Fibre optics

Glossary

The language of
optical networking.

85 terms — standards, form factors, fibre types, connectors, diagnostics and procurement vocabulary — defined as the industry uses them, not as a product pitch.

Terms

A-C

1000BASE-LX
The IEEE 802.3 Gigabit Ethernet standard for long-wavelength operation at 1310 nm over a duplex fiber pair. The standard specifies a reach of 5 km on single-mode fiber, although commercial 1000BASE-LX modules are widely offered at 10 km and above under names such as LX, LH, EX and ZX. It can also run about 550 m on multimode fiber when a mode conditioning patch cord is used to suppress differential mode delay. See also: 1000BASE-SX, SMF (Single-Mode Fiber), SFP (Small Form-factor Pluggable).
1000BASE-SX
The IEEE 802.3 Gigabit Ethernet standard for short-wavelength operation at 850 nm over duplex multimode fiber, normally with an LC or SC connector. Reach depends on the modal bandwidth of the fiber: roughly 275 m on OM1, 550 m on OM2 and further on OM3 and OM4. It is the usual choice for building risers and campus links where multimode cabling is already installed. See also: 1000BASE-LX, MMF (Multimode Fiber), Modal Bandwidth.
1000BASE-T
The IEEE 802.3ab standard carrying Gigabit Ethernet over four pairs of balanced twisted-pair copper cabling for up to 100 m. It uses PAM-5 line coding with all four pairs transmitting simultaneously in both directions, and requires Cat5e or better. A 1000BASE-T SFP places this copper PHY inside an SFP housing so that a fiber port can serve an RJ45 device with auto-negotiation at 10, 100 or 1000 Mbps. See also: Cat5e, RJ45, SFP (Small Form-factor Pluggable).
100GBASE-BiDi
An industry term rather than a single IEEE standard, covering 100G links that use wavelength division to reduce fiber count. In data centers it commonly describes a QSFP28 that carries two 50 Gb/s wavelengths over a duplex multimode pair. In access and metro networks it describes single-fiber QSFP28 modules that pair 1271 nm and 1331 nm in transmit and receive directions. Exact reach and lane structure are vendor defined, so both ends must match. See also: BiDi / BX (Bidirectional Transceiver), Wavelength Pairing (BiDi U and D), QSFP28.
100GBASE-CWDM4
A multi-source agreement rather than an IEEE standard, defining a 100G QSFP28 that multiplexes four 25 Gb/s CWDM lanes near 1271, 1291, 1311 and 1331 nm onto a duplex single-mode pair with LC connectors. It targets 2 km, filling the gap between 100 m SR4 and 10 km LR4 at lower cost than LR4. CWDM4 links require RS-FEC in the host, as CWDM4 assumes forward error correction. See also: 100GBASE-LR4, CWDM (Coarse Wavelength Division Multiplexing), FEC (Forward Error Correction).
100GBASE-LR4
The IEEE 802.3 standard for 100 Gigabit Ethernet over 10 km of single-mode fiber using a duplex LC pair. Four 25 Gb/s lanes are multiplexed on the LAN-WDM grid near 1295.56, 1300.05, 1304.58 and 1309.14 nm. Because the lanes sit close together in the 1310 nm window, chromatic dispersion stays low over the full distance. It is the common choice for building-to-building and campus 100G links. See also: 100GBASE-CWDM4, Chromatic Dispersion, QSFP28.
100GBASE-SR4
The IEEE 802.3bm standard for 100 Gigabit Ethernet over multimode fiber, using four parallel 25 Gb/s lanes at 850 nm through an MPO-12 connector with eight active fibers. Reach is about 70 m on OM3 and 100 m on OM4, and RS-FEC in the host is mandatory. It is the standard short-reach interface for spine-leaf fabrics and top-of-rack aggregation inside a data center. See also: MPO / MTP Connector, OM4 Fiber, FEC (Forward Error Correction).
10GBASE-ER and 10GBASE-ZR
Two long-reach 10 Gigabit Ethernet variants at 1550 nm. 10GBASE-ER is defined by IEEE 802.3 for 40 km of single-mode fiber. 10GBASE-ZR is not an IEEE standard but an industry convention for roughly 80 km using higher launch power and a more sensitive receiver, often an avalanche photodiode. Both usually need attention to chromatic dispersion and to receiver overload on unexpectedly short links. See also: APD (Avalanche Photodiode), Saturation (Receiver Overload), Chromatic Dispersion.
10GBASE-LR
The IEEE 802.3 standard for 10 Gigabit Ethernet over 10 km of single-mode fiber at 1310 nm, using a duplex LC connector and normally a DFB laser. Receive sensitivity is specified down to about minus 14.4 dBm, which leaves comfortable margin on typical enterprise spans. It is the most widely deployed 10G single-mode interface and the usual reference point when comparing longer-reach variants. See also: 10GBASE-ER and 10GBASE-ZR, DFB Laser, Link Budget.
10GBASE-LRM
The IEEE 802.3aq standard that carries 10 Gigabit Ethernet about 220 m over legacy FDDI-grade, OM1 and OM2 multimode fiber at 1310 nm. It uses electronic dispersion compensation in the receiver to cope with differential mode delay. Some installations still require a mode conditioning patch cord. LRM exists to extend the life of older multimode plant where OM3 or OM4 re-cabling is not practical. See also: OM1 and OM2 Fiber, Modal Bandwidth, 10GBASE-SR.
10GBASE-SR
The IEEE 802.3 standard for 10 Gigabit Ethernet over multimode fiber at 850 nm with an 850 nm VCSEL and a duplex LC connector. Reach is about 300 m on OM3 and 400 m on OM4, and far shorter on OM1 or OM2. It is the default in-rack and intra-building 10G interface, and the lowest-power and lowest-cost 10G optical option for short distances. See also: VCSEL, OM3 Fiber, OM4 Fiber.
10GBASE-T
The IEEE 802.3an standard carrying 10 Gigabit Ethernet over balanced twisted-pair copper, reaching 100 m on Cat6a or Cat7 and typically around 55 m on Cat6 depending on alien crosstalk. It draws more power and adds more latency than short-reach optics because of the heavy digital signal processing involved. A 10GBASE-T SFP+ places this PHY in an SFP+ cage, and host port power budgets should be checked before use. See also: Cat6, Cat6a and Cat7, RJ45, SFP+.
25GBASE-LR
The IEEE 802.3cc standard for 25 Gigabit Ethernet over 10 km of single-mode fiber at 1310 nm with a duplex LC connector. It is generally implemented with a DFB laser and a PIN or APD receiver, and normally operated with RS-FEC. It suits 25G uplinks between buildings and to aggregation sites where the span exceeds what multimode can carry. See also: 25GBASE-SR, SFP28, FEC (Forward Error Correction).
25GBASE-SR
The IEEE 802.3by standard for 25 Gigabit Ethernet over duplex multimode fiber at 850 nm, reaching about 70 m on OM3 and 100 m on OM4 when RS-FEC is enabled. It is delivered in the SFP28 form factor with an LC connector and is the usual server-to-top-of-rack interface in 25G fabrics, matching the lane rate used by 100GBASE-SR4 breakout. See also: SFP28, 100GBASE-SR4, FEC (Forward Error Correction).
40GBASE-LR4
The IEEE 802.3ba standard for 40 Gigabit Ethernet over 10 km of single-mode fiber, multiplexing four 10 Gb/s CWDM lanes near 1271, 1291, 1311 and 1331 nm onto a duplex LC pair. Because it uses a single fiber pair rather than parallel ribbon, it fits existing single-mode plant without MPO trunks. Longer 40G variants such as ER4 extend the same lane scheme to 40 km. See also: 40GBASE-SR4, CWDM (Coarse Wavelength Division Multiplexing), QSFP+.
40GBASE-SR4
The IEEE 802.3ba standard for 40 Gigabit Ethernet over multimode fiber, using four parallel 10 Gb/s lanes at 850 nm through an MPO-12 connector with eight active fibers. Reach is about 100 m on OM3 and 150 m on OM4. The same module can often be broken out into four independent 10G links with an MPO-to-LC fan-out cable when the host supports breakout mode. See also: MPO / MTP Connector, QSFP+, Base-8 and Base-12 Cabling.
AOC (Active Optical Cable)
A fixed-length assembly with transceiver electronics permanently attached to both ends of a multimode fiber pair or ribbon. Because the optics and fiber are matched and sealed at the factory, an AOC needs no connector cleaning and no separate patch cord. It is lighter and reaches further than direct attach copper, typically from a few metres to tens of metres, and is common for switch-to-server and switch-to-switch links inside a rack row. See also: DAC (Direct Attach Copper Cable), VCSEL, ToR (Top of Rack).
APD (Avalanche Photodiode)
A photodetector that applies a high reverse bias so that each absorbed photon releases multiple carriers, giving internal gain and therefore better sensitivity than a PIN diode. APDs are used in long-reach receivers such as 40 km and 80 km modules where every decibel of sensitivity matters. The trade-off is higher cost, greater temperature dependence and a lower overload point, so short links may need an attenuator. See also: PIN Photodiode, Receiver Sensitivity, Saturation (Receiver Overload).
Attenuation
The loss of optical power as light travels through fiber, connectors and splices, expressed in decibels. Single-mode fiber loses roughly 0.35 dB per kilometre at 1310 nm and about 0.2 dB per kilometre at 1550 nm, while multimode fiber at 850 nm loses around 3 dB per kilometre. Each mated connector pair and each fusion splice adds further loss that must be counted in the link budget. See also: Link Budget, Optical Margin, SMF (Single-Mode Fiber).
Bail Latch
The wire or moulded lever on the front of a pluggable transceiver that locks it into the host cage and releases it when pulled down. Bail colour is often used as a quick visual code for wavelength or reach, and on single-fiber BiDi modules it commonly distinguishes the upstream and downstream halves of a matched pair. The latch should always be used rather than pulling on the cable. See also: Hot-Pluggable, Wavelength Pairing (BiDi U and D), SFP (Small Form-factor Pluggable).
Base-8 and Base-12 Cabling
Two ways of organising MPO trunk cabling. Base-12 uses twelve-fiber MPO connectors, which suits duplex breakout but leaves four fibers idle when feeding an eight-fiber interface such as SR4. Base-8 uses eight-fiber MPO connectors so that every fiber is used by a 40GBASE-SR4 or 100GBASE-SR4 port. Choosing one scheme and keeping to it avoids stranded fibers and mismatched trunk inventory. See also: MPO / MTP Connector, Polarity (Method A, B and C), 100GBASE-SR4.
BER (Bit Error Ratio)
The proportion of received bits that are wrong, used as the primary measure of link quality. Ethernet optical interfaces are normally specified to a bit error ratio of ten to the minus twelve or better. Where forward error correction is used, a much higher raw error ratio is corrected down to that target, so the pre-correction figure alone does not indicate a fault. See also: FEC (Forward Error Correction), Receiver Sensitivity, Optical Margin.
BiDi / BX (Bidirectional Transceiver)
A transceiver that sends and receives on different wavelengths over a single fiber strand, using an internal wavelength filter and a simplex connector. Common Gigabit pairings are 1310 nm with 1490 nm or 1550 nm, and 10G and 25G variants often use 1270 nm with 1330 nm. Modules are always supplied as matched pairs, since the transmit wavelength of one end must equal the receive wavelength of the other. See also: Wavelength Pairing (BiDi U and D), WDM (Wavelength Division Multiplexing), Duplex and Simplex Fiber.
BOM (Bill of Materials)
The itemised list of parts that make up a project or an order, giving for each line the description, quantity and any coding or labelling requirement. In optical procurement a clear bill of materials states form factor, standard, wavelength, reach, connector, temperature grade and the host platform each module is coded for, which removes most of the back-and-forth from quoting and reduces the risk of a mismatched delivery. See also: RFQ (Request for Quotation), MOQ (Minimum Order Quantity), EEPROM Coding.
Cat5e
Enhanced Category 5 balanced twisted-pair cabling, specified to 100 MHz and able to carry 1000BASE-T for 100 m. It remains the minimum grade for Gigabit Ethernet and is the cabling normally assumed for a 1000BASE-T copper SFP. It is not suitable for 10GBASE-T over any useful distance, so 10G copper links call for a higher category. See also: 1000BASE-T, Cat6, Cat6a and Cat7, RJ45.
Cat6, Cat6a and Cat7
Higher grades of balanced twisted-pair cabling. Cat6 is specified to 250 MHz and supports 10GBASE-T only over a reduced distance, commonly around 55 m and subject to alien crosstalk. Cat6a is specified to 500 MHz and supports 10GBASE-T for the full 100 m. Cat7 is a shielded construction specified to 600 MHz, used mainly in Europe. Copper 10G SFP+ modules assume Cat6a or better for full reach. See also: 10GBASE-T, Cat5e, RJ45.
Chromatic Dispersion
The spreading of an optical pulse caused by different wavelengths travelling at slightly different speeds in the fiber. It grows with distance and with source spectral width, and it eventually closes the eye at the receiver. Standard G.652 fiber has near zero dispersion around 1310 nm and roughly 17 picoseconds per nanometre per kilometre at 1550 nm, which is why long 1550 nm links need narrow-linewidth sources. See also: G.652.D Fiber, DFB Laser, EML (Electro-absorption Modulated Laser).
CWDM (Coarse Wavelength Division Multiplexing)
A wavelength multiplexing scheme defined by ITU-T G.694.2 with channels spaced 20 nm apart from 1271 nm to 1611 nm. The wide spacing tolerates uncooled lasers and loose temperature control, which makes CWDM optics cheaper than dense alternatives. It is used both to combine separate services on one fiber pair and inside four-lane modules such as 40GBASE-LR4 and 100GBASE-CWDM4. See also: DWDM (Dense Wavelength Division Multiplexing), WDM (Wavelength Division Multiplexing), 100GBASE-CWDM4.

Terms

D-H

DAC (Direct Attach Copper Cable)
A fixed-length twinaxial copper assembly with transceiver housings moulded onto both ends, used for short in-rack connections. Passive versions contain no active electronics and typically reach up to about five metres at 10G and rather less at higher lane rates, while active versions add signal conditioning for longer runs. Gauge, length and host coding all matter, so both ends of the intended link should be stated when ordering. See also: AOC (Active Optical Cable), ToR (Top of Rack), EEPROM Coding.
DDM / DOM (Digital Diagnostics Monitoring)
The management interface defined in SFF-8472 that lets a host read real-time transceiver telemetry over I2C: module temperature, supply voltage, laser bias current, transmit optical power and received optical power, along with warning and alarm thresholds. It is the main field tool for judging whether a fault lies in the module, the fiber path or the far end, and it is read with a simple show command on most switches. See also: TX Power and RX Power, EEPROM Coding, MSA (Multi-Source Agreement).
DFB Laser
A distributed feedback laser, in which a grating etched along the active region forces the device to emit on a single longitudinal mode with a narrow linewidth. DFB sources are used at 1310 nm and 1550 nm in most single-mode transceivers because the narrow spectrum limits chromatic dispersion penalty over long spans. They are more expensive than VCSELs and are normally directly modulated at 10G and below. See also: EML (Electro-absorption Modulated Laser), VCSEL, Chromatic Dispersion.
Duplex and Simplex Fiber
Duplex means the link uses two fiber strands, one carrying each direction, which is the normal arrangement for interfaces such as 1000BASE-LX and 10GBASE-LR. Simplex means a single strand carries both directions, which requires bidirectional optics that transmit and receive on different wavelengths. Simplex working halves fiber consumption and is common in access networks and surveillance, at the cost of needing matched module pairs. See also: BiDi / BX (Bidirectional Transceiver), LC Connector, Wavelength Pairing (BiDi U and D).
DWDM (Dense Wavelength Division Multiplexing)
A wavelength multiplexing scheme defined by ITU-T G.694.1, using a frequency grid anchored at 193.1 THz with typical spacing of 100 GHz, about 0.8 nm, or 50 GHz in the C-band around 1550 nm. The tight spacing requires temperature-stabilised lasers and wavelength-selective filters, but allows dozens of channels on one fiber pair, which is why it dominates metro and long-haul transport. See also: CWDM (Coarse Wavelength Division Multiplexing), WDM (Wavelength Division Multiplexing), SOA (Semiconductor Optical Amplifier).
EEPROM Coding
The vendor information written into a transceiver memory map, read by the host at insertion. The SFF-8472 A0h page holds fields such as vendor name, part number, revision, serial number, date code, nominal bit rate, wavelength and supported media, protected by checksums. Host platforms read these fields to decide how to present or accept the module, which is why third-party optics are programmed to match the intended host. See also: MSA (Multi-Source Agreement), DDM / DOM (Digital Diagnostics Monitoring), OEM and ODM.
EML (Electro-absorption Modulated Laser)
A device that combines a continuous-wave DFB laser with an integrated electro-absorption modulator on the same chip. Because the laser itself is not switched, the output has far less frequency chirp than a directly modulated laser, which greatly reduces dispersion penalty. EMLs are therefore used in longer-reach and higher-rate single-mode optics such as 40 km and 80 km modules and many 25G and 100G lanes. See also: DFB Laser, Chromatic Dispersion, 10GBASE-ER and 10GBASE-ZR.
Extinction Ratio
The ratio between the average optical power of a logic one and a logic zero, expressed in decibels. A higher ratio opens the eye and improves sensitivity, but too high a value can reduce average launch power and stress the laser. Standards set a minimum per interface, ranging from around 3 dB for some 10G types up to 9 dB for 1000BASE-SX. See also: BER (Bit Error Ratio), TX Power and RX Power, Receiver Sensitivity.
FEC (Forward Error Correction)
A coding scheme that adds redundancy to the transmitted stream so the receiver can correct errors without retransmission. Reed-Solomon FEC as defined in IEEE 802.3 Clause 91 is assumed by 25G and 100G interfaces such as 100GBASE-SR4 and CWDM4. It buys several decibels of effective margin at the cost of a small fixed latency, and both ends must be configured consistently or the link will not come up cleanly. See also: BER (Bit Error Ratio), 100GBASE-SR4, 25GBASE-SR.
FTTx
A collective term for fiber access architectures, distinguished by how close the fiber runs to the subscriber: fiber to the home, to the building, to the premises, to the curb or to the node. The closer the fiber terminates to the end user, the more bandwidth is available and the less copper remains in the path. FTTx deployments are the main use of single-fiber bidirectional optics and passive optical networks. See also: PON (Passive Optical Network), BiDi / BX (Bidirectional Transceiver), Media Converter.
G.652.D Fiber
The ITU-T recommendation describing standard single-mode fiber with a low water peak, so that the full spectrum from about 1260 nm to 1625 nm is usable rather than only the 1310 nm and 1550 nm windows. It has a zero dispersion wavelength near 1310 nm and is the default fiber type in enterprise, metro and long-haul plant, which is what most single-mode transceivers assume. See also: SMF (Single-Mode Fiber), G.657 Fiber, Chromatic Dispersion.
G.657 Fiber
The ITU-T recommendation for bend-insensitive single-mode fiber, divided into categories that tolerate progressively tighter bends. It keeps macrobend loss low at radii around 10 mm for the A1 category and smaller still for the more tolerant categories, which suits patch panels, riser ducts and in-building drops. The A categories remain compatible with G.652.D so they can be spliced into standard plant. See also: G.652.D Fiber, SMF (Single-Mode Fiber), FTTx.
GBIC (Gigabit Interface Converter)
The first widely used hot-swappable Gigabit Ethernet optical module, defined by a multi-source agreement and fitted with a duplex SC connector. It was superseded by the much smaller SFP, which is sometimes called mini-GBIC for that reason. GBIC ports are now found mainly on legacy equipment, where the larger housing and SC interface remain the only option for replacement. See also: SFP (Small Form-factor Pluggable), SC Connector, MSA (Multi-Source Agreement).
Hot-Pluggable
The property of a module that allows it to be inserted or removed while the host is powered and running, without damage and without rebooting the system. Pluggable optics achieve this with staggered contact lengths on the edge connector so that ground and power make contact before the signal pins. Good practice is still to disable the port before removal and to fit dust caps immediately afterwards. See also: Bail Latch, SFP (Small Form-factor Pluggable), MSA (Multi-Source Agreement).

Terms

I-M

Incoterms (EXW, FOB, CIF)
Standard trade terms published by the International Chamber of Commerce that define where risk and cost pass from seller to buyer. Under EXW the goods are made available at the seller premises and the buyer arranges everything onward. Under FOB the seller delivers on board the vessel at a named port and risk passes there. Under CIF the seller pays carriage and insurance to a named destination port, although risk still passes at shipment. See also: RFQ (Request for Quotation), MOQ (Minimum Order Quantity), BOM (Bill of Materials).
Industrial Temperature Range
A hardware grade rated for operation from minus 40 to plus 85 degrees Celsius, as opposed to the commercial grade of 0 to plus 70 degrees Celsius. Industrial grading matters in outdoor cabinets, trackside and roadside enclosures, unconditioned plant rooms and vehicles. The grade affects laser and receiver selection inside a transceiver, so it must be stated at the time of enquiry rather than assumed. See also: MTBF (Mean Time Between Failures), Surge Protection, Media Converter.
LC Connector
A small form factor optical connector with a 1.25 mm ferrule and a latching clip, supplied either as a duplex pair or as a simplex unit for single-fiber bidirectional working. Its compact size is what allows two fiber ports to fit on an SFP front face, and it is the dominant connector on modern transceivers. Endface cleanliness is the most common cause of unexplained loss on LC links. See also: SC Connector, MPO / MTP Connector, Duplex and Simplex Fiber.
Media Converter
A standalone device that converts between two physical media, most often copper twisted pair and optical fiber, so that an Ethernet segment can be extended beyond the 100 m copper limit. Units may have a fixed optical port or an SFP slot, and some add power over Ethernet on the copper side. They operate at the physical or data link layer and are transparent to higher-layer protocols. See also: FTTx, PoE (Power over Ethernet), SFP (Small Form-factor Pluggable).
MMF (Multimode Fiber)
Optical fiber with a core large enough, typically 50 or 62.5 microns, to guide many propagation modes. The larger core makes alignment and connectorisation easier and allows low-cost 850 nm VCSEL sources, but differential mode delay limits distance. Reach therefore depends on the fiber grade and the data rate, ranging from hundreds of metres at 1G to around 100 m at 100G. See also: SMF (Single-Mode Fiber), Modal Bandwidth, OM3 Fiber.
Modal Bandwidth
The bandwidth-distance product of a multimode fiber, expressed in megahertz-kilometres, which sets how far a given data rate can travel before pulse spreading closes the eye. Effective modal bandwidth at 850 nm is about 200 MHz per kilometre for OM1, 500 for OM2, 2000 for OM3 and 4700 for OM4. This single figure explains why the same transceiver reaches very different distances on different fiber grades. See also: MMF (Multimode Fiber), OM3 Fiber, OM4 Fiber.
MOQ (Minimum Order Quantity)
The smallest quantity a supplier will accept for a given item or configuration. It usually differs between stock items and built-to-order configurations, because custom coding, custom labelling or a non-standard temperature grade involves setup work that is uneconomic in very small volumes. Buyers should confirm the applicable minimum for each line of a bill of materials rather than for the order as a whole. See also: BOM (Bill of Materials), RFQ (Request for Quotation), OEM and ODM.
MPO / MTP Connector
A multi-fiber push-on connector that carries 8, 12, 16 or 24 fibers in a single rectangular ferrule, used by parallel optical interfaces such as 40GBASE-SR4 and 100GBASE-SR4. MTP is a particular manufacturer implementation of the MPO standard with tighter mechanical tolerances. Correct key orientation and polarity are essential, and the many fiber endfaces make inspection and cleaning more important than on duplex connectors. See also: Polarity (Method A, B and C), Base-8 and Base-12 Cabling, 100GBASE-SR4.
MSA (Multi-Source Agreement)
An agreement among manufacturers that fixes the mechanical envelope, electrical interface and management memory map of a module so that products from different suppliers are interchangeable in the same host cage. The SFP, SFP+, QSFP+ and QSFP28 families all rest on such agreements, together with management documents such as SFF-8472 and SFF-8636. MSA compliance is what makes a pluggable optics market possible. See also: EEPROM Coding, DDM / DOM (Digital Diagnostics Monitoring), SFP (Small Form-factor Pluggable).
MTBF (Mean Time Between Failures)
A statistical reliability figure, usually quoted in hours, describing the average interval between failures across a large population of units under stated conditions. It is a population estimate derived from modelling or accelerated testing, not a guarantee for any individual unit and not a service life. It is most useful for comparing designs and for sizing spares holdings across a large deployment. See also: Industrial Temperature Range, Surge Protection, BOM (Bill of Materials).

Terms

N-R

OEM and ODM
Two supply models. Under OEM the buyer specifies an existing product and has it labelled, coded or packaged under their own brand. Under ODM the supplier also carries out design or configuration work to meet a specification the buyer defines. In optics the practical difference usually shows up in labelling, EEPROM coding, packaging artwork and how much non-standard engineering a configuration requires. See also: EEPROM Coding, BOM (Bill of Materials), MOQ (Minimum Order Quantity).
OM1 and OM2 Fiber
The two legacy multimode grades. OM1 has a 62.5 micron core with about 200 MHz per kilometre of effective modal bandwidth at 850 nm, and OM2 has a 50 micron core with about 500. Both carry Gigabit Ethernet over useful campus distances but support 10G only over very short spans, which is why 10GBASE-LRM exists for older plant. See also: Modal Bandwidth, 10GBASE-LRM, OM3 Fiber.
OM3 Fiber
Laser-optimised 50 micron multimode fiber with about 2000 MHz per kilometre of effective modal bandwidth at 850 nm, usually jacketed in aqua. It supports 10GBASE-SR to about 300 m, 40GBASE-SR4 to about 100 m and 100GBASE-SR4 to about 70 m. It remains very common in installed data center cabling and is the practical baseline for short-reach parallel optics. See also: OM4 Fiber, Modal Bandwidth, 10GBASE-SR.
OM4 Fiber
Laser-optimised 50 micron multimode fiber with about 4700 MHz per kilometre of effective modal bandwidth at 850 nm, also normally aqua jacketed. It extends 10GBASE-SR to about 400 m, 40GBASE-SR4 to about 150 m and 100GBASE-SR4 to about 100 m. It is the usual choice for new multimode structured cabling where parallel optics are planned. See also: OM3 Fiber, OM5 Fiber, 100GBASE-SR4.
OM5 Fiber
Wideband multimode fiber, normally lime green jacketed, which keeps the OM4 modal bandwidth at 850 nm and adds specified bandwidth across a wider window up to about 950 nm. The intent is to allow short-wavelength division multiplexing so that several wavelengths share one duplex multimode pair. It is fully backward compatible with OM4 applications on the same 50 micron core. See also: OM4 Fiber, WDM (Wavelength Division Multiplexing), Modal Bandwidth.
Optical Margin
The decibels left over once a link budget has been calculated with worst-case figures, representing headroom for connector ageing, additional patching, fiber repairs and component drift. A commissioned link with only a fraction of a decibel to spare is fragile even if it passes on the day. Three decibels is a common working target, with more on long or hard-to-reach spans. See also: Link Budget, Attenuation, Receiver Sensitivity.
PIN Photodiode
A photodetector with an intrinsic region between p-type and n-type material, giving low noise, low bias voltage and a wide dynamic range without internal gain. It is the standard receiver for short and medium reach interfaces such as SR and LR optics, where sensitivity requirements are modest. Where a longer span demands several more decibels of sensitivity, an avalanche photodiode is used instead. See also: APD (Avalanche Photodiode), Receiver Sensitivity, Saturation (Receiver Overload).
PoE (Power over Ethernet)
The delivery of DC power alongside data on the same twisted-pair cabling, so that cameras, access points and phones need no local power outlet. The original IEEE 802.3af standard provides up to 15.4 W at the sourcing equipment and about 12.95 W at the powered device, after cable loss. Powered devices declare their needs through a classification handshake before full power is applied. See also: PoE+ and PoE++ (Power Classes), Media Converter, RJ45.
PoE+ and PoE++ (Power Classes)
Later extensions of power over Ethernet. IEEE 802.3at, known as PoE+, raises sourcing power to 30 W per port, about 25.5 W at the device. IEEE 802.3bt adds Type 3 at up to 60 W and Type 4 at up to about 90 W per port, using all four pairs and extending classification to class 8. Switch total power budget, not just per-port rating, determines how many devices can be powered. See also: PoE (Power over Ethernet), Cat5e, Media Converter.
Polarity (Method A, B and C)
The TIA cabling methods that ensure a transmit fiber at one end arrives at a receive fiber at the other across MPO trunks and modules. Method A uses straight-through trunks with a key-up to key-down alignment, Method B reverses the fiber positions with key-up to key-up, and Method C flips fibers in pairs. Mixing methods within one channel is a frequent cause of links that never come up. See also: MPO / MTP Connector, Base-8 and Base-12 Cabling, 40GBASE-SR4.
PON (Passive Optical Network)
A point-to-multipoint access architecture in which one line terminal feeds many subscriber units through passive optical splitters, with no powered equipment in the outside plant. GPON as defined in ITU-T G.984 offers about 2.488 Gb/s downstream and 1.244 Gb/s upstream shared across a split of typically 1:32 or 1:64, and later generations raise this to 10 Gb/s. Traffic is separated by wavelength in each direction. See also: FTTx, WDM (Wavelength Division Multiplexing), BiDi / BX (Bidirectional Transceiver).
QSFP+
The quad small form-factor pluggable module used for 40 Gigabit Ethernet, carrying four electrical lanes of 10 Gb/s. Optical variants include SR4 over parallel multimode with an MPO connector and LR4 or ER4 over a duplex single-mode pair using four multiplexed wavelengths. Many hosts can also break a QSFP+ port out into four independent 10G interfaces when the port is configured for breakout. See also: QSFP28, 40GBASE-SR4, 40GBASE-LR4.
QSFP28
The quad small form-factor pluggable module used for 100 Gigabit Ethernet, carrying four electrical lanes of 25 Gb/s in the same mechanical envelope as QSFP+. Variants include SR4 over parallel multimode, CWDM4 and PSM4 at 2 km, LR4 at 10 km and longer single-mode types. Power draw and host cage cooling matter more than at 40G, particularly for long-reach variants in densely populated line cards. See also: QSFP+, 100GBASE-SR4, 100GBASE-LR4.
Receiver Sensitivity
The lowest average received optical power at which a receiver still meets its target bit error ratio, quoted in dBm as a negative number, where a more negative figure is better. A 10GBASE-LR receiver, for example, is specified to work down to about minus 14.4 dBm. Sensitivity sets one end of the operating window, and the saturation point sets the other. See also: Saturation (Receiver Overload), Link Budget, APD (Avalanche Photodiode).
RFQ (Request for Quotation)
A structured enquiry sent to a supplier asking for price, availability and terms against a defined specification. A useful request states quantities per line, the host platform and software version each module must work with, temperature grade, connector and reach, packaging and labelling requirements, destination and the trade term wanted. The more of this is fixed in the enquiry, the fewer assumptions appear in the quotation. See also: BOM (Bill of Materials), MOQ (Minimum Order Quantity), Incoterms (EXW, FOB, CIF).
RJ45
The eight-position, eight-contact modular connector used for twisted-pair Ethernet, formally described as 8P8C. It terminates Cat5e and higher cabling under the T568A or T568B pin assignments and carries power over Ethernet where that is used. Copper SFP and SFP+ modules present an RJ45 face so that a fiber cage can accept a standard patch lead. See also: Cat5e, Cat6, Cat6a and Cat7, 1000BASE-T.
RoHS
The European Union directive restricting hazardous substances in electrical and electronic equipment, covering lead, mercury, cadmium, hexavalent chromium, two brominated flame retardants and, since a later amendment, four phthalates. Compliance concerns the materials used in the product and its assembly. Whether a given item is declared compliant, and what documentation can accompany a shipment, should be confirmed in writing for the specific part numbers ordered. See also: BOM (Bill of Materials), RFQ (Request for Quotation), MTBF (Mean Time Between Failures).

Terms

S-Z

Saturation (Receiver Overload)
The highest average received optical power a receiver can accept before errors begin to rise again, sometimes called the overload point. It is the upper limit of the operating window, as sensitivity is the lower one. High-sensitivity long-reach receivers saturate at comparatively low input power, which is why running an 80 km module over a short patch lead needs a fixed optical attenuator. See also: Receiver Sensitivity, APD (Avalanche Photodiode), TX Power and RX Power.
SC Connector
A square-bodied optical connector with a 2.5 mm ferrule and a push-pull latching action, available in simplex and duplex forms. It predates the LC connector and is still widely used on media converters, patch panels, access equipment and older GBIC optics. Its larger footprint is why high-density transceivers moved to LC, but SC remains common in outside plant and FTTx cabinets. See also: LC Connector, GBIC (Gigabit Interface Converter), Media Converter.
SFP (Small Form-factor Pluggable)
The compact hot-pluggable transceiver form factor that replaced the GBIC, defined by a multi-source agreement and normally fitted with a duplex or simplex LC interface. Gigabit SFP optics cover everything from 850 nm short reach to 1550 nm long haul, and copper variants present an RJ45 face. The same cage also accepts SFP modules at 100 Mb/s and 2.5 Gb/s where the host supports those rates. See also: SFP+, GBIC (Gigabit Interface Converter), MSA (Multi-Source Agreement).
SFP+
The enhanced small form-factor pluggable module for 10 Gigabit Ethernet, mechanically the same size as an SFP but with an electrical interface specified for 10 Gb/s serial operation. Most SFP+ cages also accept a Gigabit SFP module, though not the reverse. The family covers SR, LR, ER, ZR, LRM, bidirectional and copper 10GBASE-T variants as well as direct attach and active optical cables. See also: SFP (Small Form-factor Pluggable), SFP28, 10GBASE-SR.
SFP28
The 25 Gb/s member of the SFP family, keeping the SFP mechanical envelope while specifying the electrical interface and signal integrity needed for a single 25 Gb/s lane. It is used for 25GBASE-SR, LR and ER optics, bidirectional variants and 25G direct attach cables, and it matches the lane rate of 100GBASE-SR4, which makes four-way breakout from a QSFP28 port straightforward. See also: SFP+, 25GBASE-SR, QSFP28.
SMF (Single-Mode Fiber)
Optical fiber with a core of roughly 9 microns, small enough that only one propagation mode is guided, which removes modal dispersion entirely. It is used with 1310 nm and 1550 nm sources and supports distances from a few hundred metres to many tens of kilometres. Tolerances on connector alignment and cleanliness are tighter than on multimode, and the same fiber serves every rate from 155 Mb/s upward. See also: MMF (Multimode Fiber), G.652.D Fiber, Attenuation.
SOA (Semiconductor Optical Amplifier)
A semiconductor device that amplifies an optical signal directly, without converting it to electrical form. In pluggable optics it appears mainly inside extended-reach modules, acting as a booster after the transmitter or as a pre-amplifier before the detector to add several decibels of system gain. It adds noise and consumes power, so it is used only where the link budget genuinely requires it. See also: Link Budget, APD (Avalanche Photodiode), DWDM (Dense Wavelength Division Multiplexing).
Spine-Leaf Architecture
A two-tier data center topology in which every leaf switch connects to every spine switch and no leaf connects directly to another leaf. Any two servers are therefore separated by the same small number of hops, which gives predictable latency and easy horizontal scaling. It creates a large population of identical uplinks, which is why parallel and multiplexed 40G and 100G optics are so heavily used in these fabrics. See also: ToR (Top of Rack), 100GBASE-SR4, QSFP28.
Surge Protection
Design measures that divert or absorb transient overvoltage from lightning or switching events, protecting Ethernet ports and power inputs. Equipment intended for outdoor cabinets, tower sites and industrial plant is usually assessed against the surge immunity test described in IEC 61000-4-5. The protection level that applies to a particular product is stated on its own datasheet and should be confirmed for the installation environment. See also: Industrial Temperature Range, Media Converter, MTBF (Mean Time Between Failures).
ToR (Top of Rack)
A cabling model in which each rack holds its own access switch, so servers connect with short copper or fiber leads inside the rack and only the switch uplinks leave it. This keeps horizontal cabling short and predictable, and it is the main reason direct attach copper and active optical cables exist in the lengths they do. See also: Spine-Leaf Architecture, DAC (Direct Attach Copper Cable), AOC (Active Optical Cable).
TX Power and RX Power
The average optical power leaving a transmitter and arriving at a receiver, expressed in dBm, where 0 dBm equals one milliwatt and negative values are below that. Both are readable through digital diagnostics on most modules. Comparing the two across a span gives the total path loss, which is the quickest way to separate a fiber problem from a module problem in the field. See also: DDM / DOM (Digital Diagnostics Monitoring), Link Budget, Attenuation.
VCSEL
A vertical-cavity surface-emitting laser, which emits perpendicular to the wafer surface and can therefore be tested on wafer and made in high volume at low cost. VCSELs operate at 850 nm in nearly all multimode transceivers, including SR, SR4 and active optical cables. They are efficient and reliable at short reach but their spectral width and wavelength restrict them to multimode fiber over short distances. See also: 10GBASE-SR, MMF (Multimode Fiber), AOC (Active Optical Cable).
Wavelength Pairing (BiDi U and D)
The rule that single-fiber bidirectional modules are deployed as matched pairs, one upstream and one downstream. If one end transmits at 1310 nm and receives at 1550 nm, the far end must transmit at 1550 nm and receive at 1310 nm. Suppliers usually distinguish the two halves by part number suffix and bail colour, and ordering a quantity of one half alone leaves the link unusable. See also: BiDi / BX (Bidirectional Transceiver), Bail Latch, Duplex and Simplex Fiber.
WDM (Wavelength Division Multiplexing)
The technique of carrying several independent signals on one fiber by giving each its own wavelength, separated at the far end by optical filters. It appears at three scales in Ethernet optics: inside four-lane modules such as LR4 and CWDM4, in single-fiber bidirectional modules that use one wavelength per direction, and in external multiplexers that combine many services onto one fiber pair. See also: CWDM (Coarse Wavelength Division Multiplexing), DWDM (Dense Wavelength Division Multiplexing), BiDi / BX (Bidirectional Transceiver).
XFP
A 10 Gigabit transceiver form factor defined by its own multi-source agreement, larger than SFP+ and using a serial XFI electrical interface with the clock and data recovery function inside the module. It preceded SFP+ and was displaced by it because SFP+ offers higher port density and lower power. XFP cages are still found on older transport and routing equipment. See also: SFP+, MSA (Multi-Source Agreement), 10GBASE-LR.

Next step

Need the term applied to your network?

Send the link requirement and we will translate it into a configuration you can quote.

Ask our team