Small form-factor pluggable transceiver
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. 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. 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. The SFP replaced the larger GBIC in most applications, and has been referred to as a Mini-GBIC by some vendors.
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. There are inexpensive adapters allowing SFP transceivers to be placed in a QSFP port.
Both a SFP-DD, which allows for 100 Gbit/s over two lanes, as well as a QSFP-DD 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. 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. Its proponents say a low cost adapter will allow for QSFP module compatibility.
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 SFPEdit
- 1 Gbit/s multi-mode fiber, LC connector, with black or beige extraction lever
- 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.
- 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. 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
- 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. 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.
- 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.
- 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 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.
- Although it is not mentioned in any official specification document the maximum data rate of the original SFP standard is 5 Gbit/s. 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. 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.[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. Through the use of an active electronic adapter, SFP+ modules may be used in older equipment with XENPAK ports  and X2 ports.
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.
25 Gbit/s SFP28Edit
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 accommodating 25 Gbit/s of data with encoding overhead.
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.
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."
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 QSFPEdit
- The original QSFP document specified four channels carrying Gigabit Ethernet, 4GFC (FiberChannel), or DDR InfiniBand.
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. The 4 channels can also be combined into a single 40 Gigabit Ethernet link.
50 Gbit/s QSFP14Edit
100 Gbit/s QSFP28Edit
- The QSFP28 standard 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" for sake of simplicity.
200 Gbit/s QSFP56Edit
- QSFP56 is designed to carry 200 Gigabit Ethernet, HDR InfiniBand, or 64G Fibre Channel. The biggest enhancement is that QSFP56 uses PAM-4 encoding instead of NRZ. As of April 2019[update], this new standard has not been published, but transceivers already are in use. It uses the same physical specifications as QSFP28 (SFF-8665), with electrical specifications from SFF-8024 and the still unpublished revision 3.0 of SFF-8636. Sometimes this transceiver type is referred to as "200G QSFP" for sake of simplicity.
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. There also exist fanout cables to adapt a single QSFP28 port to four independent 25 gigabit ethernet SFP28 ports (QSFP28-to-4×SFP28) as well as cables to adapt a single QSFP56 port to four independent 50 gigabit ethernet SFP56 ports (QSFP56-to-4×QSFP56).
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.
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. Third-party SFP manufacturers have introduced SFPs with "blank" programmable EEPROMs which may be reprogrammed to match any vendor ID.
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).
|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|
|12||RD-||Inverted received data|
|15||VccR||Receiver power (3.3 V, max. 300 mA)|
|16||VccT||Transmitter power (3.3 V, max. 300 mA)|
|19||TD-||Inverted transmit data|
|2||Tx2n||Transmitter Inverted Data Input|
|3||Tx2p||Transmitter Non-inverted Data Input|
|5||Tx4n||Transmitter Inverted Data Input|
|6||Tx4p||Transmitter Non-inverted Data Input|
|10||Vcc-Rx||+3.3V Power Supply Receiver|
|11||SCL||Two-wire Serial Interface Clock|
|12||SDA||Two-wire Serial Interface Data|
|14||Rx3p||Receiver Non-Inverted Data Output|
|15||Rx3n||Receiver Inverted Data Output|
|17||Rx1p||Receiver Non-Inverted Data Output|
|18||Rx1n||Receiver Inverted Data Output|
|21||Rx2n||Receiver Inverted Data Output|
|22||Rx2p||Receiver Non-Inverted Data Output|
|24||Rx4n||Receiver Inverted Data Output|
|25||Rx4p||Receiver Non-Inverted Data Output|
|29||Vcc-Tx||+3.3V Power Supply Transmitter|
|30||Vcc1||+3.3V Power Supply|
|31||LPMode||Low Power Mode|
|33||Tx3p||Transmitter Non-inverted Data Input|
|34||Tx3n||Transmitter Inverted Data Input|
|36||Tx1p||Transmitter Non-inverted Data Input|
|37||Tx1n||Transmitter Inverted Data Input|
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.
|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)|
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 monitoringEdit
Modern optical SFP transceivers support standard digital diagnostics monitoring (DDM) functions. 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.
- 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.
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