Optical Module Memory Map

Optical module memory maps define how identification, diagnostics, and control data are stored and accessed in SFP, QSFP, and advanced CMIS modules.

SFP/SFP+ Memory Map (SFF-8472)

SFP and SFP+ modules use a small EEPROM accessible via the I²C interface. The memory map is divided into two main areas :

• Base EEPROM (A0h / 0xA0): Contains static information such as module identifier, connector type, vendor name (bytes 20–35), vendor OUI, part number, and revision. This area is typically 96–128 bytes depending on the form factor.
• Digital Diagnostic Monitoring (DDM/DOM, A2h / 0xA2): Stores real-time operational parameters including temperature, supply voltage, laser bias current, TX power, RX power, and alarm/warning thresholds. Not all modules implement all DDM fields, but when present, they allow host systems to monitor module health and performance.

QSFP/QSFP28 Memory Map (SFF-8636)

QSFP family modules extend the SFP concept with multi-page memory maps to support higher data rates and multiple lanes :

• Page 0 (Base ID): Similar to SFP, contains vendor and module identification.
• Page 1 (Extended ID / DDM): Real-time diagnostics for each lane, including TX/RX power, bias, and temperature.
• Pages 2–3: Optional extended features, calibration constants, and lane mapping.
• QSFP28 modules typically use four memory pages, sufficient for 100G per lane monitoring and control.

CMIS and CMIS-VCS Memory Map (QSFP-DD / OSFP / QSFP112 / QSFP-DD800)

Advanced high-speed modules (400G/800G) use the Common Management Interface Specification (CMIS), which introduces a more flexible and extensive memory map :

• Banked Pages: Memory is organized into banks to support multiple data lanes and applications. For example, a 4x100GE module may have four banks of alarms and performance monitoring pages.
• Extended Payload (EPL, A0h–AFh): Stores bulk data such as firmware updates, diagnostics, and performance metrics.
• Versatile Control Set (VCS): Allows per-lane configuration, signal integrity controls, and explicit host/module ownership of parameters. VCS parameters are TLV-encoded and populate Stage and Active Control Set memory spaces.
• Coherent-CMIS: Provides extensions for coherent optical modules, including additional telemetry and control parameters.

Key Concepts Across Memory Maps

• I²C Access: SFP/SFP+ and QSFP modules use standard two-wire I²C communication for reading/writing memory.
• Static vs Dynamic Data: Base EEPROM pages store static identification data, while DDM/CMIS pages store dynamic operational data.
• Multi-Lane Support: QSFP-DD and CMIS modules use banked pages to handle multiple lanes and applications.
• Extensibility: CMIS-VCS allows new parameters to be added without impacting existing memory layout, supporting future high-speed modules. Understanding these memory maps is essential for module identification, diagnostics, performance monitoring, and configuration, ensuring interoperability and efficient network management.