Personal Project/Research - Mobile Devices

A little about me, I m a C/C++/C# fanatic and i do most of my coding in the latter two. I have also done a little bit of Java and VB.

Recently i have come up with a thought of desiging an app which basicaly communicates with Smart Phones.

Currently the Smartphones are dominating the mobile phone market and every 1/3 people carry a smart phone.

I have come up with a thought of designing a an Application with a Database linked. The Database could contain basically anything and eveyrthng (thats not a problem). The problem is that, i want this application to communicate with Smartphones (or any portable devices like tablets or iPads), either via WiFi or maybe a 2.5G - 4G Network.

So my question is, if i were to develop such an application will it be neccesarry to develop Mobile apps for the different types of Smartphones OR is it possible to achieve the above without a Mobile app or knowing the number of the phones?

what would be the most effective and cost efficient way to achieve it?

When i say cost effective, i m mean on the development side of things.

Someone please help me on this. This is not a homework question so there is no due date.

iOS, Andriod and Windows Phone 8 have distinct API's and dinstinct deployment platforms. If you want to support more than one, you'll need to write specific versions for each platform.
You might be able to get your thing done with a pure web based UI, but that will restrict what you can do on the device (and how your device gets 'installed' on the device).

Typically speaking. your server doesn't talk to phones (you can't push to phones)... Phones will (periodically) connect to your server if they're set up that way. Note that internet bandwidth may be limited, restricted, throttled, periodically unavailable and/or (very) costly.

Will it be possible to the same without the use of internet. Say by plugging a Wireless Router (WiFi) on a LAN port and configuring the mobiles to the Router

if you manually configure the phones to have an open wifi connection and connect to the server and keep the connection open. then yes, probably.

it's still the phones that will need to have the app running at all time, you can't just "push" stuff to any random Phone that doesn't have your app already running (other than methods that will be marked as hacks/malware/virus/etc).

What if a login token is given to whoever is interested in using this app.

When their Wifi is turned on and they are in the specific range, then?