The document discusses digital diagnostic monitoring (DDM), represented by the "D" in Cisco's GLC-SX-MMD transceiver. DDM allows users to monitor real-time parameters of fiber optic transceivers like power, temperature, and current. It enables component monitoring, fault isolation, and failure prediction to help ensure network uptime. Modern transceivers generally include DDM, making GLC-SX-MMD compatible with systems designed for its predecessor GLC-SX-MM.

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Cisco announced the end of sale of its SFP transceiver Cisco GLC-SX-MM transceiver and published a
replacement—Cisco GLC-SX-MMD transceiver. The module numbers of the two transceiver only
differ in one letter “D”. This “D” mainly represents a function of Cisco GLC-SX-MMD transceiver and
is inherited by most of optical transceivers offered today. It is the main reason why Cisco
GLC-SX-MMD transceiver can replace its predecessor. What is this “D”? Why the GLC-SX-MMD can
replace GLC-SX-MM? This article is to offer the answers to these questions.
What is Digital Diagnostic Monitoring (DDM)?
“D” in GLC-SX-MMD represents the DDM function which is short for digital diagnostic monitoring
according to the industry standard MSA (Multi-Source Agreement) SFF-8472 and is also known as
DOM (Digital Optical Monitoring). When buy fiber optic transceiver today, you will have the option
with or without DDM/DOM. And most of the modern transceivers are with the DDM function. This
technology allows the user to monitor real-time parameters of the fiber optic transceivers, like
optical input/output power, temperature, laser bias current, and transceiver supply voltage, etc.
What Can Digital Diagnostic Monitoring Do?
Literally, DDM function can provide component monitoring on transceiver applications in details.
However, DDM’s application is not limited to this. The SFF-8472 added DDM interface and outlined
that DDM interface is an extension of the serial ID interface defined in GBIC specification, as well as
the SFP MSA. DDM interface includes a system of alarm and warning flags which alert the host
system when particular operating parameters are outside of a factory set normal operating. Thus,
DDM interface can also enable the end user with the capabilities of fault isolation and failure
prediction. This part is to illustrate what can be done with DDM.
Component Monitoring: The DDM enables the end user to monitor key parameters in the
performance of the fiber optic transceiver including the following:
 Transceiver temperature
 Transceiver supply voltage
 Laser bias current
 Transmit average optical power
 Received optical modulation amplitude (OMA) or Average Optical Power
Digital Diagnostic
Monitoring Introduction

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Fiberstore White Paper | Digital Diagnostic Monitoring Introduction
The real-time diagnostic parameters can be monitored to alert the system when the transceiver’s
specified operating limits are exceeded and compliance cannot be ensured.
Fault isolation: The DDM function can be used to isolate the particular location of fault in fiber optic
network system. Combining the DDM interface status flags, transceiver hard pins and diagnostic
parametric monitor data the specific location and cause of a link failure can be pinpointed.
Failure prediction: The DDM can also be used to help in failure prediction on fiber optic links, which
is based on the transceiver parametric performance. Although, this application is not yet fully
mature, but there is still room for improvement. There are two basic types of failure conditions that
can be seen on fiber optic transceivers:
 Device faults—A device non-operation or malfunction. Typically applied more to transmitter
performance, due to nature of semiconductor lasers.
 High error rate conditions—Operating conditions are such that transceiver is operating at its
signal-to-noise limit. Applies more to fiber optic receiver performance.
Providing parameter monitoring, fault isolation, and failure prediction, fiber optic transceivers with
DDM help to ensure that the business can be proactive in preventative maintenance of the network
and ensure business continuity. And it would easy to explain why modern transceivers are with
DDM and why GLC-SX-MMD can replace GLC-SX-MM. It is irresistible development trend of industry
and technology.
What should be mentioned is that although optical transceivers with DDM are much popular than
those without DDM, some user still use the older optical transceivers in consideration of the
upgrading costs. To satisfy the customers’ needs, optical transceivers with DDM and without DDM

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Fiberstore White Paper | Digital Diagnostic Monitoring Introduction
are provided by Fiberstore, welcome to contact us for more detailed information.
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Notice: This document is for informational purposes only and does not set forth any warranty, expressed or
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