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The growing popularity of 40G ports of servers and switches stimulates demand for 40G short-distance interconnection optics. The mainstream 40G MMF transceiver solutions are QSFP-40G-SR-BD and QSFP-40G-SR4. This post will introduce them in detail from the aspects of their working principle, specification, cabling, and connection solution.
QSFP-40G-SR-BD and QSFP-40G-SR4 Introduction
Both QSFP-40G-SR-BD and QSFP-40G-SR4 support high-speed and high-bandwidth 40G short-distance links and characterize the high-density QSFP form factor and pluggable. Then, what are their differences? It will be explained next.
What is QSFP-40G-SR4 and How Does it Work?
QSFP-40G-SR4 is designed with 4 parallel and individual to transmit and receive channels. Each channel supports 10Gbps data rate transmission at the wavelength of 850nm, up to a total of 40Gbps bandwidth. QSFP SR4 supports a link length up to 150m over OM4 and up to 100m over OM3 with an MTP/MPO connector. The module structures with two interfaces, electric interface and optical interface. The electric interface is designed for 4-lane individual and parallel electric signals input and output while the optical interface is used to input and output optical signals with MTP/MPO connector. The transceiver is able to convert the input electric signals into optical signals, and then transmit the optical signals via a multimode fiber, in the meantime, the other side receives the optical signals and transfers them into electric signals, the working principle of QSFP-40G-SR4 is shown in figure 1.
Figure 1 – QSFP-40G-SR4 Working Principle
What is QSFP-40G-SR-BD? and How Does it Work?
QSFP-40G-SR-BD also stands for QSFP 40G BiDirectional (BiDi) transceiver
Similar to QSFP-40G-SR4, QSFP BiDi also supports a link length up to 150m over OM4 and up to 100m over OM3. However, QSFP BiDi structures with LC duplex connector instead of MTP/MPO connector. Another major difference is QSFP-40G-SR-BD enables a 40G data rate link with only two 20Gbps of transmission and receive channels, each of which enables different wavelengths of 832nm – 918nm over only one multimode fiber. As mentioned above, to achieve an equatorial 40G data rate link, the QSFP-40G-SR4 transceiver needs a total of 8-lane 10G channels. The transmission and receive channels of QSFP SR4 and QSFP BiDi are shown in figure 2.
Figure 2 – Transmission and Receive Link of QSFP-40G-SR4 and QSFP-40G-SR-BD
Specification Comparison: QSFP-40G-SR-BD vs QSFP-40G-SR4
As mentioned above, the major common and differences between QSFP BiDi transceiver and 40G SR4 can be concluded as the following table:
| Transceiver Type | Wavelength | Connector | Cable Type | Reach Distance | Power Consumption |
| QSFP-40G-SR-BD | 832 – 918nm | LC duplex | MMF | 100m(OM3)/150m(OM4) | 3.5W |
| QSFP-40G-SR4 | 850nm | MTP/MPO-12 | MMF | 100m(OM3)/150m(OM4) | 1.5W |
Table 1 – Specification Comparison of QSFP-40G-SR-BD and QSFP-40G-SR4
QSFP-40G-SR-BD vs QSFP-40G-SR4, How to Choose?
QSFP-40G-SR4 Cabling and Link Solution
As mentioned above, QSFP SR4 structures with MTP/MPO connector, so it can work with 12-fiber MTP/MPO OM4 multimode trunk cable to fulfill a 40G link, as shown in figure 3. Besides, it can also support 4x10G breakout connectivity by working with 8-fiber MTP to 4 LC OM4 multimode breakout cables to connect 4x 10G SFP+ SR transceivers, as shown in the figure 4. The QSFP to 4x SFP+ breakout connection is widely applied in 10G to 40G network upgrading.
Figure 2: QSFP-40G-SR4 Cabling Solution 1 – QSFP-40G-SR4 to QSFP-40G-SR4
Figure 3: QSFP-40G-SR4 Cabling Solution 2 – QSFP-40G-SR4 to 4x SFP-10G-SR Breakout Connection
Product of mentioned in above figures:
12-Fiber MTP/MPO OM4 Multimode Trunk Cable
8-Fiber MTP to 4 LC OM4 Multimode Breakout Cable
QSFP-40G-SR-BD Cabling and Link Solution
Unlike QSFP SR4, which must work with MTP fiber cables, QSFP BiDi enables duplex LC multimode fiber (MMF) connection with the duplex LC interfaces. So QSFP BiDi brings a great benefit in making full use of the existing 10G MMF cabling construction. When upgrading the current 10G network to 40G, you are able to simply replace the 10G SR transceivers at both ends without any fiber cable changing, which can save a lot of budget on redeploying optical cables.
However, the price of QSFP BiDi is much more expensive than QSFP SR4, so the cost of MTP fiber cable construction needs to be compared against the price of the QSFP BiDi transceiver in the actual network environment.
Conclusion
From those mentioned above, we are able to learn that each of the two transceivers has pros and cons. For QSFP-40G-SR-BD, it enables the most convenient and cost-saving cabling solution based on the existing 10G cable structure, but QSFP BiDi itself is much more expensive than QSFP SR4. For QSFP-40G-SR4, it provides a flexible 4x10G breakout connection between QSFP SR4 and SFP+ SR, and QSFP SR4 enjoys price advantages owing to mature production technology and high-volume manufacturing, but it refers to an entire rip-and-replace of existing multimode cabling.
If you can’t make up your mind which to choose between QSFP BiDi and QSFP SR4, please feel free to consult the IT engineer of QSFPTEK by email at [email protected] or visit qsfptek.com to buy quality-assured QSFP optics at best price. QSFPTEK always loves to help if you need it.
