How to Read Transceiver Names: DR4, FR4, OSFP, QSFP-DD, MPO

Scope

A datacenter optical module is sold under a name such as "800G OSFP 2xFR4" or "400G QSFP-DD DR4". The name carries five facts: the aggregate data rate, the mechanical form factor, the reach class, the number of optical lanes, and, by implication, the fiber type and connector the module needs. This article explains how each part of the name is built, following the IEEE 802.3 physical-layer (PHY) names on which most module names are based and the multi-source agreement (MSA) names that fill the gaps. It covers intensity-modulated modules for Ethernet and InfiniBand-class links from 100G to 1.6T. Coherent modules are touched on only where their names overlap; the 400ZR entry and What the Coherent DSP Actually Does cover them in depth. The architectural choice between retimed, linear and co-packaged modules is the subject of Pluggable, LRO, LPO, CPO.

Anatomy of an IEEE PHY name

IEEE 802.3 names each optical PHY as rate, the word BASE, and a suffix. For 400GBASE-FR4:

Rate. The aggregate payload rate of the Ethernet interface: 100G, 200G, 400G, 800G or 1.6T. It is the MAC data rate, not the rate on the fiber; each lane runs somewhat faster because of line coding and forward error correction overhead.

BASE. Baseband signaling. Every current optical Ethernet PHY is baseband, so the word carries no information for a buyer.

Reach letters. One or two letters give the reach class, and through it the fiber type and wavelength band (next section). In older names the second letter described the line coding: in 10GBASE-LR, L is the long-wavelength reach class and R is 64B/66B coding. From 40G onward the pair is read as a single reach label.

Lane count. The trailing digit is the number of optical lanes in each direction. A lane is one modulated optical signal, and it can travel either on its own fiber or on its own wavelength sharing a fiber with others. A missing digit means one lane (100GBASE-DR, 100GBASE-FR1 and 100GBASE-LR1 are all single-lane PHYs).

Further suffixes. A second number after a hyphen marks a variant, usually a different reach: 400GBASE-LR4-6 is a four-lane 6 km PHY, and in the 200G-per-lane generation a "-2" suffix marks the 2 km variants such as 800GBASE-DR4-2 (see 200G per lane). A number after a decimal point counts wavelengths per fiber: 400GBASE-SR4.2 uses four fiber pairs with two wavelengths on each fiber.

The per-lane rate follows from dividing the aggregate by the lane count, and the generation of a module is easiest to place from that quotient:

Reach classes

The reach is a conformance distance, not a physical ceiling: a module passes its specification over that length of standard fiber with the specified number of connectors, and the margin behind that statement is what a link budget accounts for. SR and VR modules use vertical-cavity lasers on graded-index multimode fiber; everything from DR upward is single-mode. The single-mode classes up to LR sit in the O-band because standard G.652 fiber has its zero-dispersion wavelength there, which lets directly detected PAM4 cross kilometres without dispersion compensation. The exceptions to the table are marked by suffixes, as above (LR4-6 at 6 km, DR4-2 at 2 km), and by vendor labels: a plus sign, as in "DR4+", usually means a reach longer than the standard's, and what it guarantees is set by the vendor's datasheet rather than by any standard.

Parallel fiber versus wavelength multiplexing

The same lane count can be carried in two ways, and the reach letters say which.

Parallel single-mode (DR, and SR on multimode). Each lane has its own fiber, all lanes on the same wavelength. A DR4 module therefore needs four transmit fibers and four receive fibers, eight in all, and uses a multi-fiber MPO connector. The optics are simple (one laser design repeated four times, or one laser split four ways) and the cabling is expensive per metre. Because the lanes are independent fibers, a parallel module can be broken out: a 400G-DR4 port with an MPO-to-four-duplex-LC breakout cable feeds four 100G-DR single-lane modules.

Wavelength-multiplexed (FR, LR). All lanes share one fiber in each direction, each on its own wavelength, combined and separated inside the module by a WDM multiplexer (technologies compared in WDM Mux and Demux Technologies). The module needs one fiber pair and a duplex LC connector, and the cost moves from the cable into the module. Two wavelength grids are in use, contrasted generally in CWDM vs DWDM:

The two grids trade temperature control against dispersion. A DFB laser moves by about 0.09 nm/K (see wavelength temperature coefficient), so across a 0 to 70 °C case range it drifts about 6.3 nm: more than the 4.5 nm LAN-WDM spacing, less than the 20 nm CWDM spacing. CWDM lasers can therefore run uncooled, while LAN-WDM lasers are usually held by a thermoelectric cooler. In exchange, the CWDM4 lanes span 60 nm around the dispersion zero: on a worst-case G.652 fiber the 1271 nm lane sees 5.19 ps/(nm·km), against 2.70 ps/(nm·km) for the worst lane of the four-wavelength LAN-WDM grid used by LR4 (the fiber-standard formula is worked in the zero-dispersion wavelength entry, and the Dispersion and Pulse Broadening Calculator turns either figure into broadening).

Module names built on PHY names

A module name usually borrows the PHY suffix and adds a form factor. Three patterns go beyond the IEEE name.

Two PHYs in one module. "2xFR4" and "2xDR4" describe a module that carries two independent 400G PHYs, each with its own optical port. An 800G OSFP 2xFR4 is two 400G-FR4 links side by side, with two duplex LC connectors (or a very-small-form-factor connector such as CS, SN or MDC where two LC receptacles do not fit the faceplate). It is not the same as a single 800G PHY, and each half can connect to a separate 400G-FR4 module at the far end.

MSA names. Some widely deployed modules were defined by industry agreements before or instead of IEEE. 100G PSM4 is a parallel single-mode 500 m interface on NRZ lanes; 100G CWDM4 is a 2 km, four-wavelength interface on the CWDM4 grid; the 100G Lambda MSA defined 400G-FR4 before IEEE adopted it and adds a 10 km 400G-LR4-10. The names look like IEEE names but are specified in the MSA documents.

Electrical and optical lanes need not match. The host side of a module has its own lane count and rate, named in IEEE's attachment-unit-interface convention (400GAUI-8 is eight 50G electrical lanes; 800GAUI-8 is eight 100G lanes). An early 400G-FR4 in QSFP-DD takes eight 50G electrical lanes from the host SerDes and drives four 100G optical lanes, and the DSP inside the module performs the conversion (a gearbox). Whether the module has that DSP at all is what separates a retimed module from LPO and LRO designs, which often carry those three letters in the product name.

Copper and cable products share the naming scheme. CR is a direct-attach copper cable (100GBASE-CR4), KR a backplane channel, and an active optical cable (AOC) is a pair of modules with the fiber permanently attached, sold by rate and form factor with no reach letters at all.

Form factors

The form factor is the mechanical and electrical envelope defined by an MSA (QSFP-DD MSA, OSFP MSA), and its name encodes the number of electrical lanes. The pluggable optics entry lists dimensions and power classes.

The number after SFP or QSFP loosely names the lane generation: 28 for 25G NRZ lanes, 56 for 50G PAM4, 112 for 100G PAM4. QSFP-DD ("double density") adds a second row of contacts to the QSFP footprint, and a QSFP-DD cage accepts older QSFP modules. OSFP ("octal") is wider and taller, with a larger heat-dissipation allowance, and does not accept QSFP modules without an adapter. The two carry the same 400G and 800G PHYs, so "QSFP-DD DR4" and "OSFP DR4" describe the same optical link in different housings, and they interoperate across the fiber.

Connectors and fiber for each name

The PHY suffix determines the optical connector, and the connector determines which patch cord will work.

A four-lane parallel module on an MPO-12 transmits on fiber positions 1 to 4, receives on positions 9 to 12, and leaves the middle four dark. Single-mode MPO connectors on these modules are normally polished at 8° for return loss, the same reason simplex connectors come in APC form (see PC, UPC and APC connectors), and an angled MPO will not mate correctly with a flat one. The module's receptacle carries the MPO guide pins, so the patch cord that plugs into it must be unpinned. Between two parallel modules each transmit fiber must arrive at a receive position; structured-cabling standards (TIA-568) define MPO polarity types A, B and C for this, and a direct module-to-module trunk is normally type B.

The connector end faces on these modules are the most frequent cause of a link that comes up with low margin or not at all, and a 16-fiber MPO has sixteen of them. Fiber Connector Inspection and Cleaning gives the procedure.

Worked examples

Two names that differ only in form factor interoperate; two that differ in reach class or lane structure generally do not, even at the same rate and on the same connector. A 400G-FR4 and a 400G-DR4 carry the same rate but share neither a connector nor a lane structure. The fleet-scale power consequences of these choices can be explored in the Datacenter Link Budget Explorer.

References: IEEE Std 802.3-2022, IEEE Standard for Ethernet (clause naming conventions and the 100G, 200G and 400G PHYs); IEEE Std 802.3bs-2017 (200 Gb/s and 400 Gb/s Ethernet); IEEE Std 802.3cu-2021 (100 Gb/s and 400 Gb/s over single-mode fiber at 100 Gb/s per wavelength); IEEE Std 802.3cm-2020 (400 Gb/s over multimode fiber); IEEE Std 802.3db-2022 (multimode PHYs at 100 Gb/s per lane); IEEE Std 802.3df-2024 (800 Gb/s Ethernet); IEEE P802.3dj Task Force project documents (2023 to 2026); 100G PSM4 MSA specification; 100G CWDM4 MSA technical specification; 100G Lambda MSA specifications; QSFP-DD MSA hardware specification; OSFP MSA specification; ITU-T Recommendation G.694.2, Spectral grids for WDM applications: CWDM wavelength grid; TIA-568.3, Optical Fiber Cabling and Components Standard.