14 | 11 | 2018

Seamless Roaming

Seamless Roaming of mobile network is a must in our living when we are walking or traveling in a vehicle. It makes sure our phone call without any interruption, delay or disconnection.

It is the same when apply to Wi-Fi. While Wi-Fi plays more and more important role in our life, studying, doing business, a seamless roaming Wi-Fi is crucial to some applications such as voice-over-ip, real time broadcast and some industrial purpose which require least data buffering and at the same time high bandwidth.

Unlike our cell phones and mobile network, Wi-Fi network has no mechanism yet to actively full control of Wi-Fi client connectivity because Wi-Fi was designed, at the first place, for bridging wireless and wire network. Roaming was not concerned or even not included in IEEE 802.11 standards. So, 100% seamless roaming is impossible to a middle to large scale Wi-Fi network. How about 99%,98%....?

When disconnection is unavoidable, there are ways to minimize the time of disconnected so as to maintain certain kind of link quality which enough for fulfilling application's requirement. For example, a VoIP call. It is acceptable if the call didn't disconnect and conversation still clear and smooth.

Same SSID, Channel and Security is most common practice for homes and offices although this is not a seamless roaming solution. When roaming, the client is completely disconnect from original AP before connect to second AP. Few seconds timeout is expected which applications may hang up, disconnected.

WDS (wireless distribution system) is a concept to wirelessly bridge the network to a wire unreachable location and the AP at the same time provides Wi-Fi service to clients. Some product's WDS has a roaming ability because the AP will broadcast Wi-Fi service on behalf of the master AP so client likely to connected to same AP. WDS has a bandwidth issue because speed is cut into half when it bridge once. Two levels WDS turns out quarter bandwidth left. WDS is also not a standard so no guarantee.

One Radio
solution is a 100% seamless but limited to small area which likely as large as coverage by serval AccessPoints. The idea is pretty simple, a client won't disconnect if it didn't leave the coverage of the AccessPoint. Do this by extending the coverage by cable which radio frequency (RF) is running along the cable and end up by antenna. We call this DAS (Distributed Antenna System).


Tunneling
is now the only option capable for large implementation and has no bandwidth issue like WDS. When a client connected to an AP, the data frames are actually tunneled to a controller for processing. The connected AP only provide radio-related processing such as wireless connectivity and encryption. From network layer point of view, clients are always behind the controller, avoid re-broadcasting and learning activities. Hence save a lot of time and so likely be the same as seamless roaming.

Below table is a capture of controller that same device is registered to different interfaces (AP or more specific, radio). The registration helps controller quickly pick the strongest signal AP (radio) for the client and force disconnection to previous AP (radio).

Tunneling + One Radio

Benefits of combining tunneling and DAS solution are:

1) Minimize infrastructure scale (cabling and no. of AP, controller size)
2) Minimize roaming activities (no roaming under one radio), best performance
3) Highest system reliability (DAS can run decade without a single problem), lowest maintenance cost