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Forward Error Correction (FEC)

Introduction

With forward error correction (FEC), redundant data is sent by the source (transmitter) and only the parts of the data that appear to be mistake-free are recognized by the destination (receiver). This technique allows error control during data transmission. A finite number of errors can be detected and corrected by the receiver when FEC is employed in data transmissions. The sender must retransmit the packets containing the errors if there are too many mistakes.

Data transmission between the source and the destination does not require handshaking thanks to FEC, hence there is no need for the two systems to connect beforehand. This enables data to be broadcast simultaneously from a single source to several destinations. Furthermore, since FEC may fix errors, less data needs to be retransmitted, using less bandwidth and power.

FEC is appropriate for both wired and wireless communications because of its error-correcting capabilities, which are often used for sending data via erratic or noisy communication channels. For instance, FEC offers a productive way to support cellular networks or broadcast video content. Additionally, it supports cable and fiber optic connections, eases one-way data exchanges, and is utilized by routers and modems.

What is the process of forward error correction?

In the simplest form of FEC, the transmitter sends each character numerous times to provide a safeguard against lost or damaged data. In the event that there are differences between the character instances, the receiver applies a "majority rules" method to retrieve the data. For instance, the capital letter W, which has the binary value 01010111, may be included in the transferred data. The receiver would compare the bits in each byte instance if the transmitter sent the W byte three times in order to identify which bits are accurate.

When the data in this instance reaches the recipient, it contains two mistakes. In the second character instance, the fifth bit is distinct from the other two bits at that place; similarly, in the third case, the second bit is distinct from the other two bits at that same position. The receiver compares the bits for each bit location. The value is regarded as the proper bit if all three are the same. The two majority values are regarded as the proper bit if one value differs from the other two.

Different systems employ different procedures for FEC-based communications. On the whole, though, they usually take a similar tack. To add the required parity data to the original data, the transmitter uses an encoder of some kind, and the receiver uses a decoder of some kind to extract the original data from the transmitted data while fixing errors along the way.

FEC error-correcting codes

When implementing FEC on their systems, the communication and storage industries use a range of error-correcting codes. Among the more popular codes are the following ones:

  • Chaudhuri, Bose, and Hocquenghem (BCH). This is a broad class of cyclic codes with random error correction that are built using polynomial formulas. The codes are straightforward to encode and decode using algebraic techniques, and they are rather simple to implement in hardware or software.
  • Hamming: This kind of error-correcting code is frequently used for data that is stored on NAND flash storage as well as data that is sent. One kind of block code is hamming code. Blocks of the data are divided, and each block is given parity information. Hamming code is limited to correcting one-bit faults; it is not capable of detecting two-bit errors. When consistency is more important than transmission economy, the code is frequently utilized.
  • Low-density parity check (LDPC): For noisy data channels, this class of linear error-correcting block codes is frequently employed. LDPC codes are used for wired and wireless communication and are regarded as being extremely efficient. A low-density parity-check matrix is used by LDPC to characterize the code.
  • Reed- Solomon (RS): Many storage and communication devices, such as disk players, digital TVs, cell phones, high-speed modems, and communication satellites, use this kind of widely used error-correcting code. RS code is a block code similar to Hamming code, except codewords are defined in multi-bit symbols since RS code is non-binary. Both one-bit and multi-bit faults can be found and fixed using RS coding.

For FEC-based communications, these are just a handful of the several varieties of error-correcting codes that are available. Businesses using FEC should choose the strategy that best suits their needs. They should bear in mind one crucial requirement, though: when encoding and decoding data, the transmitter and receiver must utilize the same FEC type and adhere to the same set of guidelines. They won't know for sure that the decoder is correctly detecting and fixing mistakes until then.

Types of FECs

FEC codes don't need to retransmit the data stream in order to identify and fix a certain amount of faults. Block codes and convolution codes are the two categories into which FEC codes fall. Convolution codes are categorized as Soft-Decision FECs, while block codes are categorized as Hard-Decision FECs. Block codes use fixed-size blocks to rectify problems. Reed-Solomon block codes are the most widely used type. The Hard-Decision FEC algorithm code decides whether each symbol corresponds to 0 or 1 using a predetermined length of code.

FEC methods that use Soft-Decision employ convolutional codes. They introduce confidence factors for decisions of 0 or 1, and they operate with streams of symbols of varied length. This means that the receiver can interpret a bit as 0 or 1 based on the amplitude of the signal if it is in the 0 confidence interval or the 1 confidence interval. These codes add 30–40% to an optical transmission system's total reachability over longer distances. Therefore, Soft-Decision FEC has a drawback: it results in a 15–30% increase in overhead. Block codes with hard decisions are three times larger. Trellis error correcting codes are part of the algorithm branch of the Soft-Decision FEC.

In today's telecommunications sectors, Reed-Solomon error correcting codes are the most commonly employed error detection technology. Reed-Solomon codes function on a data block that is represented by a collection of symbols, or finite-field elements. Numerous symbol errors can be found and fixed via Reed-Solomon codes. In today's telecommunications lines, RS-FEC (528, 514) and RS-FEC (544, 514) are the two most used FEC systems. Whereas RS-FEC (528, 514) is utilized on 100G NRZ connections, the RS(544,514) FEC is utilized on 400G PAM4 transceiver lines and 100G PAM4 (CAUI-2) links. The two RS-FEC schemes differ in the following ways: – To create a 528-symbol encoded codeword, RS-FEC(528,514) encoding starts with a 514-symbol data field with 10 bits per symbol and adds 14 parity symbols. On the other hand, the RS-FEC (544, 514) encodes a 544 symbol codeword using 30 parity symbols. PAM-4 transmissions are slightly larger and need more overhead since they have a smaller eye amplitude, one-third that of an NRZ signal with a similar voltage level spacing. The PAM-4 signal's signal-to-noise ratio (SNR) decreases as a result, making it more vulnerable to noise. KP-FEC has a larger coding gain by design to make up for the lower SNR. While KR-FEC can only correct up to seven symbols each codeword, KP-FEC can potentially correct up to fifteen symbols.

FEC Advantages and Drawbacks

We shall examine the benefits and drawbacks of FEC(Forward Error Correction)

Pros:

  • FEC is cost-effective. The techniques’ major goal is to repair transmission mistakes, so we can obtain better outcomes with the same hardware components and avoid the need for more expensive lasers and receivers.
  • FEC uses straightforward methods to act in real time and correct code in a matter of seconds.
  • Extend the lengths between connections. While FEC corrects the code, it also increases the signal's reception range. For instance, utilizing SD-FEC on 100G lines can enhance the signal's reception range by up to 30–40%.
  • The Bit Error Rate (BER) is decreased by the method (BER).
  • If a mistake is found, FEC does not require retransmission of the full frame. Only the superfluous bits are identified and corrected using the method. It spares bandwidth that would have been needed for retransmission as a result.

Cons:

  • A rise in delay; The FEC method reduces the payload by adding overhead bytes, which causes the transfer of data from point A to point B to take longer.
  • Because a same sort of FEC must be used on both sides of the link, link configuration may need to be adjusted. Keep this in mind when connecting equipment from different brands.
  • In most cases, employing forward error correction has more advantages than disadvantages, albeit not all transceivers benefit from it. The type of module and the system in which the transceivers are utilized usually dictate the implementation and use of FEC.

Important Information Regarding FEC Matching FEC on Both Ends of the Link:

One easy thing to remember when utilizing FEC is that switches and transceivers on both ends of the link must utilize the same FEC Type. For instance, if the transceiver supports RS-FEC, the host device into which it is connected must likewise have this capability, and the link configuration on the other end must adhere to the same guidelines. However, the FEC capability will not function and the link will not function with FEC set on if you have equipment supporting RS-FEC on one side of the link and SD-FEC on the other. In a similar vein, a link cannot work if FEC is enabled on one side but not the other.

RS-FEC 25G

Many 25G SFP28 transceivers use Reed-Solomon Forward Error Correction to boost range in 25G-CSR, 25G-LR, 25G-ER, and BIDI applications.

NRZ 100G FEC

With the exception of 100GBASE-LR4 and 100GBASE-ER4, which employ LAN-WDM transmitters and may reach the necessary lengths of 10 km and 40 km without requiring FEC, all 100G NRZ modules require the host platform to have RS(528,514) FEC activated in order to reach their full reach.

100G PAM4 FEC

The digital signal processor (DSP) chip of the optical transceiver contains an inbuilt RS(544, 514) PAM4 FEC (KP1) module known as 100G PAM4 (100GBASE-DR, 100GBASE-FR, 100GBASE-LR, and 100G-ER). The host platform disables FEC upon detecting these modules.

400G FEC PAM4

In order to optimize performance, 400G QSFP-DD modules based on PAM4 require the host device to enable FEC (544,514).

Conclusion

In today's communication systems, Forward Error Correction (FEC) plays a crucial role in resolving issues caused by transmission errors and channel impairments. All in all, FEC assumes an essential part in guaranteeing solid information move by consolidating repetitive data inside the sent information stream. FEC enables receivers to reconstruct the original message even when certain bits are corrupted or lost during transmission by proactively introducing error-correcting codes.

One prominent benefit of FEC lies in its capacity to upgrade correspondence vigor without the requirement for extra retransmissions. This diminishes dormancy as well as is especially gainful in situations where constant or low-idleness correspondence is basic. Additionally, FEC significantly enhances the effectiveness and dependability of wireless, satellite, and optical communication systems thanks to its widespread application in these fields.

Generally, FEC remains as a foundation in present day correspondence conventions, finding some kind of harmony between blunder versatility and proficiency. FEC algorithms continue to evolve in tandem with technological advancements, ensuring their continued relevance in the ever-evolving landscape of communication systems.

To summarize, with current events increasing system bandwidth and total network speed, error correction must be considered while transferring over greater distances. Forward Error Correction can extend the transmission distance of lower-cost components and allow you to receive a high-quality signal uninterrupted by noise, which distorts the signal. Although the FEC technique is already well-known, as speed and optical modulation techniques improve, we may anticipate that its popularity will grow even further.