Tag Archives: 40GBASE-SR4

Differences Between QSFP-40G-SR4, QSFP-40G-CSR4 and QSFP-40G-CR4

When selecting 40G fiber optic transceivers, customers are easily confused by the standards and models of the 40G modules. This is because the standards of transceiver are generally the combinations of letters and numbers. Some of the standards and models are really similar to each other. Thus, this post is to offer an illustration of three 40G modules whose standards and models are easily misidentified. These 40G modules are QSFP-40G-SR4, QSFP-40G-CSR4 and QSFP-40G-CR4. Details of the three modules will be firstly introduced separately. Then comparison in specific applications among them will be offered.

QSFP-40G-SR4 (40Gbase-SR4)

The three standards look similar. However, they actually have great differences from each other. Here starts from 40Gbase-SR4. Many vendors use “QSFP-40G-SR4” to identify 40G QSFP+ modules that support by 40Gbase-SR4 standards. 40Gbase-SR4 QSFP+ is the most popular 40G fiber optic transceiver for transmission in short distances. 40GBASE-SR4 QSFP+ module supports link lengths of 100m and 150m, respectively, on OM3 and OM4 multimode fibers. It is a parallel fiber optic transceiver containing a MTP/MPO interfaces, which use 4*10G transmission pattern. With four 10G SFP+ lane in one module, QSFP-40G-SR4 can achieve 40G optical signal transmitting and receiving at the same time via a ribbon fiber cable. The following picture shows how signals are transmitted over a MTP ribbon fiber cable.

40G-SR4 transmission

QSFP-40G-CSR4 (40Gbase-CSR4)

The working principle of QSFP-40G-CSR4 module is similar to QSFP-40G-SR4 module. Both QSFP-40G-CSR4 module and QSFP-40G-SR4 module has MTP/MPO interface. However, 40Gbase-CSR4 module can be regarded as an upgraded version of the 40Gbase-SR4 module. It can support longer 40G transmission up to 300m on OM3 and 400m on OM4. Transmission distance is the biggest different form the two 40G fiber optic transceivers.

QSFP-40G-CR4 (40Gbase-CR4)

QSFP-40G-CR4 module is very much different from the above mentioned two 40G modules. It is a pre-terminated copper cable with both ends terminated with QSFP+ connector. It is known as 40G QSFP+ cable or 40G QSFP+ direct attach cable. Limited by the property of copper cable, QSFP-40G-CR4 can only reach a transmission distance of 7 meters.


Which One Should You Choose QSFP-40G-SR4, QSFP-40G-CSR4 or QSFP-40G-CR4?

As the above mentioned, QSFP-40G-SR4, QSFP-40G-CSR4 and QSFP-40G-CR4 are all designed for 40G transmission in short distances. Which one is better for your applications? This should be decided by the specific applications. Generally, there are two types of transmission in 40G network: 40G to 40G transmission and 40G to 10G transmission. All the three modules can accomplish the two applications. However, there are some differences.

40Gbase SR4

QSFP-40G-SR4 and QSFP-40G-CSR4 uses the same connection methods for 40G-40G and 40G-10G transmission. For 40G to 40G connections, a length of MTP trunk cable can be used to connect two QSFP-40G-SR4 or QSFP-40G-CSR4 modules to form the whole link. The following picture takes Cisco Nexus 9396PX 40G connection as an example. For 40G to 10G connection, a MTP-8LC harness cable can be used to connect a QSFP-40G-SR4 or QSFP-40G-CSR4 module with other 10G devices.

40G to 40G connection with QSFP-40G-CSR4

For 40G QSFP+ to QSFP+ direct attach cable (DAC), no additional fiber patch cables are need for 40G to 40G connection. Directly insert the two QSFP+ connector in to the devices, 40G transmission can be achieved. For 40G to 10G transmission, another version of QSFP+ DAC can be used, which is known as QSFP+ to 4 SFP+ direct attach cable. The following picture shows how to connect a 40G port to four 10G port by using a QSFP+ to 4 SFP+ copper cable.

40G to 10G connection

If your transmission distance is less than 150 meters but longer than 7 meters, then QSFP-40G-SR4 module would be the best choice. If your transmission distance is longer than 150 meters, QSFP-40G-CSR4 module is suggested for better network performance. If the transmission distance is less than 7 meters, then QSFP-40G-CR4 modules would be the most cost-effective solutions.

Related Articles: How to Connect 40G QSFP+ SR4 Transceivers to Network?

                              40G Transceiver Module: QSFP+ Module And CFP Module


40GBASE-SR4 QSFP+ transceivers is now being widely used for 40G network interconnection in short distance. This is a highly integrated component which has small size and uses four channels to support conversions between optical signals and electrical signals over a high data rate up to 40G. However, another component which can replace this transceiver in network interconnection becomes popular in data center application. It is 40GBASE QSFP+ AOC (active optical cable).

AOC is a kind of direct attached cable (DAC), which is available in 10G, 40G and even 120G. 40GBASE QSFP+ AOC contains a QSFP+ connector on one end and one QSFP+ connector on the other end which are linked by a length of fiber optic cable. There are also fanout versions of 40GBASE QSFP+ AOC with one end connected with a QSFP+ connector and the other end with several SFP+/XFP connectors. It looks like a fiber optic patch cable, but, it has a similar function of optical transceiver, and also can transmission signals with its fiber optic cable. 40G AOC removes the process of two modules, which must be done in a 40G interconnection using 40G SR4 QSFP+ transceiver. It seems that both 40G AOC and 40G transceiver are good solutions for interconnection. However, everything has it’s Pros and Cons. Figure out the one that fits your application is the most reasonable way. The following is to offer comparison between these two components for your references.


Transmission Distance: The first aspect to be considered is the transmission distance of these two components. Currently the 40G transmission is usually used in backbone network. Thus, to assure the transmission quality, distance should be ensured. Both 40GBASE-SR4 QSFP+ and 40GBASE QSFP+ AOC are designed for 40G transmission in short distance. Generally, when the distance is shorter than 100 meters, the two have similar performance. However, when it’s longer than 100 meters, AOC cannot perform as good as transceiver. Currently most 40G AOC provided by the manufacturers are less than 100 meters. However, Fiberstore can provide 40G AOC up to 300 meters.

Reliability: In work state, both of the components should be inserted into a switch or server. And the repeating plug of them are necessary for daily use and maintenance. It is known to us that, these actions might affect the performances of the component. Thus reliability of these components should be considered. The connectors of 40G AOC are factory pre-terminated, while QSFP+ SR4 transceivers are connected by additional MPO connectors and fiber optic cable. Thus, compared with QSFP+ SR4 transceiver, AOC is less affected by the repeating plug during daily use. It has been proved that AOC has better reliability than that of transceivers.

Installation and Maintenance: it has been clear that 40G AOC is much easier during installation, as the connectors have already terminated in factory. Customers just need to plug the two connectors in the switches, then can start working. While, for 40G SR4 QSFP+, additional patch cords with MPO connectors are used to finish the link. If there is a fault in the interconnection, for AOC, you can just replace it with another AOC. However, for interconnection using 40G QSFP+ SR4 transceivers, you have to locate the fault firstly by testing the patch cords and optics.

40GBASE-SR4 QSFP+ transceiver under test

Digital Diagnostic Monitoring (DDM): to achieve the best working state, most modern transceivers are armed with DDM function. With it, the working states and performance of the optics can be visually controlled. No wonder that 40GBASE-SR4 QSFP+ has such function. However, 40G AOCs that are provided by the market now do not have it.

Cost: two main aspects should be considered in selection a product in data center. One is the material cost. The other is the maintenance cost in the daily use. AOC has advantages over transceivers on both aspects. The price for 40G AOC is generally cheaper than 40G QSFP+ SR4. In addition, the interconnections reply on 40G transceivers also need additional fiber optic cables. The latter aspect has been clearly illustrated in the above. AOC can save more in general.

In conclusion, 40GBASE-SR4 QSFP+ can achieve best working status by using DDM and it has better performance when the transmission distance is longer than 100 meters. While 40GBASE QSFP+ AOC is cheaper, easy to manage and test, and it has similar performance as the former does over transmission distance less than 100 meters. For your reference, Fiberstore offer a wide range of both 40GBASE-SR4 QSFP+ and 40GBASE QSFP+ AOC. You can trust all of them cause they are all be tested before they go to the marker. Kindly contact sale@fs.com or visit FS.COM for more detail, if you are interested.

Upgrade to High Data Rate Transmission With Parallel Optic

Parallel optic represents a type of optical communication technology as well as the devices on either end of the link that transmit and receive information which are also known as parallel optical transceivers. Compared with traditional optical communication, parallel optic communication employs a different cabling structure for signal transmitting aiming at high-data transmission for short reach multimode fibers that are less than 300 meters. Traditional fiber optic transceivers cannot satisfy the increasing demand for high speed transmission, like 40GbE, while parallel optics technology can be a cost effective solution for 40/100GbE transmission.

Comparison between parallel optic technology and the traditional serial optical communication would better explain what parallel optic is and the reason why it is a cost effective solution to high data rate transmission. The following of this article will offer the comparison between the two optical communication technology from two aspects: connectivity method and key components.

Connectivity Method of Parallel Optic

Literally, parallel optics and serial optics transmit signals in different ways. In traditional serial optical communication, on each end of the link, there are one transmitter and one receiver. For example, the transmitter on End A communicates to the receiver on End B, sending a single stream of data over a single optical fiber. And a separate fiber is connected between the transmitter on End B and the receiver on End A. In this way, a duplex channel is achieved by two fibers.

2-fiber duplex connection

While in parallel optic communication, duplex transmission is achieved in a different way. A signal is transmitted and received through multiple paths, thus, the parallel optical communication can support higher data rate than the traditional optical communication. This is because, the devices for parallel optic communication on either end of the link contain multiple transmitters and receivers. For instance, in 2010 IEEE 802.3ba approved the 40GBASE-SR4 physical-medium-dependent multimode parallel optical solution, which uses eight fibers to transmit four duplex channels each at 10 Gigabit Ethernet. In this case, four 10Gbps transmitters on End A communicate with four 10Gbps receivers on End B, spreading a single stream of data over four optical fibers at a total data rate of 40Gbps.

Key Components of Parallel Optic

The parallel optical communication transmitting signals over multiple fibers, which has great advantages over traditional serial optical communication. It also means that it requires different components to support its high data rate transmission.

Connector: As previously mentioned, duplex transmission in serial optical communication uses 2-fiber duplex connectors, like duplex LC connectors to link the optics with other devices, while in parallel optical communication, multi-fibers are used to reach a higher data rate. Thus, multi-fiber connectors, like 12-fiber MPO connectors are used to connect with other devices. MPO connector is one key technology support parallel optical communication. This connectivity method is showed in the following picture?(Tx stands for transmit; Rx stands for receive).

12-fiber MTP parallel connection

Optical transceiver light source: Another complementary technology for parallel transmission is the light source of parallel optics—VCSELs (Vertical Cavity Surface Emission Lasers). Comparing with the edge-emitting semiconductor lasers in the traditional optics, VCSELs have better formed optical output which enables them to couple that energy into optical fibers more efficiently. In addition, VCSELs emit from the top surface, they may be tested while they are part of a large production batch (wafer), before they are cut into individual devices, which dramatically lowers the cost of the lasers. The following chart is about the comparison between VCSELs and edge-emitting semiconductor lasers. Cheaper to manufacture, easier to test, less electrical current required, supporting higher data rate, parallel optics using VCSELs could be a better choice to reach 40/100GbE transmission compared with traditional serial optics.

VCSEL vs Edge-Emitting Laser
Feature VCSEL Edge-Emitting Laser
Power consumption 2-3 mW 20 mW
Beam quality/ease of coupling Better, round low divergence Fine, asymmetric
Speed 10 Gbps 1 Gbps
Temperature stability 0.06 nm/oC 0.25 nm/oC
Specral width 1 nm 1-2 nm
Speckle Low in an array High


Parallel Optic for 40/100GbE Transmission

IEEE has already included physical layer specifications and management parameters for 40Gbps and 100Gbps operation over fiber optic cable. Two popular parallel optic solutions for 40Gbps and 100Gbps over multimode fibers are introduced here. For 40G, 40GBASE-SR4 transceiver is usually used, which requires a minimum of eight OM3/OM4 fibers for a transmit and receive link (4 fibers for Tx and 4 fibers for Rx). 100GBASE-SR10 transceiver is for 100Gbps transmission, which requires a minimum of 20 OM3/OM4 fibers for a Tx/Rx link, 10 fibers are used for Tx and the other 10 are for Rx.

40BASE-SR4 and 100BASE-SR10


The capabilities and uses of parallel optic and MPO technology continue to evolve and take shape as higher-speed fiber optic transmission, including 40/100GbE. It is uncertain that parallel optical communication would be the trend in the future. However, many cabling and network experts have pointed out that parallel optical communication supported with MPO technology is currently a way to equip an environment well prepared for the 40/100GbE transmission.