40G Ethernet is rolling full steam ahead with the rapid development of data centers and cloud computing, which raises the demand for high-speed 40G interconnection cables and fibers. This post will introduce two of the most popular QSFP interconnection cabling solutions, QSFP DAC Cable, and QSFP+ SR4, the latter of which operate with fibers. Both of them are designed for 40G short-distance range transmission links. What are the differences and how to choose between them? This post will tell.
What is QSFP+ DAC Cable?
QSFP+ DAC Overview
QSFP+ DAC Cable (Direct Attach Cable) is also known as 40G QSFP+ DAC Twinax copper cable. QSFP DAC assembles two QSFP form factor modules at both ends of a Twinax copper cable. Both ends of QSFP modules are used to connect 40G QSFP ports of switches, routers, and servers. The max cable distance of 40G DAC reaches up to 10m (Passive QSFP+ DAC up to 7m, Active QSFP DAC up to 10m). Consequently, 40G Twinax copper cable is typically used within racks and across adjacent racks between servers and Top-of-Rack (ToR) switches. Since the cable material is copper, as we all know copper is cheap, 40G QSFP+ Twinax copper cable is naturally considered a cost-effective way to establish a 40-gigabit link.
How Many Types of QSFP+ DAC Cable
QSFP+ DAC consists of two types, passive QSFP+ DAC and active QSFP+ DAC, the main difference of which is that the Active QSFP DAC contains electronic components for signal conditioning while Passive QSFP DAC doesn’t. That’s why Active QSFP DAC is more costly than Passive DAC.
Besides, QSFP+ DAC also can be divided as QSFP+ to QSFP+ DAC, and QSFP+ DAC breakout cable (QSFP+ to 4x 10G SFP+ DAC breakout cable). They are used in different scenarios. The former is used to connect both ends of 40G ports of different network switches, while QSFP+ DAC breakout cable connects one QSFP port of one end with four SFP+ ports of the other end, which is mainly applied in the migration from 10G to 40G.
What is QSFP SR4 Transceiver?
QSFP-40G-SR transceiver is an optics used to achieve photoelectric conversion in a 40G optical link. In the transmitter, four-lane parallel 10 Gbp/s electrical signals are input through MTP/MPO fiber connector and converted into optical signals through the laser driver chip. Then, the optical signals are transmitted over the transmission media of the MTP multimode optical fiber cable to the receiver. At the receiver side, the optical signals are finally recovered as electrical signals via photodetector. The working diagram of QSFP+ SR4 is shown below.
QSFP+ SR4 Working Principle
Different from 40G DAC cable operating with copper transmission media, the QSFP+ SR4 uses fiber cable as a transmission media, which supports a link length of 150m over OM4 fiber cable, and up 100m over OM3 fiber cable with MTP/MPO fiber connectors.
40G QSFP DAC Cable vs QSFP SR4 Transceiver
Transmission Media Comparison
One of the most important differences between the 40G DAC cable and the QSFP SR4 transceiver is the transmission media. The 40G DAC cable uses copper while QSFP SR4 uses fiber. Is fiber better than cable? The post will compare the 40G DAC copper cable and fiber below.
Cost Comparison
We investigated the price of QSFP DAC cable and QSFP SR4 transceiver and fiber cable from two manufacturers of QSFPTEK and FS.COM. By comparison, it is a truth that both of QSFPTEK and FS.COM, 40G QSFP DAC cable solution is cheaper than QSFP SR4 transceiver and fiber solution.
| 40G DAC Cable (3M) | 40G SR4*2+Fiber(3M) | |
| QSFPTEK | $18.6 | $97.7 |
| FS.COM | $35 | $132 |
* Transceiver cabling solution includes two transceivers and one fiber cable.
Transmission Distance Comparison
The QSFP28 SR4 and fiber cable combination support a link length of 150m, which is much longer than the max cable length of 40G DAC cable. Does it mean QSFP SR4 and fiber are better than 40G DAC copper cable? Not exactly. Transceiver and fiber combination does have a wider range of applications than DAC cable, and they are more suitable for backbone cabling of data centers.
However, besides backbone cabling, there is horizontal cabling in data centers. DAC copper cable is usually used in horizontal cabling of the access layer for its advantages of low cost and easy-to-deploy. So which option is better between 40G DAC cable and fiber depends on the application and connected devices (switch, router, rack, etc.)
Conclusion
By comparison, 40G DAC cable is better than 40G optical transceiver and fiber from the aspect of cost and easy deployment and management. 40G optical transceiver and fiber is better than 40G DAC cable with the latter advantages of a wider range of transmission distance, stable transmission. QSFPTEK provides cost-effective 40G DAC cables and QSFP optics. If you are interested, you are welcome to consult via sales@qsfptek.com.
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