1 QoS-Aware Channel Scanning for IEEE 802.11 Wireless LAN Sang-Jo Yoo*, Nada Golmie, Haolang Xu National Institute of Standards and Technology, Gaithersburg, MD {sangjo.yoo, nada.golmie, haolangx}@nist.gov Abstract – Channel scanning is an important aspect of seamless handovers since it is required in order to find a target point of attachment (PoA). In the IEEE 802.11 WLAN, scanning of other channels causes service disruptions with the current AP so that the provided quality of service (QoS) will be degraded seriously during the handoff. In this paper, we propose a QoS supported dynamic channel scanning algorithm. The scanning period is scheduled to guarantee the user’s QoS requirements while the scan progresses. The simulation results show that the proposed mechanism reduces service disruptions and provides the desired quality of service to users during the scanning period. * I. INTRODUCTION During WLAN [1] handoffs, a certain level of service disruption in terms of delay and packet loss is simply unavoidable due to the channel scanning and link switching procedures that need to be performed in order to find a target AP and transition the connection to it. Since channel scanning can be a relatively time consuming procedure, the quality of service (QoS) degradation during a scan is a critical issue. Therefore, to support user QoS requirements and seamless handovers, service disruptions during the scanning procedure should be controlled and scheduled effectively. Most previous scanning algorithms focused on reducing the scanning time [2-4]. Using either the previous channel selection history or information regarding the neighboring network topology, they aim at scanning fewer channels in an effort to reduce the handover latency. Few other scanning algorithms [5][6] were proposed for VoIP applications, in which scanning and VoIP traffic transmission are simply interleaved with VoIP delay requirements. In this paper, we propose a new QoS-aware channel scanning mechanism for WLAN networks. To provide the desired QoS level to user applications while minimizing MAC layer delays and the packet loss ratio, the entire scanning period is scheduled based on the scanning policy and measured QoS level. The proposed scheduling method divides the entire required scanning period into several smaller scan times termed service interruption times. After each short scan during a SIT (service interruption time), the MS reverts back to the normal data transmission mode. The remainder of this paper is organized as follows: in Section II, we propose a new QoS-aware channel scanning mechanism. In Section III, * sjyoo@inha.ac.kr , Associate professor of Inha University, Korea. "This research was supported by the NIST/Office of Law Enforcement Standards (OLES)" simulation results are shown. We conclude this paper in Section IV. II. QOS-AWARE CHANNEL SCANNING A. QoS Supported Handoff Procedure For QoS-supported scanning and handoff, a ‘QoS satisfaction degree’ is defined as a link quality metric in this paper. It is a function of QoS metrics as defined in (1). Each QoS component is a ratio between required and measured. The QoS satisfaction degree for class c can be defined as a minimum value from two QoS components or a weighted average depending on the user requirements. () 1 ≥ t QoS c is desired. () () () ⎟ ⎟ ⎠ ⎞ ⎜ ⎜ ⎝ ⎛ = t M_loss R_loss , t M_delay R_delay F t QoS c (1) where, R_delay and R_loss are the required delay and loss ratio, respectively; M_delay(t) and M_loss(t) are the measured delay and loss ratio at time t. In our approach, we separate the actual handoff execution that is most time critical behavior with QoS supported scanning. To start the QoS supported scanning, we have defined the QoS_Scan_Start link layer trigger. Fig. 1 shows the time sequence of the proposed approach. If the current measured QoS satisfaction degree is less than the QoS_Scan_Start threshold, then the MS sends query messages to the IEEE 802.21 information server (IS) [7] to obtain neighbor network information. The neighbor information also can be obtained from the current serving AP using neighbor advertisement messages in WLAN. Neighbor report frame from the current AP includes the list of the neighbor APs. This neighbor AP information is stored in MS’s scanning list. After obtaining neighbor AP information, the MS performs QoS supported scanning. To provide the desired QoS in terms of delay and loss ratio during the entire QoS supported scanning period, short time scanning and normal data transmission are interleaved. QoS SD QoS_Scan_Start QoS_LGD QoS_LinkDown HO Decision and L3 Handoff HD FMIPv6 Service New AP Association SII LS QoS Support Scanning SIT: Service Interruption Time for Scanning SII: Service Interruption Interval : Service Disruption Time HD: Handoff Decision SIT: Service Interruption Time for Scanning SII: Service Interruption Interval : Service Disruption Time HD: Handoff Decision Neighbor Discovery & Scanning Desired QoS Level Non-QoS Scanning SIT Neighbor Info 978-1-4244-2644-7/08/$25.00 © 2008 IEEE