Sagemcom F@st 3896

Sagemcom F@st 3896

Overview

The Sagemcom F@st 3896 is a DOCSIS 3.1 cable modem/gateway deployed by several European ISPs. DOCSight supports it via the sagemcom driver.

Property	Value
Driver Key	sagemcom
Auth Method	SHA-512 session (XMO JSON-RPC)
DOCSIS Version	3.1
Channels	SC-QAM + OFDM (downstream), ATDMA + OFDMA (upstream)

Confirmed ISPs

ISP	Country	Firmware	Status	Reporter
Tele2	Sweden	-	Working	thewho2018
DNA	Finland	23.83.19.25g	Working	pjoksanen

The same sagemcom driver works across ISP-specific firmware variants.

Setup

1. Set Modem Type to Sagemcom F@st 3896 in DOCSight settings
2. Enter the modem's admin password
3. Default modem IP: 192.168.100.1 (may vary by ISP)

The driver connects via the XMO JSON-RPC protocol over HTTP, fetching DOCSIS data from the Device/Docsis/CableModem/Downstreams and Upstreams xpaths.

Known Observations

OFDMA Upstream Power Reporting

The Sagemcom F@st 3896 reports OFDMA upstream power values that differ significantly from the expected DOCSIS MIB range. This has been observed on multiple ISPs.

What we see:

• SC-QAM/ATDMA upstream power: 41-42 dBmV (normal, within DOCSIS spec)
• OFDMA upstream power: 88-89 dBmV (as returned by the modem API)

DOCSIS reference (CableLabs CM-SP-PHYv3.1):

Parameter	Value
OFDMA power measurement standard	P1.6r_n (power per 1.6 MHz reference bandwidth)
MIB valid range (docsIf31CmUsOfdmaChanTxPower)	17-53 dBmV (in QuarterdBmV units)
Practical operating range	40-50 dBmV
Maximum total CM output (Pmax)	65 dBmV

The reported 88-89 dBmV values exceed the maximum valid MIB range (53 dBmV) and the total maximum CM output power (65 dBmV).

Possible explanations:

1. The firmware reports aggregate total power across the full OFDMA bandwidth instead of the standard P1.6 reference per 1.6 MHz
2. The XMO API uses a different power measurement reference than the DOCSIS MIB standard
3. A firmware display/reporting issue specific to the Sagemcom platform
4. The modem's own web UI shows the same values, suggesting this is intentional behavior by the firmware rather than a parsing error

Impact on DOCSight:

• The OFDMA power values are displayed as-is from the modem API - DOCSight does not modify raw values
• The diagnostic notes in PDF exports may flag these values as outside the expected range
• SC-QAM channel data is unaffected and works correctly with standard thresholds

What we do not know:

• Whether the values represent a different but valid measurement (e.g., total power vs per-1.6 MHz)
• Whether all Sagemcom F@st 3896 firmware versions behave this way
• What conversion (if any) would produce the correct P1.6 reference value

If you have a Sagemcom F@st 3896 and can share your OFDMA upstream power readings, please comment on Issue #163.

References:

• CableLabs DOCSIS 3.1 Transmit Power
• OFDMA TxPower MIB Discussion
• CM Upstream Transmit Headroom

Related

• Adding Modem Support
• Reference Values
• Bridge Mode Compatibility
• Issue #163: Sagemcom F@st 3896 Support