Small form-factor pluggable transceiver

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

The small form-factor pluggable (SFP) is a compact, hot-pluggable optical module transceiver used for both telecommunication and data communications applications. The form factor and electrical interface are specified by a multi-source agreement (MSA) under the auspices of the Small Form Factor Committee.[1] It is a popular industry format jointly developed and supported by many network component vendors.

An SFP interface on networking hardware is a modular (plug-and-play) slot for a variable, media-specific transceiver in order to connect a fiber optic cable or sometimes a copper cable.[2] SFP transceivers exist supporting SONET, Gigabit Ethernet, Fibre Channel, and other communications standards. At introduction, speeds were limited to 1Gib/s, but the published SFP28 iteration is designed for speeds of 25 Gbit/s.[3] The SFP replaced the larger GBIC in most applications, and has been referred to as a Mini-GBIC by some vendors.[4]

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. The latest published variant is QSFP28 variant allowing speeds up to 100 Gbit/s[5]. There are inexpensive adapters allowing SFP transceivers to be placed in a QSFP port.

Both a SFP-DD[6], which allows for 100 Gbit/s over two lanes, as well as a QSFP-DD[7] specifications, which allows for 400 Gbit/s over eight lanes, have been published. These use a formfactor which is backwardly compatible to their respective predecessors. An alternative competing solution, the OSFP (Octal Small Format Pluggable) transceiver is also intended for 400Gbps fiber optic links between network equipment via 8 x 50 Gbps electrical data lanes.[8] 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.[9] Its proponents say a low cost adapter will allow for QSFP module compatibility[10].

SFP Types[edit]

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 reach over the available optical fiber type (e.g. multi-mode fiber or single-mode fiber). Transceivers are also designated by their transmission speed. SFP modules are commonly available in several different categories.

1 Gbit/s SFP[edit]

  • 1 Gbit/s multi-mode fiber, LC connector, with black or beige extraction lever[1]
    • 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.[11]
  • 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.[12] 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[1]
    • 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 [13]
    • ZX – 1550 nm, for distances up to 80 km (depending on fiber path loss), with green extraction lever (see GLC-ZX-SM1)[13]
    • EZX – 1550 nm, for distances up to 160 km (depending on fiber path loss)[13]
    • 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.[14][15] 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.[16][17]
    • CWDM and DWDM transceivers at various wavelengths achieving various maximum distances. CWDM and DWDM transceiver usually support 40 km, 80 km and 120 km link distance.[18][19]
  • 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[20] and can only be used for gigabit Ethernet because of the specific line code. They are not compatible with (or rather: do not have equivalents for) Fiber 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 1 Gbit/s SFPs.[21]
  • Although it is not mentioned in any official specification document the maximum data rate of the original SFP standard is 5 Gbit/s.[22]. This was eventually used by the DDR Infiniband especially in its four lane QSFP form.

10 Gbit/s SFP+[edit]

The enhanced small form-factor pluggable (SFP+) 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.[23] 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.[24][a]

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.[25] Through the use of an active electronic adapter, SFP+ modules may be used in older equipment with XENPAK ports [26] and X2 ports. [27]

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.[28]

25 Gbit/s SFP28[edit]

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[29] accommodating 25 Gbit/s of data with encoding overhead.[30]

SFP28 modules exist supporting single-[31] or multi-mode[32] fiber connections, active optical cable[33] and direct attach copper.[34][35]

cSFP[edit]

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.[36][37]

SFP-DD[edit]

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."[38]

QSFP types[edit]

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[edit]

The original QSFP document specified four channels carrying Gigabit Ethernet, 4GFC (FiberChannel), or DDR InfiniBand.[39]


40 Gbit/s QSFP+[edit]

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

50 Gbit/s QSFP14[edit]

The QSFP14 standard is designed to carry FDR InfiniBand, SAS-3.[41] or 16G Fibre Channel

100 Gbit/s QSFP28[edit]

The QSFP28 standard is designed to carry 100 Gigabit Ethernet, EDR InfiniBand.[5] or 32G Fibre Channel. This transceiver type is also used with direct-attach breakout cables to adapt a single 100GbE port to four independent 25 gigabit ethernet ports (QSFP28-to-4x-SFP28).[42] Sometimes this transceiver type is also referred to as "QSFP100" or "100G QSFP" [43] for sake of simplicity.

Fanout[edit]

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 in a 1U height 24-port switch (24 40Gb ports x 4 = 96 10GbE).[44][45][46]

Compatibility[edit]

Many manufacturers restrict their devices to accept only original SFP modules of the same brand, as identified by their vendor ID. Due to sometimes significant price differences between original and generic modules, there is a large market of "compatible" or "third party" modules with their EEPROMs programmed to show the appropriate vendor ID.

It is possible to design an SFP+ slot that can accept a standard SFP module. Some routing and Ethernet switch equipment allows for the use of a 10 Gbit/s SFP+ slots with lower Gigabit Ethernet SFPs, such as a 100BASE-LX SFP.[47][48]

Applications[edit]

Ethernet switch with two empty SFP slots (lower left)

SFP sockets are found in Ethernet switches, routers, firewalls and network interface cards. Storage interface cards, also called HBAs or Fibre Channel storage switches, also make use of these modules, supporting different speeds such as 2Gb, 4Gb, and 8Gb. 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[edit]

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 cm along the edge of a mother board) than the GBIC, which is why SFP is also known as mini-GBIC. The related Small Form Factor transceiver is similar in size to the SFP, but is soldered to the host board as a through-hole device, rather than plugged into an edge-card socket.[citation needed]

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.[49] Third-party SFP manufacturers have introduced SFPs with "blank" programmable EEPROMs which may be reprogrammed to match any vendor ID.[50]

Signals[edit]

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 PCB with 20 pads that mates on the rear with the SFP electrical connector in the host system. QSFP has 38 pins including 4 high-speed TX pairs (CML-I) and 4 high-speed RX pairs (CML-O).[39][40]

SFP electrical pin-out[1]
Pin 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
5 SCL 2-wire Serial Interface Clock
6 Mod_ABS Module Absent, connected to VeeT or VeeR in the module
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[39]
Pad Symbol Name/Description
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 +3.3V Power Supply Receiver
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 +3.3V Power Supply Transmitter
30 Vcc1 +3.3V 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[edit]

Side view of SFP module (length is 6 cm).

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[1] QSFP[39] XFP[51]
Height 8.5 mm (0.33 inches) 8.5 mm (0.33 inches) 8.5 mm (0.33 inches)
Width 13.4 mm (0.53 inches) 18.35 mm (0.72 inches) 18.35 mm (0.72 inches)
Depth 56.5 mm (2.22 inches) 72.4 mm (2.85 inches) 78.0 mm (3.10 inches)

EEPROM information[edit]

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 an I²C interface at the 8-bit address 1010000X (A0h).

Digital diagnostics monitoring[edit]

Modern optical SFP transceivers support standard digital diagnostics monitoring (DDM) functions.[52] This feature is also known as digital optical monitoring (DOM). Modules with this capability enable the end user to monitor parameters of the SFP, such as optical output power, optical input power, temperature, laser bias current, and transceiver supply voltage, in real time. This functionality is commonly implemented for monitoring on routers, switches and optical transport equipment via SNMP.

A DDM interface allows end users to display diagnostics data and alarms for fiber optical transceivers and can be used to diagnose why a transceiver optics is not working, increasing popularity of transceiver optics with DDM. Generally, the transceiver vendor sets the thresholds that trigger a high alarm, low alarm, high warning, or low warning before shipment. In order to be able to take advantage of DDM/DOM capability, most of the modern pluggable transceiver optics support DDM/DOM interfaces.[citation needed]

See also[edit]

Notes[edit]

  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[edit]

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  2. ^ "SFP Definition from PC Magazine Encyclopedia". www.pcmag.com. Retrieved 2018-05-10.
  3. ^ "SFF-8402: SFP+ 1X 28 Gb/s Pluggable Transceiver Solution (SFP28)". 1.9. SNIA SFF Committee. 2014-09-13. Retrieved 26 March 2019.
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  7. ^ "QSFP-DD MSA".
  8. ^ "Lightwave Online news article re: 400Gb".
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  10. ^ https://osfpmsa.org/assets/pdf/OSFP-to-QSFP-Adapter.pdf
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  38. ^ http://sfp-dd.com/
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  40. ^ a b SFF Committee. "QSFP+ 10 Gbs 4X Pluggable Transceiver (SFF-8436)" (PDF). p. 13. Retrieved 22 June 2016.
  41. ^ SFF Committee. "QSFP+ 14 Gb/s 4X Pluggable Transceiver Solution (QSFP14)" (PDF). p. 5. Retrieved 22 June 2016.
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  47. ^ SFF-8432, Abstract, Page 1: "The mechanical dimensioning allows backwards compatibility between IPF modules plugged into most SFP cages which have been implemented to SFF-8074i. It is anticipated that when the application requires it, manufacturers will be able to supply cages that accept SFP style modules. In both cases the EMI leakage is expected to be similar to that when SFP modules and cages are mated."
  48. ^ SFF-8431, Chapter 2 Low Speed Electrical and Power Specifications, 2.1 Introduction, Page 4: "The SFP+ low speed electrical interface has several enhancements over the classic SFP interface (INF-8074i), but the SFP+ host can be designed to also support most legacy SFP modules."
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