Depending on which EFB app you use, you may need to start AvTraffic before your EFB when you are using a GDL90 device to avoid the problems described below.
The GDL90 protocol was developed by Garmin as a standard means for sending traffic information between devices.
It is supported by most EFB apps and AvTraffic.
AvTraffic uses it to receive traffic from a GDL90 device and also to send traffic to your EFB.
There are plenty of ADS-B receivers to choose from and most of them use GDL90. If you are reading this you probably already have one.
However, some receivers such as the Garmin GDL52 and ForeFlight Sentry do not use the GDL90 protocol.
Almost all of them expose a WiFi hotspot that you connect your tablet or phone to before the device can send messages.
Once you are connected to the device's WiFi, your EFB opens a 'port' to receive the messages. GDL90 uses port 4000.
Some EFBs open port 4000 exclusively and others allow shared access. When AvTraffic starts it also opens port 4000 but specifies that it can be shared.
NB The following only applies to EFBs that open port 4000 for exclusive access.
If you start AvTraffic before your EFB, access to the port succeeds for both apps.
If you start AvTraffic after your EFB, AvTraffic's access to the port is denied if your EFB already has exclusive access. A warning message is shown to that effect on the Status View. When this happens AvTraffic will not receive traffic from the device. However it will still receive traffic from its other sources and forward it as shown on the Status View.
If you see this message, terminate your EFB and restart it so that the connection is freed (see below).
The best way to make sure AvTraffic is started first is to use the auto start feature and always start your EFB from AvTraffic.
To terminate an app, swipe it up and remove it from the task view. The task view is opened by double clicking the home button on older Apple devices or swiping up from the bottom of the screen on newer devices.
Simply closing an app doesn't necessarily terminate it. If an EFB has exclusive access to port 4000 it won't be freed by closing it. You need to terminate it as described above.
Unfortunately this is a limitation of the network protocols being used by these devices. The only way to avoid it is for all parties to share access to the port.
You could contact your EFB app provider and request that they change their app to share the port instead of making it exclusive. This is a very simple change and there should be no objection. AvTraffic support will be happy to provide details on request.