PAM4 for 100G and 400G Applications

Hyper-scale data centers have been seeking for transceiver solutions with higher port densities and lower cost per bit, which has driven the development of PAM4 (Four-Level Pulse Amplitude Modulation) technology. Compared to the expensive multi-state coherent modulation scheme, simple PAM4 can deliver the right combination of speed, low cost, and low power consumption in data centers. This article is intended to introduce PAM4 for 100G and 400G applications.

What Is PAM4?

PAM4 is a technology that uses four different signal levels for signal transmission and each symbol period represents 2 bits of logic information (0, 1, 2, 3). By transmitting two bits in one symbol slot, PAM4 halves the signal bandwidth. With half of the bandwidth, PAM4 can achieve 50Gb/s data rate transmission in the 25Gb/s electrical tolerance environment. Also, PAM4 can minimize signal degradation and double the data rate. PAM4 allows us to put more data onto the existing fiber. In other words, if you want to increase bandwidth, you don’t have to reconfigure the data center with more fibers, just using advanced modulation PAM4 technology to increase the data rate. These components for single-lambda 100G can be extended to 400Gbps transceivers with four-channel drivers and CWDM4 wavelengths. However, these advanced modulation techniques impose additional requirements on the optical components used, especially consume higher amounts of electrical power.

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PAM4 or CWDM4 for 100G and 400G Applications

Although speed is important in the data center, economics and special constraints make cost and complexity more important than speed. Most of the data centers have already worked toward 100G, 200G, and even 400G with the technology of PAM4 and CWDM4, so which is the best for 100G and 400G application?

PAM4 for 100G and 400G Applications

PAM4 is considered to be a cost-effective and efficient alternative solution for 100G and 400G construction. For 100G transceiver modules, single-wavelength PAM4 technology reduces the number of lasers to one and eliminates the need for optical multiplexing. For 400G, the largest cost is expected to be optical components and related RF packages. PAM4 technology uses four different signal levels for signal transmission. It can transmit two bits of logic information per clock cycle and double the transmission bandwidth, thus effectively reducing transmission costs. This effectively solves the problem of high cost while meeting bandwidth improvements.

CWDM4 for 100G and 400G Applications

CWDM4 (Coarse Wavelength Division Multiplexing) technology is another cost-effective solution for large-scale deployment and migration in data centers. For 100G and 400G networks, the network architecture uses four lanes of 25 Gb/s, using CWDM technology to transport 100G and 400G optical traffic on duplex single mode fiber (SMF). WDM reduces the number of fibers required to achieve this type of transmission, ultimately reducing the cost of the entire board.


As a popular signal transmission technology for high-speed signal interconnection in next-generation data centers, PAM4 signals are widely used for electrical and optical signal transmission on 100G, 200G, and 400G interfaces. There are also a large number of PAM4 QSFP28 and PAM4 SFP28 modules available on the market to help you build your network.

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25G WDM-PON for 5G Fronthaul Transmission

With the rapid development of mobile Internet, mobile data traffic and connected devices grow explosively. In response to the challenges brought by the network growth, 5G is under construction. 25G WDM-PON, combines the WDM technology and the PON topology, effectively bring high bandwidth, low latency, plug-and-play ONU (Optical Network Unit), simple OAM (Operation Administration and Maintenance) and low-cost advantages to 5G fronthaul transmission. Therefore, it has attracted widespread attention in the industry. This article describes 25G WDM-PON for 5G fronthaul transmission.

Technical Requirements for 5G Fronthaul Transmission

Compared to 4G architecture, the 5G RAN (Radio Access Network) architecture will evolve from the formal BBU (Base Band Unit) and RRU (Radio Remote Unit) two-level architecture of LTE to the three-level structure of CU (Centralization Unit), DU (Distribution Unit) and AAU (Active Antenna Unit). Therefore, the bearer network is also divided into front haul, middle haul, and backhaul. 5G base station fronthaul requires a bandwidth up to 25Gbps. However, as the number of base station increases, the cost of the base station to the fiber infrastructure networks and deployment will be higher. In this case, employing a Centralized Radio Access Network (C-RAN) architecture for 5G fronthaul holds great appeal for operators, but the current dark fiber solution for the C-RAN architecture requires a large number of mobile backhaul fibers. Most operators are still seeking an optimal solution to meet the needs of 5G fronthaul.

altTechnical Requirements for 5G Fronthaul Transmission

25G WDM-PON Solution for 5G Fronthaul Transmission

In the C-RAN architecture of 5G network fronthaul, the functionality of the 5G BBU will be reconstructed into two functional entities — CU and DU. The CU primarily includes the non-real-time part of wireless upper-layer protocol stacks and supports the distribution of some core network functions and the deployment of applications at the network edge. The DU mainly handles physical-layer functions and real-time transmission-layer functions. In order to save the transmission cost and reduce the transmission bandwidth between the RRU and the DU, some functions of the physical layer are moved down to the RRU implementation. 25G WDM-PON is a solution for transmission between DU and RRU. In the network architecture of WDM-PON 5G fronthaul, WDM-PON OLT and ONU are connected with DU and RRU respectively. The transparent business transmission between DU and RRU is realized by the adoption of the wavelength position multiplexing technology and AMCC (Auxiliary Management and Control Channel) technology. The OLT devices carry the midhaul service between DU and RRU at the same time that the OLT devices realize the fronthaul service between DU and RRU.

alt 25G WDM-PON Solution for 5G Fronthaul Transmission

Advantages of 25G WDM-PON for 5G Fronthaul

25G WDM-PON has advantages like high bandwidth, low latency, and low-cost. These all make 25G WDM-PON a better choice to satisfy 5G fronthaul requirements.

Reduce Capex

In the 5G construction, operators are faced with the pressure to reduce the number of sites and leased equipment rooms. In this case, employing a centralized radio access network (C-RAN) architecture can significantly reduce capital expenditure (Capex) on sites and equipment rooms. The 25G WDM-PON OLT can utilize an Access Office (AO) to deploy a centralized distributed unit (DU) pool. Thus, wireline and wireless AOs can be co-located if the condition permits.

Reduce Deployment Cost

The 25G WDM-PON 5G fronthaul solution allows for sharing of the existing fiber infrastructure, making it suitable for densely populated urban residential areas. The 5G network requires a large number of fiber resources. A network architecture based on the point-to-multipoint tree topology of a 25G WDM-PON can save a lot of fibers. The existing FTTx networks have rich lines and port resources in a wide range of deployments. 25G WDM-PON can completely reuse these resources to reduce 5G network deployment costs, avoid overlapping investments and improve 5G network coverage.


The 25G WDM-PON optoelectronic devices belong to the world’s leading edge, which requires high technology, chips, research and development capabilities. Overall, the 25G WDM-PON industry chain is basically mature, but it still needs continuous investment from all aspects of chips, transceiver modules (SFP, SFP+, SFP28), equipment and systems, to develop key technologies, reduce core device costs, establish a unified standard, and accelerate the productization process.

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25G SFP28 Transceivers for Data Center and Enterprise Applications

With the rapid expansion of data centers, faster speed and higher bandwidth are required for better experience. To meet the demands, data center operators are now turning to 25G Ethernet technology. 25G SFP28 transceivers, as high-bandwidth data switching fiber optics, play an important role in 25G fiber-optic communication systems. This article will reveal the 25G SFP28 transceivers for data center and enterprise applications.

What Is the Definition and Type of 25G SFP28 Transceiver?

25G SFP28 transceiver is the enhanced version of SFP+, which is designed for 25G signal transmission. It has the same physical structure as SFP and SFP+, but its electrical interface is upgraded to handle 25Gbps per lane. Therefore, the pinouts of SFP28 and SFP+ connectors are compatible. Compared to SFP+ optics, SFP28 has higher bandwidth, superior impedance control, and less crosstalk. 25G SFP28 transceiver module can be divided into Short-Range (SR) and Long-Range (LR) transceivers. The SR SFP28 is mainly used with OM3\OM4 multi-mode fibers to transfer data over a short distance (up to 100m), while the LR SFP28 is mainly used with OS2 single-mode fibers for long distance transmission (up to 10 km). 25G SFP 28 DAC/AOC cables are also popular on the market and both used for transmission distance below 30m.

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25G SFP28 Transceivers for Data Center Applications

The data center architecture is undergoing tremendous changes. Previously, the data center mainly composed of 1/10G servers and 1/10/40G EOR-MOR-TOR (End-of-Row, Middle-of-Row, Top-of-Rack) switches. Today, data centers are built using high-performance 10/25G servers and 10/25/40/100G EOR-MOR-TOR switches. Moreover, many data center operators choose SFP28 transceivers with SMF/MMF (Single-Mode Fiber/Multi-Mode Fiber) over SFP28 DAC/AOC (Direct-Attach Cable/Active Optical Cable) cables for 25G cabling solutions. That’s because by using the pluggable modules and fibers, data center operators can easily replace transceivers or upgrade TOR, MOR and EOR switches, servers, and cables without changing the cabling infrastructure.

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25G SFP28 Transceivers for Enterprise Applications

The traditional enterprise network includes workstations, desktop computers, access points or video walls, and local computer rooms. Generally, these networks consist of a building backbone cabling based on IEEE 802.3 MMF 10G-SR or SMF 10G-LR transceivers. Due to the increase in video conferencing and business applications, the enterprise campus traffic continues to grow, requiring faster speed than traditional 10G networks. In response to the needs, the enterprise campus is transforming from a backbone that was based on a 10G network to new high-performance 25G network. Since the 25G SFP28 transceivers are compatible with 10G equipment, it enables the 10G connectivity infrastructure to remain and achieve a seamlessly upgrading from 10G to 25G. The upgrading of the 25G network not only brings higher speed and bandwidth than 10G network but also saves money for the enterprise networks.

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25G SFP28 transceivers enable data centers and enterprises to reach a higher speed, scalability and performance level. To meet your interconnection needs, FS provides a variety of 25G SFP28 transceivers include 25GBASE-SR, 25GBASE-LR, CWDM SFP28 transceivers, 25G DAC/AOC Cables, and 100G QSFP28 to 4x25G SFP28 DAC/AOC cables. For all the transceivers, FS provides a lifetime warranty to ensure product quality and they are all compatible with major brand companies.

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25GbE Ethernet and 25GbE Products FAQs

25GbE Ethernet has been an eye-catching topic ever since its introduction in 2016. After its appearance, the upgrade path to 100G has a new option — 10G-25G-100G. Although 25G has already been released, it’s still a new thing for many people. Here collect some common questions which will help you to better know 25GbE Ethernet and 25GbE Products.

alt 25GbE Ethernet and 25GbE Products FAQs

25GbE Ethernet FAQs

What is 25GbE Ethernet?

25GbE Ethernet is a standard for Ethernet connectivity that benefits cloud and enterprise data center environments. It uses technology defined for 100GbE Ethernet, which implemented as four 25 Gbit/s lanes running on four fibers or copper pairs.

25GbE VS 10GbE, which is better?

As data centers require faster speed and higher bandwidth, 10GbE is not fast enough for switch-server connections. 25GbE was proposed to provide 2.5 times the performance of 10GbE as well as offers higher bandwidth at the same power consumption. 25GbE Ethernet supports 10GbE technology advancements in packaging and silicon. All these benefits contribute the existing switch architectures to supporting link speeds faster than 10GbE without the need to add cable/trace interconnects. At the same time, the architectures can keep up with the growth of the network bandwidth and become faster and richer.

25GbE VS 40GbE, who is the winner?

Compared to 40GbE, 25GbE can provide a higher port density with maximum switch I/O performance and fabric capability, enabling the network bandwidth to be effectively scaled in cloud and web-scale data center environments. What’s more, the 25G-100G (4x25G lanes) networking migration path provides a lower cost per unit of bandwidth by fully utilizing switch port capabilities when compared to 40G-100G upgrade path.

What makes 25GbE a choice for next-generation switch-server connections?

25GbE technology has two major advantages. First, by offering a single-lane 25Gbps variant, 25GbE allows for optimization of the bandwidth available from switch-fabric design, which is typically viewed from a per-pin and therefore per-lane standpoint. Another major advantage is that it takes existing module form factors and optical plants factors, such as SFP28 and QSFP28, and allows for a breakout connection that is configurable as either 25GbE per lane or the full 100GbE without changing the port on the front of switches and the wholes physical infrastructure.

Will the upgrade path of 100GbE be 10G-25G-100G?

Before the 25GbE specification was released, the 10G-40G-100G upgrade path is predominantly adopted by data centers to upgrade to 100GbE. However, due to the appearance of 25GbE, the 10G-25G-100G upgrade path has gained more recognition by offering the cost per bit, power consumption and server rack density advantages. 25GbE provides another option for 100GbE upgrade, and its prospect is bright.

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25GbE Products FAQs

What kinds of 25GbE products are on the market today?

Currently, 25GbE products on the market are relatively complete. These 25GbE switch, 25GbE NIC, 25GbE SFP28, 25GbE DAC/AOC cable are all available on the market.

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What does SFP28 stand for?

SFP28 is the enhanced version of SFP+, designed for 25G signal transmission. It has the same physical structure as SFP and SFP+, but its electrical interface is upgraded to handle 25Gbps per lane. The pinouts of SFP28 and SFP+ connectors are compatible. Compared to SFP+ solution, SFP28 has higher bandwidth, superior impedance control, and less crosstalk. In addition, the SFP28 copper cable has greater bandwidth and lower loss than the SFP+ version.

Can SFP28 cables be plugged into SFP+ ports?

Yes. SFP28 is backward compatible with SFP+ ports and can work sufficiently. SFP+ cables can be plugged into SFP28 ports but they are not designed for 25Gb/s data rates.

Can 10G SFP+ and 1G SFP transceivers be plugged into the associated 25G SFP ports?

Yes. The associated 25G SFP ports can run multiple speeds and support 10G SFP+ and 1G SFP transceivers. However, some port level restrictions may occur during speed mixing and matching.

Are branded 25G transceivers interoperable with third-party 25G transceivers?

Yes. Third-party 25G transceivers are compliant with relevant industry standard specifications and can be fully interoperable as long as compatibility is guaranteed.

Where can customers buy split cables for 100G to 4x25G connectivity?

Large vendors can supply volumes of MTP-LC multimode fiber breakout cables to customers for 100G to 4x25G connectivity. FS also provides 100G QSFP28 to 4x25G SFP28 DAC/AOC cables for you to choose.


25GbE Ethernet is considered an incremental update of 10GbE Ethernet, which supports 100GbE Ethernet with single lane at 25Gbps. Due to the booming of 25GbE, data centers will gradually turn to 25GbE networks. Knowing the above information is essential for you to deploy 25GbE Ethernet networks and avoid some problems when purchasing 25GbE products. FS offers a wide variety of high-quality and cost-effective 25GbE Ethernet portfolios such as 25gbe switch, 25G NIC, 25G SFP28 transceiver, and 25G DAC/AOC cables to meet your needs of 25GbE devices and connectivity options.

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CFP8 VS QSFP-DD: Which 400G Transceiver to Choose?

With the large-scale deployment of 100G, the ultra 100G technology — 400G has gradually become a hot topic in the industry. There is no doubt that 400G will eventually be the next major Ethernet speed in the data centers. In fact, the world’s leading optical transceiver manufacturers have launched their own 400G optical transceiver modules such as 400G QSFP-DD, OSFP, CFP8 and COBO transceivers for 400G data transmission. This article will focus on the 400G CFP8 VS QSFP-DD transceivers.

What Is 400G QSFP-DD Transceiver?

QSFP-DD refers to Quad Small Form-factor Pluggable Double Density. It is a new 400G transceiver designed with eight lanes that operate up to 25Gb/s NRZ modulation or 50Gb/s PAM4 modulation, providing solutions up to 200Gb/s or 400Gb/s aggregate. The term “Double-Density” refers to the doubling of the number of high-speed electrical interfaces that the module supports compared to the regular QSFP28 module. 400G QSFP-DD transceiver enables up to 14.4Tb/s aggregate bandwidth in a single switch slot. By quadrupling aggregate network switch bandwidth while maintaining port density, QSFP-DD can support continuing growth in network bandwidth demand and data center traffic.

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What Is 400G CFP8 Transceiver?

CFP8 transceiver is the 400G form factor type specified by the CFP MSA (Multi-Source Agreements). It is the same size as the CFP2 transceiver but uses a new 16x25G electrical I/O connector. As for bandwidth density, it respectively supports eight times and four times the bandwidth density of CFP and CFP2 transceiver. Generally speaking, there are four types of CFP8 modules on the market— CFP8 FR8/LR8, CFP SR16, and CFP DR4. The interfaces of the four CFP8 transceivers are generally specified to allow for 8 x 50Gb/s, 16 x 25Gb/s, 4x100Gb/s modes, respectively.


CFP8 VS QSFP-DD: Which 400G Transceiver to Choose?

The upgrading path of the CFP series is CFP-CFP2-CFP4-CFP8, and QSFP-DD has also experienced hardware specifications from 1.0 to 4.0. These are all long-established form-factor series. Compared to the QSFP series, the CFP series seems to be less popular for obvious reasons — large size and high power consumption. Below we will detail the differences between CFP8 VS QSFP-DD.

CFP8 VS QSFP-DD: Form Factor

In general, the width, length, and thickness of QSFP-DD are 18.35mm, 89.4mm and 8.5mm, while those of CFP8 are 41.5mm, 107.5mm and 9.5mm. The size of CFP8 transceiver is larger than QSFP-DD, and the volume is more than three times that of QSFP-DD. However, since the CFP8 transceivers are applied for telecommunication applications, and the port density requirements are not as high as in the data center, so the size is acceptable.

CFP8 VS QSFP-DD: Bandwidth

The maximum bandwidth of CFP8 and QSFP-DD is 400Gb/s, but CFP8 only supports 400Gb/s (16x25G, 8x50G, 4x100G), while QSFP-DD supports both 200Gb/s (8x25G) and 400Gb/s (8x50G). Thus, QSFP-DD works better than CFP8 in the performance.

CFP8 VS QSFP-DD: Thermal Capacity and Power Consumption

QSFP-DD has a thermal capacity of 7 to 12 watts, while CFP8 allows up 24 watts power consumption. The larger the thermal capacity, the greater the power consumption that the optical transceiver can withstand. With the advancement of technology, some industry-leading manufacturers have been able to reduce the power consumption of optical modules far below the upper limit of thermal capacity specified by MSA, so the larger thermal capacity does not seem to be a real advantage. However, as we all know, the lower the power consumption, the better.

CFP8 VS QSFP-DD: Backwards Compatibility

There is not any mention of backward compatibility in the hardware specification of CFP8. In fact, the entire CFP series transceiver does not seem to be backward compatible. However, 400G QSFP-DD transceiver has the advantage of backward compatible with QSFP, QSFP+, and QSFP28, which provides flexibility for end users and system designers.


After introducing and comparing QSFP-DD and CFP8, we find that QSFP-DD has unparalleled advantages in 400G applications. It is expected that when the world’s leading hyper-scale data centers begin to deploy 400G, QSFP-DD will become the mainstream form-factor of 400G optical transceivers. It is undeniable that QSFP-DD transceiver is the best choice for your network.

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