Metamorphosis of Transceivers (Part Ⅱ)

Posted on by FS.COM

Last week, we talked about the metamorphosis of transceivers. And the topic made reference to 10G transceivers. Though SFP+ has become so brilliant in the market, people still strive to explore new technology. One of an outstanding achievements called “Parallel” has come on stage. Today, we are going to acquaint with this guy.

People’s desire for bandwidth is always endless. In the near past, a movie of RMVB format would make us happy. But later, we preferred 1080P rather than 720P. Nowadays, super 4K is commonplace. The increasing bandwidth consumption for these applications boosts the demand for transceivers with higher rates. Since the time division multiplexing (TDM) technology has its limitation (10Gbps), the transceiver manufacturers start to design smaller transceivers and put these transceivers into a “box” which is as large as the original one. Parallel was born at that time.


Parallel Modules: QSFP & CXP
The operational mode of traditional XFP and SFP+ transceivers is like a pair of shoe in a box, one shoe is called transmitter and the other one is called receiver. But how about four shoes in a box? Can it realized? The answer is absolutely yes. We will call this as QSFP+ (it is usually called QSFP, but appears as QSFP+ in the literature. “+” means that its rate is over 8Gbps). And how about twelve shoes in a box? CXP is such thing to achieve it. “C” represents for 12 in hex, and the Roman number “X” means that each channel has a transmission rate of 10 Gbps. “P” refers to pluggable that supports the hot swap. Thus, CXP is a kind of hot-pluggable transceiver with data rate up to 12×10 Gbps.

Those modules, such as QSFP+, CXP, are called parallel transceiver. They work as if multi-transceivers working simultaneously in one box.

Does it mean that traditional SFP+ transceivers are out-dated as parallel transceivers are developed? The answer is no. This is because what is sold in the market is decided by customers’ demand and the products’ cost-performance ratio. Though SFP+ is no longer as popular as past, it is still the favor of 10G applications. Moreover, by improved to 28Gbps, it has a new development direction to meet the migration to 40/100 G.

MPO Optical Interface
The traditional XFP/SFP+ transceivers have two optical interfaces, one for transmitting and the other for receiving. In QSFP transceiver, there are four Tx and four Rx. While in CXP transceiver, 12 Tx and 12 Rx are desinged. Then how the light is transmitted in them? MPO optical interfaces is the key to achieve this. MPO, short for multiple-fiber push-on/pull-off, can provide connection for multiple optical fibers. MPO can be divided into 12-fiber MPO and 24-fiber MPO. 12-fiber MPO connection contains 12 optical fibers and 24-fiber MPO contains 24 optical fibers.

MPO interface

In short-haul transmission, the cost of fiber is not the matter. But it is quite different in long-haul transmission. In long-haul transmission, fiber cost is a main consideration so that we always want to find solution that help us save more cost on fiber. The basic version of QSFP is SR4 and LR4. QSFP SR4 is designed for short-haul transmission and QSFP LR4 is designed for long-haul transmission . SR is short for short reach and LR represents Long Reach. 4 means 4 Tx and Rx. Additionally, there are two prisms in the QSFP transceiver for long-distance transmission. One is used for MUX and the other one for DeMUX.



According to the above content, we know that the first-generation QSFP transceivers are equipped with four Tx and Rx and each channel has a rate of 10 Gbps. As the technology develops, now each channel of QSFP can transmit and receive data up to 28 Gbps. We called it QSFP28 which is a new trend for 100G applications.


Actually, 40G and 100G appeared almost at the same time. QSFP is defined to meet the demand for 40G applications. And for 100G demands, CFP is the pioneer. In CFP, the letter C is the abbreviation of Centum. Then how can it achieve 100 G? The following pictures give the answer:

CFP Solution 1:


CFP Solution 2:


Although CFP can achieve 100G application but its big size can no longer meet the demands of high-density data center. In this case, CFP2 and CFP4 with smaller sizes are developed. The following picture shows the size comparison of CFP, CFP2 and CFP4 modules.

Size comparison of CFP/CFP2/CFP4

CFP2 generally provides two solutions for 100G applications, as shown in the following picture:

CFP2 block


CFP4 is half the width of CFP2, which is half again the width of CFP. It’s more suitable for high-density applications. And now, people like to compare CFP4 with QSFP28 as they are both small size 100G module. In fact, QSFP28 has the same footprint and faceplate density as QSFP+ and is just slightly smaller than CFP4. QSFP28 seems to have the density advantage over CFP4, but CFP4’s higher maximum power consumption gives it the advantage on longer reach optical distances. Only time will tell how this all plays out, but in the meantime, there is a richness of choice in the 100G interconnect market.


In addition, there is another 100G module called CPAK in the market. CPAK is newcomer to this year’s demonstration. This is a proprietary form factor from Cisco but the interfaces demonstrated are IEEE standards and will interoperate with the same interfaces supported by other form-factors.

The following picture shows the comparison between 100G solutions which may help you to get a deeper understanding of this paper.


The metamorphosis of transceivers is coming to an end. More stories about transceiver and its friends in optical network will be introduced in the coming days. Please stay tuned for the rest of our articles.

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