A Comprehensive Introduction to 100G Optics


These years, the deployment of the 100G network has been the development focus of the optical interconnect industry, which contributes to the fast development of 100G optics in data center markets. Currently, the 100G optical module mainly consists of CXP optics, CFP, CFP2, CFP4optical module and QSFP28 optical transceiver. Then this post will have a comprehensive introduction to 100G optic fiber modules.

CXP Optical Transceiver

CXP is targeted at the clustering and high-speed computing markets. Therefore,  it is also can be called as high-density CXP. Technically, the CFP will work with multimode fiber for short-reach applications, but it is not actually optimized in size for the multimode fiber market, which is due to that the multimode fiber market requires high panel density to a large extent. The CXP is created to satisfy the high-density requirements of the data center, targeting parallel interconnections for 12x QDR InfiniBand (120Gbps), 100GbE, and links between systems collocated in the same facility. CXP is currently standardized by the InfiniBand Trade Association.

Moreover, in structure, the CXP is 45mm in length and 27mm in width, slightly larger than that of an XFP. It includes 12 transmitting/receiving channels in its compact form factor, which can be realized via a connector configuration similar to that of the CFP. In some sense, the CXP enables a front panel density higher than that of an SFP+ to run at 10Gbps. In applications, typical applications of CXP in the data center include 100GE over Copper (DAC with CXP connectors): 7m (23ft) and 100GE over multimode fiber: CXP for short reach applications (CFP is used for longer reach applications).

CFP Optical Transceiver

CFP, short for C form-factor pluggable, is a multi-source agreement to produce a common form-factor for the transmission of high-speed digital signals. The C stands for the Latin letter C used to express the number 100 (centum), since the standard is primarily developed for 100 Gigabit Ethernet systems. In fact, CFP also supports 40GbE. When it comes to CFP, it can be defined as multipurpose CFP.

CFP is designed after the Small Form-factor Pluggable transceiver (SFP) interface but is significantly larger to support 100Gbps. The electrical connection of a CFP uses 10 x 10Gbps lanes in each direction (RX, TX). The optical connection can support both 10 x 10Gbps and 4 x 25Gbps variants. CFP transceivers can support a single 100Gbps signal like 100GE or OTU4 or one or more 40Gbps signals like 40GE, OTU3, or STM-256/OC-768.

CFP vs CFP2 vs CFP4Furthermore, CFP form factor, detailedly defined in the CFP MSA(Multi-Source Agreement), is available for both single-mode and multi-mode fiber and various data rates, protocols, and link lengths, including all the Physical Media-Dependent (PMD) interfaces included in the IEEE 802.3ba standard. For 40GE, its target optical interfaces include the 40GBase-SR4 for 100 meters and the 40GBase-LR4 for 10 kilometers. While with respect to 100GE, there are three PMDs for it: 100GBase-SR10 for 100 m, 100GBase-LR4 for 10 km, and 100GBase-ER4 for 40 km.

Note: about CFP form factors, the CFP-MSA Committee has defined three types as follows:

  • CFP – Currently at standard revision 1.4 and is widely available in the market
  • CFP2 – Currently at draft revision 0.3, half the size of the CFP transceiver; recently available in the market
  • CFP4 – Standard is not yet available, half the size of a CFP2 transceiver, not yet available

QSFP28 transceiver

For QSFP28 optical module, except that it’s a hot-pluggable optical module designed for 100G data rate, it integrates 4 transmitting and 4 receiving channels. And “28” means each lane carries up to a 28G data rate. Meanwhile, it can do the 4x25G/2x50G/1x100G connection.

In addition, the QSFP28 transceiver and interconnect cable are a high-density, high-speed product solution designed for applications in the telecommunications, data center and networking markets. The interconnect offers four channels of high-speed differential signals with data rates ranging from 25Gbps up to potentially 40Gbps and will meet 100Gbps Ethernet (4x25Gbps) and 100Gbps 4X InfiniBand Enhanced Data Rate (EDR) requirements. QSFP28 is fast becoming the universal data center form factor. QSFP28 transceiver is typically available in two standards—100GBASE-SR4 and 100G QSFP28 LR4. 100G QSFP28 SR4 operates at multimode fiber for a distance of 100m. 100GBASE-LR4 QSFP28 can support a much longer distance of 10 km.


Above all is about the introductions to the 100G optical transceiver. Except for the above three types, there is another 100G optical transceiver called CPAK in the market. The CPAK optical transceiver is the new module type popular this year.


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