Cisco announced the end of sale of its SFP transceiver Cisco GLC-SX-MM transceiver and published a replacement—Cisco GLC-SX-MMD transceiver in 2012. 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? MMD-digital diagnostic monitoring will be introduced in this article.
“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.
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
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.The following picture shows the eye diagram illustrating optical figures of merit.
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 are provided by the vendors like Fiberstore.