srsRAN experiments with SDR

• Setting up Debian networking for srsRAN depends on which component you're running (srsRAN Project for 5G, or srsRAN 4G)
• The core networking requirements are similar. Here's a comprehensive guide:

Network Interfaces & Configuration

1. Install Required Packages

sudo apt update
sudo apt install iproute2 net-tools iptables tun-tools bridge-utils

2. TUN/TAP Interface (for EPC/Core)

srsRAN uses a TUN interface (srs_spgw_sgi or ogstun) to route UE traffic:

# Create TUN interface (usually done automatically by srsEPC or Open5GS)
sudo ip tuntap add name srs_spgw_sgi mode tun
sudo ip addr add 45.45.0.1/24 dev srs_spgw_sgi
sudo ip link set srs_spgw_sgi up

3. Enable IP Forwarding

# Temporary
sudo sysctl -w net.ipv4.ip_forward=1

# Permanent — add to /etc/sysctl.conf or /etc/sysctl.d/99-srsran.conf:
net.ipv4.ip_forward = 1
sudo sysctl -p

4. NAT / Masquerading

So UEs can reach the internet through your host:

# Replace eth0 with your actual uplink interface
sudo iptables -t nat -A POSTROUTING -o eth0 -j MASQUERADE
sudo iptables -A FORWARD -i srs_spgw_sgi -j ACCEPT
sudo iptables -A FORWARD -o srs_spgw_sgi -j ACCEPT

To make iptables rules persist across reboots:

sudo apt install iptables-persistent
sudo netfilter-persistent save

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Interface Between Components (S1/E1/F1/N2/N3)

srsRAN 4G (enb + epc on same host)

Communication happens over loopback by default — no special config needed. Just confirm:

# epc.conf
[spgw]
gtpu_bind_addr = 127.0.0.1

# enb.conf
[epc]
mme_addr = 127.0.0.1

srsRAN Project 5G (gNB + Open5GS/free5GC on same or different host)

# gnb.yml — AMF N2 interface
amf:
  addr: 127.0.0.5       # AMF address
  bind_addr: 127.0.0.1  # gNB local address

# N3 (user plane GTP-U)
gtpu:
  bind_addr: 127.0.0.1

If running on separate machines, replace loopback addresses with actual interface IPs and ensure the firewall allows:

• N2: SCTP port 38412
• N3: UDP port 2152 (GTP-U)
• S1-MME: SCTP port 36412

# Open SCTP/UDP ports if using ufw
sudo ufw allow 38412/sctp
sudo ufw allow 2152/udp

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Real-Time & Performance Tuning

srsRAN is latency-sensitive — network stack tuning helps:

# Increase socket buffer sizes in /etc/sysctl.d/99-srsran.conf
net.core.rmem_max = 25165824
net.core.wmem_max = 25165824
net.core.rmem_default = 25165824
net.core.wmem_default = 25165824

sudo sysctl -p /etc/sysctl.d/99-srsran.conf

Also disable offloading on your uplink NIC to reduce latency variance:

sudo ethtool -K eth0 gso off gro off tso off

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Verifying Connectivity

# Check TUN is up
ip link show srs_spgw_sgi

# Watch GTP-U tunnel traffic
sudo tcpdump -i srs_spgw_sgi

# Verify forwarding rules
sudo iptables -t nat -L -v -n
sudo iptables -L FORWARD -v -n

# Check SCTP support (needed for N2/S1)
modprobe sctp
lsmod | grep sctp

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Common Issues

Problem	Fix
UE gets IP but no internet	Check NAT masquerade rule and IP forwarding
gNB can't reach AMF	Check SCTP is loaded (modprobe sctp)
High latency / packet loss	Tune socket buffers; disable NIC offloading
TUN device missing	Run srsEPC/Open5GS as root or with CAP_NET_ADMIN