One of the things a well configured GSM, UMTS or LTE network does is to give the mobile device clear and precise instructions of when it should select another cell or even performs a handover to a better suited cell during an active communication session. There are plenty of standardized parameters and algorithms based on the signal strength of the current cell, the neighboring cells, offsets before a lower speed technology network is selected, interference, etc. etc. When a mobile ends up on the Wi-Fi layer, this kind of sophistication abruptly ends as I recently experienced.
On the Wi-Fi layer it's completely up to the device to decide when it is time to reselect from one Wi-Fi access point to something else. The device I played around with clung to the Wi-Fi access point right down to the last dbm where communication was hardly possible anymore, despite an excellent other Wi-Fi network with a different known SSID in range. I manually had to reselect to the other Wi-Fi to continue working. Also, reselection from Wi-Fi to the cellular layer is probably also only done once connectivity with the Wi-Fi network is lost, which often happens much later than moving out of the "usable range" of the network where data rates are still acceptable.
Sure, Wi-Fi was never designed to include that kind of functionality and 99% of home users would likely be unable to make the required settings. Also, the break incurred in terms of IP connectivity and a different pricing between cheap home Wi-Fi and a more expensive cellular layer makes the decision to move from Wi-Fi to cellular as late as possible understandable. Nevertheless from a usability point of view it's far from ideal. In other words, the user has to make sure the Wi-Fi signal is strong enough everywhere in the house or appartment so devices never leave the usable range.
Annoying isn’t it. These boundary conditions are outright hostile to the average user.
A mix of the WiFi Alliance’s Passpoint and 3GPP’s ANDSF should help in the medium to longer term.
Shorter term, Apple’s new WiFi Plus Cellular in the iOS6 beta looks useful.
For Android it appears some equivalent form of connectivity manager would need to be developed by each OEM, as thus far it doesn’t seem to be forthcoming from Google
The situation with 2 or more Wi-Fi access points is similar to 2 or more BTSs without neighbor relations. In this case the MS will stick to the serving BTS until the call is dropped, because of coverage for instance. To perform a handover a controller that analyses measurements and neighbor relationships is needed.
I’ve heard of Wi-Fi controllers that do the aforementioned and make the handoffs between APs possible, but I’ve never encountered them in my life and have no such experience. I also don’t know whether those controllers can only automatically change an access point (like idle mode in GSM) or they can perform a handover (downloading a file while moving from one AP to another). There is a mention on the internet about 802.11r (roaming) which is an amendment to the 802.11 that is aimed to make outage of a connection less than 50 msec, so that VoWiFi calls are possible while moving. However, this feature seems to be present on enterprise APs, which are usually quite expensive and are used with a controller. Wi-Fi controllers are quite expensive and will set you back in the region of 500$ or more http://www.itworld.com/answers/topic/mobile-wireless/question/what-wi-fi-controller-and-do-i-need-one. The article also mentions the “software controller”, and if anybody has some practical experience with this stuff or with enterprise hardware controllers, it would be fascinating to hear – the topic itself is very interesting.
Until those controllers are cheap, you have to “be a controller” and switch from one AP to another manually. It’s not the worst idea, considering the pretty affordable price of your own Wi-Fi-BTS. Wi-Fi controllers aren’t cheap, but GSM or UMTS networks that can perform handovers aren’t cheap as well.
On the other hand, if you have several know SSIDs that you have already registered to and you can switch between them manually, why this “switching” can’t be done automatically? Some kind of software or OS facility could compare the received level from the known SSIDs and based on some algorithm could switch between them, at least in “idle mode”.
Speaking of weak Wi-Fi, I’ve come across this website http://www.freeantennas.com/projects/template2/index.html that offers a pretty interesting and easy way to boost the coverage of a Wi-Fi access point. I’ve tried it myself with an A4 sheet and a piece of foil from a chocolate bar. The result was 9 dB boost in the direction of the main lobe and -12 dB in the direction of the opposite side lobe. A very nice way to increase Wi-Fi coverage, considering easiness and cheapness of such solution. And don’t forget about a bonus – a chocolate bar that comes wrapped into the antenna reflector.