Pluggable optics
Optical transceiver modules in standardized, hot-swappable form factors (SFP, QSFP, OSFP, etc.) that plug into networking equipment via a defined electrical interface. The architectural standard for all current datacom and most telecom optical interfaces.
Pluggable optics are optical transceiver modules in standardized form factors that plug into a host networking device (switch, router, server) through a defined electrical and mechanical interface. They are the dominant interface architecture for optical connectivity in current networking equipment.
Why pluggable. Originally, optical interfaces were soldered directly to switch line cards, an inflexible, expensive, and slow-to-deploy approach. The pluggable architecture decouples the optical interface from the host electronics:
- Flexibility: any compliant module from any vendor works in any compliant host
- Field-serviceability: failed modules are replaced in seconds without rebooting or rewiring the host
- Mix-and-match: choose short-reach copper-DAC modules in some ports, long-haul coherent in others, all in the same host
- Independent upgrade cycle: networking hardware lifecycle is decoupled from optics lifecycle
Standard form factors (MSA-defined):
| Form factor | Dimensions (LWH mm) | Lanes × Speed | Max power | Year introduced |
|---|---|---|---|---|
| SFP+ | 56 × 14 × 9 | 1 × 10G | 1 W | 2006 |
| SFP28 | 56 × 14 × 9 | 1 × 25G | 1 W | 2014 |
| QSFP+ | 72 × 18 × 9 | 4 × 10G | 1.5 W | 2009 |
| QSFP28 | 72 × 18 × 9 | 4 × 25G NRZ | 3.5 W | 2013 |
| QSFP56 | 72 × 18 × 9 | 4 × 50G PAM4 | 5 W | 2018 |
| CFP2-DCO | 107 × 41 × 13 | 200G coherent | 20 W | 2016 |
| QSFP-DD | 89 × 18 × 9 | 8 × 50G or 8 × 100G | 12 – 18 W | 2019 |
| OSFP | 100 × 23 × 13 | 8 × 50G or 8 × 100G PAM4 | 15 – 25 W | 2018 |
| OSFP-RHS | 100 × 23 × 13 | up to 1.6T | 20 – 25 W | 2023 |
| OSFP-XD | 138 × 41 × 13 | 1.6T+ | 30 W+ | 2025 |
Wavelength designations. Pluggable modules are typically labeled by the optical wavelength technology and the reach class (decoded in full, with lane counts and connectors, in How to Read Optical Transceiver Names):
- SR (short reach): 850 nm multimode VCSEL, up to 100 m on OM4 fiber
- DR (data center reach): 1310 nm single-mode laser per lane, 500 m
- FR (far reach): 1310 nm, 2 km
- LR (long reach): 1310 nm, 10 km
- ER (extended reach): 40 km; 1550 nm at 10G, 1310 nm LAN-WDM with an SOA preamplifier at 100G
- ZR (very long reach): 1550 nm coherent, 80 – 120 km
- ZR+: Coherent with additional FEC, 400 – 1000 km
Electrical interface. Modern pluggables use SerDes (serializer/deserializer) electrical interfaces at the host. The electrical lanes operate at 25 Gbps NRZ or 50, 100, or 200 Gbps PAM4 per lane, with multiple lanes aggregated for higher total bandwidth. The host platform's serial-link technology (typically silicon CMOS SerDes) sets the per-lane data rate.
Management interface. Pluggables include an internal microcontroller exposed to the host over an I²C bus, using the CMIS (Common Management Interface Specification) protocol on QSFP-DD and OSFP modules, and SFF-8472 or SFF-8636 on older SFP and QSFP modules. The host can read module type, capabilities, temperature, current drive levels, and optical power; the host can also control wavelength tuning, transmit-power adjustment, and other parameters.
Power density problem. As data rates scale (400G → 800G → 1.6T in pluggable), power consumption scales roughly linearly while the module envelope cannot grow proportionally. Pluggable thermal limits become the dominant design constraint. OSFP-XD (138 mm long, 30 W) is approximately the limit of what passive air cooling can handle in standard 1U rack space. Future scaling to 3.2T per port requires either liquid cooling or moving optics off the pluggable.
Co-packaged optics (CPO) is the proposed long-term alternative: move optical engines onto the same package substrate as the switch ASIC, eliminating the long electrical SerDes traces and thermal contention. CPO promises power reductions of about half or more at high speeds, at the cost of breaking the pluggability model. The optical interface becomes part of the switch and must be replaced as a unit.
The architecture ladder away from the fully retimed pluggable, LRO, LPO, and co-packaged optics, is compared in Pluggable, LRO, LPO, CPO.
References: OIF Common Electrical Interface (CEI) specifications; QSFP-DD MSA; OSFP MSA; Cisco Optical Pluggables Reference Architecture.