The Handoff Process below the MAC layer

As described in [1], the time spent by a mobile station to search APs stands for 90% of the handoff latency in IEEE 802.11-based wireless LANs. So, to achieve fast handoff, it is essential to improve the search phase.

3.2.1 A Two-Stage Handoff Procedure

In an 802.11 WLAN, a mobile station always tries to stay on its associated channel until the transmission is almost disrupted due to weak signal. At this moment, the channel is

unstable and has a high probability to be disconnected. Therefore, the mobile station may suffer sudden disassociation and packet losses occur. When this happens, the mobile station would first spend lots of time to scan all legitimate channels, and then re-associate with one AP according to the scanning results to complete the handoff procedure.

Instead of doing all work at the very last moment, we believe the handoff procedure should be divided into two stages so that the disconnection time and the packet loss can be minimized. In the first stage, instead of scanning all 11 channels at a time, only a selected channel is scanned each time. This selective channel scanning can be done stealthy without causing noticeable quality degradation of an on-going VoIP conversation. A stealthy channel scanning algorithm will be presented in Section 3.2.3. In the second stage, a mobile station can directly associate with the AP that is chosen based on the stealthy scanning results of the first stage.

3.2.2 Selective Active Scanning using System Channel Map

It’s common that a network deploys two, or even three, overlapped BSSs at radio signal’s boundary. If there are too many overlapped BSSs, the radio quality worsens due to the interference from the neighbors. From this point of view, it’s unnecessary to scan all possible channels when a mobile station performs the search phase. But, the problem is which channels need to be scanned. As far as we know, there are two possible methods. The fist, known as Neighbor Graph [5], uses dynamic learning process from all mobile stations’

mobility patterns. The second uses static information which may be available before the deployment of wireless LANs because careful planning and a meticulous site survey are required for the nature of the radio link. We prefer the second method as the answer because the first method requires renewing the existing access points which stands for extraneous expenses needed to offer by operators.

We store the APs’ topology at the network side and a mobile station can request for these information stored in XML format. When a mobile station needs to know the APs’ topology, it can convey the current associated access point as a key to the mobility server. Then, the mobility server will reply information, called System Channel Map, about access points around the current associated access point. The XML schema of System Channel Map is shown in Figure 7, where BSSID, CHANNEL and SSID describe parameters of an access point, and SCENARIO describes the type of handoff scenarios. Currently, there are three types of handoff scenarios in our design. Scenario one indicates that the target AP for the handoff has the same SSID as the current one. Scenario two indicates that the target AP for the handoff has a different SSID but it locates at the same network segment. Scenario three indicates that the target AP for the handoff has a different SSID and it also locates at a different network segment.

Figure 7: XML Schema of the System Channel Map

3.2.3 Stealthy Channel Scanning

To further reduce scanning overhead, we propose a stealthy channel scanning algorithm that can be used to decide the access point to hand off to in an effective way. The followings are three aspects that we considered in this algorithm.

„ Even though we can reduce the scanned channels to a subset of the whole channels according to the system channel map, it may still be inefficient. It would be best that we only scan one channel at each time.

„ We observe that not all access points in the system channel map are equivalent. For example, the handoff between two access points located in the same ESS is always faster than those access points located in different ESSs. Therefore, we need to give different preference to each access point in the system channel map according to different handoff scenarios.

„ Because the IEEE 802.11b operates in the unlicensed ISM band, the radio link is unstable. Brief period of interference should not cause unnecessary scanning actions.

Our experiment of the selective active scanning, which will be presented in more detail in Chapter 5, shows that it takes 75ms in which period the mobile station can’t receive voice packets. This means about two to three voice packets can’t be delivered on time if packets are sent at an interval of 20ms, and thus, we can’t perform selective active scanning too frequently; otherwise the on-going VoIP session is disrupted. Therefore we propose an efficient algorithm to determine when to perform selective active scanning and maintain the QoS of VoIP session at an acceptable level.

Before a mobile station decides to perform selective active scanning, it must have observed the signal strength of a sequence of received data frames. We call this observation period the tuning period and give it a formal definition here. As indicated in Figure 8, T

denotes the tuning period, and the interval consists of a series of slots, each of which represents a voice packets received. If the voice packets arrive at an interval of R ms, the number of slots in the tuning period will be T/R. There are also a pointer, denoted by pos, pointing to the first empty slot within the tuning period and a counter, denoted by ScanCounter, recording the number of selective active scanning consecutively performed.

Figure 8: The Tuning Period of Stealthy Channel Scanning Algorithm

Our rules of determining when to perform selective active scanning is listed below, Rule 1: We use two level thresholds to divide the space of signal strength into three

regions and data frames of signal strength in the same region have the same degree of influence. When a frame’s signal strength is larger than S1 (-87dbm), the slot pointed by pos is assigned value 1 and pos points to the next slot.

When a frame’s signal strength is lesser than S1, but larger than S2 (-92dbm), the slot pointed by pos is assigned value 0 and pos points to the next slot.

When a frame’s signal strength is lower than S2, the slot pointed by pos and the next slot are both assigned value 0, and then pos points to the next empty slot.

Rule 2: When all slots within the tuning period are filled, the tuning period slides one slot to the right, and the slot slid out of the tuning period becomes stale information. The value of ScanCounter resets to zero.

Rule 3: When over 80% of total slots in the tuning period are with value 0, the mobile performs selective active scanning, as described in Figure 9, and all records in

the tuning period discard.

Figure 9: The Selective Active Scanning Procedure

3.2.4 Direct Channel Handoff

Whenever the following conditions occur, mobile stations need to associate with another access point:

Condition 1: The value of ScanCounter is equal to, or greater than two, which may mean that a long interference is on the current radio channel or the mobile station is really near the edge of the AP’s coverage area.

Condition 2: As the research in [2] suggests, three consecutive frame transmission errors can be used as a trigger to initiate the handoff.

Condition 3: The association is forced to terminate. This condition may result from the access point crash caused by an accident or other reasons.

To handoff, a mobile station can bypass the search phase and directly enter the execution phase because there is sufficient information collected by the stealthy channel scanning for the MS to decide a target AP. If the MS can’t find a target AP under conditions 1 and 2, it will ignore this handoff trigger. However, under condition 3, the MS needs to perform the search phase with those channels specified in the system channel map, and then the execution phase according to the results of the search phase.