Tunable DWDM memory maps: SFF-8690, SFF-TA-1004

A tunable DWDM transceiver can be set to any channel on the ITU grid instead of being fixed to one wavelength. The channel is chosen by writing to registers in module memory — and two SFF documents define where those registers are.

The documents

CMIS-managed modules (QSFP-DD, OSFP) handle tuning through CMIS advertisement and control pages instead.

What the maps define

Both documents follow the same model, adapted to their form factor's paging:

• Capabilities — whether the module tunes by channel number, by frequency, or by

wavelength; supported grid spacings (e.g. 100 GHz, 50 GHz); the first and last frequency the laser can reach.

• Set registers — the channel number (or frequency in THz/GHz parts, or wavelength)

the host writes to select a channel.

• Status and fault — whether the laser has settled on the requested channel

(tuning in progress / locked), and error flags such as an unsupported channel.

• Current values — read-back of the wavelength/frequency the module is actually on.

In SFF-8690 these live in an upper page of the SFP+ A2h block (page 02h). In SFF-TA-1004 they occupy an upper page of the QSFP paged memory, selected via byte 127 as with the rest of SFF-8636.

The tuning sequence

1. Read capabilities: tuning mode, grid spacing, frequency range.
2. Write the desired channel (or frequency/wavelength).
3. Poll the status until the laser reports locked.
4. Read back the current wavelength to confirm — it should match the

ITU grid value for that channel.

In CodingBox

CodingBox implements these maps to set the channel number and wavelength on tunable DWDM transceivers and to read the current setting back — one of the application's core functions for DWDM work. The values are visible in the EEPROM editor and on the Check transceiver screen.

Source

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