Rate & application selection: SFF-8079, SFF-8089

Many SFP-class modules can operate at more than one data rate or in more than one application (for example 1/2/4GFC, or Ethernet and Fibre Channel). Two small SFF documents define how a host learns what a module supports and how it tells the module which mode to use.

The documents

SFF-8079 — how selection works

Rate selection has two mechanisms, and the module advertises which it supports:

• Hardware rate select — the RS0/RS1 pins on the SFP connector. The host drives

them high or low to switch the module's receiver (and, on later modules, transmitter) bandwidth between rate groups.

• Soft rate select — bits in the A2h control/status area that do the same thing

over the two-wire interface, so a host without RS pins wired can still switch rates.

The module's SFF-8472 identity bytes declare the rate-select scheme it implements (rate identifier byte) so the host knows whether to use pins, bits, or neither.

SFF-8089 — the codes

SFF-8089 defines the rate and application code values written into module memory — a small table that says "this module supports application X at rate Y". Hosts read the codes to decide whether the module fits the port's configuration before attempting to link. The identifier, connector and compliance byte values themselves are shared via SFF-8024.

Why this matters for coding

Rate and application codes are among the identity bytes a switch checks. A module coded with codes that do not match its hardware, or missing the application the port expects, either refuses to link or negotiates the wrong rate. When recoding a multi-rate module in the EEPROM editor, keep the rate identifier and application codes consistent with what the module actually supports.

Related

• SFF-8472 — where the identity and rate

identifier bytes live.

• SFP family hardware specs — the RS pins

and low-speed interface.

Source

• sfptotal.ru — List of MSA standards and specifications; SNIA SFF library