INTRO

Interconnections made using fiber optic cables are currently the most popular type of medium for use in telecommunications networks. The need for additional bandwidth is rising because of the growing popularity of streaming media, social networking, and cloud computing. This demand is driving networking speeds to greater horizons. When communicating at high speeds, such as 100G Ethernet and above, it is possible for transmission errors to arise because optical receivers struggle to differentiate between signal and noise as the amount of noise in the environment grows. The process of controlling errors that take place during the transmission of data through unreliable or noisy communication channels is referred to as Forward Error Correction or FEC. In this article, we will go deeper into the topic by answering questions such as “What is FEC?”, “What are the pros and cons of using FEC?” and in the “Good to Know About FEC” section, we’ll talk about what you need to pay attention to when using FEC, including whether 100G FEC is different from 400G FEC, how PAM4 FEC is different from NRZ FEC, what RS-FEC 25G is, and whether FEC processing is done on the module or in the host device. All of this should be relevant to modern optical communication links.

What is FEC?

Our world is full of noise. Noise affects everything, including data transmission and communication systems, and there is no way to escape it. The receivers of optical communication systems are directly impacted by noise, which makes it more difficult to understand the information that has been received. Technically speaking, noise has an effect on the optical intensity of the wave as it is transmitted through an optical fiber, and optical dispersion creates noticeable defects in the signal when it is transmitted over a long distance. Whenever there is an impact from noise or optical dispersion distortion, the optical pulse suffers degradation and loses its meaning as either 0 or 1. The receiver takes the optical pulse it receives and converts it to voltage. When the receiver does so with too much noise it interprets the data incorrectly, reading either 0 as 1 or 1 as 0 bit. Obviously, in a perfect system, there would be no noise, and the receiver would be able to correctly recognize bits 0 and 1 as they were transmitted. However, this is not the case in the systems that exist in the real world.

FEC, which stands for Forward Error Correction, comes into play at this point because it lessens the effect that noise has on the transmission quality of optical transmission systems. By adding overhead information to a stream of bits before transmission, this method is able to detect and correct a portion of the errors that may be present in a stream of bits. The data block is subjected to specialized functions, and the output of those functions is the generation of parity bits. Overhead consists of redundant bits, which also contain parity bits. After that, the initial data block and these new data are joined together, and the FEC codeword is produced as a result. Following that, this FEC codeword is sent along the transmission line.

The same FEC mode is configured on the equipment at the receiver end so that the receiver FEC decoder mechanism knows what kind of functions to apply to the FEC codeword. This allows the receiver FEC decoder mechanism to choose the functions to regenerate the data and remove the FEC overhead with a high degree of accuracy. As a result, an initial data bit stream is produced, which is then sent to higher network layers.

FEC Types

FEC codes can detect and correct a limited number of errors without having to retransmit the data stream. FEC codes are classified into two types: block codes and convolution codes. Block codes are classified as Hard-Decision FECs, whereas convolution codes are classified as Soft-Decision FECs. To correct errors, block codes employ fixed-size blocks. The most common block code type is Reed-Solomon. Hard-Decision FEC algorithm code uses a fixed length of code and determines whether each symbol corresponds to 0 or 1.

Convolutional codes are used in Soft-Decision FEC algorithms. They work with variable length symbol streams and introduce confidence factors for 0 or 1 decisions. 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 increase the overall distance reachability of an optical transmission system by 30-40%. Thus, Soft-Decision FEC has a disadvantage: it increases overhead by 15-30%. Hard-decision block codes are three times as large. The Soft-Decision FEC algorithm branch includes Trellis error correction codes.

Reed-Solomon error correction codes are the most widely used error detection mechanism in today’s telecommunications industries. Reed-Solomon codes operate on a data block represented as a set of finite-field elements known as symbols. Reed-Solomon codes can detect and correct a variety of symbol errors. The two most common FEC schemes in today’s telecommunications links are RS-FEC (528, 514) and RS-FEC (544, 514). The RS(544,514) FEC is used on 400G PAM4 transceiver links and 100G PAM4 (CAUI-2) links; while RS-FEC (528, 514) is being used on 100G NRZ links. The following is the distinction between the two RS-FEC schemes: – RS-FEC(528,514) encoding begins with a 514-symbol data field with 10 bits per symbol and adds 14 parity symbols to form a 528-symbol encoded codeword. However, the RS-FEC (544, 514) uses 30 parity symbols to form a 544 symbol encoded codeword. Because PAM-4 signals have a tighter spacing between voltage levels, the eye amplitude is one-third that of a similar NRZ signal, it is slightly larger and uses more overhead. As a result, the SNR of the PAM-4 signal is reduced and it is more susceptible to noise. To compensate for the lower SNR, KP-FEC is designed with a higher coding gain. KP-FEC has the potential to correct up to 15 symbols per codeword, whereas KR-FEC can only correct up to seven symbols.

Good to Know About FEC

Matching FEC on both sides of the Link

When using FEC, one simple factor to consider is that matching FEC Type must be used across switches and transceivers on both sides of the link. For example, if the transceiver supports RS-FEC, the host device into which it will be plugged must also support RS-FEC, and the other side of the link setup must follow the same principle. If, however, you have equipment on one side of the link supporting RS-FEC and equipment on the other side of the link supporting SD-FEC, the FEC functionality will not work, and the link will not work with FEC turned on. Similarly, if FEC is turned on on one side of the link but not on the other, the link will not function..

RS-FEC 25G

Reed-Solomon Forward Error Correction is used in many 25G SFP28 transceivers to increase the range in 25G-CSR, 25G-LR, 25G-ER, and BIDI scenarios.

NRZ 100G FEC

Except for 100GBASE-LR4 and 100GBASE-ER4, which use LAN-WDM transmitters and can achieve required distances of 10km and 40km without the need for FEC, all 100G NRZ modules to achieve maximum reach require RS(528,514) FEC enabled on the host platform.

100G PAM4 FEC

100G PAM4 modules (100GBASE-DR, 100GBASE-FR, 100GBASE-LR, and 100G-ER) have RS(544, 514) PAM4 FEC (KP1) built into the optical transceivers digital signal processor (DSP) chip internally, and when these modules are detected by the host, FEC on the host platform is disabled.

PAM4 400G FEC

To achieve best performance FEC (544,514) must be enabled on the host device for 400G QSFP-DD modules based on PAM4.

BIDI transceivers – The Future in 100GE

Introduction

BIDI transceivers – The Future in 100G Era, where cloud computing and 5G networks, data centers have the tendency to develop at a higher speed, we see that the demand for 100G optical solutions is increasing and now it is the base of any operation. 100G currently provides the most energy, space, and cost-efficient way to meet this demand. In recent years, transceiver costs for 100G networks have also dropped significantly, including the initial outlay and ongoing associated costs. Moreover BIDI SFP, 10G to 25G to 100G provides a clearer migration path to upgrade to higher speeds with a roadmap to higher data rates in the coming years. These are just a few facts that 100G can meet the leads of most links. From 2022 data more than 90% of fiber links in small and medium-sized data centers are less than 100 meters in length. 70% of fiber links in large data centers are less than 100 meters in length, and more than 80% of them are less than 125 meters in length. To sum up, for 100G modules, the largest demand is for the 100G multimode optics.

Short History of Bidirectional transceivers:

First BIDI transceivers (BIDI) were introduced more than 20 years ago. In mid 2001 the first 100M and 1G optical BIDI transceivers were developed and standardized. Bidirectional transceivers got their abbreviation BX and firstly they had one SC connector. Latter it was replaced with much smaller form LC type connector. Bidirectional fiber optic communication offers greatly lower expenses towards optical fiber link resources because transmission can be achieved through one optical fiber strand. The optical fiber pair as required with the Double Fiber transceiver technology was no longer required.

Bidirectional technology was introduced as well in 10G and as time went by also in 25G, 40G and 100G optical transceivers. Most modern BIDI transceivers offers even two LC ports where each of them works as Bidirectional entity.

Bidirectional 100G optics

For short range optics over multimode it provide BIDI-100G-QSFP28-SR and BIDI-40G/100G-QSFP28-SR.

BIDI-100G-QSFP28-SR is a Bidirectional QSFP28 module. The module integrates four host electrical data into two optical lanes (by Dual Wavelength VCSEL Bi-Directional Optical Interface, 850nm and 900nm) to allow optical communication over a 2-fiber duplex LC optical multimode fiber. Reversely, on the receiver side, the module De-multiplexes 2 sets of optical input signals and converts them to 4 channels of electrical data. Module has a minimum guaranteed optical budget of 1.9 dB which in most cases is enough to reach 150 meters distance over OM5 multimode optical cable. The product is designed with form factor, optical/electrical connection and digital diagnostic interface according to the QSFP28 Multi-Source Agreement (MSA). 

BIDI-40G/100G-QSFP28-SR is a more advanced BIDI transceivers than BIDI-100G-QSFP28-SR, because it is not just a 100GBASE Bidirectional SR QSFP28 module, but also is compliant to 40GBASE. Its function is just as same as the BIDI-100G-QSFP28-SR, which is mentioned in the above paragraph. As well it has the same 1.9 dB of optical budget guarantee and can reach up to 150 meters distance over OM5 multi-mode optical cable.

The only difference between the two modules is only the data rate advantages. The schematic image of both modules can be seen in the image below.

Short Wavelength Division Multiplexing 4 optics

Short Wavelength Division Multiplexing 4 is similar to 100G BIDI transceivers, both have a duplex LC interface, where SWDM (Short Wavelength Division Multiplexing) refers to short wavelength division multiplexing technology. The MUX/DMUX tech enables the transmission of 4 (hence the name SWDM4) bands of optical signals on a single core of multimode fiber. The four bands are in the wavelengths 850nm, 880nm, 910nm and 940nm.

For SWDM4 modules it exist 40G data rates, 100G data rates and also both 40G & 100G three types.

Bidirectional 400G optics

Bidirectional 400G SR4 module has an MPO interface and ir is designed for QSFP-DD SR4.2 to QSFP- DD SR4.2 point to point communication and QSFP-DD SR4.2 to 4x QSFP28 SR1.2 breakout communication. The 400G BIDI transceivers accepts 8x 50G (400GAUI-8) host electrical data and transmits them in two groups of optical Bi-Directional lanes (each group contains 4 pairs of optical lanes) to allow optical communication over optical Multi-Mode fibers. Reversely, on the receiver side , the module accepts 8 sets of optical input signals and converts them to 8 channels of electrical data.

Understanding Video SFP Transceiver

Video SFP Transceiver Basics

Video SFP transceivers are also referred to as digital video transceivers or SDI (short for Serial Digital Interface) video transceivers. They are small, hot-pluggable transceiver modules, working with fiber optic cables. Why do we need to use SDI video SFP transceiver? As the rapid evolution of the broadcast video transport for high-capacity HD and ultra high-definition (UHD) digital transmission, it is necessary to produce a kind of fiber optic transceiver that can achieve high performance level in video image transmission. With SDI interface, video SFP transceiver is able to support SDI video pathological signal and ensure the quality of video transmission.

Types of Video SFP Transceiver

Video SFP transceivers can be classified into various types according to different aspects. Divided by operating rate, there are 3G-SDI SFP, 6G-SDI SFP, 12G-SDI SFP; by transceiver type, there are dual transmitters, dual receivers, single transmitter, single receiver, or a transceiver; by compliant standards, there are MSA and non-MSA; by operating wavelength, there are 1310nm, 1490nm, 1550nm and CWDM wavelength; by transmission distance, there are 300m, 2km, 10km, 20km, 40km.

How to Use Video SFP Transceiver

Video SFP modules complete optical signal to digital signal conversion or vice versa in the applications. They are usually used for HD camera or monitor system and broadcast video transport application. This part takes 3G-SDI SFP for example.

Video SFP Module Used in HD Camera or Monitor System

Usually there are multiple HD end-devices in HD camera or monitor system, so we can use one HD video matrix as one end, which provides multiple video SFP ports, and multiple HD-SDI equipment as the other end. 3G-SDI transceivers are plugged into the equipment respectively, then SDI SFP transceivers on both ends are connected via fiber optic cables. The cabling diagram is shown in the following figure.

Video SFP Module Used for Broadcast Video Transport Application

Broadcast video transport application needs high-density cabling, so we use HD-SDI equipment with high-density video SFP ports. As the following figure shows, multiple 3G-SDI SFP transceivers are plugged into the HD-SDI equipment on the two sides respectively, then these video SDI SFP transceivers are connected through optical cables.

Be Careful When Choosing Video SFP Transceiver

As video SFP transceiver can be designed with dual transmitters, dual receivers, single transmitter or single receiver, you should be careful when choosing video SFP transceiver. If you need unidirectional transmission, then you can use single SDI SFP transmitter and single SDI SFP receiver for single signal, dual transmitters and dual receivers for dual signals. If bidirectional transmission is required, you can use video SFP transceiver which has transmitter and receiver built in one module. Besides, there are SDI CWDM SFP and SDI BiDi SFP. The SDI CWDM SFP can transmit signals at different wavelengths, and SDI BiDi SFP can mix and transmit multiple wavelengths simultaneously over the same fiber. Both of them are cost-effective solutions. What’s more, the choice of fiber optic cable must be compatible to the type of video SFP transceiver.

Common Questions about Video SFP Transceiver

1. What is SDI?

SDI (Serial Digital Interface) is a digital video interface standard made by SMPTE organization. This serial interface transmits every bit of data word and corresponding data through single channel. Due to the high data rate of serial digital signal (a kind of digital baseband signal), it must be processed before transmission.

2. What's the difference between 3G-SDI SFP and 6G-SDI SFP?

3G-SDI video SFP transceiver supports 3Gbps data rate, while 6G-SDI video SFP transceiver can deliver a payload of 6Gbps. Both of them are specifically designed for robust performance in the presence of SDI pathological pattern for SMPTE 259M, SMPTE 344M, SMPTE 292M and SMPTE 424M serial rates, but 6G-SDI SFP is also designed for SMPTE 2081. For application, 3G-SDI SFP is generally used for security monitoring application and television broadcasting, while 6G-SDI SFP is often used for 4K /HDTV/SDTV service.

3. What’s the different between video SFP transmitter and video SFP receiver?

Video SFP transmitter converts digital signal into optical signal, while video SFP receiver converts optical signal into digital signal. Therefore, they must be used together in application. What’s more, video SFP transceiver also needs to be used in pair to complete the link.

4. What’s the difference between MSA and Non-MSA standards of 3G-SDI SFP optical transceiver modules? What will be the consequence if they match wrongly to host machine?

3G-SDI SFP transceiver has 20 pins, MSA and Non-MSA differ in I2C pin definition: MSA standard defines I2C definition on the fourth pin (SDA) and the fifth pin (SCL); while Non-MSA standard defines it on the fifth and sixths pins. The I2C pins failing to match the host machine can directly lead to video SFP module communication connecting error to host machine I2C. It comes out the SFP DDM function is unavailable and EEPROM unreadable by host machine.

5. What's the difference between video SFP transceiver and fiber video converter? How to make choice between them?

Both video SFP transceiver and fiber video converter are used for video transmission, and they can realize optical signal to digital signal conversion. Fiber video converter consists of fiber optic transmitter set and fiber optic receiver set, so when in use, they must work together.

The main difference between them is that video SFP transceiver is not only connected with fiber patch cable to function, but also has to be plugged into HD-SDI equipment video SFP port; while fiber video converter can just work with fiber optic cable. Also, fiber video converter usually supports 20km transmission distance over singlemode fiber; while video SFP transceiver can support short reach and long reach, over multimode fiber and singlemode fiber respectively. For cost, fiber video converter is more expensive than video transceiver. As for which one to choose, it all depends on your specific requirement.

Conclusion

Video transmission becomes more and more common in our daily life. Being able to ensure the high performance level of video digital transmission, video SFP transceiver gains great popularity among network engineers. Infioptics provides different types of 3G-SDI video SFP and 12G-SDI SFP, and custom service is also available.