Multiple Async calls in iOS

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iOS

I want to call a web service to upload some image data to the server. I have to send the data 5 times to the server. This piece of code is written in a function which is called after 10 seconds duration by a timer. Now the problem is that the response of Web service might be late and second call to web service might initiate. I want to keep them in queue so that when one finishes other is called. I think I am not going in right way. I just want to maintain a queue in which I can call the web service multiple times and make different async calls to the server. Basically how I could call multiple async tasks.

Any help will be appreciated.

dispatch_queue_t myQueue;
myQueue = dispatch_queue_create("My Queue",NULL);
 dispatch_async(myQueue, ^{
                               [self uploadDataToServer];
                                dispatch_async(dispatch_get_main_queue(), ^{
                                    // Update the UI

                                });
                            });