40G QSFP+ LH4 vs. LR4, What is the Difference?

40G QSFP+ LR4

As a standard solution for 10km transmission, the core of the 40G QSFP+ LR4 module lies in the application of CWDM (coarse wavelength division multiplexing) technology. The 40G module converts four 10Gbps electrical signals into four independent wavelengths of 1271nm, 1291nm, 1311nm and 1331nm, and realizes 40G aggregation transmission on a single LC duplex single-mode fiber through an internally integrated multiplexer/demultiplexer. Its design uses a combination of DFB laser and PIN photodiode, and the power consumption is controlled within 3.5W, which is cost-effective. It strictly follows the IEEE 802.3ba standard to ensure plug-and-play compatibility with mainstream devices.

40G QSFP+ LH4

The 40G QSFP+ LH4 module is enhanced for the 20km ultra-distance requirement. To double the transmission distance, the solution adopts three key technology upgrades: first, high-power EML lasers are used to replace DFB lasers, and the output optical power is increased by 3dB to compensate for long-distance losses; second, an avalanche photodiode (APD) receiver is equipped to increase the receiving sensitivity from -11dBm of LR4 to -18dBm, significantly enhancing the weak signal capture capability; finally, a forward error correction (FEC) mechanism is introduced to suppress the bit error rate. These improvements increase the power consumption to 4.5W, and the cost increases by 30%-50% accordingly. (Note: 40G QSFP+ LH4 modules may vary from vendor to vendor, please refer to the manufacturers’ optics specification.) Because it exceeds the scope of IEEE standards, it is necessary to rely on the manufacturer’s custom specifications (such as Cisco’s LR4-Lite) to achieve interoperability.

Selection Guide

In actual deployment, the LR4 module has become the first choice for scenarios within 10km due to its high cost performance, and is typically used in data center campus building interconnection or enterprise backbone network construction. Its core advantage is that it can directly reuse the existing 10G-LR fiber infrastructure, and the LC interface design supports high-density port deployment, and the operation and maintenance cost is reduced by 40% compared with similar solutions. For example, a bank data center is connected to a disaster recovery center 8 kilometers away through LR4. The power consumption of a single module is only 3.2W, and the failure rate is less than 0.5% in three years.

The LH4 module specializes in special scenarios of 10-20km, especially for links where edge data centers access core computer rooms or need to pass through electromagnetic interference sensitive areas (such as subway tunnels and substations). In a smart city project, LH4 successfully achieved 18 kilometers of cross-river transmission, and its APD receiver effectively resisted signal fluctuations caused by water surface reflection. However, it should be noted that this solution requires the fiber link loss to be ≤0.25dB/km, and the unit price is 1.8 times that of LR4, so it needs to be carefully evaluated when the budget is limited. When the transmission distance is close to the critical value, it is recommended to perform OTDR fiber quality detection-if the measured loss exceeds 6dB, LH4 must be used to avoid the complexity and failure points caused by signal relay equipment.