Optical transceivers convert electrical signals to optical signals (and back), enabling fiber optic communication in switches, routers, and servers. The market offers a confusing array of form factors — SFP, SFP+, SFP28, QSFP+, QSFP28, QSFP-DD, XFP, CFP — each designed for specific speeds and applications.
This guide explains every major transceiver type, their speeds, and how to choose the right one.
Transceiver Form Factor Overview
| Form Factor | Max Speed | Lanes | Typical Use |
|---|---|---|---|
| SFP | 1 Gbps | 1 | GbE, SONET/SDH |
| SFP+ | 10 Gbps | 1 | 10GbE, 8G FC |
| SFP28 | 25 Gbps | 1 | 25GbE |
| SFP56 | 50 Gbps | 1 | 50GbE |
| QSFP+ | 40 Gbps | 4×10G | 40GbE |
| QSFP28 | 100 Gbps | 4×25G | 100GbE |
| QSFP56 | 200 Gbps | 4×50G | 200GbE |
| QSFP-DD | 400 Gbps | 8×50G | 400GbE |
| OSFP | 400 Gbps | 8×50G | 400GbE |
| XFP | 10 Gbps | 1 | 10GbE (older) |
| CFP | 100 Gbps | 10×10G | 100GbE (older) |
| CFP2 | 200 Gbps | — | 100/200GbE |
| CFP4 | 100 Gbps | — | 100GbE |
SFP (Small Form-factor Pluggable)
The original SFP standard supports up to 1 Gbps over a single fiber lane. SFP modules are used for:
- 1000BASE-SX: Multimode fiber, up to 550 m
- 1000BASE-LX: Single-mode fiber, up to 10 km
- 1000BASE-ZX: Single-mode fiber, up to 80 km
- 1000BASE-T: Copper (RJ45), up to 100 m
- SONET/SDH: OC-3, OC-12, OC-48
SFP modules are still widely used in access switches, routers, and legacy equipment. The SFP cage is the same physical size as SFP+, so SFP+ ports can accept SFP modules (at reduced speed).
SFP+ (Enhanced SFP)
SFP+ is the most widely deployed transceiver form factor in enterprise and data center networks. It supports 10 Gbps over a single lane.
Common SFP+ types:
- 10GBASE-SR: Multimode fiber, 300 m (OM3) or 400 m (OM4)
- 10GBASE-LR: Single-mode fiber, 10 km
- 10GBASE-ER: Single-mode fiber, 40 km
- 10GBASE-ZR: Single-mode fiber, 80 km
- 10GBASE-T: Copper (RJ45), up to 30 m
- 10G DWDM: Tunable or fixed wavelength for WDM networks
SFP+ is backward compatible with SFP ports — an SFP module works in an SFP+ port, but an SFP+ module does not work in an SFP-only port.
SFP28
SFP28 uses the same physical form factor as SFP/SFP+ but supports 25 Gbps per lane. It's the standard for 25GbE server connections in modern data centers.
Common SFP28 types:
- 25GBASE-SR: Multimode fiber, 70 m (OM3) or 100 m (OM4)
- 25GBASE-LR: Single-mode fiber, 10 km
- 25GBASE-ER: Single-mode fiber, 40 km
SFP28 ports are backward compatible with SFP+ and SFP modules.
QSFP+ (Quad SFP+)
QSFP+ combines four 10G lanes for a total of 40 Gbps. It's larger than SFP+ but smaller than older 40G form factors.
Common QSFP+ types:
- 40GBASE-SR4: Multimode fiber (MPO-12 connector), 150 m (OM3)
- 40GBASE-LR4: Single-mode fiber (LC duplex), 10 km — uses 4 CWDM wavelengths
- 40GBASE-CR4: Direct attach copper (DAC), up to 7 m
QSFP+ can also be used with breakout cables to connect to four SFP+ ports (4×10G).
QSFP28
QSFP28 uses the same physical form factor as QSFP+ but supports 100 Gbps (4×25G lanes). It's the dominant form factor for 100GbE in data centers.
Common QSFP28 types:
- 100GBASE-SR4: Multimode fiber (MPO-12), 70 m (OM3) or 100 m (OM4)
- 100GBASE-LR4: Single-mode fiber (LC duplex), 10 km — 4 CWDM wavelengths
- 100GBASE-ER4: Single-mode fiber, 40 km
- 100GBASE-CR4: Direct attach copper, up to 5 m
- 100G CWDM4: Single-mode fiber, 2 km — 4 CWDM wavelengths
- 100G PSM4: Single-mode fiber (MPO-12), 500 m — 4 parallel lanes
QSFP28 ports accept QSFP+ modules at 40G speed.
QSFP56 and QSFP-DD
QSFP56 supports 200 Gbps (4×50G PAM4 lanes) in the same QSFP form factor.
QSFP-DD (Double Density) adds a second row of electrical contacts, doubling the lane count to 8×50G for 400 Gbps total. QSFP-DD ports are backward compatible with QSFP28, QSFP56, and QSFP+ modules.
OSFP (Octal SFP)
OSFP is a competing 400G form factor with 8×50G lanes. It's slightly larger than QSFP-DD and offers better thermal performance. Used in high-density 400G switches.
XFP (10 Gigabit SFP)
XFP was an early 10G form factor, larger than SFP+. It has been largely replaced by SFP+ in new designs. XFP is still found in older SONET/SDH equipment and some telecom applications.
CFP Family (C Form-factor Pluggable)
CFP modules are large, high-power form factors designed for 100G and beyond:
- CFP: Original 100G form factor (10×10G lanes) — now obsolete in new designs
- CFP2: Smaller, supports 100G and 200G
- CFP4: Smaller still, supports 100G
- CFP8: Supports 400G
CFP modules are primarily used in telecom/carrier equipment and long-haul DWDM systems.
Choosing the Right Transceiver
By speed requirement:
- 1G: SFP
- 10G: SFP+
- 25G: SFP28
- 40G: QSFP+
- 100G: QSFP28
- 200G: QSFP56
- 400G: QSFP-DD or OSFP
By distance:
- Up to 100 m: Multimode SR (OM3/OM4)
- Up to 2 km: CWDM4 or PSM4 (single-mode)
- Up to 10 km: LR (single-mode)
- Up to 40 km: ER (single-mode)
- Up to 80 km: ZR (single-mode)
- Metro/long-haul: DWDM tunable
Third-party vs OEM:
Third-party transceivers (compatible modules) offer significant cost savings — typically 60–80% less than OEM pricing. They use the same optical components and meet the same IEEE standards. Most network equipment supports third-party modules; some vendors require disabling compatibility checks.
Summary
The transceiver landscape has evolved from 1G SFP to 400G QSFP-DD, with each generation using the same or similar form factors at higher speeds. Key takeaways:
- SFP/SFP+/SFP28: Single-lane modules for 1G/10G/25G
- QSFP+/QSFP28/QSFP56: Quad-lane modules for 40G/100G/200G
- QSFP-DD/OSFP: 8-lane modules for 400G
- Form factor backward compatibility allows gradual upgrades
CZT supplies SFP, SFP+, QSFP28, and QSFP-DD transceivers for data center, enterprise, and telecom applications. Browse our fiber optic transceiver range for compatible modules.
