A Study on Dynamic Load Balance for IEEE 802.11b Wireless LANby Ioannis Papanikos and Michael LogothetisEE 228a - Fall 2003 Dennis ChangIntroduction• IEEE 802.11 defines two types of wireless networks for different communication needs:• Independent Basic Service Set (IBSS) or Ad Hoc mode:• Limited in its range – all stations need to “see” or “hear” each other• Peer-to-peer communication among the stations• No fixed wired infrastructure for stations to communicate with each otherIntroduction• Extended Service Set (ESS) or Infrastructure mode:AP APWire stationWireless station(WS)• Wireless Stations are associated to an Access Point and through AP communicate with other wire or wireless stationsIntroduction• WS can be supported by more than 1 AP in the same region• Operation functions of the AP (Channel, ESSID, WEP, etc.) can differ among the AP while they support the same network• For roaming function, all AP in the same region need the same key functions as ESSID and WEP, but can operate in different channels (14, 11 in U.S., 3 non-overlapping)• Maximum number of associated stations per AP is 2007• No function specifying the AP selected by a WSLoad Balance Problem• One AP may support many WS, while some neighbor AP may support few or no stationsAP 1AP 2AP 3• this asymmetry in load between AP causes a high probability of packet loss in AP 1 and thus an overall network degradation compared to AP 2, 3• Can be avoided by balancing the number of associated stations among APLoad Balance ProblemAP 1AP 2AP 3AP 1AP 2AP 3• Whether applying a WS distribution algorithm results in a load balance for each AP depends on the nature of the wireless LAN• Problem becomes more difficult due to dynamic network topology changes when the WS are roaming around• Would be impractical to consider a station rearrangement after every new connection/disconnectionLoad Balance Problem• Classical Approach• Proposed Dynamic Load Balance Algorithm – 3 levels:– AP Channel Autoselection Level – tries to best distribute the APs to the available channels– Station Join Decision Level – method by which the WS select the AP to associate with– Link Observation Level – determines when the WS leaves its associated AP and the roaming function is performed (seeks a newAP)• ResultsClassical Approach• Currently, most manufacturers implement this procedure for association of a WS to an AP:– WS scans the available channels of each AP in the region (activescan with Probe Request)– Listens to the Beacon or Probe Response Frames– WS stores the Received Signal Strength Indicator (RSSI) of the received frames and other info. (ESSID, encryption (on/off), etc.)– After scanning, WS selects AP with the maximum RSSI, given that the AP covers the other requirements (ESSID, WEP) also– WS leaves AP when the RSSI falls under a predefined thresholdClassical Approach• Procedure is based on the hope that the quality of service of the selected AP is the best• But this typically results in many stations being connected to a few AP, and other neighboring AP being idle, as mentioned before• Overloaded AP leads to performance degradation• A proper load balance algorithm needs to consider the status of each AP and its already associated WS, in order to associate new WS to an APDynamic Load Balance – Level 1• AP Channel Autoselection Level– At start-up phase of each AP, the AP is informed of the existence of other APs in the same region by the Inter Access Point Protocol (IAPP), which transfers information proving that the AP service the same LAN.– AP active scans channels to discover which other AP are neighbors– Also it discovers the operation channels of the neighboring AP– Starts using the operation channel where the interference from neighboring AP is minimized– This level is initialization step to normalize operation conditions of the network– At the time of paper (2001) there is no AP with such initialization level, due to difficulties in implementation– So initial operation channel is chosen to be 1, 6, or 11 in U.S.Dynamic Load Balance – Level 2• Station Join Decision Level– WS sends a Probe Request to all channels in order to localize the AP, AP answer with Probe Response containing extra info. than already defined in protocol:• Ni: Number of stations associated to AP i• Si: RSSI value of the Probe Request received by AP i• Mi: Mean RSSI value for the set of stations associated to AP i (including RSSI for the new WS)– After the scanning procedure, WS determines the best AP to associate with, and sends an Association Request to that APDynamic Load Balance – Level 2Wireless Station Access Point iProbe RequestProbe Response (Ni, Si, Mi)Save Ni,Si, MiAssociation RequestDetermine best AP to joinAP iDynamic Load Balance – Level 2• So how does the WS determine the best AP to associate with? Authors provide this formulation:– Select AP that maximizes the weighted function Wi= Di* Pwi* Pi– Di denotes the difference between RSSI received from AP I, Si, and the mean RSSI– Pwi is a weight proportional function that considers not only differences from mean value but also the absolute value of the mean value– Pi is the weight proportional to the number of already associated WS to AP i≤−≥+=0/10/1iiiiiiwiDifSMDifSMPiii SMD −=region in the AP ofnumber total theisn where,0∑==njjiiNNPDynamic Load Balance – Level 2• Corrected formulation (perhaps):– Select AP that Maximizes Wi = -Di * Pwi * Pi– Or Minimizes Wi = Di * Pwi * Pi– Where∑−==≤−≥+=−=100/10/1njjiiiiiiiiwiiiiNNPDifSMDifSMPMSDDynamic Load Balance – Level 3• Link Observation Level– Determines when the WS leaves the AP and the roaming function is performed (seeks a new AP)– Each AP updates the mean RSSI value, Mi, and the number of associated stations, Ni, in each Beacon or Probe Response Frame– WS periodically probes the AP and updates RSSI in the side of APi, Si, Mi, and Ni, or monitors the Miand Nithrough the Beacon frames– Each time the WS detects a worse case than that calculated previously, it increases a Handover Counter (HC)– Once HC reaches a predefined threshold, the WS seeks a new AP– Threshold depends on manufacturing characteristics of WS and AP (sensitivity, transmission power)Protocol Modifications• New fields are needed in Beacon and Probe Response frames for transmitting extra information
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