40GBASE-SR4 vs. LR4 vs. CSR4 vs. CR4

Posted on December 13, 2019
August 12, 2020

When selecting a QSFP+ transceiver, customers are easily confused by the models of the 40G transceivers. Some of the models are extremely similar to each other, just like 40GBASE-SR4 and 40GBASE-CSR4. They seem to have the same number and letters. How should we distinguish them? In this post, we will tell you how to differentiate the four most bewildering 40G QSFP+ modules as well as some frequent puzzles about them.

What Is 40GBASE-SR4?

40GBASE-SR4, also known as QSFP-40G-SR4, is used for 40G Ethernet connectivity in short reach (SR). Complying with the IEEE 802.3ba 40GBASE-SR4 standard, QSFP+ SR4 optics, with an MPO/MTP fiber ribbon connector, can transmit up to 100 meters over OM3 cable and 150 meters over OM4 cable separately.

Inside the QSFP-40G-SR4, it has independent transmitter and receiver sections, each with 4 parallel fiber strands. When the SR4 optics are used in a 40G connection, all the 8 fiber strands are required and the optical cable with 12-fiber MTP/MPO connector is needed. Consequently, 4 fiber strands in each connection are idle as shown in the following figure.

Figure 1: 40GBASE-SR4 for 40G Connection

What Is 40GBASE-LR4?

Just like the 40GBASE-SR4, 40GBASE-LR4 (QSFP-40G-LR4) is also used for the 40G Ethernet transmission. Conforming to the IEEE 802.3ba 40GBASE-LR4 standard, QSFP+ LR4 optics, with a duplex LC connector, can support long reach (LR) connectivity up to 10 km over single mode fiber. Using the same form factor as 40GBASE-SR4, QSFP-40G-LR4 transceiver also offers four independent transmit and receive channels, with a bandwidth of 10 Gbps for each one.

To carry the 4-channel signals over a single mode fiber, the 40G LR4 optics has to multiplex/demultiplex optical signals via the MUX/DEMUX within it. You can refer to the following figure for the detailed working mode of 40GBASE-LR4.

Figure 2: Working Mode of 40GBASE-LR4

What Is 40GBASE-CSR4?

Based on the IEEE 802.3ba 40GBASE-SR4 standard, 40GBASE-CSR4 (QSFP-40G-CSR4) is an extended version of 40GBASE-SR4 with the same working principle. Besides, each 10-Gigabit lane of the 40GBASE-CSR4 module is compliant with IEEE 10GBASE-SR specifications. Equipped with an MTP/MPO interface, QSFP-40G-CSR4 supports a longer 40G transmission up to 300 m over OM3 and 400 m over OM4 separately.

What Is 40GBASE-CR4?

40GBASE-CR4(QSFP-40G-CR4) is a pre-terminated copper cable with both ends terminated with a QSFP+ connector. It is very different from the three 40G modules mentioned in previous parts.40GBASE-CR4 is known as 40G QSFP+ direct attach cable (DAC), but 40GBASE--SR4/LR4/CSR4 are optical transceivers. Limited by the property of copper cable, 40GBASE-CR4 can only reach a transmission distance up to 7 meters for passive DAC type and 10 meters for active DAC type.

40GBASE-SR4 vs. LR4 vs. CSR4 vs. CR4: How to Choose?

40GBASE-SR4/LR4/CSR4/CR4 modules are all designed for 40G Ethernet transmissions. You may wonder which one is better. And the answer will definitely depend on the specific applications and whether the transceiver’s performance can meet your application. In the table below, the major differences between these three QSFP+ transceivers and a QSFP+ cable have been listed for your reference.

Parameter QSFP-40G-SR4 QSFP-40G-LR4 QSFP-40G-CSR4 QSFP-40G-CR4
Wavelength 850 nm 1310 nm 850 nm N/A
Connector MTP/MPO LC duplex MTP/MPO QSFP+ to 4 SFP+/duplex LC
Max. Cable Distance 100 m (OM3)
150 m (OM4)
10 km 300 m (OM3)
400 m (OM4)
7 m (passive)
10 m (active)

Figure 3:40GBASE-SR4 vs. LR4 vs. CSR4 vs. CR4

FAQs About 40GBASE-SR4/LR4

40GBASE-SR4 vs. iSR4/eiSR4: what are their differences?

40GBASE-iSR4 (QSFP-40G-iSR4) is Huawei's product name for 40GBASE-SR4 transceiver. And 40GBASE-eiSR4 (QSFP-40G-eiSR4), also owned by Huawei, refers to the 40GBASE-CSR4 module. Actually, the features of 40GBASE-iSR4 and 40GBASE-eiSR4 are the same with 40GBASE-SR4 and 40GBASE-CSR4 respectively.

How to distinguish between 40GBASE-ER4 and 40GBASE-LR4?

Similar to 40GBASE-LR4, 40GBASE-ER4 (QSFP-40G-ER4), with a duplex LC connector, is also used for long-distance transmission. But these two 40G transceivers show differences in transmission distance and IEEE standard. Complying with the IEEE 802.3bm 40GBASE-ER4 standard, the link distance of QSFP-40G-ER4 is up to 40 km over the single mode cable.

What’s the difference between QSFP-40G-LR4 and QSFP-40G-PLR4?

QSFP-40G-PLR4 is also named as QSFP-40G-LR4 PSM by some IT peers in an informal way. The major differences between 40GBASE-LR4 and 40GBASE-PLR4 lie in their optical interface and supported protocols. Based on IEEE 802.3ae 10GBASE-LR standard, 40GBASE-PLR4 module comes with an MTP/MPO connector, but the 40GBASE-LR4, based on IEEE 802.3ba 40GBASE-LR4 standard, is equipped with a duplex LC connector. To get more details about the difference, you can move to the post: QSFP-40G-LR4 Transceiver: QSFP CWDM vs PSM.

What’s the difference for 40GBASE-SR4 vs. AOC?

Like the 40GBASE-SR4 transceiver, 40G AOC (active optical cable) is also used for 40GbE applications in a short distance. But the difference is that the QSFP+ AOC is a pre-terminated optical cable, terminated with 40GBASE-QSFP+ on one end, while on the other end, besides QSFP+ connector, it can be terminated with SFP+ connector, LC connector, etc. Conventionally, the link distance of 40G AOC is no farther than 100 meters.