Small form-factor pluggable transceiver

The small form-factor pluggable (SFP) is a compact, hot-pluggable network interface module used for both telecommunication and data communications applications. An SFP interface on networking hardware is a modular slot for a media-specific transceiver in order to connect a fiber-optic cable or sometimes a copper cable.[1] The advantage of using SFPs compared to fixed interfaces (e.g. modular connectors in Ethernet switches) is that individual ports can be equipped with any suitable type of transceiver as needed.

Small Form-factor Pluggable connected to a pair of fiber optic cables.

The form factor and electrical interface are specified by a multi-source agreement (MSA) under the auspices of the Small Form Factor Committee.[2] The SFP replaced the larger GBIC in most applications, and has been referred to as a Mini-GBIC by some vendors.[3]

SFP transceivers exist supporting synchronous optical networking (SONET), Gigabit Ethernet, Fibre Channel, PON, and other communications standards. At introduction, typical speeds were 1 Gbit/s for Ethernet SFPs and up to 4 Gbit/s for Fibre Channel SFP modules.[4] In 2006, SFP+ specification brought speeds up to 10 Gbit/s and the SFP28 iteration is designed for speeds of 25 Gbit/s.[5]

A slightly larger sibling is the four-lane Quad Small Form-factor Pluggable (QSFP). The additional lanes allow for speeds 4 times their corresponding SFP. In 2014, the QSFP28 variant was published allowing speeds up to 100 Gbit/s.[6] In 2019, the closely related QSFP56 was standardized[7] doubling the top speeds to 200 Gbit/s with products already selling from major vendors.[8] There are inexpensive adapters allowing SFP transceivers to be placed in a QSFP port.

Both a SFP-DD,[9] which allows for 100 Gbit/s over two lanes, as well as a QSFP-DD[10] specifications, which allows for 400 Gbit/s over eight lanes, have been published. These use a form factor which is backward compatible to their respective predecessors. An alternative competing solution, the OSFP (Octal Small Format Pluggable) transceiver is also intended for 400 Gbit/s fiber optic links between network equipment via 8 × 50 Gbit/s electrical data lanes.[11] It is slightly larger version than the QSFP formfactor which is capable of handling larger power outputs. The OSFP standard was initially announced on November 15, 2016.[12] Its proponents say a low cost adapter will allow for QSFP module compatibility.[13]

SFP types

SFP transceivers are available with a variety of transmitter and receiver specifications, allowing users to select the appropriate transceiver for each link to provide the required optical or electrical reach over the available media type (e.g. twisted pair copper cables, multi-mode or single-mode fiber cables). Transceivers are also designated by their transmission speed. SFP modules are commonly available in several different categories.

Comparison of SFP types
Name Standard Introduced Status Size Backward compatible MAC block to a PHY chip Media Connector Max channels Notes
100 Mbit/s SFP SFF INF-8074i 2001-05-01 current 113.9 mm2 none MII Fiber, copper LC, RJ45 1
1 Gbit/s SFP SFF INF-8074i 2001-05-01 current 113.9 mm2 100 Mbit/s SFP* SGMII Fiber, copper LC, RJ45 1
1 Gbit/s cSFP current 113.9 mm2 Fiber LC 2
10 Gbit/s SFP+ SFF SFF-8431 4.1 2009-07-06 current 113.9 mm2 1 Gbit/s SFP XGMII Fiber, copper, DAC LC, RJ45 1
25 Gbit/s SFP28 SFF SFF-8402 2014-09-13 current 113.9 mm2 10 Gbit/s SFP+ Fiber, DAC LC 1
50 Gbit/s SFP56 current 113.9 mm2 Fiber, DAC LC 1
4 Gbit/s QSFP SFF INF-8438 2006-11-01 current 156 mm2 none GMII 4
40 Gbit/s QSFP+ SFF SFF-8683 2012-04-01 current 156 mm2 none XGMII Fiber. DAC LC, MTP/MPO 4 CWDM
50 Gbit/s QSFP28 SFF SFF-8665 2014-09-13 current 156 mm2 QSFP+ Fiber, DAC LC 2
100 Gbit/s QSFP28 SFF SFF-8665 2014-09-13 current 156 mm2 none Fiber, DAC LC, MTP/MPO-12 4 CWDM
200 Gbit/s QSFP56 SFF SFF-8665 2015-06-29 current 156 mm2 none Fiber, DAC LC, MTP/MPO-12 4
400 Gbit/s QSFP-DD SFF INF-8628 2016-06-27 current 156 mm2 QSFP+, QSFP28[14] Fiber, DAC LC, MTP/MPO-16 8 CWDM

100 Mbit/s SFP


  • Multi-mode fiber, LC connector, with black or Beige color coding
    • SX – 850 nm, for a maximum of 550 m
  • Multi-mode fiber, LC connector, with blue color coding
    • FS  – 1300 nm, for a distance up to 5 km.
    • LFX (name dependent on manufacturer) – 1310 nm, for a distance up to 5 km.
  • Single-mode fiber, LC connector, with blue color coding
    • LX – 1310 nm, for distances up to 10 km
    • EX – 1310 nm, for distances up to 40 km
  • Single-mode fiber, LC connector, with green color coding
    • ZX – 1550 nm, for distances up to 80 km, (depending on fiber path loss)
    • EZX – 1550 nm, for distances up to 160 km (depending on fiber path loss)
  • Single-mode fiber, LC connector, Bi-Directional, with blue and yellow color coding
    • BX (officially BX10) – 1550 nm/1310 nm, Single Fiber Bi-Directional 100 Mbit SFP Transceivers, paired as BX-U (blue) and BX-D (yellow) for uplink and downlink respectively, also for distances up to 10 km. Variations of bidirectional SFPs are also manufactured which higher transmit power versions with link length capabilities up to 40 km.
  • Copper twisted-pair cabling, 8P8C (RJ-45) connector

1 Gbit/s SFP
  • 1 Gbit/s multi-mode fiber, LC connector, with black or beige extraction lever[2]
    • SX – 850 nm, for a maximum of 550 m at 1.25 Gbit/s (gigabit Ethernet). Other multi-mode SFP applications support even higher rates at shorter distances.[15]
  • 1.25 Gbit/s multi-mode fiber, LC connector, extraction lever colors not standardised
    • SX+/MX/LSX (name dependent on manufacturer) – 1310 nm, for a distance up to 2 km.[16] Not compatible with SX or 100BASE-FX. Based on LX but engineered to work with a multi-mode fiber using a standard multi-mode patch cable rather than a mode-conditioning cable commonly used to adapt LX to multi-mode.
  • 1 to 2.5 Gbit/s single-mode fiber, LC connector, with blue extraction lever[2]
    • LX – 1310 nm, for distances up to 10 km (originally, LX just covered 5 km and LX10 for 10 km followed later)
    • EX – 1310 nm, for distances up to 40 km
    • ZX – 1550 nm, for distances up to 80 km (depending on fiber path loss), with green extraction lever (see GLC-ZX-SM1)
    • EZX – 1550 nm, for distances up to 160 km (depending on fiber path loss)
    • BX (officially BX10) – 1490 nm/1310 nm, Single Fiber Bi-Directional Gigabit SFP Transceivers, paired as BX-U and BX-D for uplink and downlink respectively, also for distances up to 10 km.[17][18] Variations of bidirectional SFPs are also manufactured which use 1550 nm in one direction, and higher transmit power versions with link length capabilities up to 80 km.
    • 1550 nm 40 km (XD), 80 km (ZX), 120 km (EX or EZX)
    • SFSW – single-fiber single-wavelength transceivers, for bi-directional traffic on a single fiber. Coupled with CWDM, these double the traffic density of fiber links.[19][20]
    • Coarse wavelength-division multiplexing (CWDM) and dense wavelength-division multiplexing (DWDM) transceivers at various wavelengths achieving various maximum distances. CWDM and DWDM transceivers usually support link distances of 40 km, 80 km and 120 km.
  • 1 Gbit/s for copper twisted-pair cabling, 8P8C (RJ-45) connector
    • 1000BASE-T – these modules incorporate significant interface circuitry for Physical Coding Sublayer recoding[21] and can be used only for gigabit Ethernet because of the specific line code. They are not compatible with (or rather: do not have equivalents for) Fibre Channel or SONET. Unlike non-SFP, copper 1000BASE-T ports integrated into most routers and switches, 1000BASE-T SFPs usually cannot operate at 100BASE-TX speeds.
  • 100 Mbit/s copper and optical – some vendors have shipped 100 Mbit/s limited SFPs for fiber-to-the-home applications and drop-in replacement of legacy 100BASE-FX circuits. These are relatively uncommon and can be easily confused with 100 Mbit/s SFPs.[22]
  • Although it is not mentioned in any official specification document the maximum data rate of the original SFP standard is 5 Gbit/s.[23] This was eventually used by both 4GFC Fibre Channel and the DDR Infiniband especially in its four lane QSFP form.
  • In recent years, SFP transceivers have been created that will allow 2.5 Gbit/s and 5 Gbit/s Ethernet speeds with SFPs with 2.5GBASE-T[24] and 5GBASE-T.[25]

10 Gbit/s SFP+
A 10 Gigabit Ethernet XFP transceiver, top, and a SFP+ transceiver, bottom

The SFP+ (enhanced small form-factor pluggable) is an enhanced version of the SFP that supports data rates up to 16 Gbit/s. The SFP+ specification was first published on May 9, 2006, and version 4.1 published on July 6, 2009.[26] SFP+ supports 8 Gbit/s Fibre Channel, 10 Gigabit Ethernet and Optical Transport Network standard OTU2. It is a popular industry format supported by many network component vendors. Although the SFP+ standard does not include mention of 16 Gbit/s Fibre Channel, it can be used at this speed.[27][lower-alpha 1]

SFP+ also introduces direct attach for connecting two SFP+ ports without dedicated transceivers. Direct attach cables (DAC) exist in passive (up to 7 m), active (up to 15 m), and active optical (AOC, up to 100 m) variants.

10 Gbit/s SFP+ modules are exactly the same dimensions as regular SFPs, allowing the equipment manufacturer to re-use existing physical designs for 24 and 48-port switches and modular line cards. In comparison to earlier XENPAK or XFP modules, SFP+ modules leave more circuitry to be implemented on the host board instead of inside the module.[28] Through the use of an active electronic adapter, SFP+ modules may be used in older equipment with XENPAK ports [29] and X2 ports.[30][31]

SFP+ modules can be described as limiting or linear types; this describes the functionality of the inbuilt electronics. Limiting SFP+ modules include a signal amplifier to re-shape the (degraded) received signal whereas linear ones do not. Linear modules are mainly used with the low bandwidth standards such as 10GBASE-LRM; otherwise, limiting modules are preferred.[32]

25 Gbit/s SFP28

SFP28 is a 25 Gbit/s interface which evolved from the 100 Gigabit Ethernet interface which is typically implemented with 4 by 25 Gbit/s data lanes. Identical in mechanical dimensions to SFP and SFP+, SFP28 implements one 28 Gbit/s lane[33] accommodating 25 Gbit/s of data with encoding overhead.[34]

SFP28 modules exist supporting single-[35] or multi-mode[36] fiber connections, active optical cable[37] and direct attach copper.[38][39]

cSFP

The compact small form-factor pluggable (cSFP) is a version of SFP with the same mechanical form factor allowing two independent bidirectional channels per port. It is used primarily to increase port density and decrease fiber usage per port.[40][41]

SFP-DD

The small form-factor pluggable double density (SFP-DD) multi source agreement is a new standard for doubling port density. According to the SFD-DD MSA website: "Network equipment based on the SFP-DD will support legacy SFP modules and cables, and new double density products."[42]

QSFP types
QSFP+ 40 Gb Transceiver

Quad Small Form-factor Pluggable (QSFP) transceivers are available with a variety of transmitter and receiver types, allowing users to select the appropriate transceiver for each link to provide the required optical reach over multi-mode or single-mode fiber.

4 Gbit/s QSFP
The original QSFP document specified four channels carrying Gigabit Ethernet, 4GFC (FiberChannel), or DDR InfiniBand.[43]

40 Gbit/s QSFP+
QSFP+ is an evolution of QSFP to support four 10 Gbit/s channels carrying 10 Gigabit Ethernet, 10GFC FiberChannel, or QDR InfiniBand.[44] The 4 channels can also be combined into a single 40 Gigabit Ethernet link.

50 Gbit/s QSFP14
The QSFP14 standard is designed to carry FDR InfiniBand, SAS-3.[45] or 16G Fibre Channel

100 Gbit/s QSFP28
The QSFP28 standard[6] is designed to carry 100 Gigabit Ethernet, EDR InfiniBand, or 32G Fibre Channel. Sometimes this transceiver type is also referred to as "QSFP100" or "100G QSFP"[46] for sake of simplicity.

200 Gbit/s QSFP56
QSFP56 is designed to carry 200 Gigabit Ethernet, HDR InfiniBand, or 64G Fibre Channel. The biggest enhancement is that QSFP56 uses four-level pulse-amplitude modulation (PAM-4) instead of non-return-to-zero (NRZ). It uses the same physical specifications as QSFP28 (SFF-8665), with electrical specifications from SFF-8024[47] and revision 2.10a of SFF-8636.[7] Sometimes this transceiver type is referred to as "200G QSFP"[48] for sake of simplicity.

Fanout or breakout

Switch and router manufacturers implementing QSFP+ ports in their products frequently allow for the use of a single QSFP+ port as four independent 10 gigabit ethernet connections, greatly increasing port density. For example, a typical 24-port QSFP+ 1U switch would be able to service 96x10GbE connections.[49][50][51] There also exist fanout cables to adapt a single QSFP28 port to four independent 25 gigabit ethernet SFP28 ports (QSFP28-to-4×SFP28)[52] as well as cables to adapt a single QSFP56 port to four independent 50 gigabit ethernet SFP56 ports (QSFP56-to-4×SFP56).[53]

Applications
Ethernet switch with two empty SFP slots (lower left)

SFP sockets are found in Ethernet switches, routers, firewalls and network interface cards. They are used in Fibre Channel host adapters and storage equipment. Because of their low cost, low profile, and ability to provide a connection to different types of optical fiber, SFP provides such equipment with enhanced flexibility.

Standardization

The SFP transceiver is not standardized by any official standards body, but rather is specified by a multi-source agreement (MSA) among competing manufacturers. The SFP was designed after the GBIC interface, and allows greater port density (number of transceivers per given area) than the GBIC, which is why SFP is also known as mini-GBIC.

However, as a practical matter, some networking equipment manufacturers engage in vendor lock-in practices whereby they deliberately break compatibility with "generic" SFPs by adding a check in the device's firmware that will enable only the vendor's own modules.[54] Third-party SFP manufacturers have introduced SFPs with EEPROMs which may be programmed to match any vendor ID.[55]

Color coding of SFP

Color coding of SFP
Color Standard Media wavelength Notes
black INF-8074 Multimode 850 nm
Beige INF-8074 Multimode 850 nm
black INF-8074 Multimode 1310 nm
Blue INF-8074 Singlemode 1310 nm
Red proprietary
(non SFF)		 Singlemode 1310 nm Used on 25GBASE-ER[56]
Green proprietary
(non SFF)		 Singlemode 1550 nm Used on 100BASE-ZE
Red proprietary
(non SFF)		 Singlemode 1550 nm Used on 10GBASE-ER
White proprietary
(non SFF)		 Singlemode 1550 nm Used on 10GBASE-ZR

Color coding of CWDM SFP [57]
Color Standard wavelength Notes
Grey 1270 nm
Grey 1290 nm
Grey 1310 nm
Violet 1330 nm
Blue 1350 nm
Green 1370 nm
Yellow 1390 nm
Orange 1410 nm
Red 1430 nm
Brown 1450 nm
Grey 1470 nm
Violet 1490 nm
Blue 1510 nm
Green 1530 nm
Yellow 1550 nm
Orange 1570 nm
Red 1590 nm
Brown 1610 nm

Color coding of BiDi SFP
Name Standard Side A Color TX Side A wavelength TX Side B Color TX Side B wavelength TX Notes
1000BASE-BX Blue 1310 nm Purple 1490 nm
1000BASE-BX Blue 1310 nm Yellow 1550 nm
10GBASE-BX
25GBASE-BX		 Blue 1270 nm Red 1330 nm
10GBASE-BX White 1490 nm White 1550 nm



Color coding of QSFP
Color Standard wavelength Multiplexing Notes
Beige INF-8438 850 nm No
Blue INF-8438 1310 nm No
White INF-8438 1550 nm No

Signals
Front view of SFP module with integrated LC connector. The blue extraction lever indicates the module is designed for use with single-mode optical fiber.
OC-3 SFP internal. The top, metal canister is the transmitting laser diode, the bottom, plastic canister is the receiving photo diode.

The SFP transceiver contains a printed circuit board with an edge connector with 20 pads that mate on the rear with the SFP electrical connector in the host system. The QSFP has 38 pads including 4 high-speed transmit data pairs and 4 high-speed receive data pairs.[43][44]

SFP electrical pin-out[2]
Pad Name Function
1 VeeT Transmitter ground
2 Tx_Fault Transmitter fault indication
3 Tx_Disable Optical output disabled when high
4 SDA 2-wire serial interface data line (using the CMOS EEPROM protocol defined for the ATMEL AT24C01A/02/04 family[58])
5 SCL 2-wire serial interface clock
6 Mod_ABS Module absent, connection to VeeT or VeeR in the module indicates module presence to host
7 RS0 Rate select 0
8 Rx_LOS Receiver loss of signal indication
9 RS1 Rate select 1
10 VeeR Receiver ground
11 VeeR Receiver ground
12 RD- Inverted received data
13 RD+ Received data
14 VeeR Receiver ground
15 VccR Receiver power (3.3 V, max. 300 mA)
16 VccT Transmitter power (3.3 V, max. 300 mA)
17 VeeT Transmitter ground
18 TD+ Transmit data
19 TD- Inverted transmit data
20 VeeT Transmitter ground
QSFP electrical pin-out[43]
Pad Name Function
1 GND Ground
2 Tx2n Transmitter inverted data input
3 Tx2p Transmitter non-inverted data input
4 GND Ground
5 Tx4n Transmitter inverted data input
6 Tx4p Transmitter non-inverted data input
7 GND Ground
8 ModSelL Module select
9 ResetL Module reset
10 Vcc-Rx +3.3 V receiver power supply
11 SCL Two-wire serial interface clock
12 SDA Two-wire serial interface data
13 GND Ground
14 Rx3p Receiver non-inverted data output
15 Rx3n Receiver inverted data output
16 GND Ground
17 Rx1p Receiver non-inverted data output
18 Rx1n Receiver inverted data output
19 GND Ground
20 GND Ground
21 Rx2n Receiver inverted data output
22 Rx2p Receiver non-inverted data output
23 GND Ground
24 Rx4n Receiver inverted data output
25 Rx4p Receiver non-inverted data output
26 GND Ground
27 ModPrsL Module present
28 IntL Interrupt
29 Vcc-Tx +3.3 V transmitter power supply
30 Vcc1 +3.3 V power supply
31 LPMode Low power mode
32 GND Ground
33 Tx3p Transmitter non-inverted data input
34 Tx3n Transmitter inverted data input
35 GND Ground
36 Tx1p Transmitter non-inverted data input
37 Tx1n Transmitter inverted data input
38 GND Ground

Mechanical dimensions
Side view of SFP module. Depth, the longest dimension, is 56.5 mm (2.22 in).

The physical dimensions of the SFP transceiver (and its subsequent faster variants) are narrower than the later QSFP counterparts, which allows for SFP transceivers to be placed in QSFP ports via an inexpensive adapter. Both are smaller than the XFP transceiver.

Dimensions
SFP[2] QSFP[43] XFP[59]
mminmminmmin
Height 8.5 0.33 8.5 0.33 8.5 0.33
Width 13.4 0.53 18.35 0.722 18.35 0.722
Depth 56.5 2.22 72.4 2.85 78.0 3.07

EEPROM information

The SFP MSA defines a 256-byte memory map into an EEPROM describing the transceiver's capabilities, standard interfaces, manufacturer, and other information, which is accessible over a serial I²C interface at the 8-bit address 1010000X (A0h).

Digital diagnostics monitoring

Modern optical SFP transceivers support standard digital diagnostics monitoring (DDM) functions.[60] This feature is also known as digital optical monitoring (DOM). This capability allows monitoring of the SFP operating parameters in real time. Parameters include optical output power, optical input power, temperature, laser bias current, and transceiver supply voltage. In network equipment, this information is typically made available via Simple Network Management Protocol (SNMP). A DDM interface allows end users to display diagnostics data and alarms for optical fiber transceivers and can be used to diagnose why a transceiver is not working.

See also

Notes
  1. Besides the data rate, the major difference between 8 and 16 Gbit/s Fibre Channel is the encoding method. 64b/66b encoding used for 16 Gbit/s is a more efficient encoding mechanism than 8b/10b used for 8 Gbit/s, and allows for the data rate to double without doubling the line rate. The result is a 14.025 Gbit/s line rate for 16 Gbit/s Fibre Channel.

References
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