100G BiDi vs 100G CWDM4: Fiber Usage, Wavelengths, Distance and Applications丨C-LIGHT

100G network upgrades require more than higher-capacity optical modules. Fiber availability, cabling infrastructure, transmission distance, and deployment costs all influence the choice of optical technology. Two common QSFP28 solutions are 100G BiDi and 100G CWDM4, both designed to provide 100Gbps-class connectivity over single-mode fiber.

The main architectural difference is how they use optical fibers. 100G BiDi uses bidirectional wavelength transmission over a single fiber strand, while 100G CWDM4 multiplexes four wavelengths and transmits them over a duplex single-mode fiber pair. This difference affects cabling requirements, installation planning, and network expansion.

1. What Is a 100G BiDi Optical Transceiver?

A 100G BiDi optical transceiver uses bidirectional optical transmission to send and receive data through one single-mode fiber strand. It uses different wavelengths for the transmit and receive directions, allowing both directions of communication to share the same fiber.

At the two ends of a link, complementary BiDi modules are required. One module transmits on one wavelength and receives on the other, while the opposite module uses the corresponding reverse wavelength arrangement. This wavelength pairing enables full-duplex communication over a single fiber.

Typical 100G BiDi characteristics:

Data rate: 100Gbps-class, depending on the module specification
Fiber type: Single-mode fiber (SMF)
Fiber requirement: One strand for bidirectional transmission
Optical interface: Depends on the specific BiDi implementation; C-LIGHT's listed QSFP28 BiDi products use LC interfaces
Wavelengths: Paired transmit and receive wavelengths
Applications: Data center interconnects, enterprise networks, and metro links

C-LIGHT offers a range of 100G BiDi QSFP28 optical transceivers. Its listed options cover multiple transmission distances, including 2km, 10km, 20km, 30km, 40km, and 80km configurations. The exact wavelength pair and reach should be selected according to the specific part number.

2. What Is a 100G CWDM4 Optical Transceiver?

A 100G CWDM4 optical transceiver uses four optical wavelengths to carry four data channels within the 100G link. The four wavelength channels are multiplexed onto one fiber for transmission and separated at the receiving end.

Typical CWDM4 wavelength channels are centered around 1271nm, 1291nm, 1311nm, and 1331nm. The module uses a duplex single-mode fiber connection: one strand carries the combined transmit channels, and the other carries the receive channels.

Typical 100G CWDM4 characteristics:

Data rate: 100Gbps-class
Optical channels: Four wavelength channels
Fiber type: Single-mode fiber (SMF)
Fiber requirement: Two strands for duplex transmission
Connector: Duplex LC
Common reach: Up to 2km for standard 100GBASE-CWDM4 implementations
Applications: Data center interconnects, campus networks, and enterprise aggregation

C-LIGHT provides a 100G QSFP28 CWDM4 2km solution, designed for 100Gbps transmission over single-mode fiber. The published configuration uses four CWDM wavelengths, a duplex LC interface, and supports link distances up to 2km.

3. 100G BiDi vs 100G CWDM4: Technical Comparison

Both technologies support high-speed 100G networking over single-mode fiber, but their optical architectures and fiber requirements are different.

Parameter	100G BiDi	100G CWDM4
Data rate	100Gbps-class	100Gbps-class
Transmission method	Bidirectional wavelength transmission	Four-wavelength multiplexing
Optical channels	Transmit and receive wavelength pair	Four multiplexed wavelengths per direction
Fiber type	Single-mode fiber	Single-mode fiber
Fiber strands	One	Two
Typical connector	Depends on implementation; C-LIGHT listed QSFP28 BiDi options use LC	Duplex LC
Wavelength arrangement	Paired Tx/Rx wavelengths	1271nm, 1291nm, 1311nm, 1331nm nominal channels
Common reach	Varies by product; C-LIGHT lists multiple reach options	2km for standard CWDM4
Main advantage	Uses one fiber strand for both directions	Four wavelengths provide 100G transmission over a duplex fiber pair
Typical application	Fiber-constrained links and DCI	Short-reach SMF links and data center interconnects

4. Fiber Infrastructure: One Strand vs. Two Strands

Fiber availability is one of the most important factors when choosing between BiDi and CWDM4. The two technologies use different fiber arrangements, which can affect installation cost and the ability to reuse existing cabling.

100G BiDi: Single-Strand Transmission

BiDi technology combines transmission and reception on one fiber strand by using different wavelengths for the two directions. This can be particularly useful when fiber resources are limited or when an existing cable route has only one available strand.

Because the two ends use complementary wavelength configurations, the correct A-side and B-side modules must be paired. Two identical wavelength configurations should not be assumed to form a valid link.

100G CWDM4: Duplex Fiber Transmission

CWDM4 uses one fiber strand for transmission and another for reception. Four wavelengths are multiplexed within each direction, allowing the module to carry the required aggregate bandwidth over a duplex single-mode fiber connection.

For sites that already have duplex LC single-mode cabling, CWDM4 can provide a straightforward deployment option without requiring a parallel-fiber MPO infrastructure.

Deployment consideration: BiDi reduces the number of fiber strands required for a link, but it does not eliminate the need to verify wavelength pairing and link compatibility. CWDM4 requires a duplex fiber pair, but its two-ended connection uses a familiar duplex LC arrangement.

5. Transmission Distance and Network Applications

Transmission distance should be evaluated using the exact module specification rather than the technology name alone. Standard CWDM4 is commonly associated with a 2km reach, while BiDi products are available in different reach categories depending on their optical design.

Network Scenario	100G BiDi	100G CWDM4
Short data center interconnect	Suitable when single-fiber connectivity is preferred and the selected reach matches the link	Suitable for duplex SMF links within the supported reach
Existing single-strand fiber route	Useful when the available fiber infrastructure supports the required wavelength plan	Requires two available fiber strands
Enterprise campus links	Useful where fiber availability is limited	Suitable for duplex SMF connections within the module's distance limit
Longer metro links	Longer-reach BiDi variants may be available, subject to exact specifications	Standard 2km CWDM4 is not intended for longer metro distances
Data center expansion	Can reduce fiber-strand consumption	Provides a four-wavelength solution over duplex SMF

6. Cost and Deployment Considerations

The total cost of a 100G optical link includes more than the transceiver price. Fiber availability, patch panels, optical distribution frames, cable installation, and the need to replace or extend existing infrastructure can all influence the final deployment cost.

Fiber availability: BiDi can be advantageous when only one usable fiber strand is available. CWDM4 requires a duplex pair.
Module pairing: BiDi links require complementary modules with matching wavelength arrangements at both ends.
Existing cabling: CWDM4 is convenient when duplex LC single-mode fiber is already installed.
Distance: The selected module must support the actual link distance and optical loss budget.
Equipment compatibility: Both ends must support the QSFP28 interface, required data rate, and module coding.

There is no universal cost winner. A BiDi solution may reduce fiber infrastructure requirements, while CWDM4 may be more convenient for a site that already has duplex LC cabling. The complete link cost should be evaluated before making a decision.

7. C-LIGHT 100G BiDi and CWDM4 Product Options

C-LIGHT offers both BiDi and CWDM4-related 100G QSFP28 solutions for different fiber infrastructure and transmission requirements.

100G BiDi QSFP28 Optical Transceiver

The C-LIGHT 100G BiDi QSFP28 product family includes single-fiber bidirectional options covering multiple transmission distances. Listed configurations include 2km, 10km, 20km, 30km, 40km, and 80km variants, depending on the part number.

Typical selection factors: Required reach, wavelength pairing, fiber availability, connector type, and host equipment compatibility.

100G QSFP28 CWDM4 2km Optical Transceiver

The C-LIGHT 100G QSFP28 CWDM4 2km solution uses four CWDM wavelength channels and a duplex LC interface for single-mode fiber connections.

Published configuration: 100G-class aggregate rate, four CWDM channels, up to 2km over SMF, and duplex LC connectivity.

For additional 100G optical module options, visit the C-LIGHT optical transceiver portfolio. Product selection and compatibility can be evaluated according to the switch model, required distance, fiber type, and network application.

8. How to Choose Between 100G BiDi and 100G CWDM4

The best option depends on the available fiber infrastructure and the required transmission distance.

Choose 100G BiDi When:

Only one fiber strand is available for the link.
Reducing fiber consumption is a major deployment objective.
The required distance is covered by an available BiDi product variant.
The network can use the correct complementary wavelength modules at both ends.

Choose 100G CWDM4 When:

A duplex single-mode fiber pair is available.
The link distance is within the supported CWDM4 reach.
A duplex LC connection is preferred.
The application requires a conventional four-wavelength 100G solution over SMF.

Before ordering either module, confirm:

Switch and router model at both ends.
Required 100G Ethernet operating mode.
Actual fiber type and number of available strands.
Link distance and measured optical loss.
Connector type and wavelength requirements.
Module coding and platform compatibility.

9. Conclusion

100G BiDi and 100G CWDM4 both provide 100Gbps-class connectivity over single-mode fiber, but they are designed around different fiber architectures. BiDi transmits and receives over one fiber strand using paired wavelengths, while CWDM4 multiplexes four wavelengths and uses a duplex fiber pair.

BiDi is a practical option when fiber availability is limited or reducing fiber consumption is important. CWDM4 is a suitable choice for duplex single-mode links within its supported reach, particularly where duplex LC infrastructure is already available.

The final selection should consider fiber availability, transmission distance, optical budget, wavelength pairing, and equipment compatibility. For 100G BiDi and CWDM4 product selection, technical assistance, or compatibility verification, Contact C-LIGHT.