Methods for Channel Assignment in Infrastructure based WLAN-A Survey

Transcription

1 Methods for Channel Assignment in Infrastructure based WLAN-A Survey Mrs. Sandhya Tanpure Prof. Sujata Kadam Mrs. Mrunalini Gavfale Abstract: Wireless Local Area Networks (WLANs) are having tremendous growth & becoming increasingly popular. The use of the unlicensed frequency spectrum & the inexpensive network equipment has encouraged the deployment of WLANs. The planning of wireless local area network (WLAN) infrastructures that supply large buildings or areas requires the consideration of many aspects (coverage, different traffic densities, interference, cost minimization, network throughput) and therefore is a difficult task if done manually. In this paper we present a survey on the Channel Assignment techniques in IEEE We then conclude the survey with several research issues open for further investigation. General Terms :Channel Assignment Schemes Keywords :Channel allocation, IEEE , wireless Networks, Channel Assignment, WLAN, interference 1. INTRODUCTION Due to use of the unlicensed frequency spectrum & the inexpensive network equipment has encouraged the deployment of WLANs. For smaller scenarios with only a few access points (AP) to be installed, no complex network planning is needed. However, network solutions supplying larger areas like public hot-spots, university campus, office buildings etc. need much more sophisticated planning. Based on how WLAN are managed, wireless LANs (WLAN) can be categorized into one of the following: 1) centrally managed [1], [2] or 2) uncoordinated [3], [4], [5]. The sensational evolution of the wireless/mobile user populace, coupled with the bandwidth requirements of Audio/video applications, requires efficient reuse of the inadequate radio spectrum allocated to wireless/mobile technology. A total radio spectrum is to be divided into a set of disjointed channels that can be used simultaneously while minimizing interference in adjacent channel by allocating channels appropriately. A good WLAN design should assign different channels to APs to cause less interference. Generally interference from devices that operate in the same frequency band reduces data transmission rate. WLANs are heavily deployed in areas such as university campuses, corporate offices, hospitals, coffee shops, airports and shopping malls. 2. System under Consideration 2.1, Topology for Network In this survey we focused only on an IEEE WLANs with an infrastructure network topology as shown in Fig 1, where APs and client resort to existing communication infrastructures such as legacy LAN to facilitate their communication. All communication accomplishment must be facilitated via this AP. This survey focuses only on an IEEE WLAN with an infrastructure network topology as shown in Fig. 1, where APs and clients. Basic Service Set (BSS) or cell is a single instance of topology. Extended Service Set (ESS) is comprises of multiple BSS in same infrastructure. A WLAN can configure in two basic modes: 1. Peer-to Peer (Ad-hoc) mode: This mode consists of two or more clients are equipped with wireless network adapter with no connection to wired network. 2. Client/Server (infrastructure networking): This mode offer fully distributed data connectivity. This mode generally consists of multiple stations liked with central hub. This survey focuses only on an IEEE WLAN with an infrastructure network topology as shown in fig. 1, where APs and client. Fig. 1: Arrangement of Infrastructure based IEEE WLANs 76

2 2.2, Channels in IEEE WLANs have seen tremendous growth in recent years as a last-hop connectivity solution. Due to such growth, network administrators are faced with an emerging challenge of efficiently managing bandwidth resources to provide better service to clients. In general, WLANs operate in the two unlicensed frequency spectrum bands as follows: 1) 2.4 GHz Industrial, Scientific, and Medical (ISM) band, and 2) 5 GHz Unlicensed National Information Infrastructure (UNII) band [6] and [7]. Both bands are available internationally. The number of allowable channels however varies from country to country due to each country s regulations on radio spectrum allocation. Only three channels are non-overlapping Fig. 2: Channels in for 2.4 GHz ISM band. The 5-GHz UNII band contains three sub bands referred to as low, middle, and high, each of which contains four non-overlapping channels as shown in Fig. 3. Each channel occupies a bandwidth of 20 MHz. Fig. 3: Two lower sub bands of the 5 GHz UNII band. 2.3, Co-ordination function of MAC: BSS are having wireless terminals which uses wireless medium for transmission. Only one terminal can transmit at a time. Therefore, if multiple terminals can transmit simultaneously, a collision may occur and packet to be transmitted will be delayed. The IEEE standard specifies a contention-based Medium Access Control protocol, called Carrier Sense multiple access/collision avoidance (CSMA/CA) technique or distributed co-ordination function (DCF), to coordinate wireless terminal transmission within BSS/ESS. The aim of this protocol is to achieve co-ordination between wireless terminals throughout the control messages travelling in the medium. The information in control messages is provided implicitly by the channel itself through the use of carrier sensing mechanism before each transmission to check if the channel is either active or idle. Fig. 4 shows the process in CSMA/CA. Fig. 4: Process of CSMA/CA 3. Channel Assignment Schemes 3.1, PACA (Peer-Assisted Channel Assignment for Home Wireless LANs): PACA [8] is scalable as it is completely distributed. Each AP collects the local information from its peers to do channel assignment. PACA is an algorithm which helps AP continuously gather channels information and switches [9] channel when a better channel is needed. When a client in network becomes idle, i.e., it has no communication with the AP, it enters a process called channel utilization query process, which is shown in Algorithm 1. When the client enters the process, it randomly selects a channel (including its current operating channel) and switches to that channel to gather the channel utilization information. In this paper, we assume that AP can only operate in non-overlapping. Algorithm 1 Channel Utilization Query Process of Client if (No data communication with AP) then if (Enters faked PSM and operates in a random channel) then Sends CUQuery; if (Receives CUReply) then Update channel utilization information; if (Time-out) then Switches back to original channel; Exits faked PSM; Receives all the buffered packets; Feeds the channel utilization information to AP; The switching node broadcasts a CUQuery (Channel Utilization Query) when it enters the visited channel. Nodes or APs receiving this query reply the visited node with a CUReply (Channel Utilization Reply). The format of the CUReply is <Peer Type, Load>. 77

3 3.2, MCACAO (Multihop Client Assisted Channel Assignment optimization): MCACAO is scalable and completely distributed. In, MCACAO mobile clients help their APs to detect interference on networks. Each APs simultaneously query their associative clients to collect traffic information on each channel, based on this traffic information it chooses a interference free path to operate on a best channel to reduce interference among Basic service set(bss). Figure 5 shows the process flow of MCACAO [11]. MCACAO follows the following flow to assign channels into the network. The channel assignment has two routines. 1. Initialization routine 2. Optimization routine. During the initialization routine APs initially assign the k non-overlapping channels randomly. In Optimization routine it has three phases. 1. GATHER STATICS () 2. COMPUTE INTERFERENCE () 3.SWITCH TO () (1) GATHER STATICS () In this phase all APs assist their associative clients to collect the traffic information about the various paths. Due to this, all APs collect the traffic information and storing it to the database. (2) COMPUTE INTERFERENCE () Based on the Traffic information the interference level will be calculated for each path, and the interference will be compared with previous path. With this information the path can be allocated. (3) SWITCH TO () With this interference value the interference free path can be calculated and the channel will be switch to that path. Fig. 5: Process flow of MCACAO 3.3, TDCS mechanism TDCS is new dynamic selection mechanism which focuses on the restrictions imposed by scenarios with independent IEEE networks and adapts faster to the interference pattern variations i.e. allows the channel allocation in a faster way to the changes in the interference patterns that affects WLANs[10]. The Figure 6 shows the TDCS mechanism. In this mechanism the AP requests its client stations to generate reports in time intervals of D, in the order of seconds, about the occupation level in the BSS operating channel. These measurements do not prevent nodes from functioning normally. According to the k standard, it is not required that nodes stop sending and acknowledging packets to perform measurements on the operating channel. Thus, the AP can verify by these periodic reports whether the mean occupation level in the operating channel surpassed the threshold of tolerance α. When the threshold is exceeded, the process of looking for a new operation channel is started. So, the AP requests its client stations to perform measurements of the occupation and noise levels on all available channels for allocation. Once these data is gathered, the AP executes the channel selection algorithm, which now has only two steps: it selects the n channels with the smallest mean occupation level; and then, among these n channels, it selects the one with the smallest mean noise level. Fig. 6: Operations of the TDCS mechanism 3.4, Measurement Based Algorithms The three proposed algorithms [12] on Measurement based all have an iterative nature and weighted interference is a metric is used to capture the overall interference in the cell.. At each point in time (predefined, randomly chosen, or determined at runtime), say every 1, 2, or 5 minutes, one iteration of channel switching takes place where one or more APs switch their frequency channels according to mechanisms that are specific to the proposed algorithms, while other APs stay on their current channels. The channel switching time in hardware is several milliseconds and is thus negligible as compared to the interval between two iterations. APs and clients measure and average their in-situ interference between very two successive iterations. Iterations keep taking place on different AP(s) until the channel allocations converge. Below we describe the different conditions of the three algorithms that a representative AP am can switch from channel k = fm to k = f m. The denotes a vector of channels selected by APs after the representative AP am moves from channel fm to f m. Hence differs from in only the m-th element. The a m denotes the identity of access point. 78

4 3.4.1,. The No-Coord Algorithm: If a m switches from its current channel fm to f m only if the weighted interference on the new channel f m is lower, i.e., the following condition holds: No-Coord Condition: This algorithm is denoted No-Coord, because am makes a greedy channel selection without coordination with other APs , The Local-Coord Algorithm: A representative AP a m switches from channel k to k if the max weighted interference seen by these cells decreases after the channel switching, i.e., the following condition holds: (1) uncoordinated fashion by different network administrators, the scalability of the implementation of channel assignment algorithms becomes even more important issue. In such scenarios, a channel assignment scheme of choice should be cooperative and Scalable enough to orchestrate channel switching across the entire network without creating significant interference to the neighbors. Being aware of the neighboring networks located in different administrative domains, the scheme should also be able to interact and exchange necessary information with its neighbors in order to allocate appropriate channels to the APs. Table I shows the summary of channel assignment schemes in WLAN. Local-Coord Condition: (2) TABLE I SUMMARY OF CHANNEL ASSIGNMENT SCHEMES IN WLAN This algorithm is denoted Local-Coord, since am needs to locally coordinate with the APs indexed by Gm;k(f) and Gm;k (f) via wired backbone network for the channel switching , The Global-Coord Algorithms: AP am will switch to a new channel only if the sum interference on the new channel is lower (after am switches there) than the sum interference on its current channel, i.e. the following condition holds. Global-Coord Condition: (3) This algorithm requires global coordination among APs using a central network controller that communicate with all APs, and is thus denoted Global-Coord. 4. CLASSIFICATION OF VARIOUS CHANNEL ASSIGNMENT TECHNIQUES IN IEEE In summary of channel assignment schemes various aspects are considered: 1) how often channel assignment is triggered (static or adaptive), 2)to which type of deployment a channel assignment is applicable (uncoordinated or centrally managed), 3) the type of frequency channels used (overlapping or non- overlapping channels), 4)the procedure in obtaining channel assignment solutions (heuristic or integer linear programming). The main challenge would be how to capture the network dynamics as much as possible while maintaining the complexity of implementation of channel assignment algorithm at a practical level. Furthermore, when WLANs are deployed in an 5. CONCLUSION AND OPEN RESEARCH ISSUES Channel assignment is one mechanism to improve the performance of WLANs. In this survey we have discussed several existing channel assignment schemes applicable to either centrally managed or uncoordinated environments. Several possible future research directions and open issues with regard to channel assignment in WLANs are outlined below: The challenge would be how to capture the network dynamics as much as possible while maintaining the complexity of implementation of channel assignment algorithm at a practical level. Furthermore, when WLANs are deployed in an uncoordinated fashion by different network administrators, the scalability of the implementation of channel assignment algorithms becomes even a more important issue. The research direction tends to shift toward adaptive channel assignment in uncoordinated environments, in which network dynamics is incorporated into the problem formulation. 79

5 Continually monitoring the network dynamics, say on a daily basis, at a particular location may lead to a discovery of traffic pattern. Channel assignment can then be performed at a particular location during a particular period of time based on the prediction (or self-learning experience) as well as the application requirements. The schemes discussed in this survey assume either Uplink or downlink traffic. To be more realistic, traffic in both directions should be considered. This is reasonable as peer-to-peer communications become more popular. REFERENCES [1] Y. Bejerano, S. Han, and L. Li, Fairness and Load Balancing in Wireless LANs Using Association Control, in Proc. ACM Mobicom, [2] Y. Bejerano, S. Han, and L. Li, Fairness and Load Balancing in Wireless LANs Using Association Control, in Proc. ACM Mobicom, [3] A. Vasan, R. Ramjee, and T. Woo, Echos: Enhanced Capacity Hotspots, in Proc. IEEE Infocom, [4] A. Akella, G. Judd, S. Seshan, and P. Steenkiste, Self-Management in Chaotic Wireless Deployments, in Proc. ACM Mobicom, [8] C.Wong S.-H. Gary Chan Jiancong Chen PACA: Peer-Assisted Channel Assignment for Home Wireless LANs Department of Computer Science The Hong Kong University of Science and Technology Clear Water Bay, Kowloon, Hong Kong, 2006 [9] A.Nithya 1, M. Jothimani Improving communication efficiency with adaptable transmissions for Uncoordinated WLANs K. S. R. College Of Engineering, Tiruchengode, Namakkal Dt Tamil Nadu, India [10] M W Rocha da Silva and Jos e Ferreira de Rezende TDCS: A New Mechanism for Automatic Channel Assignment for Independent IEEE Networks [11] J. K. Chen, G. D. Veciana, and T. S. Rappaport, Improved Measurement-based Frequency Allocation Algorithms for Wireless Networks, in Proc. IEEE GLOBECOM 07, Washington, DC, USA, Nov [12] R. Chandra, P. Bahl, and P. Bahl, Multinet: Connecting to multiple IEEE networks using a single wireless card, in Proceedings of IEEE INFOCOM 04, vol. 2, Mar 2004, pp [5] B. Alawieh, Y. Zhang, C. Assi, and H. Mouftah, Improving Spatial Reuse in Multihop Wireless Networks - A Survey, IEEE Commun. Surveys Tutorials, vol. 11, no. 3, [6] IEEE Standard for Information Technology- Telecommunications and Information Exchange between Systems-Local and Metropolitan Area Networks-specific Requirements - Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications, IEEE STD (Revision of IEEE Std ), pp. C1 1184, June [7] Supplement to IEEE Standard for Information Technology Telecommunications and Information Exchange between Systems - Local and Metropolitan Area Networks - Specific Requirements. Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) Specifications: High-Speed Physical Layer in the 5 GHz Band, IEEE STD a-1999,