UE no longer connects with latest srsRAN 5G (Benetel RU v1.0.4) — works on May build

[ismagom]

Discussed in #1374

^{Originally posted by ujjwalpawar September 29, 2025}
Hi team,

After updating to the latest srsRAN_5G (current main as of this week), our UE no longer connects. The exact same setup and config work reliably on a build from May.

RU / split: Benetel RU v1.0.4 (O-RAN 7.2)
DU/CU: srsRAN 5G (RU OFH + gnb), standalone core
OS: Ubuntu 22.04
Bandwidth / band: n77, 100 MHz, SCS 30 kHz
Antennas: 4x4 MIMO
Fronthaul: DPDK (EAL args below)
UE can see the cell but never does the RACH successfully (no uplink activity at RU side)
On the latest build, RU-side counters (e.g., RX_TOTAL / RX_ON_TIME) are consistently lower than on the May build under configuration

UE logs

RU logs with latest version

RU logs with may version

Configuration (works on May build, fails on latest)

cu_cp:
  amf:
    addr: 172.30.129.35
    port: 38412
    bind_addr: 10.0.0.3
    supported_tracking_areas:
      - tac: 1
        plmn_list:
          - plmn: "99999"
            tai_slice_support_list:
              - sst: 1

ru_ofh:
  t1a_max_cp_dl: 535
  t1a_min_cp_dl: 286
  t1a_max_cp_ul: 535
  t1a_min_cp_ul: 286
  t1a_max_up: 390
  t1a_min_up: 80
  ta4_max: 500
  ta4_min: 25
  is_prach_cp_enabled: false
  compr_method_ul: bfp
  compr_bitwidth_ul: 9
  compr_method_dl: bfp
  compr_bitwidth_dl: 9
  compr_method_prach: bfp
  compr_bitwidth_prach: 9
  enable_ul_static_compr_hdr: true
  enable_dl_static_compr_hdr: true
  iq_scaling: 5.5
  cells:
    - network_interface: 0000:86:01.0
      ru_mac_addr: 70:b3:d5:e1:5f:e9
      du_mac_addr: 00:11:22:33:77:88
      vlan_tag_cp: 11
      vlan_tag_up: 11
      prach_port_id: [4,5,6,7]
      dl_port_id: [0, 1,2,3]
      ul_port_id: [0, 1,2,3]

cell_cfg:
  dl_arfcn: 660000
  band: 77
  channel_bandwidth_MHz: 100
  common_scs: 30
  plmn: "99999"
  tac: 1
  pci: 11
  nof_antennas_dl: 4
  nof_antennas_ul: 4
  prach:
    prach_config_index: 7
    prach_root_sequence_index: 11
    zero_correlation_zone: 0
    prach_frequency_start: 0
    preamble_trans_max: 4
  ssb:
    ssb_block_power_dbm: 0
  tdd_ul_dl_cfg:
    dl_ul_tx_period: 10
    nof_dl_slots: 7
    nof_dl_symbols: 6
    nof_ul_slots: 2
    nof_ul_symbols: 4

hal:
  eal_args: "--lcores (0-1)@(2-55) -a 0000:86:01.0"

expert_phy:
  allow_request_on_empty_uplink_slot: false

log:
  filename: stdout
  all_level: warning
  config_level: debug

pcap:
  mac_enable: false
  mac_filename: /tmp/gnb_mac.pcap
  ngap_enable: false
  ngap_filename: /tmp/gnb_ngap.pcap
  
  
</div>

[sauka]

Hi @ujjwalpawar,

I see you are using the long format PRACH. Could you check the lf_prach_slot_id parameter in Benetel's ru_config file? It should be set to 0. In the old RU FW releases this parameter didn't exist but the behaviour of the RU was corresponding to lf_prach_slot_id=1. In the new FW releases the default value is lf_prach_slot_id=0, and latest srsRAN expects this configuration too.

[ujjwalpawar]

@sauka Thanks you for your attention. But as i mentioned I am using RU firmware 1.0.4. Which doesnt have lf_prach_slot_id parameter.
Here is my RU config file.

# Config file for Benetel RANx50 radio units
# Please place this file in /etc

# MIMO mode - the radio can operate in 1_3, 2_4, 1_2_3_4_4x2 or 1_2_3_4_4x4 mode
# Variable name : mimo_mode
# Valid options : 1_3
#                 2_4
#                 1_2_3_4_4x2
#                 1_2_3_4_4x4
mimo_mode=1_2_3_4_4x4

# FPGA downlink scaling in steps of 6 dB (0, 6, 12 and 18 can be set)
# Variable name : downlink_scaling
# Valid options : 0
#                 6
#                 12
#                 18
downlink_scaling=6

# PRACH format
# Variable name : prach_format
# Valid options : short
#                 long
prach_format=long

# Short form PRACH compression settings (note uncompressed mode is not supported when RU is in 4x2 or 4x4 mode)
# Variable name : compression
# Valid options : static_uncompressed
#                 dynamic_uncompressed
#                 static_compressed
#                 dynamic_compressed
compression=static_compressed

# Long form PRACH compression settings
# Variable name : lf_prach_compression_enable
# Valid options : true
#                 false
lf_prach_compression_enable=true

# C-Plane per-symbol workaround
# Variable name : cplane_per_symbol_workaround
# Valid options : enabled
#                 disabled
cplane_per_symbol_workaround=disabled

# CUPLANE dl couplling sectionID
# Variable name : cuplane_dl_coupling_sectionI
# Valid options : enabled
#                 disabled
cuplane_dl_coupling_sectionID=disabled

# FlexRAN PRACH workaround
# Variable name : flexran_prach_workaround
# Valid options : enabled
#                 disabled
flexran_prach_workaround=disabled

#DL_UL special slot tunning
#Variable name : dl_ul_tuning_special_slot
#Valid options : 0x0
#                0xfd00000
dl_ul_tuning_special_slot=0xfd00000

[ujjwalpawar]

Is there any configuration parameter i can try to fix this. Or any change to the code?

[sauka]

There is no config option to force an old behaviour. Could you upgrade the FW of your Betenel RU?

[ujjwalpawar]

Only if it is the last option. Can you point me to the code where this is handled?

[sauka]

I would recommend to upgrade the FW as Benetel fixed a few important issues since the version 1.0.4. If it it is not an option, you could try reverting this commit c427ebf7. Then you will be able to use enable_lf_prach_slot_autoderivation param in ru_ofh section of the config file. Setting it to true will force an old behaviour. However it is non standard - that is why we dropped it.

[ujjwalpawar]

Using short prach format won't create this issue i suppose. But never got short prach to work with benetel I have tried this before.

  prach:
    prach_config_index: 159
    prach_root_sequence_index: 1
    zero_correlation_zone: 0
    prach_frequency_start: 0
    preamble_trans_max: 4

Do have any suggestion for short prach ?

[sauka]

Using short prach format won't create this issue i suppose. But never got short prach to work with benetel I have tried this before.

  prach:
    prach_config_index: 159
    prach_root_sequence_index: 1
    zero_correlation_zone: 0
    prach_frequency_start: 0
    preamble_trans_max: 4

Do have any suggestion for short prach ?

I don't have the complete ru_config on hand for v1.0.4. For the latest Benetel FW release you can find it here: https://docs.srsran.com/projects/project/en/latest/tutorials/source/oranRU/source/rus/r550.html , the ru config file in the tutorial specifically targets the short PRACH.

Basically you need to set prach_format=short and compression=dynamic_compressed in the RU.

On the gNB side you need these OFH settings (I am not including timing window and other params):

ru_ofh:
  is_prach_cp_enabled: true
  compr_method_ul: bfp
  compr_bitwidth_ul: 9
  compr_method_dl: bfp
  compr_bitwidth_dl: 9
  compr_method_prach: bfp
  compr_bitwidth_prach: 9
  enable_ul_static_compr_hdr: false
  enable_dl_static_compr_hdr: false

And the PRACH parameters in the cell config:

cell_cfg:
  prach:
    prach_config_index: 159
    prach_root_sequence_index: 1
    zero_correlation_zone: 0

Let me know please if you have any further questions

[ujjwalpawar]

Sadly, it is not working with short prach in our setup.
I am using latest build. Here is my config. I don't see any error/warning. Do you see anything wrong with the config?

ru_config

mimo_mode=1_2_3_4_4x4

downlink_scaling=0

prach_format=short

compression=dynamic_compressed

lf_prach_compression_enable=false

cplane_per_symbol_workaround=disabled

cuplane_dl_coupling_sectionID=disabled

flexran_prach_workaround=disabled

dl_ul_tuning_special_slot=0xfd00000

gnb_config

  is_prach_cp_enabled: true
  compr_method_ul: bfp
  compr_bitwidth_ul: 9
  compr_method_dl: bfp
  compr_bitwidth_dl: 9
  compr_method_prach: bfp
  compr_bitwidth_prach: 9
  enable_ul_static_compr_hdr: false
  enable_dl_static_compr_hdr: false
  iq_scaling: 20

cell_cfg:
  dl_arfcn: 660000
  band: 77
  channel_bandwidth_MHz: 100
  common_scs: 30
  plmn: "99999"
  tac: 1
  pci: 11
  nof_antennas_dl: 4
  nof_antennas_ul: 4
  prach:
    prach_config_index: 152
    prach_root_sequence_index: 1
    zero_correlation_zone: 0
    #prach_frequency_start: 0
    #preamble_trans_max: 4
#    ports: 1
  ssb:
    ssb_block_power_dbm: 0
  tdd_ul_dl_cfg:
    dl_ul_tx_period: 10
    nof_dl_slots: 7
    nof_dl_symbols: 6
    nof_ul_slots: 2
    nof_ul_symbols: 4

[sauka]

Could you post here the exact version of the FW you have? You can do it with cat /etc/benetel-rootfs-version.
The RU config looks correct, except for the last parameter. Benetel User Guide for v1.0.4 says the default value is dl_tuning_special_slot=0x13b6, but it may depend on whether it is R550/R650 and the exact hash in the version.

In the gNB config please stick to PRACH index 159, this one was tested a lot and confirmed to work with R550/R650.

From the initial description it seems the RU receives the traffic on-time, but it is worth checking the output of oru_vlan_mac_info - its output should match the gNB configuration.

Please also check the output of oru_dpd_stats - the reported dpdErrorCode should be 0. If it is not 0, please lower the iq_scaling parameter in the gNB config to 10 or lower.

The KPIs you posted show 0 TX packets, but this doesn't necessarily mean the RU didn't send any PRACH U-Plane, because PRACH is not included in this stats. I'd advise to run the gNB without DPDK and capture traffic on the NIC interface. You should see there the PRACH C-Plane sent by the gNB, and the PRACH U-Plane sent by the RU in response to C-Plane. If it is not the case, there is still something wrong with the configs. However if you see those packets but the UE can't attach, please set the ofh_level: debug in the gNB log configuration and share the log file here.

[ujjwalpawar]

oru_vlan_mac_info

PCAP with non dpdk setup (C-plane and U-plane messages are getting exchanged)

gnb.log with ofh_level in debug
gnb.tar.gz

[sauka]

From the OFH communication point of view everything looks good, the PRACH packets are received, decompressed and forwarded to the PHY and MAC with no warnings. Could you additionally set phy_level and mac_level to debug in the log configuration and attach the new log? Also share the pcap file you previously captured.

oru_vlan_mac_info output matches the gNB config, all good here. Please provide other information requested in my last message.

What UE are you using?