Application MobilityManager

The code is here.

It is an application that performs a reactive management of the STA mobility.

When the STA moves, this application uses the statistics obtained with the auxiliary wireless interface, for selecting the best suited AP for it.

The figure shows the handoff scheme, in which we suppose that an STA is associated to AP1 (in channel A). AP2 is in channel B. Besides this, an Ethernet control plane connection is used to communicate between the controller and the APs.

The handoff scheme follows the next process:

Initially, the wi5 controller establishes different “subscriptions” in the APs, in order to raise an event whenever a power threshold (noise level, power, etc.) is reached by an STA associated to an AP. Thus, the entity that starts the handoff process is the origin AP, when it "feels" that the STA is going away.

• When the STA moves (1) away from the Origin AP (AP1 in channel A), it detects that the signal is below a threshold and sends a PUBLISH message to the controller (2).

• According to its AP map, the controller sends a Scan Request message to the neighbouring APs (3).

• For a short period of time, all neighbouring APs switch their auxiliary interfaces to channel A and listen to packets originated by the STA. If an AP successfully hears the STA’s packets, it sends a Scan Response message to the controller (4).

• Once the controller has received the Scan Response messages from the APs, it runs its algorithms (5) and selects the best suited one for the STA. The decision is: “move STA to AP2.” Next, the controller tells AP1 (6) to send a series of CSAs to the STA (7). They are understood by the STA like a countdown, meaning “after N beacons, switch to channel B”.

• When the countdown ends, three events must occur in a specific order: a) the STA switches to channel B (while AP1 does not) (8); b) the controller sends an Add LVAP message to AP2, which starts sending beacons to the STA in channel B (9), and c) a Remove LVAP message is sent to AP1 (10).

After that moment, the STA starts receiving beacons from AP2 in channel B. The synchronization of these events is the most critical part of the handoff, as we will see in the subsequent tests and measurements. It should be noted that the STA interprets this as a channel switch carried out by the same AP, so Layer 3 and upwards are not aware of the handoff. Therefore, on-going communications are only interrupted briefly due to the channel switching.