Importance of FEC Test and Measurement Procedure

Stable 400G network operation requires measurement of FEC error margins. This explains the importance of performing 400G Ethernet FEC tests and the procedure using the MT1040A tester with both quantitative and visual FEC analysis functions.

400G Ethernet Trend

400G Ethernet standardization is progressing to support higher speeds and larger capacities for networks and data centers. Following-on from the SR4 and SRm.n standards for data centers, the FR4 and LR4 standards for network connections have been decided and widespread future adoption is anticipated.
These 400G Ethernet standards use PAM4 technology instead of NRZ used by 100G Ethernet, enabling 400G Ethernet to achieve a high bandwidth at low cost while cutting wavelength multiplexing and power consumption.

400G Ethernet and PAM4

Compared to 25G NRZ for 100G Ethernet, 50-Gbaud PAM4 for 400G Ethernet achieves a four times higher bit rate using two times frequencies carrying 2 bits of data. However, in comparison to NRZ, PAM4 suffers from degraded SNR due to the smaller amplitude, making it difficult to achieve the error-free implementation of 100G Ethernet. Consequently, to assure error-free communications, FEC technology is being introduced for autonomous bit-error correction.

FEC used for 400G Ethernet

RS-FEC (Reed Solomon-Forward Error Correction) bit-error correction is always performed by 400G Ethernet systems. Instead of appending parity bits, autonomous bit-error correction is enabled.
FEC is performed in units called codewords (CW) and performance is expressed as RS(x, y, a, b), where:

x: Number of symbols in one codeword (message symbols + parity symbols)
y: Number of message symbols in one codeword
a: Maximum number of correctable errors in one codeword
b: Number of bits per symbol

400G Ethernet FEC uses RS(544, 514, 15, 10). It can correct symbol errors up to 2.7E-02 (= 15 / 544) assuming bit errors occur at random.

Impact of Errors on 400G Ethernet Communications

The IEEE802.3 standard defines the High SER alarm to detect the occurrence of excess network errors and assure stable network operation. When this alarm occurs, the 400G Ethernet Link is down due to alignment resynchronization. Detection of the High SER alarm is specified as occurring when 5560 or more Symbol Errors occur in 8192 codewords (1.2E-03 = 5560/8192), but since the FEC correctable error limit for 400G Ethernet is 2.7E-02 Symbol Errors, there is a risk of overlooking line and equipment faults because operation can be error-free when operating close to the High SER alarm limit. Additionally, since FEC also corrects burst errors, there is a risk of overlooking early signs of failing and aging equipment.

Consequently, continued operation that fails to pay attention to increasing error rates or signs of equipment failure, in the worst-case, cause a fatal network fault, such as sudden Link Down.

Error Measurement at 400G Ethernet Communications

Achieving stable 400G Ethernet communications not only requires Layer 2 BER tests, which is the main test for speeds up to 100G Ethernet, but also requires quantitative measurements of FEC correction status. At these measurements, it is useful to measure the number of FEC-corrected Symbol Errors per codeword and the distribution.

MT1040A FEC Symbol Error Measurement

The MT1040A suppoprts both visual monitoring and quantitative measurement of FEC Symbol Errors while communicating using Ethernet Frames. In addition, it supports two types of threshold setting and continuous saving of measurement results at any interval for stability evaluation and analysis of correlations with other events.

FEC Measurement Using MT1040A Automatic Test Tools

Optical module performance tests can be executed easily using the MT1040A SEEK (Scenario Edit Environment Kit) conversational dialog-type test tool. With its all-in- one support, the MT1040A is ideal for on-site installation inspections and troubleshooting.

• Easy on-site work using conversational dialog-type sequences
• In addition to BER tests and optical-module internal warning/alarm evaluations, also supports key Symbol Error tests for FEC performance evaluation
• Autonomous test execution, and results and report output

The MX100003A dedicated software can be used to open and edit this scenario description. This software can be downloaded from the Anritsu homepage. MX100003A download page