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In current optical components markets, cost-effective 100G QSFP28 optical transceiver modules are in great demand, especially QSFP28 optical transceivers with low-cost form factor tech, widely utilized in the data center, LAN, WAN, Ethernet switches. However, among these optic fiber modules, which one is the most suitable for 100G Ethernet? Well, this article will introduce tips on choosing QSFP28 optical transceivers.

For QSFP28 optical module, except that it’s a hot-pluggable optical module designed for 100G data rate, it integrates 4 transmitting and 4 receiving channels. And “28” means each line carries up to the 28G data rate. Meanwhile, it can do the 4x25G/2x50G/1x100G connection.

To some degree, the selection of 100G QSFP28 products depends on transmission distance. Therefore, it would be better to choose it from this perspective. For different applications, there are different requirements for transmission distance. The below is about its specific conditions and feasible measures:

1. When transmission distance is 100m or under 100m, QSFP28 SR4 is highly recommended. The QSFP28 SR4 supports links of 70 m (OM3) and 100m (OM4) over multimode fiber with MPO connectors. It offers 4 independent transmitting and receiving channels, and each is with 25Gbps able to be aggregated into 100Gbps. Meanwhile, the QSFP28 SR4 optic module is also ideal for the connections from rack to rack in the data center.

2. When transmission distance is over 100m but under 10 km, QSFP28 LR4 is preferred. The QSFP28 LR4 is a fully integrated 4 × 25Gbit/s optical transceiver module, supporting distance up to 10 km. So for long spans 100G deployment, such as cabling between two buildings, QSFP28 LR4 with duplex LC and single-mode fiber cable is the perfect option.

3. When transmission distance exceeds 10 km, QSFP28 ER4 is ideal for very long transmission distance. It provides superior performance for 100G Ethernet applications up to 30km links and converts 4 input channels of 25Gb/s electrical data to 4 channels of LAN WDM optical signals and then multiplexes them into a single channel for 100Gb/s optical transmission.

Of course, the above ways are just for the different transmission distance. But for the same transmission distance, how to choose it? Maybe we can get the answer from the example of QSFP28 CWDM4 and QSFP28 PSM4.

The QSFP28 CWDM4 provides a 100G Ethernet high-speed link with a maximum transmission distance of 2 km, which interfaces with LC duplex connectors, and uses Mux/Demux technologies with 4 lanes of 25Gbps optically multiplexed into and demultiplexed from a duplex single-mode fiber.

While PSM4 does not need a MUX/DEMUX for each laser but it does need either a directly modulated DFB laser (DML) or an external modulator for each fiber. Besides, with an MTP interface, PSM4 modules can transmit data at 100Gb/s from point to point over 2 km or can be divided into dual 50Gb/s or quad 25Gb/s links for linking to servers, storage, and other subsystems.

It’s seen from that both of QSFP28 CWDM4 and QSFP28 PSM4 are designed to meet the requirement for intermediate or mid-reaches for datacenter applications (500 m to 2 km). And they both use WDM and parallel single mode fiber technologies and support transmission distance up to 2 km.

When faced with such a situation, maybe we can make a decision from the two aspects. For one thing, from the aspect of an inner transceiver module structure, by comparison, PSM4 can be more cost-effective due to its lower component costs. For another thing, from the infrastructure viewpoint, PSM4 will be more expensive when the link distance is long, because PSM4 uses 8 optical single-mode fibers while CWDM uses only 2 optical single-mode fibers.

While choosing one QSFP28 optical transceiver for 100G Ethernet, one can take different solutions according to the practical demands in different situations.  After all, the most suitable is the best.