Small form-factor pluggable transceiver

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

The small form-factor pluggable (SFP) is a compact, hot-pluggable 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 SFF Committee. It is a popular industry format jointly developed and supported by many network component vendors.^[1]

The SFP interfaces a network device motherboard (for a switch, router, media converter or similar device) to a fiber optic or copper networking cable. SFP transceivers are designed to support SONET, gigabit Ethernet, Fibre Channel, and other communications standards. Due to its smaller size, SFP obsolesces the formerly ubiquitous gigabit interface converter (GBIC); the SFP is sometimes referred to as a Mini-GBIC. In fact, no device with this name has ever been defined in the MSAs.

Types

SFP 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 the available optical fiber type (e.g. multi-mode fiber or single-mode fiber). SFP modules are commonly available in several different categories:

1 and 2.5 Gbit/s SFP

• 1 to 2.5 Gbit/s multi-mode fiber, LC connector, with black or beige^[1] extraction lever
  • SX - 850 nm, for a maximum of 550 m at 1.25 Gbit/s (gigabit Ethernet) or 150m at 4.25 Gbit/s (Fibre Channel)^[2]
• 1 to 2.5 Gbit/s single-mode fiber, LC connector, with blue^[1] extraction lever
  • LX - 1310 nm, for distances up to 10 km
  • EX - 1310 nm, for distances up to 40 km ^[3]
  • ZX - 1550 nm, for distances up to 80 km (depending on fiber path loss), with green extraction lever (see GLC-ZX-SM1) ^[3]
  • EZX - 1550 nm, for distances up to 160 km (depending on fiber path loss) ^[3]
  • BX - 1490 nm/1310 nm, Single Fiber Bi-Directional Gigabit SFP Transceivers, paired as BS-U and BS-D for Uplink and Downlink respectively, also for distances up to 10 km.^[4][5] 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.^[6][7]
  • CWDM and DWDM transceivers at various wavelengths achieving various maximum distances
• 1 Gbit/s for copper twisted pair cabling, 8P8C (RJ-45) connector
  • 1000BASE-T - these modules incorporate significant interface circuitry^[8] and can only be used for gigabit Ethernet, as that is the interface they implement. 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.^[9]

10 Gbit/s SFP+

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.^[10] 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.

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 linecards.

Although the SFP+ standard does not include mention of 16G Fibre Channel it can be used at this speed.^[11] Besides the data rate, the big difference between 8G Fibre Channel and 16G Fibre Channel is the encoding method. 64b/66b encoding used for 16G is a more efficient encoding mechanism than 8b/10b used for 8G, and allows for the data rate to double without doubling the line rate. The result is the 14.025 Gbit/s line rate for 16G Fibre Channel.

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.^[12] Through the use of an active electronic adapter, SFP+ modules may be used in older equipment with XENPAK ports.^[13]

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.^[14]

SFP+ also introduces Direct Attach for connecting two SFP+ ports without dedicated transceivers.

25 Gbit/s SFP28

SFP28 is a 25 Gbit/s interface which has evolved from 100 Gigabit Ethernet, which is typically implemented with 4 × 25 Gbit/s data lanes. Identical in mechanical dimensions to SFP and SFP+, SFP28 implements one 28 Gbit/s lane^[15] (25 Gbit/s + error correction) for top-of-rack switch to server connectivity.^[16] SFP28 may also be used to "break out" a single 100GbE port in a top-of-rack switch into four 25 Gbit/s individual server connections. SFP28 functions with both optical and copper interconnects.

For very short distances of 5 meters or less, as with 10 Gbit/s SFP+ "direct attach" cables, passive copper SFP28 modules integrate cable and transceivers into a single fixed-configuration module.

25 Gbit/s interfaces are also implemented using the QSFP transceiver form factor.

• 25 Gbit/s copper
  • Direct attach cables, 1 to 5 meters in length.^[17]
• 25 Gbit/s fiber
  • 850 nm SR using two strands of multimode fiber, distances up to 100 meters on OM4 grade multimode cable.^[18]
  • 1310 nm LR using two strands of singlemode fiber,^[19] distances from 5 to 20 km depending on optical link budget.

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.

Compatibility

Vendor specific modules

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 that are programmed to show the appropriate vendor ID.^[20]

SFP/SFP+

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 transceiver at lower gigabit ethernet speed, such as with a 1 Gbit/s 1310 nm LX SFP.^[21][22]

Applications

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

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.

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.^[23] Third-party SFP manufacturers have introduced SFPs with "blank" programmable EEPROMs which may be reprogrammed to match any vendor ID.^[24]

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.

The front of the SFP features a duplex LC connector; one connection for transmit and the other for receive.

The SFP transceiver contains a PCB that mates on the rear with the SFP electrical connector in the host system.

SFP electrical pin-out^[1]

Pin	Name	Function
1	VeeT	Transmitter ground
2	TxFault	Transmitter fault indication
3	TxDisable	Optical output disabled when high
4	MOD-DEF(2)	Data for serial ID interface
5	MOD-DEF(1)	Clock for serial ID interface
6	MOD-DEF(0)	Grounded by the module to indicate module presence
7	RateSelect	Low selects reduced bandwidth
8	LOS	When high, indicates received optical power below worst-case receiver sensitivity
9	VeeR	Receiver ground
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

Mechanical dimensions

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

The physical dimensions of the SFP transceiver are slightly smaller than the later XFP transceiver.

Dimensions

	SFP[25]	XFP[26]
Height	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)
Depth	56.5 mm (2.22 inches)	78.0 mm (3.10 inches)

Although it is not mentioned in any official specification document the maximum data rate of the original SFP standard is 5 Gbit/s.^[27]

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

Digital diagnostics monitoring

Modern optical SFP transceivers support standard digital diagnostics monitoring (DDM) functions.^[28] This feature is also known as digital optical monitoring (DOM). Modules with this capability give the end user the ability 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.

See also

• Interconnect bottleneck
• Optical communication
• Optical fiber cable
• Optical interconnect
• Parallel optical interface
• Quad Small Form Factor Pluggable
• Serial digital interface

References

1. SFF Committee (2001-05-01), INF-8074i Specification for SFP (Small Formfactor Pluggable) Transceiver (PDF), retrieved 2012-08-12
2. Agilestar/Finisar FTLF8524P2BNV specification (PDF)
3. 1000BASE Gigabit Ethernet SFP Transceiver, Optcore, retrieved March 26, 2013
4. Single Fiber Bidirectional SFP Transceiver (PDF), MRV, retrieved June 16, 2010
5. Gigabit Bidirectional SFPs, Yamasaki Optical Technology, archived from the original on February 3, 2010, retrieved June 16, 2010
6. "Single-fiber single-wavelength gigabit transceivers". Lightwave. Retrieved September 5, 2002.
7. "The principle of Single Wavelength BiDi Transceiver". Gigalight. Archived from the original on May 3, 2015.
8. VSC8211 media converter/physical layer specification
9. "Fiberstore: 100 M SFPs".
10. "SFF-8431 Specifications for Enhanced Small Form Factor Pluggable Module SFP+ Revision 4.1" (PDF). July 6, 2009. Retrieved May 9, 2011.
11. Tektronix (November 2013). "Characterizing an SFP+ Transceiver at the 16G Fibre Channel Rate".
12. "10-Gigabit Ethernet camp eyes SFP+". LightWave. April 2006.
13. "SFP+ to XENPAK adapter".
14. Ryan Latchman and Bharat Tailor (January 22, 2008). "The road to SFP+: Examining module and system architectures". Lightwave. Retrieved July 26, 2011.
15. "Ethernet Summit SFP28 examples" (PDF).
16. "Cisco SFP28 product examples".
17. "Cisco SFP28 direct attach cables" (PDF).
18. "SFP28 850nm example product" (PDF).
19. "SFP28 LR 1310nm transceivers".
20. "FAQs About Third-Party SFP". 2015-08-27. Retrieved 2016-11-19.
21. 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."
22. 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."
23. John Gilmore. "Gigabit Ethernet fiber SFP slots and lock-in". Retrieved December 21, 2010.
24. "Reprogrammable SFPs".
25. INF-8074i Specification for SFP (Small Formfactor Pluggable) Transceiver (pdf), SFF Committee, May 12, 2001, p. 6
26. "INF-8077i: 10 Gigabit Small Form Factor Pluggable Module" (PDF). Small Form Factor Committee. August 31, 2005. Retrieved June 16, 2011.
27. "FAQs for SFP+". The Siemon Company. 2010-08-20. Retrieved 2016-02-22.
28. SFF-8472 (PDF), 21 November 2014, retrieved 2016-01-04