The primary difference between BiDi (Bidirectional) SFP (Small Form-Factor Pluggable) and normal SFP lies in their approach to optical communication and the number of fibers used for transmission. 

1. BiDi SFP:

• Bidirectional Communication: BiDi SFP modules enable bidirectional communication over a single fiber optic cable. This is achieved by using two different wavelengths for upstream and downstream data transmission on the same fiber. One wavelength is used for transmitting data, and the other is used for receiving data.

• Wavelength Division Multiplexing (WDM): BiDi SFPs use Wavelength Division Multiplexing (WDM) technology to separate the different wavelengths. Typically, one wavelength operates in the 1310 nm range, and the other operates in the 1490 nm to 1610 nm range.

• Single Fiber: Since BiDi SFPs use a single fiber for bidirectional communication, they are often referred to as "single-fiber" optics.

2. Normal SFP:

• Two Fibers: Standard or "normal" SFP modules use two separate fibers for data transmission—one for transmitting data (Tx) and another for receiving data (Rx). This is often referred to as "dual-fiber" communication.

• WDM Optional: In dual-fiber SFP configurations, WDM can also be used to enable multiple wavelengths on each fiber, but in a different way than BiDi. In WDM-based dual-fiber configurations, each fiber can carry multiple independent data streams with distinct wavelengths.

Key Considerations:

• Cabling Infrastructure: BiDi SFPs are particularly useful when the number of available fibers is limited or when upgrading an existing infrastructure where running additional fibers is challenging. Normal SFPs may be preferred when there is no constraint on the number of fibers.

• Compatibility: While BiDi SFPs can be compatible with existing SFP ports, they require specific configurations and wavelengths. Normal SFPs follow standard dual-fiber configurations and are widely compatible.

• Distance and Speed: Both BiDi and normal SFPs are available in various configurations supporting different data rates and transmission distances. The specific capabilities depend on the wavelength and other specifications of the module.

In summary, the main difference lies in the number of fibers used for communication—BiDi SFPs use a single fiber for bidirectional communication, while normal SFPs use two fibers for separate transmission and reception. The choice between them depends on the specific requirements of the networking environment and the available fiber infrastructure.