Domain 1 of 5 · Chapter 7 of 9

Transceivers and Connectors

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Included in this chapter:

  • Transceiver form factors and their speeds
  • Connectors by media: fiber, copper, and coax
  • Matching optic to fiber, connector, and polish
  • Exam-pattern recognition

Transceiver form factors and their connectors

Form factorSpeedChannelsTypical connector
GBIC (legacy)1 Gbps1SC (predecessor of SFP)
SFP1 Gbps1Duplex LC (fiber) or RJ45 (copper)
SFP+10 Gbps1Duplex LC, or DAC twinax
SFP2825 Gbps1Duplex LC, or DAC twinax
QSFP+40 Gbps4MPO/MTP (SR4) or duplex LC
QSFP28100 Gbps4MPO/MTP (SR4) or duplex LC

Decision tree

Need 40G or 100G?i.e. four parallel lanesQSFP+ 40G / QSFP28 100GMPO/MTP ribbon connectorFew meters in a rack?cost is the driverDAC twinax cableoptics molded on each endWhat single-lane speed?picks the SFP tierSFP (1G)duplex LC; GBIC is legacySFP+ (10G)duplex LCSFP28 (25G)duplex LCAlways: both ends match fiber mode, standard, and polishyesnoyesno1G10G25G
Cheat sheet

Cheat sheet

  • A transceiver is a hot-pluggable optic that converts a port's electrical signal to the cable's signal
  • SFP is the 1 Gbps single-channel optic; it replaced the legacy GBIC
  • SFP+ is the 10 Gbps optic in the same cage size as SFP
  • SFP28 is the 25 Gbps single-lane optic
  • QSFP+ delivers 40 Gbps as four 10G lanes
  • QSFP28 delivers 100 Gbps as four 25G lanes
  • Read the form factor as a speed: SFP 1G, SFP+ 10G, SFP28 25G, QSFP+ 40G, QSFP28 100G
  • Both ends must run the identical fiber mode and standard, not just the same form factor
  • A dark fiber link with good cable usually means a transceiver/connector mismatch
  • A reversed transmit/receive fiber pair leaves a link dark even with a clean cable
  • LC is the small-form-factor duplex connector used on SFP-family optics
  • SC is the square push-pull fiber connector with a 2.5 mm ferrule
  • ST is the bayonet twist-lock fiber connector
  • MPO/MTP packs 12 or 24 fibers into one connector for parallel 40G/100G optics
  • RJ45 (8P8C) is the eight-pin connector for twisted-pair Ethernet, never fiber
  • RJ11 is the small 6-position telephone/DSL connector, not Ethernet
  • Coax uses F-type for cable internet and BNC for legacy/video
  • Read the connector to read the media: LC/SC/ST/MPO fiber, RJ45/RJ11 copper, F-type/BNC coax
  • Never mate UPC to APC fiber; APC connectors are green
  • Use a DAC cable for cheap, short switch-to-server links

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References

  1. Cisco Transceiver Modules (SFP, SFP+, SFP28, QSFP+, QSFP28) product family
  2. Gigabit interface converter (GBIC) — legacy 1G module superseded by SFP (mini-GBIC) Community
  3. Small Form-factor Pluggable (SFP / SFP+ / SFP28 / QSFP+ / QSFP28) data rates and lanes Community
  4. Optical fiber connector — LC/SC/ST ferrule sizes, MPO/MTP fiber counts, UPC vs APC polish Community
  5. Modular connector — 8P8C/RJ45 misnomer and RJ11 6-position telephone jack Community