An optical transceiver is the component in a switch, router, server or carrier device that converts electrical signals to light, and light back to electrical signals, so data can travel over fiber. Transceivers can be board-mounted or integrated with the switching chip (co-packaged optics), but the kind buyers usually deal with is a small, usually hot-swappable pluggable module in standard form factors such as SFP, SFP+ and QSFP28. Each is built for a particular speed, fiber type and distance. Both ends of a link need compatible optics.
At a glance
- A transceiver (“optic”) sets the speed, wavelength and reach of a fiber link; most are pluggable modules, and the port sets which form factors fit.
- Common form factors are SFP (1G), SFP+ (10G), SFP28 (25G), QSFP+ (40G) and QSFP28 (100G).
- Names like SR, LR and ER indicate reach and fiber type; both ends must use compatible types.
- Form factors follow industry multi-source agreements (MSAs), but equipment vendors may restrict which coded optics they accept.
- Direct attach copper (DAC) and active optical cables (AOC) are alternatives for short links.
What problem it solves
Switches and routers move data as electrical signals. Fiber carries light. The transceiver sits between them. Some equipment has optics built onto the board or integrated with the switching chip, but where it is a pluggable module, one switch port can be fitted for a short multimode link, a 10 km single-mode link or a copper cable, depending on what you plug in. That flexibility is why carriers, data centers and IT teams commonly standardize on pluggable ports instead of fixed optical interfaces.
How it works
Inside a transceiver, a laser or other light source turns the outgoing electrical signal into light at a specific wavelength, and a photodetector turns incoming light back into an electrical signal. A pluggable module has an electrical edge that plugs into the port and an optical receptacle, usually a duplex LC or an MPO fiber optic connector.
| Form factor | Typical speeds | Notes |
|---|---|---|
| SFP | 1 Gbps | Also used for some copper (RJ-45) 1G modules |
| SFP+ | 10 Gbps | Same size as SFP; many SFP+ ports also accept SFP |
| SFP28 | 25 Gbps | Same size as SFP+; often backward compatible with 10G in the same port |
| QSFP+ | 40 Gbps | Four lanes; can often be split into 4 x 10G |
| QSFP28 | 100 Gbps | Four lanes; can often be split into 4 x 25G |
| QSFP56, QSFP-DD, OSFP | 200 to 400 Gbps and higher | Newer high-speed data center and carrier ports |
Older form factors such as GBIC and XFP still appear on legacy equipment.
Reach and fiber type. Optic names usually include a reach code. SR (short reach) runs over multimode fiber for distances inside a building or data center. LR (long reach) runs over single-mode fiber and is commonly rated around 10 km. ER and ZR are longer single-mode reaches. At 1G the equivalents are SX (multimode) and LX (single-mode). Exact distances depend on the speed and standard, so read the datasheet. Both ends of a link need compatible optics: same speed, same standard and compatible wavelengths (BiDi optics come in matched pairs).
Coding and compatibility. Form factors are defined by multi-source agreements, so modules from different makers fit the same port. Each module also stores identifying data that the host equipment reads. Some equipment accepts only modules coded for that vendor, or warns on others, and support policies for third-party optics vary. Third-party suppliers commonly sell optics coded for specific equipment brands.
Monitoring. Many optics report transmit and receive light levels and temperature (often called digital optical monitoring or DOM). Reading these is the first step when a link will not come up or shows errors.
Alternatives. Direct attach copper (DAC) cables have modules fixed to both ends of a twinax copper cable and suit very short links, typically within a rack. Active optical cables (AOC) have optics fixed to both ends of a fiber cable and reach farther. Both avoid separate optics and patch cords for short runs.
When it matters for buyers
- Circuit orders. Agree the handoff and the optic in writing, for example “SMF, LC/UPC, 10GBASE-LR, customer supplies optic for customer router.” A wrong or missing optic is a common reason a turn-up slips.
- Colocation cross-connects. Each side of a cross-connect in a colocation facility supplies its own optic. Confirm speed, reach type and fiber type with the carrier or cloud on-ramp.
- Dark fiber and wavelength services. With dark fiber you choose and buy the optics, based on the route’s distance and loss. With wavelength services the handoff is a specific optical interface you must match.
- Switch refreshes. Budget for optics; on high-speed ports they can be a significant share of the cost. Decide early whether you will use the equipment vendor’s optics or coded third-party optics.
For high-capacity links between sites where you supply the optics, see our dark fiber solution page.
Questions to ask vendors
- What exact optic type is required at our end (speed, standard such as 10GBASE-LR, and wavelength)?
- Who supplies the optic at each end, and who replaces it if it fails?
- Is the handoff single-mode or multimode, and what connector and polish?
- Will your equipment accept third-party coded optics, and does using them affect support?
- What light levels should we expect at turn-up, and will you share your side’s readings?
- For short in-rack links, would DAC or AOC cables be cheaper and simpler than separate optics?
How it differs from a fiber optic connector
A fiber optic connector is the passive plug on a cable that aligns the fiber with a port. The transceiver is the active module that generates and receives the light and determines speed and reach. A link needs both to match: the right optic at each end, and cabling with the right fiber type, connector and polish between them.