FAQs in O-RAN FH Testing & Integration

Section 1: C-Plane (Control Plane)

1. What is the primary role of the C-Plane in O-RAN fronthaul architecture?

   To carry control information between O-DU and O-RU such as scheduling, beamforming, and uplink/downlink symbol configuration.

Scheduling Info

Tells the O-RU when to transmit or receive specific IQ data. Includes frame ID, subframe ID, and symbol timing. Enables precise time-domain mapping for TDD systems.

Resource Grid Mapping

Specifies how IQ data maps to frequency-time grid (via subcarrier offset and number of subcarriers). Ensures frequency-domain alignment in FFT/IFFT processing.

Beamforming Instructions

Contains beam IDs, antenna port mapping, and possibly precoding matrix info. Tells the RU how to direct energy in space using digital or hybrid beamforming.

Uplink/Downlink Configuration

Indicates symbol-level direction in TDD (DL, UL, guard symbols). Allows the RU to switch roles dynamically on a per-slot basis.

Section Management

The C-Plane divides IQ data into logical “sections” with their own metadata (Section ID, Start Symbol ID, etc.). Each section maps to a specific antenna port or beam.

2. What protocol is used in the C-Plane over fronthaul?

   
   

   Enhanced CPRI (eCPRI) is typically used to transport control messages over Ethernet. In O-RAN WG4 architecture, the C-Plane messages that instruct the O-RU how to map, process, and transmit the IQ data are encapsulated in eCPRI messages, specifically:

·       eCPRI Message Type 2: C-Plane Data Message

Each eCPRI packet includes:

·       eCPRI Header (with message type, sequence number, payload size)

·       Radio Application Header (specific to O-RAN, includes Section ID, Frame ID, Subcarrier Offset, etc.)

·       Control Payload (carries configuration for beamforming, timing, etc.)

3.      What is Section Type 0 in O-RAN C-Plane?

Indicates to O-RU that certain Resource Blocks or symbols will not be used (idle periods, guard periods). Likewise, there are no associated U-Plane messages containing IQ data for this Section Type. The purpose is to inform the O-RU that transmissions may be halted during the specified idle interval for e.g. power-savings or to provide an interval for calibration.

4. What are some of the key parameters in a C-Plane message?

   
   

   Section ID, Start Symbol ID, Subcarrier Offset, Number of Subcarriers, Frame ID, Subframe ID, Symbol ID.

Field	Description
Section ID	Logical identifier for this section (to match with U-Plane eAxC)
Start Symbol ID	Indicates the symbol number in the subframe where transmission should start
Subcarrier Offset	Starting point in the frequency grid (e.g., subcarrier 200)
Number of Subcarriers	How many subcarriers to span from the offset
RE Mask	Optional bitmask indicating which resource elements to activate
Time Advance	Optional timing shift for synchronization
CP Length	Cyclic prefix length
IQ Width	Width of IQ samples (e.g., 15 bits)

5. Explain how symbol-level granularity is achieved using C-Plane messages.

   
   

   By specifying Symbol ID, Start PRB, and Num PRBs in control sections, symbol-level scheduling can be achieved.

6. What is Section Type 1 and how is it different from Type 0?

   
   

   Section Type 1 adds dynamic beamforming, frequency-domain parameters, and more granularity. Most DL/UL radio channels use Section Type 1.

7. How does the C-Plane support beamforming?

   
   

   Beam IDs and precoding matrix indicators (PMIs) are used in Section Type 1 and 3 to enable beamforming. It uses Section Extension commands in C-plane to support Beamforming.

ExtType=1: Beamforming Weights Extension Type

bfwCompHdr - the compression method and IQ bit width for the beamforming weights

bfwCompParam - applies to the compression method specified by the associated bfwCompMeth, For ex: for BFP, bfwCompMeth=0001b

bfwI (for TRX 0) - In-phase beamforming weight value

bfwQ (for TRX0) - Quadrature beamforming weight value.

remaining beamforming weights bfwI and bfwQ up to K TRXs

ExtType=2: Beamforming Attributes Extension Type

This section extension applies only to section types 1 and 3.

bfaCompHdr - the bit width for the beamforming attributes extension parameters

bfAzPt - the azimuth beamforming pointing angle in degrees.

bfZePt - the zenith beamforming pointing angle in degrees

bfAz3dd - the azimuth beamforming beamwidth in degrees

bfZe3dd - the zenith beamforming beamwidth in degrees

8. What is meant by Control Packet Fragmentation in the C-Plane?

   
   

   Breaking large control messages into multiple Ethernet frames; crucial for symbol-level tight latency.

9. How are eAxC IDs used in the C-Plane context?

   
   

   Used to identify logical antenna carriers between O-DU and O-RU. It identifies the specific data flow associated with each C-Plane (ecpriRtcid) or U-Plane (ecpriPcid) message.

One eAxC identifier (eAxC ID) comprises of,

·       DU_Port_ID: Used to differentiate processing units at O-DU (e.g., different baseband cards). It is expected the O-DU will assign these bits, and the O-RU will attach the same value to the UL U-Plane messages carrying the same sectionId data.

·       BandSector_ID: Aggregated cell identifier (distinguishes bands and sectors supported by the O-RU).

·       CC_ID: distinguishes Carrier Components supported by the O-RU.

·       RU_Port_ID: designates logical flows such as data layers or spatial streams, and logical flows such as separate 20 numerologies (e.g. PRACH)

10. How does the C-Plane achieve tight latency constraints?

    
    

    Through timestamping, synchronized clocks (S-Plane), and real-time prioritization in transport networks.

11. Explain the role of the U-plane and C-plane time alignment.

    
    

    C-Plane must reach O-RU before U-Plane data; synchronization is managed to ensure real-time operation.

12. What challenges occur in multi-vendor DU-RU C-Plane integration?

    
    

    eCPRI version mismatches, Section TLV parsing errors, latency budget violations.

13. What are common defects in C-Plane during DU-RU integration testing?

    Symbol ID mismatch, missed DL/UL slots, section drop errors, invalid beam IDs.

14. How do you simulate and validate C-Plane in lab setups?

    Using DU simulators, protocol analyzers, eCPRI packet injectors, and loopback configurations.

15. What test tools are used for C-Plane validation?

    
    

    Keysight/Viavi test solutions, Wireshark with eCPRI dissector, Intel FlexRAN.

16. What is the role of VLAN tagging in C-Plane traffic isolation?

    
    

    Used to differentiate U/C/S/M traffic on the same physical link.

17. Can a single Ethernet link carry both U and C plane traffic?

    
    

    Yes, using VLAN tags and prioritization (IEEE 802.1p) for deterministic latency.

18. How is scheduling info handled in C-Plane packets?

    
    

    Section headers include parameters for timing, beam ID, and PRB allocations.

19. How do you debug dropped C-Plane sections at the RU?

    
    

    Correlate timestamps, verify Section ID decoding, monitor eCPRI packet counters.

20. What is the function of the section extension field in Section Type 1?

    
    

    Allows additional fields like beam index, CSI-RS mapping, and special scheduling.

21. What tools can decode C-Plane packets?

    
    

    Customized Wireshark plugins, vendor-specific analyzers, and traffic loggers.

22. What is the overhead of C-Plane packets and its impact?

    
    

    Introduces control overhead which must be minimized to optimize fronthaul bandwidth.

23. How is round-trip latency for C-Plane tested?

    
    

    Through timestamped traffic and loopback verification.

24. What logs are critical for analyzing C-Plane issues?

    
    

    DU scheduler logs, RU control message logs, transport packet counters.

25. How does C-Plane interact with the M-Plane and S-Plane?

    
    

    M-Plane handles configuration; S-Plane ensures synchronization; C-Plane uses both for real-time delivery.

    
    

Section 2: U-Plane (User Plane)

26. What is the role of the U-Plane in O-RAN architecture?

    
    

    It transports user data (IQ samples) between the O-DU and O-RU.

27. Which protocol is used for U-Plane in O-RAN fronthaul?

    
    

    eCPRI Protocol Message Type 0 is used for user plane data.

28. What is an eAxC ID in the U-Plane?

    
    

    It identifies a specific logical antenna carrier mapping.

    Refer to Q9 response.

29. How does the U-Plane ensure data reaches the correct antenna port?

    
    

    Through the eAxC ID in the eCPRI header.

30. What is IQ compression in U-Plane?

    
    

    It reduces bandwidth usage by applying compression algorithms to IQ data.

Ex: Block Floating point compression

31. How are U-Plane packets timestamped?

    
    

    Each packet includes a timestamp to align with radio frame timing.

32. How is latency measured for U-Plane packets?

    
    

    Using timestamp analysis at ingress and egress points.

33. What are common U-Plane testing KPIs?

    
    

    Throughput, latency, jitter, packet loss, and IQ integrity.

34. What is IQ sample width and how does it affect U-Plane bandwidth?

    
    

    Higher sample widths (e.g., 16-bit vs 8-bit) increase bandwidth requirements.

35. What tool is used to visualize IQ samples?

    
    

    Vector Signal Analyzers (VSA) or MATLAB scripts.

36. How do you test dynamic bandwidth adaptation in the U-Plane?

    
    

    By adjusting traffic profiles in the DU and observing IQ throughput.

37. What is the effect of jitter in U-Plane transmission?

    
    

    Leads to demodulation errors and degraded RF performance.

38. Can U-Plane and C-Plane coexist on the same physical link?

    
    

    Yes, using different VLAN tags or DSCP values.

39. How do you validate packet sequence integrity in U-Plane?

    
    

    Sequence numbers in eCPRI headers or custom test traffic patterns.

40. What causes IQ data corruption?

    
    

    Transport errors, buffer overflows, or compression algorithm mismatches.

41. How are U-Plane packets prioritized?

    
    

    Through traffic shaping, QoS settings, and real-time scheduling.

42. What is the impact of packet loss on U-Plane?

    
    

    Causes RF signal distortion and link quality degradation.

43. How do you emulate U-Plane load in test scenarios?

    
    

    Using IQ data generators or traffic simulators.

44. What encoding schemes are used for IQ data?

    
    

    Typically, 15-bit, 16-bit fixed-point representation.

45. How are radio frame and subframe boundaries maintained in U-Plane?

    
    

    Through precise timestamp alignment using S-Plane synchronization.

46. What is functional split 7.2 and how does it relate to U-Plane?

    
    

    It defines PHY split between DU and RU, requiring IQ data exchange over U-Plane.

47. What does U-Plane packet format include?

    
    

    eCPRI header, sequence ID, timestamp, and IQ data payload.

48. How do you validate compression/decompression accuracy?

    
    

    Compare original and received IQ samples using error vector magnitude (EVM).

49. Which tools can generate U-Plane test traffic?

    
    

    Keysight UeSIM, O-RAN test tools, or proprietary traffic generators.

50. What are best practices for U-Plane fronthaul testing?

    
    

    Ensure lossless transport, accurate timestamping, and IQ fidelity.

    
    

Section 3: S-Plane (Synchronization Plane)

51. What is the purpose of the S-Plane in O-RAN?

    
    

    To provide synchronization (time and frequency) between O-DU and O-RU.

52. Which protocol is used in S-Plane?

    
    

    Precision Time Protocol (PTP), IEEE 1588v2.

53. What hardware element is key to S-Plane?

    
    

    Grandmaster clock, boundary clock, and slave clock devices.

54. What is SyncE and how is it related to S-Plane?

    
    

    Synchronous Ethernet used for frequency sync as a backup to PTP.

55. What is the required timing accuracy for 5G O-RAN?

    
    

    Typically ±65ns for RU timing alignment.

56. What is the role of a Telecom Profile in PTP?

    
    

    Defines parameters for time sync in telecom networks (G.8275.1/2).

57. How do you test time synchronization between DU and RU?

    
    

    By comparing 1PPS signals or using timestamp analysis tools.

58. What are common sync issues in O-RAN?

    
    

    PTP packet loss, delay asymmetry, incorrect profile config.

59. How does S-Plane impact C/U plane performance?

    
    

    Desync causes symbol misalignment and RF transmission issues.

60. What is Transparent Clocking in PTP?

    
    

    Switches/stations add transit time to packets to ensure accuracy.

61. How is jitter handled in S-Plane?

    
    

    Filtered using phase-locked loops (PLLs) or averaging algorithms.

62. What is the function of Sync Status Messages (SSM)?

    
    

    Indicates synchronization quality on the link.

63. What is the difference between Boundary Clock and Transparent Clock?

    
    

    Boundary clocks regenerate timing; transparent clocks update delay.

64. What are key metrics to monitor in S-Plane testing?

    
    

    Mean Path Delay, offset from master, packet delay variation.

65. What tools help in sync testing?

    
    

    Calnex Paragon, Spirent, VIAVI PTP analyzers.

66. Can sync be tested without live RF?

    
    

    Yes, using timestamp validation and loopback setups.

67. What is holdover in synchronization?

    
    

    Maintaining sync temporarily after losing timing source.

68. How do sync failures manifest at RU?

    
    

    IQ corruption, missed symbols, beamforming failures.

69. How is S-Plane redundancy handled?

    
    

    Using multiple Grandmasters and failover mechanisms.

70. What is T-BC and T-TC in sync architecture?

    
    

    Telecom Boundary Clock and Telecom Transparent Clock.

71. What alarms indicate S-Plane failure?

    
    

    Loss of Sync, PTP Sync Fail, Timing Source Alarm.

72. What is the impact of asymmetrical delay in sync?

    
    

    Causes timestamp drift leading to out-of-phase signals.

73. Can VLANs impact S-Plane performance?

    
    

    Yes, improper QoS can introduce jitter or delay.

74. What is phase alignment and why is it critical?

    
    

    Required for coordinated multipoint transmission (CoMP).

75. What is frequency sync vs time sync?

    
    

    Frequency aligns oscillators; time sync aligns absolute time.

    
    

Section 4: M-Plane (Management Plane)

76. What is the role of the M-Plane in O-RAN?

    
    

    Provides configuration, fault management, and performance monitoring.

77. Which protocols are used in M-Plane?

    
    

    NETCONF over SSH with YANG models.

78. What are O-RAN defined YANG models?

    
    

    Data models for configuring O-RU/O-DU based on O-RAN standard.

79. What is O1 and O2 interface?

    
    

    O1: SMO to O-RU/DU; O2: SMO to Cloud Infrastructure.

80. How is fault management performed in M-Plane?

    
    

    Through alarms, logs, and event notifications (e.g., syslog, SNMP traps).

81. How are software upgrades handled in M-Plane?

    
    

    Via defined workflows using NETCONF/YANG and file transfer mechanisms.

82. What is the difference between configuration and provisioning in M-Plane?

    
    

    Provisioning is initial setup; configuration may happen dynamically post-deployment.

83. How do you perform O-RU configuration via M-Plane?

    
    

    Using NETCONF client commands with appropriate YANG modules.

84. What is the role of SMO in M-Plane?

    
    

    Acts as the central orchestrator for managing, monitoring, and automating O-RAN nodes.

85. How is performance management executed in M-Plane?

    
    

    Collection of KPIs, statistics, and health reports through NETCONF or file-based mechanisms.

86. What security measures are in place for M-Plane?

    
    

    Secure channels (SSH), authentication, role-based access, and encryption.

87. What is the typical test setup for M-Plane validation?

    
    

    NETCONF client (e.g., Yuma123), test YANG files, simulator/emulator for O-RU.

88. What are common test cases in M-Plane testing?

    
    

    Alarm generation, parameter reconfiguration, software update, reset flows.

89. How is rollback handled in M-Plane during config change failure?

    
    

    Through transaction-based config application supported by NETCONF.

90. What is the purpose of file management functions in M-Plane?

    
    

    Upload/download of logs, software packages, certificates.

91. How do M-Plane operations vary between O-DU and O-RU?

    
    

    O-RU M-Plane is more static, O-DU may involve dynamic changes per slice or use case.

92. What is the role of TLS in M-Plane?

    
    

    Secures NETCONF over SSH or HTTPS if using RESTCONF.

93. How are multiple RUs managed by a single DU using M-Plane?

    
    

    Through logical instances identified by unique resource IDs within SMO.

94. How are events and traps tested in M-Plane?

    
    

    By inducing faults or triggers and validating SMO reception and handling.

95. What are common YANG validation issues?

    Schema mismatch, namespace issues, unsupported leaf nodes.

96. What tool validates NETCONF/YANG compliance?

    pyang, yanglint, confd or vendor-provided validation suites.

97. How are managed objects (MOs) structured in M-Plane?

    Hierarchical, based on classes, attributes, and instances defined in YANG.

98. How is heartbeat or keepalive managed in M-Plane?

    Through periodic notifications or polling via NETCONF sessions.

Ex: Supervision notification between O-DU and O-RU

99. What challenges arise during interoperability testing for M-Plane?

    Vendor-specific YANG extensions, version mismatches, unsupported features.