Editor's Note: This is the last of a four-part deep dive series into High Density Experience (HDX), Cisco's latest solution suite designed for high density environments and next-generation wireless technologies. For more on Cisco HDX, visit www.cisco.com/go/80211ac.  Read part 1 here. Read part 2 here. Read part 3 here.

If you've been a long time user of Wi-Fi, at some point you have either observed someone encounter (or have personally suffered from) so called "sticky client syndrome". In this circumstance, a client device tenaciously, doggedly, persistently, and stubbornly stays connected to an AP that it connected to earlier even though the client has physically moved closer to another AP.

Surprisingly, the reason for this is not entirely...errr...ummm...unreasonable. After all, if you are at home, you don't want to be accidentally connecting to your neighbor's AP just because the Wi-Fi device you're using happens to be closer to your neighbor's AP than to your own.

However, this behavior is completely unacceptable in an enterprise or public Wi-Fi environment where multiple APs are used in support of a wireless LAN and where portability, nomadicity, or mobility is the norm. In this case, the client should typically be regularly attempting to seek the best possible Wi-Fi connection.

Some may argue that regularly scanning for a better Wi-Fi connection unnecessarily consumes battery life for the client device and will interrupt ongoing connectivity. Therefore the "cure is worse than the disease". But this is true only if the client is very aggressively scanning and actually creates the complete opposite of being "sticky".

The fundamental issue with "stickiness" is that many client devices simply wait too long to initiate scanning and therefore seeking a better connection. These devices simply insist on maintaining an existing Wi-Fi connection even though that connection may be virtually unusable for anything but the most basic functionality.

This problem also applies to and affects devices that are capable of a cellular connection. In other words, if a device can achieve a better connection over cellular than that over an existing Wi-Fi connection, then it should switch from Wi-Fi to cellular when Wi-Fi degrades to near unusability.

So what is the solution? Enter Cisco's "Optimized Roaming". In this case a Cisco Access Point is continuously evaluating the quality of the Wi-Fi connection for each associated device and can actively disconnect a device upon detecting that the device is moving into a region of poor coverage. This detection is accomplished by measuring data frames received  from the client device. The advantages to to relying on data traffic are two fold, First, more reliable measurements can be madefor active clients that are in motion. Second, idle clients that could be in satisfactory coverage are not disconnected (it would not be advisable to penalize a client and disconnect it simply for being quiet). Furthermore, the Cisco AP can perform this detection before the device goes into a "dead zone" where typically that device can only hear beacons. Note that beacons are typically transmitted at lowest data rate and highest power and therefore have the greatest range.  Therefore, and unfortunately, many devices make their scanning decision based solely upon beacons thereby creating a "sticky client"

The result of actively disconnecting the client via "Optimized Roaming" is that it forces the client to enter its scanning mode much earlier than it ordinarily would. Some may think this method is extreme, but in reality it is merely terminating a connection that is becoming essentially unusable.

The benefit here is not only to the end user but to the overall performance of the AP and therefore to other users as well. Consider that a client which is gradually degrading in performance (and possibly degrading faster than it can dynamically rate adapt or compensate to) may cause a large number of retransmissions (either frames it is receiving from the AP or frames it is transmitting to the AP). This consumes airtime. If enough clients are being sticky, then it can degrade the performance of the AP. Therefore"sticky" clients that maintain poor connections can easily impact well behaved clients.

In conclusion, Cisco's "Optimized Roaming" helps prevent a negative experience for Wi-Fi users by monitoring the connection quality of all devices and proactively prompting poorly performing client devices to seek a better connection much sooner.

FAQ for Optimized Roaming *Thanks to Brian Levin for providing these questions and answers

What is a "Client behaving badly"?

Clients have the option to join either 3G/4G or Wi-Fi, in addition clients can also select the access point that they choose to join.   Many times clients remain joined to a distant access point long after they have changed location or select the non-optimal connection medium.  Two common behaviors are the "Sticky Client" problem and the client's inability to decide when to roam from Wi-Fi to 3G/4G.   Cisco has developed the Optimized Roaming feature to assist clients to make the correct decision every time.

What is the Sticky Client problem?

The sticky client problem refers to the case in which a client associates to and/or remains associated with an AP that is far from the optimal AP candidate for association in a deployment. Clients implement varying association or roaming algorithms and steadily have been improving such algorithms. However there remain cases where clients are seen associated with an AP far away, when closer by APs are available. Such clients, while possibly suffering low data rates and airtime access, generate unwanted interference and clogging in larger areas of Wi-Fi deployment. The sticky client problem is more pronounced in the HD scenarios due to both higher likelihoods of non-optimal client to AP associations as well as larger impact of the resulting interference on network quality.

When should a client device roam from Wi-Fi to 3G/4G?

When clients that have the option to use both 802.11 and 3G/4G roam on an outbound path towards the edge of the Wi-Fi deployment, they have been observed to attempt to communicate using Wi-Fi for longer than their link quality allows. For example, a client on the edge of the Wi-Fi deployment may still be able to receive the 802.11 beacons, but suffer from a link quality that is too low to support the smallest available data rate. In such cases loss of connectivity is experienced until the client either moves far enough to disconnect from Wi-Fi and pass data through the cellular network, or move backwards toward the interior of the Wi-Fi deployment.

What is Optimized Roaming?

Cisco Optimized Roaming addresses sticky client challenge and allows client devices to roam from Wi-Fi to 3G/4G by pro-actively disconnecting the clients thus enabling the clients to move sooner to a stronger, better connection. It achieves this functionality by actively monitoring Data RSSI packets and enforcing client disassociation when the RSSI is lower than the set threshold. When Optimized Roaming is enabled, a client that sustains a signal strength (as measured at the AP) lower than -80 dBm (default value) will receive disassociation and achieves the following:

• Client receives best connectivity and maintains the quality of experience
• Improves overall performance of each Access Point cell by reducing airtime errors

When should Optimized Roaming be used?

Cisco Optimized Roaming should be used in any Cisco Wireless network where mobile clients are present.  This feature will improve the client's overall performance

Thanks for reading to learn more about Cisco's HDX technology. We hope you enjoyed learning a little more about our latest innovations. We're always working on improving the user experience for our customers. To learn more about the AP 3700 and 802.11ac, visit these webpages:

• Cisco Access Point 3700 with HDX 
• Cisco 802.11ac