AP-assisted Client Mobility – Draytek

When using multiple Access Points, wireless clients moving between access points have control of which wireless access point they remain connected to. This can result in a wireless client choosing to stay connected to a distant access point, even when moving to a now nearer wireless access point could result in a better connection.

By not moving to the nearer wireless acess point, the client wouldn't benefit from the stronger signal and possibly better connectivity. The wireless access point will also need to send wireless transmissions further, spending more time transmitting to the more distant client than local ones, due to the difference in achievable wireless transmission rates.

VigorAP wireless access points improve this situation by using DrayTek's AP-assisted Client Mobility feature. DrayTek's AP-assisted Client Mobility tracks a wireless client's Signal Strength (RSSI) between the associated / connected access point. When a device moves out of optimal range for connection to a VigorAP access point, the VigorAP will check that device's achievable signal strength with other VigorAP access points.

If another VigorAP indicates that it can achieve better signal strength for that wireless client, the original VigorAP will disconnect that client, allowing the wireless client to re-associate with the nearer VigorAP, resulting in better signal strength, with potentially better wireless throughput and lower latency.

Applicable Products

VigorAP 710
VigorAP 810
VigorAP 900
VigorAP 902
VigorAP 903
VigorAP 906
VigorAP 910C
VigorAP 912C
VigorAP 918R
VigorAP 918RPD
VigorAP 920R
VigorAP 920RPD
VigorAP 960C
VigorAP 1000C
VigorAP 1060C

{tab How It Works}

How It Works

In the [Wireless LAN] > [Roaming] menu, there are multiple options to control how clients move between access points:

Mode	Description
Minimum Basic Rate	Disassociates wireless clients when their throughput rate decreases to the specified level. The Rate can decrease when signal strength decreases or where interference causes the achievable Rate to decrease. This does not check signal quality with other VigorAP access points.
Disable RSSI Requirement	Clients are allowed to remain connected to a VigorAP until the client loses signal or chooses to associate with another closer VigorAP. The VigorAP does not control the association of wireless clients to the VigorAP.
Strictly Minimum RSSI	Disassociates wireless clients when their signal strength goes below the specified value. This does not check signal quality with other VigorAP access points.
Minimum RSSI (Recommended)	When clients go below the specified signal strength value, the VigorAP checks potential signal strength with other VigorAPs and will disassociate a wireless client if that client can achieve a better signal with a nearer VigorAP.

Minimum RSSI Mode

DrayTek VigorAP wireless access points that support AP-assisted Client Roaming, when configured to help wireless clients move between access points with Minimum RSSI mode, will keep track of the Signal Strength (RSSI) of each wireless client connected to the VigorAP, along with the RSSI value of all nearby wireless clients, whether they are associated with that particular VigorAP or not.

Example

When a wireless client moves out of range of a VigorAP access point, determined by an RSSI Threshold value that can be configured on each VigorAP, the VigorAP will send a request over the wired network connecting the VigorAP access points.
This request will check whether any other VigorAP access points can achieve better Signal Strength to that wireless client.
The VigorAP access points on that network will then reply with its own detected Signal Strength (RSSI) values for that wireless client.
The VigorAP that sent the request will then determine whether to move that client off using the configured thresold values. By default, a VigorAP will not move a wireless client off to another VigorAP unless that Signal Strength value is 5 dBm higher (typically 15-20% difference).
The original VigorAP that the now distant client is associated with, can then determine that there is a nearer VigorAP that the client can connect to. The original VigorAP disassociates the wireless client and temporarily blocks that client from reassociating.

The original VigorAP passes this information to the nearer VigorAP and the wireless client can then reassociate with the closer VigorAP.
The wireless client will reconnect within a few seconds, the shorter distance lowers the risk of interference and increases potential Transmission Rates, providing a better connection experience, with typically higher throughput and less latency / packet loss over the wireless link.

{tab Minimum RSSI mode Configuration}

Minimum RSSI mode Configuration

Requirements

Each DrayTek VigorAP must support AP-assisted Client Roaming
At least one wireless network (SSID) with matching security details on each VigorAP - wireless clients can only move between access points that have the same name (SSID) and credentials
Wired (physical network) connection linking all DrayTek VigorAPs using the feature
All VigorAPs must be able to communicate via IP on their management LAN / VLAN
Firmware version 1.1.7.1 or later on all VigorAPs to enable use of the feature

To enable AP-assisted Client Roaming in the mode which communicates the Signal Strength (RSSI) values of wireless clients between VigorAP access points, go to the [Wireless LAN (2.4GHz)] / [Wireless LAN (5GHz)] > [Roaming] menu. The feature is enabled separately for each wireless Band (2.4GHz and 5GHz).

Set the mode to Minimum RSSI.

dBm Values

The dBm value will default to -66dBm, with smaller values indicating better Signal Strength. For instance -30dBm indicates very good signal, while -90dBm indicates very poor signal.

The default of -66dBm is equivalent to around 60% Signal Strength. This can be changed / tuned depending on requirements but typically should not need to be altered.

Adjacent AP RSSI Values

The Adjacent AP RSSI dBm value defaults to 5. Using higher values will result in wireless clients 'sticking' to an access point for longer. Lower values will move clients between VigorAP access points more frequently.

As an example of how this operates using the default values of -66dBm and adjacent RSSI of 5dBm:

If a client's signal reduces to -68dBm (55% signal strength), the VigorAP will not move the client off unless there is a VigorAP nearby that can achieve -63dBm or better.

If it finds a VigorAP that can achieve -66dBm (similar signal strength), the client will not be moved from the current VigorAP because the signal increase is not significant enough.
If it finds a VigorAP that can achieve -60dBm (better signal strength), the client will be moved to the nearer VigorAP.

Logging & Diagnostics

The AP-assisted Client Mobility system logs movement between VigorAP access points through Syslog as demonstrated in the example below.

In this example, the Original VigorAP detects that its client has moved out of range, checks Signal Strength with other VigorAPs and receives confirmation that the wireless client can achieve better Signal Strength with the Closest VigorAP. The access points then move the client between the two VigorAPs, keeping the wireless client connected to the VigorAP with the clearest signal:

Timestamp (hh:mm:ss)	Device	Syslog Message
16:18:03	Original VigorAP	syslog: [DrayRS] 00:1d:aa:52:07:e9's RSSI is low(-73 dbm).
16:18:03	Original VigorAP	syslog: [DrayRS] Ask if there is any AP can roam.
16:18:03	Closest VigorAP	syslog: [DrayRS] Reply 00:1d:aa:52:07:e9, the RSSI of AP with it is -62 dbm.
16:18:03	Closest VigorAP	syslog: [DrayRS] 00:1d:aa:52:07:e9 may roam from (Channel 132).
16:18:07	Original VigorAP	kernel: Send DEAUTH frame with ReasonCode(8) to 00:1d:aa:52:07:e9, called by drs_kick_and_block, 210
16:18:07	Original VigorAP	kernel: LOG#2 00:1d:aa:52:07:e9 has disassociated
16:18:07	Closest VigorAP	kernel: 00:1d:aa:52:07:e9 had associated successfully
16:18:08	Original VigorAP	syslog: [DrayRS] 00:1d:aa:52:07:e9 had disassociated by using Roaming Function.

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