100G optical links can use different transmission architectures to deliver the same aggregate bandwidth. Two common QSFP28 options are 100G BiDi and 100G SR4. Although both support high-speed Ethernet connectivity, they differ in optical lane configuration, fiber requirements, wavelength operation, connector design, and transmission distance.

100G BiDi is designed to transmit and receive signals through a shared optical fiber strand using different wavelengths. It is particularly useful where single-mode fiber availability or fiber utilization is a major consideration. In contrast, 100G SR4 uses four parallel optical lanes in each direction over multimode fiber, making it suitable for short connections inside data centers.

1. Core Architecture of 100G BiDi and 100G SR4

The main architectural difference is the way optical signals travel between two network devices. BiDi combines bidirectional transmission on one fiber strand, whereas SR4 separates transmission and reception across multiple parallel fiber paths.

100G BiDi optical architecture: A BiDi transceiver transmits and receives optical signals at different wavelengths through the same fiber strand. The two ends use complementary wavelength configurations. The local transmitter wavelength must match the remote receiver wavelength, and vice versa.

100G SR4 optical architecture: SR4 uses four transmit lanes and four receive lanes. Each optical lane operates at approximately 25.78Gbps, providing an aggregate optical signaling rate of 103.125Gbps. The four transmit and four receive channels use separate fiber paths within a parallel multimode fiber connection.

2. Fiber Utilization and Cabling Structure

Fiber availability is one of the most important considerations when selecting between these two technologies. Their different optical architectures create significantly different cabling requirements.

100G BiDi uses one fiber strand for bidirectional communication. Both transmitting and receiving signals travel through the same strand, separated by wavelength. A complete link therefore requires a compatible BiDi module at each endpoint and a single-mode fiber connection between them.

This architecture can reduce the number of fiber strands required for a point-to-point connection. It is particularly valuable in existing fiber infrastructure where spare fiber capacity is limited or where additional fiber installation would increase deployment costs.

100G SR4 uses parallel multimode fiber. Four fibers carry the transmitting channels, while another four fibers carry the receiving channels. An MPO-12 connector typically provides the physical interface, with eight active positions used for the optical channels.

SR4 is well suited to structured data center cabling systems that already use MPO trunks, cassette modules, and high-density fiber distribution panels. However, the polarity and fiber mapping of the MPO connection must match the transceiver's transmit and receive arrangement.

3. Wavelength and Optical Lane Configuration

Wavelength operation determines how each technology separates its optical signals and how the corresponding transmitters and receivers must be matched.

For 100G BiDi, the transmitting and receiving wavelengths are different. One endpoint transmits at a wavelength received by the opposite endpoint, while the reverse direction uses the complementary wavelength pair. The exact wavelengths depend on the optical implementation and transmission distance.

For example, C-LIGHT's 100G QSFP28 Single Lambda BiDi 40km implementation uses complementary A/B configurations. The A-side and B-side modules operate with opposite transmit and receive wavelength assignments, allowing bidirectional transmission through one single-mode fiber strand.

100G SR4 uses four parallel optical channels centered around 850nm. Each lane carries approximately 25.78Gbps, and the four lanes operate simultaneously to provide the aggregate link capacity. The optical engine uses VCSEL transmitters and multimode fiber to support short-reach connections.

4. Transmission Distance and Fiber Performance

100G SR4 is optimized for short-reach multimode connections. Its practical transmission distance depends on the multimode fiber category and the complete channel's insertion loss and bandwidth characteristics.

A typical 100GBASE-SR4 link supports up to 70 meters over OM3 fiber and 100 meters over OM4 fiber. These distances make it suitable for switch-to-server, switch-to-switch, and other short-distance data center connections.

100G BiDi covers a broader range of transmission distances through different optical variants. C-LIGHT's 100G BiDi QSFP28 portfolio includes options from 2km to 80km, using single-mode fiber and LC interfaces. This allows the optical configuration to be selected according to the required link distance.

For longer links, the optical power budget, receiver sensitivity, fiber attenuation, connector loss, dispersion, and engineering margin must all be considered. A module's advertised reach should not be treated as a guaranteed distance under every cabling condition.

5. C-LIGHT 100G BiDi and 100G SR4 Product Options

C-LIGHT provides both 100G BiDi and 100G SR4 QSFP28 optical transceivers for different network architectures. The product selection includes single-mode bidirectional modules for extended-reach links and multimode SR4 modules for short-distance data center connectivity.

6. Installation and Interoperability Considerations

Although both products use the QSFP28 form factor, their optical interfaces are not interchangeable. The host equipment must support the selected transceiver, and the fiber infrastructure must match the optical specification.

For 100G BiDi deployment, the following points require attention:

• Use a compatible BiDi module at each end of the link.

• Match the A/B wavelength assignments correctly.

• Use single-mode fiber with the specified LC connector interface.

• Verify the optical reach, power budget, and receiver sensitivity.

• Confirm host switch compatibility and supported transceiver coding.

For 100G SR4 deployment, installation requirements are different:

• Use OM3 or OM4 multimode fiber according to the required reach.

• Verify MPO/MTP connector type, gender, polarity, and fiber mapping.

• Ensure the four transmit and four receive channels are connected correctly.

• Inspect and clean MPO end faces before installation.

• Check channel insertion loss against the optical link budget.

7. Which 100G Optical Solution Should You Choose?

The most suitable option depends on the existing fiber infrastructure, transmission distance, and network design rather than the 100Gbps data rate alone.

Choose 100G BiDi when:

• Single-mode fiber is already installed.

• Fiber strand availability is limited.

• A single-strand bidirectional connection can simplify cabling.

• The link requires distances beyond the typical SR4 range.

• Campus, enterprise, or data center interconnections need an appropriate BiDi reach option.

Choose 100G SR4 when:

• The connection is within a data center or another short-reach environment.

• OM3 or OM4 multimode fiber is already available.

• The network uses MPO/MTP structured cabling.

• Parallel optical connectivity is supported by the switch and cabling infrastructure.

• A standardized short-reach 100G optical interface is required.

For new network deployments, the decision should also consider future bandwidth requirements, fiber distribution design, transceiver power consumption, port density, and total installation cost. Selecting an optical module that matches the existing cabling system can reduce unnecessary infrastructure changes and simplify network maintenance.

8. Conclusion

100G BiDi and 100G SR4 serve different optical networking requirements. BiDi uses wavelength-separated bidirectional transmission over a single strand of single-mode fiber, providing a practical option for fiber-constrained links and longer-distance connections. SR4 uses four parallel transmit lanes and four receive lanes over multimode fiber, delivering cost-effective short-reach connectivity for data center environments.

The key selection factors are fiber type, connector architecture, transmission distance, wavelength compatibility, and the available cabling infrastructure. Matching these parameters to the actual network design is essential for reliable 100G connectivity.