topologies in terms of hop count, i.e., inter-ap (0-hop), AP-client (1-hop) and inter-client (2-hop) [7] as shown in Fig 2. To test it, AP 1 and AP 2 send a dummy packet to C 1 and C 2, respectively. If AP 1 does not receive an ACK, it could be regarded due to the interference from C 2. This could significantly reduce the time and traffic when constructing a conflict graph but the accuracy is almost the same as the bandwidth test mechanism. B. Mitigation Based on the result of interference, one can mitigate the interference problem in the network with a large number of APs in the proximity. The spectrum of the solutions ranges from the selection of AP (association) to the frequency (channel) and time multiplexing. Multi association & packet scheduling: The knowledge of conflicting APs can be utilized to mitigate the interference problem. For example, downlink packets are queued on the central controller and then, distributed in batches to, for example, two APs that do not interfere with each other [4]. To make it more effective, they break single association between a client and an AP and adopt multi-association scheme to release the increased handoff overhead [4], [8]. -aware channel assignment: [3] uses the conflict graph to calculate a pack of transmissions, which is referred to as those with no interference with each other. The pack is calculated for each time frame. If the pack improves network throughput or capacity, that is kept in transmission schedule. Beacon Positioning: mitigation is also needed to save clients energy. When APs use power save mechanism of standard to save clients energy, it would be better to stagger the beacon intervals so that clients associated with an AP do not interfere others associated with other APs [9], [10]. In other words, APs wakeup/sleep schedule are determined to be minimally overlapped with each other. IV. TRAFFIC MONITORING AND MITIGATION A. Monitoring is important in optimizing the performance of EWLAN. For the traffic, the global view is very important to tune the parameters correctly. Unlike the interference, it is typically performed in a passive manner. Desktop-based traffic monitor: This scheme uses desktop PCs with Wifi USB, making the PCs as monitors [11]. This framework is well-suited for the traffic as well as solving many wireless management problems including detection of unauthorized access points, handling malfunctioning APs, and performance. Another advantage is

3 that it uses wired connectivity and idling CPUs to assist the. Sensor-based traffic monitor: This scheme implements additional RF sensors attached to APs to monitor the network traffic [12]. This method does not (more correctly, cannot) measure the number of packets but measures the channel busy time with the sensor. They took account not only data transmission time but also back-off delay, IFS and RTS/CTS to obtain more accurate traffic condition. B. Control Based on the traffic information, EWLAN can improve network performance. Simply speaking, if the traffic is concentrated to an specific AP, clients associated with that AP suffer from a significant performance degradation. On EWLAN, the controller can solve this problem by manipulating the association process. Furthermore, it can improve performance by supporting the traffic-aware channel assignment. AP scheduling (association): To choose an optimal AP is very important in a WLAN. Rohan et al. proposed a mechanism that takes not only channel utilization but also RSSI to estimate an optimal AP [13]. Then, the controller could enforce the association with the particular AP by deferring the response from other APs for the probe request. For this purpose, the controller lets the APs set the SSID field to null to hide SSID. -aware channel assignment: Rozner et al. proposed a channel assignment mechanism taking traffic demand into account [14]. It observes the recent sent and received traffic via SNMP and predicts the traffic demand with the central controller. This information can be utilized by the controller when it assigns channels to APs. It also makes the conflict graph to better assign the channels. A highly demanded AP gets a channel with little interference while a less demanded AP gets a more interfered channel. Dual-band traffic and control: The enormous success of smartphones has accelerated the saturation of existing WLANs at 2.4GHz. This in turn facilitates the development of standards at 5GHz such as n and ac as well as dual-band (2.4 & 5GHz) devices such as iphone 5 and Galaxy S2/S3/S4 and dual-radio or multi-radio APs such as Apple Airport and Cisco 1040 series. For example, Airport Extreme supports either 2.4GHz or 5GHz operation but not both at the same time, which means it needs to make a decision which band to operate. On the other hand, a newer Airport Utility supports dual bands simultaneously. In this case, the same network name (SSID) is used at both bands to allow dualband clients to switch the two bands dynamically without handoffs. Dual-band radio relieves the saturated 2.4GHz band based on band steering. This technique renders dual-band clients to operate at 5GHz by, for example, not responding to probe request messages at 2.4GHz, leaving 2.4GHz band for legacy devices [15]. However, there has been little work done to understand the issues and solutions with a mixture of dualand single-band clients/aps in the context of EWLAN. V. MOBILITY SUPPORT There has been a plethora of work on mobility prediction for variety of wireless networks, such as cellular, WLANs, ad hoc networks, and mesh networks, and applied to reduce handoff latency, provide efficient resource reservation, improve routing protocols, and conserve power. Mobility & Handoff prediction: Reduce the handoff delay in WLANs has been an important topic since the onset of WLAN development. An immediate solution lies in optimizing the probing or scanning process because this is the most time consuming part of a handoff [16], [17]. [18] uses multiple APs in a mesh network to monitor the connectivity quality of clients in their vicinity. [16], [19] [21] reduce the number of channels to scan by defining a directed graph that represents the topological placement of APs and the mobility patterns of clients. More specifically, for example, [22] designed a fast handoff scheme based on next AP prediction. They could eliminate scanning process with the prediction scheme. This scheme is based on statistics of handoff sequence (HS). If it is considered a time-series data, the prediction is more accurate. By preassociating, they could eliminate the scanning process. The history of HS is maintained in a central controller, called Path- Cache server in Figure 3. A global history of all the MSs in the network is maintained in the Path-Cache, Fig. 3. Mobility cache (Path-cache) Handoff virtualization: Handoffs and the corresponding delay problem for delay-sensitive applications like VoIP have been considered in [23]. It sets up a virtual environment for VoIP with the purpose of eliminating handoffs. To achieve it, it uses a single virtualized channel, the same ESSID and the same MAC address on an ESS. While a single channel could cause fatal interference problems, it avoids interference by using micro-probing and an interference map and eliminates handoffs during a VoIP call. Since handoff refers to an association as well as authentication with a new AP, it is equally important to centralize

4 the authentication process too. In [24], APs are built with the PHY and lower portions of the MAC protocol only, and the remaining functionalities of AP are implemented in and managed by a central controller. The controller allows clients to connect to any of the APs without re-authentication which facilitates the handoff virtualization mentioned above. Localization: Localization has become an indispensable technology for the future ubiquitous life. This helps people find their indoor locations with mobile devices. Recently, there introduced numerous commercial localization services such as Skyhook, WiFiSLAM, Wifarer and NAO Campus. There s a growing demand for indoor localization in a large-scale EWLAN at a reduced control overhead and latency. WLAN-based positioning system (WPS) is very popular mainly due to the wide spread deployments of Wifi infrastructures. The most popular technique estimates the location by matching fingerprint measurements at the time with priori location fingerprints stored in, for example, a central controller [25] [27]. While the estimation accuracy depends on many factors including physical environments such as building structures, materials, walls, etc. [25] as well as the noise characteristics of signal propagation [25], [28], it also depends on the desired level of WPS performance, most importantly the response time during the online phase. VI. SUMMARY AND CONCLUSIONS This article presents a comprehensive survey of existing EWLAN schemes, which is summarized in Table I. To monitor interference and traffic, some schemes use test traffic [3], [7], [11] while others use real traffic [5], [10] [12]. As for the former, [7] uses probe packets to measure loss ratio while [3] measures SINR. More specifically about the latter, [10] monitors RSSI of APs beacons and [5] uses timestamp and reception status of all packets. Most of above EWLAN schemes apply their algorithm to infrastructure (controller or AP) as shown in Table I. However, several schemes [3], [4], [22] need client modification also as presented in Table I. For example, [4] needs changes in client side so that it is allowed to associate with multiple APs. By the same token, most of EWLAN schemes focus on downlink traffic rather than uplink traffic although the latter would cause interference too. [3], [23], [29] addressed this issue by utilizing, for example, CTSto-self packets. Although existing schemes are well suited to EWLAN and improve performance of network, the potential of this area has not been exhausted yet. ACKNOWLEDGMENT This research was supported in part by the Basic Science Research Program ( ) and WCU program (R ), both through the NRF (Korea) funded by the Ministry of Education, Science, and Technology. REFERENCES [1] A. Akella, G. Judd, S. Seshan, and P. 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5 Schemes Issues Strategies Function of controller PIE [5] Micro probing [7] GMS [4] TAME [10] DAIR [11] Wintry [12] DAP [13] -aware channel assignment [14] BMP [22] OmniVoice [23] FLUID [3] mitigation mitigation balancing balancing Handoff Handoff Flexible channelization Overlapped packet Coordinated probe testing Multi-association packet scheduling & Beacon interval distributing with packed APs Desktop infrastructure based Air time using RF sensor RSSI & channel air time considering AP selection demand & interference weighted channel assignment Mobility based prediction for next handoff AP Virtualizing WLAN using single virtualized channel & same MAC address Bandwidth packing based on conflict graph - Gathering time stamp and reception status - Judging whether overlapped packets nodes are in interference range - Coordinating test-probe packets - Judging whether nodes are in interference range - Extending conflict graph with multi-association - Scheduling packets - Constructing topology map and AP clustering - Beacon time schedule - Requesting specific application to monitors - Gathering requested information - Gathering channel utilization information from RF sensor - Gathering RSSI of a client from each AP and channel air time - Choose a proper AP for handoff to keep traffic balance - Weighting traffic demand and interference - Assigning channel to weighted link first - Gathering historical information of handoff - Predicting a next AP - Storing authentication parameters of each client - Setting ESSID same for every AP to be virtualized - Generating a conflict graph taking channel width and center frequency - Scheduling packets simultaneously sent on proper spectrum not to interfere each other Usage of conflict graph TABLE I SUMMARY AND COMPARISON OF EXISTING EWLAN SCHEMES Client modification - N R N T Y Y Y N R N N Both N N R N N Y N N Y Y N Y Y T Test probing& Real traffic International Symposium on Mobile Ad Hoc Networking and Computing. ACM, 2011, p. 5. [24] P. Calhoun, M. Montemurro, D. Stanley et al., Control and provisioning of wireless access points (capwap) protocol binding for ieee , IETF RFC5416, [25] P. Bahl and V. Padmanabhan, Radar: An in-building rf-based user location and tracking system, in IEEE INFOCOM, [26] M. Youssef, A. Agrawala, and A. Shankar, Wlan location determination via clustering and probability distributions, in IEEE Percom, [27] R. Battiti, T. Nhat, and A. Villani, Location-aware computing: A neural network model for determining location in wireless lans, in University of Trento, Technical Report, DIT , [28] M. Borenovic and A. Neskovic, Comparative analysis of rssi, snr and noise level parameters appli-cability for wlan positioning purposes, in IEEE EUROCON, [29] V. Shrivastava, N. Ahmed, S. Rayanchu, S. Banerjee, S. Keshav, K. Papagiannaki, and A. Mishra, Centaur: realizing the full potential of centralized wlans through a hybrid data path, in Proceedings of the 15th annual international conference on Mobile computing and networking. ACM, 2009, pp