Starting and Connecting srsENB to the Core Network

Pre-launch Checklist

Before starting the base station, it's vital to ensure its configuration is correct and that the underlying network environment is ready. Mismatches here are the most common source of issues.

1. Network Sanity Check

From our previous setup, a few system settings are crucial for communication between the core network and the base station components:

• IP Forwarding: Ensure IP forwarding is enabled on your Linux system.
• Firewall: Ensure your firewall is not blocking the traffic between srsENB and Open5GS. The S1 link uses the Stream Control Transmission Protocol (SCTP), which might be blocked by default. For a lab environment, a simple approach is to disable the firewall, as shown in the srsRAN 4G (with 5G SA/NSA) guide you've seen previously (sudo ufw disable).

2. The enb.conf Configuration

The srsENB needs to know where to find the MME. This is configured in the enb.conf file. Let's verify the three most important parameters:

• mme_addr: This must be set to the IP address of the machine running Open5GS, where the MME service is listening.
• gtp_bind_addr: This is for the GTP-U tunnel endpoint and should be the IP address of the machine running srsENB.
• s1c_bind_addr: This is for the S1-MME control plane interface and should also be the IP address of the machine running srsENB.

The following resource provides a very clear example of these changes.

Make sure your enb.conf file accurately reflects the IP addresses of your lab setup before proceeding.

Launching the srsENB

With the configuration confirmed, you can now start the eNodeB. You will execute the srsenb binary from within its build directory, passing your configuration file as an argument.

You'll need sudo privileges because srsenb needs to configure network interfaces and interact with the radio hardware (or the ZeroMQ virtual radio).

In your terminal, navigate to the srsenb directory and run:

cd /path/to/srsRAN_4G/build/srsenb
sudo ./srsenb enb.conf

If the launch is successful, you will see a series of log messages in your terminal as srsenb initializes the radio, sets the frequencies, and prepares to connect to the MME. The output should conclude with a line like ==== eNodeB started ==== without any fatal errors.

Verifying the S1 Connection

Launching the process is one thing; confirming the connection to the core network is the real goal. We'll use logs from both sides—the srsENB and the Open5GS MME—to get a complete picture.

Method 1: Check the Open5GS MME Log

This is the most definitive check. The MME must log that it has accepted a connection from your eNodeB. Just as you did in the last lesson, you can "follow" the MME log file to see this happen in real-time.

In a separate terminal, run:

sudo tail -f /var/log/open5gs/mme.log

or sudo journalctl -u open5gs-mmed -f

After starting srsenb, you should see new lines appear in the MME log almost immediately. Look for a message indicating that an eNB S1 connection has been accepted. The guide from the s5uishida GitHub repository shows exactly what this looks like:

11/09 01:24:56.954: [mme] INFO: eNB-S1 accepted[192.168.0.121]:48452 in s1_path module
11/09 01:24:56.954: [mme] INFO: [Added] Number of eNBs is now 1

(Log snippet from the "Run srsRAN 4G ZMQ RAN" resource)

Seeing this message confirms the core network has successfully established the S1 link with your base station.

Method 2: Check the srsENB Live Logs

The srsenb terminal output will also confirm the connection. The following video shows a practical demonstration of starting the eNodeB and then checking the logs for confirmation.

The combination of the confirmation log from the MME and the corresponding message from the srsenb gives you complete confidence that the connection is active.

Method 3: Protocol-Level Analysis (Optional)

Your background in network analysis with tools like Wireshark gives you an opportunity to go a layer deeper. The S1 link is established using the S1 Application Protocol (S1AP), which runs over SCTP. You can capture this handshake.

The key exchange to look for is:

1. S1 Setup Request: Sent from the eNodeB (srsenb) to the MME (open5gs-mmed).
2. S1 Setup Response: Sent from the MME back to the eNodeB.

This video demonstrates a Wireshark analysis of this exact process.

If you run tcpdump on the network interface connecting your srsENB and Open5GS machines while starting srsenb, you can capture this handshake yourself and analyze it in Wireshark.

Conclusion

Congratulations! You have successfully started your srsENB software base station and verified its connection to the Open5GS core network. You now have a live, end-to-end 4G network infrastructure running in your lab.

Key Takeaways:

• The connection between the eNodeB (RAN) and the MME (Core) is called the S1 link.
• Correctly configuring the IP addresses (mme_addr, s1c_bind_addr) in enb.conf is critical for the connection to be established.
• Verification is a two-sided process: you must check the logs on both the srsenb application and the open5gs-mmed service to confirm the eNB-S1 accepted message.
• The underlying protocol, S1AP, uses a S1 Setup Request/Response handshake to formalize the connection.

Preview of the Next Lesson:

Your network is now broadcasting and is ready for its first user. In the next lesson, you will initiate an attach with the srsUE software client and confirm its successful registration. This will complete the loop, connecting a subscriber device through your base station to your core network.