Boston — An announcement this week involving some 14 companies will result in specifications that double the data rates for small form-factor pluggable (SFP) modules used in many datacenters. The SFP-DD Multisource Agreement (MSA) Group has set out to develop the electrical, optical and mechanical specifications to double today's top data rate without increasing size.
SFP-DD companies so far:
- Alibaba
- Broadcom
- Brocade
- Cisco
- Dell
- Finisar
- HP Enterprise
- Intel
- Juniper Networks
- Lumentum
- Mellanox
- Molex
- TE Connectivity
Currently, SFP modules can handle data rates of 25 Gbps using non-return-to-zero (NRZ) modulation or 56 Gbps using four-level pulse-amplitude modulation (PAM4). Now that PAM4 is well into deployment, it's no longer fast enough. To further increase speeds while we wait for the next technology (possibly PAM8), the double-density SFP modules will use two lanes instead of one. That will increase throughput to 50 Gbps and 112 Gbps, respectively. SFP-DD should double data rates without consuming more precious front-panel space on server blades, routers and switches. Figure 1 shows the cage on a PCB and how optical modules insert.
Figure 1. SFP-DD transceivers on PCBs convert electrical signals to optical for transmission over fiber-optic cables. Source: SFD-DD To achieve the additional lane, SFP modules will add a second set of electrical conductors. That means housings will need new connectors. The cutaway drawing (Figure 2) shows the new connector in the case. 
Figure 2. Double-density SFP modules will need new connectors to handle the additional connector row. Source: SFD-DDTo accommodate the new row of contacts and retain backward compatibility, the SFP-DD plug and receptable will be longer than the current SFP configuration. That's so the existing row of connections will make contact first, followed by the additional row. Existing SFP plug won’t make contact the the additional connector row.
In addition, PCB designs will need an additional set of traces to accommodate the second row. Connections inclue two for data plus power and control lines. The additional lines will surely design challenges when it comes to crosstalk, jitter and other issues that affect signal integrity.
The second connector row of contacts inside the case means that designers of both the modules their boards will also have to deal with additional heat issues caused by reduced airflow. One possible solution to heat in the module is to add fins as shown on a module at DesignCon 2017 (Figure 3).
Figure 3. SFP modules may need fins to improve heat transfer. Photo by Martin Rowe.Heat, signal integrity, power consumption, and power integrity are just some of the issues that the SFP-DD MSA group needs to address. The ultimate goal is to keep increasing data rates and port density without the need for additional racks of datacenter equipment.
Related articles:
- PAM4: A new measurement science
- Optimize equalization for FFE, CTLE, DFE, and crosstalk
- Fiber-optic transceivers go metro
- See the crosstalk in 100GbE
- The tradeoff between crosstalk and loss
- The philosophy of jitter
- 10GbE SFP+ PHYs: Requirements and leading solutions
- Optical transceiver standard in works