A transceiver is chosen by form factor (SFP, SFP+, SFP28, QSFP+, QSFP28 and so on), reach (SR, LR, ER), fiber type (multimode or single-mode) and the brand coding your switch or adapter expects. For short links, a direct attach copper (DAC) or active optical cable (AOC) often replaces two transceivers and a patch cable.
Form factors and speeds
| Form factor | Speed | Typical use |
|---|---|---|
| SFP | 1 Gb/s | Access switch uplinks, management, legacy links |
| SFP+ | 10 Gb/s | Server links, aggregation, older data center |
| SFP28 | 25 Gb/s | Current server links |
| QSFP+ | 40 Gb/s (4 x 10G) | Older spine and uplinks |
| QSFP28 | 100 Gb/s (4 x 25G) | Spine, leaf uplinks, high-speed server links |
| QSFP56 | 200 Gb/s | High-performance networks |
| QSFP-DD, OSFP | 400 Gb/s and up | Current data center spine |
Some ports accept slower modules: many SFP28 ports run SFP+ optics at 10G, and many SFP+ ports accept 1G SFP modules. This is model-specific, so check the switch or adapter documentation.
Reach: SR, LR, ER and others
The letters in the module name describe the reach and the fiber it uses. The values below are the standard reaches for common 10G, 25G and 100G modules.
| Type | Fiber | Wavelength | Typical reach |
|---|---|---|---|
| SR (short reach) | Multimode | 850 nm | 10G: 300 m on OM3, 400 m on OM4. 25G and 100G SR4: 70 m on OM3, 100 m on OM4 |
| LR (long reach) | Single-mode | 1310 nm | 10 km |
| ER (extended reach) | Single-mode | 1550 nm | 40 km (30 km for some 25G ER modules) |
| ZR | Single-mode | 1550 nm | Around 80 km for 10G ZR; varies by speed |
Other common names:
- SR4 and LR4: four lanes. SR4 uses parallel fibers on an MPO connector. LR4 uses four wavelengths on one duplex pair with LC connectors.
- CWDM4 and PSM4: 100G options for shorter single-mode runs.
- BiDi: sends and receives on one fiber using two wavelengths. BiDi modules are used in matched pairs.
- Copper (1000BASE-T, 10GBASE-T SFPs): RJ45 modules for copper cabling. They draw more power and run hotter than optical modules, so some switches limit how many you can install.
Multimode vs single-mode fiber
- Multimode (OM3, OM4, OM5): wider core, lower-cost optics, short distances. Usually aqua or violet jackets. Used inside rooms and buildings.
- Single-mode (OS2): narrow core, long distances. Usually yellow jackets. Used between buildings, floors and sites.
Multimode and single-mode optics cannot be mixed on the same link. Both ends of a link must use the same type, and in nearly all cases the same standard (for example, SR to SR).
Connectors
- LC duplex: two fibers, used by most SFP and many QSFP modules.
- MPO/MTP: multi-fiber connectors used by parallel optics such as 40G SR4 and 100G SR4.
- RJ45: copper SFP modules.
DAC vs AOC
| Cable | How it works | Typical length | Notes |
|---|---|---|---|
| Passive DAC | Copper twinax with fixed modules on each end | Up to about 5 m at 25G and 100G; longer at 10G | Lowest cost and power; thicker and stiffer |
| Active DAC | Copper with signal conditioning | A few meters longer than passive | Small extra power draw |
| AOC | Fiber with fixed optical modules on each end | Up to tens of meters | Thin and light; higher cost than DAC |
DACs and AOCs suit links inside a rack or between neighboring racks, such as server to top-of-rack switch. For longer runs, or when you need to patch through a panel, use transceivers with structured fiber.
Breakout cables
Breakout cables split one high-speed port into several lower-speed ones, for example QSFP28 to 4 x SFP28 (100G to 4 x 25G) or QSFP+ to 4 x SFP+. The switch port must support breakout mode.
Vendor coding and compatibility
Each transceiver has an EEPROM that stores the vendor name, part number and other data. Many switches and some network adapters read this and check whether the module is coded for their brand.
- Some platforms reject modules that are not coded for the brand, or disable the port.
- Some accept them but log warnings, or limit support.
- Some accept any standards-compliant module.
Options when buying:
- OEM-branded optics with the switch brand's part number. Matches the support matrix.
- Compatible optics coded for your switch brand. Built to the same standard and programmed to identify as compatible with your platform.
- Generic optics for platforms that accept any module.
For a link between two brands, such as a Cisco switch and a Dell server adapter, each end needs a module coded for its own device. A DAC or AOC needs coding that both ends accept, or a cable coded differently on each end.
Checklist for your request
- Switch or adapter model at each end of the link.
- Speed and form factor.
- Distance and fiber type, or the reach code (SR, LR, ER) if known.
- Connector type, if patching into existing cabling.
- Brand coding required for each end.
- Quantity, including spares.
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