Small form-factor pluggable transceiver

Front view of SFP module (LC connector). The blue extraction lever indicates operation with single-mode fiber.

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). It interfaces a network device motherboard (for a switch, router, media converter or similar device) to a fiber optic or copper networking cable. It is a popular industry format jointly developed and supported by many network component vendors.^[1] 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 although 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). Optical SFP modules are commonly available in several different categories:

• for multi-mode fiber, 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]
• for single-mode fiber, 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, with green extraction lever (see GLC-ZX-SM1) ^[3]
  • EZX - 1550 nm, for distances up to 160 km ^[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.
  • 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
• for copper twisted pair cabling
  • 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.

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.^[9] 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 16G Fibre Channel it can be used at this speed.^[10] 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.^[11]

Consideration has to be given to whether the module is linear or limiting. Linear SFP+ modules are most appropriate for 10GBASE-LRM; otherwise, limiting modules are preferred.^[12]

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

Compatibility

It is possible to design an SFP+ slot that can accept a standard SFP module.^[13][14]

Applications

Ethernet switch with 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.^[15] For example, in 2003 during a routine Internet Operating System (IOS) update on their Catalyst line of switches, Cisco added a feature that would cause the switch to reject optical modules that were not deemed ‘Cisco brand’.^[16]

Signals

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

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.

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 digital diagnostics monitoring (DDM) functions according to the industry-standard SFF-8472. 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.

The diagnostic monitoring controller is available as an I²C device at address 1010001X (A2h).

See also

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

References

Wikimedia Commons has media related to Small Form-factor Pluggable.