Attaching with srsUE and Confirming Registration

1. Configuring the UE's Identity

Before a UE can join the network, the network must be able to identify and authenticate it. In a real-world scenario, this is handled by the SIM card. In our lab, we use a "virtual SIM" configured within the srsUE software. The credentials of this virtual SIM must perfectly match a subscriber profile in the core network's Home Subscriber Server (HSS), which you created in a previous module.

The configuration for srsUE's virtual SIM is stored in the ue.conf file.

To understand these parameters, let's review the official srsRAN documentation.

As the documentation states, the three key parameters are:

• IMSI (International Mobile Subscriber Identity): The unique ID for the subscriber.
• K: The secret key shared between the SIM and the HSS.
• OPc (Operator Code): A value derived from the K and a network-wide secret (OP), used in the authentication algorithm.

These are the exact same values you entered into the Open5GS web interface when you created your test subscriber.

Now, let's look at a practical example of how these values are set in the ue.conf file.

Before proceeding, take a moment to verify that your ue.conf file contains the correct IMSI, K, and OPc for the subscriber you provisioned in Open5GS. A mismatch here is the most common reason for an attach failure.

2. Launching the srsUE Client

With the configuration verified, you are ready to start the UE. You will run the srsue executable from its build directory, providing your configuration file as an argument.

Since srsUE needs to create a virtual network interface (a tun device) upon successful connection, it requires root privileges.

In a new terminal, navigate to the srsue build directory and execute the following command:

# Navigate to the srsue build directory
cd /path/to/srsRAN_4G/build/srsue




# Launch the srsUE client with your configuration file
sudo ./srsue ue.conf

The UE will now start, initialize its virtual radio, and begin scanning for a cell to connect to—your srsENB.

3. Confirming Successful Registration

A successful attach involves a complex sequence of messages between the UE, eNB, and MME. We can confirm success by observing the logs from each component.

Viewpoint 1: The UE's Perspective

The most immediate feedback comes from the srsue terminal itself. If the attach is successful, the log output will show a clear progression.

Seeing the "Network attach successful" message and an assigned IP address is your primary confirmation that the procedure worked from the UE's point of view.

Viewpoint 2: The Core Network's Perspective

Next, let's verify that the MME in your Open5GS core network has processed the registration. You can do this by tailing the MME log file in another terminal.

sudo journalctl -u open5gs-mmed -f



# Or if logging to file:
# sudo tail -f /var/log/open5gs/mme.log

The following video demonstrates launching the UE and then checking the MME logs for the corresponding activity.

The MME log provides the authoritative record that the core network has accepted the UE.

Viewpoint 3: The Protocol Perspective (Wireshark)

Given your background with Wireshark, you can appreciate that this registration process isn't magic; it's a standardized protocol exchange. The main protocols involved are:

• RRC (Radio Resource Control): Manages the radio link between the UE and eNB.
• S1AP (S1 Application Protocol): Manages the link between the eNB and MME.
• NAS (Non-Access Stratum): Carries messages directly between the UE and MME, handling session management and mobility. The Attach Request is a NAS message.

The same video provides an excellent walkthrough of a Wireshark capture showing these messages in action.

What if It Fails?

If the srsue log does not show "Network attach successful", the most likely culprit is a configuration mismatch.

• Check Credentials: The srsRAN documentation correctly points out that misconfigured network credentials are a primary cause of failure. Meticulously re-verify that the imsi, k, and opc in ue.conf are identical to what is in your Open5GS subscriber database.
• Check Connectivity: Ensure your srsenb is still running and connected to the MME. Check the logs for all three components for any error messages.

Conclusion

Excellent work! You have successfully configured and launched the srsUE client, and it has registered with your private 4G network. You now have a complete, end-to-end mobile network running in your lab, from the core to the edge device.

Key Takeaways:

• The srsUE relies on the ue.conf file for its virtual SIM card credentials (IMSI, K, OPc).
• A successful attach requires a perfect match between the UE's credentials and the subscriber profile in the HSS (Open5GS).
• Verification is a multi-step process: you must confirm success in the srsue logs, see the "Attach complete" message in the MME logs, and (optionally) observe the NAS/S1AP message flow in a packet capture.
• A successful attach results in the UE being assigned an IP address by the core network.

Preview of the Next Lesson:

The UE is attached and has an IP address, but this doesn't guarantee it can send and receive data. In the next lesson, we will verify the UE has established a data bearer and can pass IP traffic through the test network. This will be the final step in confirming your lab is fully operational and ready for security experiments.