Following code will help you to detect the change in network availability.
Download the reachablity class from here :

https://developer.apple.com/library/ios/samplecode/Reachability/Introduction/Intro.html

#import "Reachability.h"

Then put following code in your App delegate didFinishLaunchingWithOptions method

    - (BOOL)application:(UIApplication *)application didFinishLaunchingWithOptions:(NSDictionary *)launchOptions
    { 
    Reachability *reachability = [Reachability reachabilityForInternetConnection];
     [reachability startNotifier];
NetworkStatus status = [reachability currentReachabilityStatus];

if(status == NotReachable) 
{

    //No internet
}
else if (status == ReachableViaWiFi)
{
    //WiFi
}
else if (status == ReachableViaWWAN) 
{
    //3G
}
    [[NSNotificationCenter defaultCenter] postNotificationName:notifcationstr object:status]

    }

Set Notification Observer

[[NSNotificationCenter defaultCenter] addObserver:self selector:@selector(NetworkreachablityChanging:) name:kReachabilityChangedNotification object:nil];


- (void)NetworkreachablityChanging:(NSNotification *)notification {
    Reachability *reachability = (Reachability *)[notification object];

    if ([reachability isReachable]) {
        NSLog(@"online");
    } else {
        NSLog(@"offline");
    }
}