- Synchronous

NSURL *url = [NSURL URLWithString:kJsonURL];
NSURLRequest *req = [[NSURLRequest alloc] initWithURL:url
                                          cachePolicy:NSURLRequestUseProtocolCachePolicy
                                      timeoutInterval:10.0];
NSError *error = nil;
NSURLResponse *res = nil;
NSData *json = [NSURLConnection sendSynchronousRequest:req returningResponse:&res error:&error];
NSInteger statusCode = ((NSHTTPURLResponse *) res).statusCode;
// if success
if (json && statusCode == 200) {
    NSArray* data = [[NSJSONSerialization JSONObjectWithData:json options:NSJSONReadingAllowFragments error:nil] objectForKey:@"data"];

// if failure
} else {
    
}

Asynchronous

NSURL *url = [NSURL URLWithString:kJsonURL];
NSURLRequest *req = [[NSURLRequest alloc] initWithURL:url
                                          cachePolicy:NSURLRequestUseProtocolCachePolicy
                                      timeoutInterval:10.0];
[NSURLConnection sendAsynchronousRequest:req queue:[[NSOperationQueue alloc] init]
        completionHandler:^(NSURLResponse *res, NSData *json, NSError *error) {
            NSInteger statusCode = ((NSHTTPURLResponse *) res).statusCode;
            // if success
            if (json && statusCode == 200) {
                NSArray* data = [[NSJSONSerialization JSONObjectWithData:json options:NSJSONReadingAllowFragments error:nil] objectForKey:@"data"];

            // if failure
            } else {
                
            }
        }];