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25G SFP+ 20km 1310nm (usually LR or BIDI) long-distance transceivers without CDR (clock data recovery) is a very special and non mainstream design.
Usually, 20km transmission requires extremely high signal quality, and CDR is necessary to repair the signal. Only in the specific architecture of 5G fronthaul network, the 25G SFP+ transceiver will remove it for ultimate performance.
Why is CDR removed from the 20km SFP module?
There are three main reasons, all of which serve the 5G base station environment:
- Ultra Low Latency: The CDR chip will “re time” the signal, which will result in a fixed delay of microseconds. In the URLLC (Ultra Reliable Low Latency Communication) scenario of 5G, in order to free up more latency budget for core services, the 25g SFP+ transceivers will choose not to process signals internally, allowing signals to pass through transparently.
- Reduce power consumption and heat dissipation pressure: The 20km SFP module requires a high-power laser (DML), which already generates significant heat. Removing the CDR chip can reduce power consumption by approximately 0.5W-0.7W. For 5G telecommunication boxes (AAUs) that are exposed to outdoor sunlight all year round, the savings of a few watts can effectively prevent the module from losing its lifespan due to high temperatures.
- Cost down: The demand for 5G front-end modules is in the tens of millions, and saving a high-performance CDR chip can significantly reduce the cost of large-scale deployment.
What are the general uses of modules without CDR?
25G SFP+ transceiver without CDR is not designed for ordinary switches, and its application scenarios are very narrow:
- Dedicated 5G fronthaul equipment (AAU and DU directly connected): The SoC (System on Chip) on the base station equipment side usually comes with strong signal compensation and equalization functions. Since the main device can already compensate for the loss after 20km transmission, the optical module no longer needs to repeat CDR.
- CPRI/eCPRI protocol transmission: Under certain specific protocols, devices require end-to-end clock synchronization, and modules without CDR can reduce interference with signal phase.
Potential risks (why ordinary users cannot buy SFP transceivers without CDR)
If you use a 20km SFP transceiver without CDR on a regular enterprise level switch (such as Huawei, Cisco, ZTE’s data communication switch), you usually encounter the following problems:
- Unable to Up or frequently disconnected: Ordinary switch ports usually require module feedback clock synchronization signals, and without CDR modules, they may be directly “unrecognizable”.
- Bit Error Rate (BER) Soaring: After a 25G signal is transmitted over a single-mode fiber for 20km, dispersion can cause severe waveform distortion. Without the “shaping” effect of CDR, the switch may read a bunch of garbled codes.
Conclusion
| Item | 25G SFP+ Transceiver with CDR (standard) | 25G SFP+ Transceiver without CDR (custom) |
| Application scenarios | Enterprise network, data center, metropolitan area network | Only available for 5G operator front-haul network |
| compatibility | Extremely strong, plug and play | Extremely poor, requires specific motherboard support |
| Power consumption/latency | higher | lower |
| Distance capability | Stable support for 20km | Dependent on the compensation capability of the motherboard |
Yingda recommendation:
If you are purchasing switches for the company’s data center, please make sure to choose the version with CDR. Modules without CDR are usually customized for the construction of base stations by operators such as mobile and telecommunications.
