What is SFP?

Posted on Jan 16, 2009 in Knowledge Base

Small form-factor pluggable (SFP) is a specification for a new generation of optical modular transceivers. The devices are designed for use with Small Form Factor (SFF) connectors, and offer high speed and physical compactness. They are hot-swappable. It 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. SFP Transceivers are designed to support SONET, Gigabit Ethernet, Fibre Channel, and other communications standards. Due to its smaller size, SFP obsoletes 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.

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 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)
  • for single-mode fiber, with blue extraction lever
    • LX- 1310 nm, for distances up to 10 km
    • 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.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)
    • CWDM and DWDM transceivers at various wavelengths achieving various maximum distances
  • for copper twisted pair cabling
    • 1000BASE-T – these modules incorporate significant interface circuitry 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) Fibre channel or SONET.

SFP sockets are found in Ethernet switches 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) between 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 pin 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 only enable the vendor’s own modules.

Signals

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

SFP pin-out
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)
16 VccT Transmitter power (3.3 v)
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 XFP
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)

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).