What Is Digital Diagnostic Monitoring? A Complete Guide to SFP DDM/DOM

Initial Published: April 29, 2017

Digital Diagnostic Monitoring, also known as DDM, is sometimes referred to as Digital Optical Monitoring (DOM). It is an intelligent function that enables network administrators to monitor the transceiver’s operational parameters in real time. Although there are some related articles, when you Google, you may find it challenging to locate a comprehensive guide on this topic. That’s why we wrote this post.

Now, let us dive into the topic.

What Is Digital Diagnostic Monitoring?
Key Parameters of DDM/DOM Monitoring
How to Check the Transceiver DDM Interface?
Which transceivers support DDM/DOM?
FAQs
Final Words

What Is Digital Diagnostic Monitoring?

Short for DDM, Digital Diagnostic Monitoring enables network users to check and monitor the transceiver’s real-time operational parameters easily. To put it simply, it is like a family doctor to a network administrator, capable of monitoring the network’s health at any time.

However, in most cases, some manufacturers and users also use the term “Digital Optical Monitoring (DOM)” to refer to this feature. By using these words, it focuses on the “Optical” parameters of the transceiver. But in fact, they have the same meaning.

Next, let’s talk about the specific parameters of monitoring. Per the multi-source agreement SFF-8472, the key parameters include the:

Transceiver temperature
Transceiver supply voltage
Laser bias current
Transmit average optical power (TX Power)
Received Optical Power (RX Power)

DDM/DOM is a basic feature of modern optical transceivers, including SFP/SFP+/QSFP/QSFP28/QSFP-DD/OSFP. With this built-in digital diagnostic monitoring function, it is like equipping a traditional transceiver with a smart bracelet for enhanced intelligence. As a result, it bypasses the legacy transceivers’ disadvantage that they can not access the optical network operation parameters. Therefore, the Digital Diagnostic Monitoring function enables users to efficiently manage and troubleshoot fiber optic networks, as well as visualize them.

Key Parameters of DDM/DOM Monitoring

The SFF-8472 MSA primarily standardizes DDM/DOM monitoring features; the image below shows the memory map defined in the SFF-8472. By using a 2-wire serial bus address 0x1010001X (A2h) between the hosting system and transceivers, Digital Diagnostic Monitoring provides real-time access to measurements of the five basic parameters.

#1. Transceiver temperature

The transceiver temperature diagnostic read-back allows monitoring of the DDMI transceiver’s thermal environment condition. For the transceiver temperature diagnostic monitor, the measurement point is not formally defined by SFF-8472 DDMI MSA; Per DDMI MSA, the location of the temperature sensor is to be specified by the vendor.

The temperature sensor is commonly located on the transceiver’s PCB. If situated near ICs, it will read “hotter” than if distant. With regard to external transceiver temperature measurement, data sheets typically specify temperature requirements in terms of the module case. Regarding reliability prediction, the most crucial temperature point to monitor inside the transceiver is the laser junction.

#2. Transceiver supply voltage

The transceiver voltage diagnostic read-back allows monitoring of the transceiver supply voltage. The measurement point for the transceiver voltage monitor is left undefined by the SFF-8472 MSA. The transceiver vendor typically specifies which supply (VccT or VccR) is monitored and the location of the voltage measurement point on the transceiver.

The transceiver voltage read-back will tend to run slightly lower than the host supply, possibly due to the series resistance of the connector or the transient suppression circuitry. Transceiver voltage monitor is meant to be DC detector only (cannot assess power supply noise.)

#3. Transceiver Laser bias current

The laser bias current readings generated by the diagnostic monitor are commonly measured by mirroring the bias current off the laser driver circuit. Variations in laser bias current are expected during regular operation; a closed-loop feedback circuit is typically implemented in fiber-optic transceivers, which adjust the bias current to maintain a constant laser output optical power.

Laser bias varies for normal changes over temperature such as slope efficiency, forward voltage, or threshold variations. The laser bias diagnostic monitor has limited usefulness in assessing environmental compliance; its most valuable application is for fault or failure prediction.

#4. Transmit Average Optical Power

The TX average power read-back allows monitoring of the transceiver’s launched optical power. The TX average power diagnostic monitor is not a true “coupled power” optical measurement. Mirroring the monitor photocurrent in the laser feedback circuit is the approach typically implemented to generate transceiver-level TX average power measurements. Significant variations in average optical power are not expected due to the closed-loop control of TX optical power. The primary purpose of the TX power monitor is to enable the host to verify that the TX power meets the minimum compliance requirements.

#5. Receiver Average Optical Power

The SFF-8472 MSA allows for both average and OMA types of RX power measurement for the RX diagnostic monitor. RX OMA measurements are ideal for Fibre Channel standard applications, which define required receiver sensitivity in terms of OMA.

Traditional SONET customers and Ethernet customers, though, specify RX and TX power requirements in terms of average; hence, RX average power measurement type might be preferred in those applications.

How to Check the Transceiver DDM Interface?

We have already introduced the basics of DDM (Digital Diagnostic Monitoring) earlier. However, how do you query the specific DDM parameters of a particular SFP/QSFP? In this section, we will introduce two of the most common methods.

#1. Using the testing tools provided by the SFP/QSFP manufacturer

This method is only suitable for professional enterprise users and telecom operators. It requires the optical module manufacturer to provide dedicated testing tools and matching software, or to purchase such tools from the market.

Typically, you first connect the DDM testing tool to a computer, then insert the SFP or QSFP optical module, and can then use the software to read the real-time operational parameters directly.

It is essential to note that this method is not a conventional approach, and therefore, it is used by fewer users.

#2. Through the network device’s CLI

This method is the most common and direct way to query information and is widely used by the majority of users. By inserting SFP/QSFP optical modules into devices such as switches, routers, servers, or network cards, users can obtain diagnostic information through the device manufacturer’s command-line interface (CLI) or via SNMP (Simple Network Management Protocol).

Although the CLI commands may vary slightly across different manufacturers, they are generally very similar. Specific CLI commands should be referenced in the official documentation and explanations. We have previously published a dedicated article on this topic, which you can refer to here.

For example, when entering the following commands in the management interface of a Cisco switch:

show interfaces transceiver show interfaces transceiver detail

You can view the optical module DDM parameters in the management interface.

Which transceivers support DDM/DOM?

Not all optical transceivers support digital diagnostic monitoring. Many individuals may be confused about this, so we have created a comprehensive table to cover all the transceiver types and DDM features.

From the table above, we can see that legacy transceiver types, such as 1×9 and SFF, support the DOM function. However, most of the newer form factors, such as SFP, SFP+, and SFP28, usually have these basic features built in.

Transceiver Type	DDM/DOM Function
1×9 Module	No
SFF Module	No
GBIC Module	Yes
SFP Module	Yes or No
SFP+ Module	Yes
SFP28 Module	Yes
QSFP+ Module	Yes
QSFP28 Module	Yes
QSFP-DD Module	Yes
OSFP Module	Yes
QSFP56 Module	Yes
SFP56 Module	Yes
CFP Module	Yes
CFP2 Module	Yes
CFP4 Module	Yes
CXP Module	Yes
Xenpak Module	Yes
X2 Module	Yes
SFP+ DAC	No
SFP28 DAC	No
QSFP DAC	No
QSFP28 DAC	No
QSFP-DD DAC	No
OSFP DAC	Yes
SFP+ AOC	Yes
SFP28 AOC	Yes
QSFP AOC	Yes
QSFP28 AOC	Yes
QSFP-DD AOC	Yes
OSFP AOC	Yes

FAQs

Q: What is the difference between DDM alarms and warnings?

Warning: Indicates that a parameter (such as temperature) is approaching the limit of its normal operating range but has not yet exceeded it. This is typically an early warning signal, indicating that attention or preventive measures may be required, and the issue may not immediately result in a service interruption.

Alarm: Indicates that a parameter (such as temperature) has exceeded the limits of its normal operating range. This typically signifies a real issue that may already be affecting service quality and often results in a port shutdown.

Q: What should we do if the SFP DDM Rx Power is too high?

When the SFP’s Rx Power exceeds the maximum value set by the receiver, it can cause overload, leading to packet loss and bit errors in the received signal. It may even damage the laser (e.g., APD).

Generally, ensure that long-distance SFPs are not used for transmission over excessive distances. If it must be used, ensure that an optical attenuator with sufficient attenuation is added to reduce the received optical power to within the normal range.

Q: What should we do if the SFP DDM Rx Power is too low?

Multiple factors may cause this:

The SFP’s transmitter laser is damaged, or the end face is dirty, resulting in insufficient or no received light.
There are too many fiber nodes, causing compression or excessive loss.
The fiber transmission distance exceeds the link budget of the optical module.

Generally, you should take targeted methods based on the specific cause.

Final Words

You can imagine how bad the modern fiber optic network would be without DDM functionality. If there is no diagnostic optical monitoring, we cannot determine the status of the transceiver and network, and we have no idea what the network status is after it goes down. That is his fundamental role: to help us understand the health of our transceivers and network, making our transmission more intelligent.

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