Mobile applications should work regardless of which type of wireless interface is used, and should be able to conceal unstable connections from the user to improve user experience. Therefore, network testing is important when developing mobile applications, but it is a challenge to reproduce network conditions when using real cellular networks since the test engineer has no control over the quality of the cellular network. Existing software tools can restrict bandwidth and add latency to the connection, but these tools do not accurately emulate cellular networks. This thesis proposes a system where it is possible to shape the network traffic for connected devices to mimic the network patterns of a real cellular connection when running on a WiFi connection. The design presented in this thesis is intended for testing mobile applications under diverse 3G connection parameters, such as latency, bandwidth and other characteristics. This thesis was conducted at Spotify, a company that provides a music streaming service which is a frequent user of network data traffic. The 3G emulator was evaluated using the Spotify Android application by measuring the correlation between packet traces from a real 3G connection and the 3G emulator. This correlation was compared to the correlation between packet traces from a real 3G connection and the current network emulator at Spotify. The evaluation shows that the proposed 3G emulator outperforms the current network emulator when performing tests on the Spotify application for Android. By using this emulator, we expect the network testing to become more effective as any 3G condition can be tested with repeatable results.