What is a SFP or SFP+ MSA? (Multi-Source Agreement)

A multi-source agreement (MSA) is an agreement between multiple manufacturers to make products which are compatible across vendors, acting as de facto standards, establishing a competitive market for interoperable products.

An Overview of Pluggable Optical Transceiver MSA Standards:

Both Small Form-Factor Pluggable (SFP)[i] and Small Form-factor Pluggable, enhanced (SFP+)[ii] optical transceiver devices are ‘standardized’ by multi-source agreements (MSAs). These documents strictly define sufficient characteristics of an optical transceiver so that system (e.g. Ethernet switch, Router, and media converter) vendors may implement ports on their devices so MSA compliant pluggable optical transceivers (SFP or SFP+) from any vendor will function properly. That is, transceivers may be purchased from any of the multiple sources in the open market, like FluxLight.

Pluggable optical transceivers are physically composed of a small printed circuit board (PCB) containing electronic circuity, with an electrical PCB ‘edge connector’ at one end and, typically, a  fiber optic connector (dual LC-type in the case of SFP/SFP+ modules) at the other, packaged in a metal housing including a release latch. The basic function of the device is to convert electrical transmit data from the host into an optical signal transmitted onto a connected fiber optic cable, and, in the other direct, convert a received optical signal into an electrical one to be sent to the host system over the edge connector.

SFP/SFP+ transceivers also include a management (I2C) interface, an Electrically Erasable Programmable Read-Only Memory (EEPROM) and optional Digital Diagnostics Monitoring (DDM, aka Digital Optical Monitoring or DOM). The EEPROM memory space is defined in the MSA and includes information describing the transceiver’s capabilities, standard interfaces (e.g., SX, LX, EX, ZX…), manufacturer and other data. This data is accessible by the host system over the I2C interface, as is the status of the optional DDM functions.

What is Defined by the MSA?

The major elements or characteristics defined in the SFP/SFP+ MSAs are:

• Mechanical Interface
  • Mechanical dimensions of the device (H: 8.5mm, W: 13.4mm, D: 56.5mm)
  • Transceiver edge connector to host PCB-mounted electrical connector mating
  • Host board mechanical layout (location/size of solder pads, etc.)
  • Insertion, Extraction and Retention forces
  • Labeling
  • Bezel design considerations for host systems
  • Electrical connector mechanical aspects
  • Cage assembly dimensions (hollow cage mounted in host system)

• Electrical Interface
  • Pin definitions
  • Timing requirements and Status I/O
  • Module definition interface and data field description

The Digital Diagnostics Monitoring (DDM) feature common in many modern SFP/SFP+ transceivers is defined by reference to the industry-standard SFF-8472[iii].

Why are MSAs Important?

MSAs, like most standards efforts, are important primarily because they can give customers a choice in suppliers from which they purchase products. Freedom of choice is the foundation of the efficient operation of markets. Customers in the marketplace should have the benefit of multiple independent suppliers, each competing to gain a share of the market. This behavior forces suppliers to be as efficient and creative as possible, driving down costs and offering customers the widest array of options.

It is true that some system vendors have attempted to subvert the standardizing value of the MSAs described in this paper. The most common scheme is to write a unique code into some of the undefined memory in the EEPROM of each SFP/SFP+. When the transceiver is inserted into the host switch, its EEPROM is read, and, if the code is ‘incorrect’ the module is rejected as ‘incompatible’.  Like most such efforts, these have routinely failed. There are many 3^rd party optical transceiver module suppliers that have figured out how to generate and program their parts with the proper codes such that they are indistinguishable from the subversive host system’s ‘brand name’ parts.

We recommend that all such attempts to undermine the value of these standards and subvert the free operation of the marketplace be vigorously resisted by consumers. Since there many excellent choices, network operators should specifically avoid purchasing optical transceivers directly from system vendors who attempt such coercive tactics. One reason is to save money. System vendors primarily use these tactics to protect their grossly inflated prices. The second reason is to support and encourage creation and adherence to standards and, at the same time, rebuke attempts to undermine them.

[i] SFF Committee (2001-05-01), INF-8074i Specification for SFP (Small Formfactor Pluggable) Transceiver, retrieved 2012-08-12.

[ii]  “SFF-8431 Specifications for Enhanced Small Form Factor Pluggable Module SFP+ Revision 4.1”. July 6, 2009. Retrieved May 9, 2011.

[iii] SFF Committee, SFF-8472 Specification for Diagnostic Monitoring Interface for Optical Transceivers, Rev 11.0, September, 14, 2010.