Introduction to DDM Function of GLC-LH-SMD Transceiver

Basic Introduction of GLC-LH-SMD

As is known to all, Cisco GLC-LH-SMD is the replacement of GLC-LH-SM. It consists five parts: the LD driver, the limiting amplifier, the digital diagnostic monitor, the 1310nm FP laser and the PIN photo-detector. Supporting a data rate of 1.25 Gbps, this GLC-LH-SMD SFP transceiver operates on standard single-mode fiber optic link spans of up to 10 km and up to 550 m on any multimode fibers. You may say the letter “D” is the only difference between the two, and ask what’s the meaning of “D” and why GLC-LH-SMD can take the place of GLC-LH-SM? This article will give you an answer by introducing its additional function—DDM (digital diagnostic monitoring).

What Is DDM in GLC-LH-SMD?

Actually, the letter “D” stands for the digital diagnostic monitoring function according to the industry standard multi-source agreement (MSA) SFF-8472 and is known as digital optical monitoring (DOM). A fiber optic transceiver with DDM is higher end than the one without DDM function. The DDM function gives end users the ability to monitor real-time parameters of the optical transceivers, such as the transceiver temperature, laser bias current, transmitted optical power, received optical power and transceiver supply voltage. Most of today’s fiber optic transceivers have this function.

Functions of DDM in GLC-LH-SMD

After knowing what digital diagnostic monitoring is, next we’d move to the functions of DDM or what DDM can actually do. Literally, DDM is able to provide component monitoring on transceiver applications in details like the real-time parameters listed in the previous paragraph. But that’s not all functions of DDM. The SFF-8472 added DDM interface and outlined that DDM interface is an extension of the serial ID interface defined in GBIC specification and the SFP MSA. This DDM interface defines a sophisticated system of alarm and warning flags, which alerts end users when particular operating parameters are inconsistent with the factory-set normal range. So the DDM interface is also able to let the end users have the ability to achieve fault isolation and failure prediction. The three functions (component monitoring, fault isolation, failure prediction) will be stated in more details below.

Component Monitoring

Component monitoring is the most familiar function to users. Usually the key parameters of the optical transceivers—transceiver temperature, transceiver supply voltage, laser bias current, transmit average optical power and received optical modulation amplitude (OMA) or average optical power, will be monitored. If the transceiver’s specified operating limits are exceeded and compliance cannot be ensured, these real-time diagnostic parameters will alert the system.

Fault Isolation

Fault isolation is the second function of DDM. The DDM is able to isolate the particular location of fault in an optical network system. With the combination of the DDM interface status flags, transceiver hard pins and diagnostic parametric monitor data, it’s easy to pinpoint the specific location and cause of a link failure.

Failure Prediction

Failure prediction is the last function of DDM. Based on the transceiver parametric performance, the DDM can be helpful in failure prediction on fiber optic links. Device faults and high error rate conditions are the two basic types of failure conditions which can be seen on optical transceivers. Device fault means non-operation or malfunction. With the nature of semiconductor lasers, this is usually applied to transmitter performance. High error rate conditions refer to the operating conditions that transceiver is operating at its signal-to-noise limit. This is typically applied to optical fiber performance.



After reading the above statement, have you got a better understanding about the DDM function of GLC-LH-SMD? Fiberstore’s GLC-LH-SMD small form factor pluggable (SFP) transceiver is compatible with the small form factor pluggable multi-source agreement (MSA). This transceiver is programmed to be fully compatible and functional on a wide range of Cisco equipment. All Cisco compatible GLC-LH-SMD 1000BASE-LX/LH SFP transceivers from Fiberstore are all tested on Cisco original equipment to ensure their superior quality and performance (see the picture above). Besides the GLC-LH-SMD SFP transceiver, many other Cisco compatible transceivers provided by Fiberstore, such as SFP-10G-LR and QSFP-40G-CSR4, have to be tested before arriving to customers. For more detailed information about us, please visit or contact over