Several communication-level handover strategies are possible, which mainly differ
in the event used to trigger the handover. It is possible to distinguish between
two main strategy categories, reactive and proactive. Reactive handover strategies
tend to delay handover as much as possible: handover starts only when wireless
clients completely lose their current AP signal. Reactive strategies are effective
in minimizing the number of handovers, e.g., by avoiding to trigger a handover
process when a client approaches a new wireless cell, without losing the origin
signal, and immediately returns back to the origin AP. However, reactive handovers
tend to be long because they include looking for new APs, choosing one of them,
and asking for re-association. Moreover before e reactive handover available bandwidth
is limited, since signal quality is low (in first analysis signal quality depends on
AP-wireless client distance). Proactive strategies, instead, tend to trigger
handover before the complete loss of the origin cell signal, e.g., when the new
cell RSSI overpasses the origin one. These strategies are less effective in
reducing the number of useless handovers but are usually prompter, by performing
search operations for new APs before the handover procedure starts.
By concentrating on proactive strategies, a further classification is possible.
On the one hand, Hard Proactive
(HP) strategies trigger a handover any time the RSSI of a visible
AP is greater than the RSSI of the currently associated AP plus an Hysteresis
Handover Threshold (HHT); HHT is introduced mainly to prevent heavy bouncing
effects. On the other hand, Soft Proactive
(SP) strategies are "less proactive" in the sense that
they trigger handover only if i) the HP condition applies (there is an AP with
RSSI greater than current AP RSSI plus HHT), and ii) the current AP RSSI is lower
than a Fixed Handover Threshold (FHT).
For instance, the handover strategies implemented by Cisco Aironet 350 and Orinoco
Gold Wi-Fi cards follow, respectively, the HP and SP models. More in detail, Cisco
Aironet 350 permits to configure its handover strategy with the "Scan for a Better
AP" option: if the current AP RSSI is lower than a settable threshold, the Wi-Fi
card monitors RSSI data for all visible APs; for sufficiently high threshold values,
the Cisco cards behave according to the HP model. Orinoco Gold cards exactly
implements the SP strategy, without giving any possibility to configure the used
We have designed and implemented two prediction mechanisms: Handover Prediction and Mobility Prediction.
Handover Prediction supplies the probability an handover procedure starts, Mobility Prediction if an handover
procedure is probable and which is the most probable next AP.
Our handover/mobility prediction solution is based on a two pipelined module architecture.
The first module (Filter
) is in charge of filtering RSSI sequences to mitigate RSSI fluctuations
due to signal noise. The second module (Prob
) tries to estimate the probability a handover happens
in the near future and which is the most probable next AP based on RSSI values provided at its input
Figure 1. Filter and Prob Modules.
The modular architecture of our predictor permits a completely separated implementation and deployment
of Filter and Prob, thus simplifying the exploitation and experimentation of different filtering and
handover/mobility prediction mechanisms, even dynamically composed at provision time by downloading
the needed module code. In particular we compare prediction mechanism performance when based on
RSSI or wireless client position
RSSI filtered with several low-pass filters
By delving into finer details, we have implemented two variants of the Prob module, one suitable for
communication-level HP handovers and the other for SP ones. We have decided not to work on Prob prototypes
for reactive strategies because of two reasons: first, handover prediction is less challenging in the case
of reactive handovers than of proactive ones since the triggering of a reactive handover only depends on
the RSSI data from one single AP; secondly, reactive communication-level handovers are of minor interest
for services with session continuity requirements, given their longer time needed to complete handover.
of our Prob module is in the state:
- LowProb, if the filtered value for the current AP RSSI is greater than the filtered RSSI values for any
visible AP plus a Hysteresis Prediction Threshold (HPT);
- HighProb, otherwise.
of the Prob module can assume the following states:
- Low-Prob, if the filtered RSSI value for the current AP is greater than either a Fixed Prediction Threshold
(FPT) or the filtered RSSI value for any visible AP plus HPT;
- HighProb, otherwise.
Let us stress a Prob Module with only two probability states (High/Low) is sufficient for many application scenarios;
however it is possible to develop more sophisticated Prob Modules with several intermediate states, if needed.
supplies to the application level the Prob Module output, without any further refinement.
When handover probability is High Mobility Prediction
supplies the MAC address of the most probable AP,
i.e. with the strongest RSSI.
Figure 2 depicts predicted RSSI values for the
current AP and the next one, in proximity of an HP handover. A wireless client,
moving from the origin AP to the destination AP, is first associated with the
origin (white background), then with the destination (grey background). When the
predicted RSSI of the destination AP overcomes the predicted RSSI of the origin
AP plus HHT, the handover is triggered.
Figure 2. HP-variant prediction and handover triggers.
The implemented SP-variant
of the handover predictor triggers a prediction when
the predicted RSSI value for the current AP is lower than i) a Fixed Prediction
Threshold (FPT) and ii) a predicted RSSI value for one visible AP plus HPT.
Similarly to HP, the SP-variant predictor only considers the most probable future
locality in the case of several predictions simultaneously enabled. Figures 3
and 4 show predicted RSSI values for the origin and the destination APs in
proximity of an SP handover. Figure 3 depicts a case where predicted RSSI values
change quite slowly: it is the overcoming of hysteresis thresholds that triggers
handover prediction. In Figure 4, instead, predicted RSSI values rapidly evolve,
and the passing of fixed thresholds produces handover prediction.
Figure 3. SP-variant prediction and handover triggers for relatively slow RSSI evolution.
Figure 4. SP-variant prediction and handover triggers for relatively fast RSSI evolution.