With the widespread recognition of Cisco optical products, such as Cisco switches and routers, Cisco GBIC transceivers and Small Form-Factor Pluggable (SFP) transceivers for telecommunications applications have gained more and more popularity and reputation in the market. When customers choose Cisco optical transceivers, what do you require besides basic specification? Have you encountered SFPs with or without digital diagnostic monitoring (DDM) interface, such as 1000BASE-SX SFP (eg. Cisco GLC-SX-MM or Cisco GLC-SX-MMD)? What is DDM? What can it be used for?

Digital diagnostic monitoring is also known as diagnostic optical monitoring (DOM). The diagnostic monitoring interface outlined in the SFF-8472 MSA (Multi-Source Agreement) is an extension of the serial ID interface defined in the GBIC specification, as well as in the SFP MSA. Nowadays, many modern optical SFP transceivers are designed to support digital diagnostics monitoring functions according to the industry standard MSA SFF-8472.

Actually, when customers buy Cisco optical transceivers, it is highly possible to have a dilemma between SFP with DDM or SFP without DDM. On March 8, 2013, Cisco announced the end of sale of its SFP transceiver Cisco GLC-SX-MM transceiver and published a replacement—Cisco GLC-SX-MMD transceiver. "D” in Cisco GLC-SX-MMD here just means the DDM function which is not designed in Cisco GLC-SX-MM. The following image is a product: NEW Cisco GLC-SX-MMD Compatible 1000BASE-SX SFP 850nm 550m EXT DDM Transceiver Module.

Don't just judge DDM functions from its name. Certainly, DDM interface allows for component monitoring on transceiver applications in great details. Additionally, DDM interface includes a system of alarm and warning flags which alert the host system when particular operating parameters are not in line with the normal operating parameters set by the factory. Consequently, users can find fault isolation according to outcomes of DDM, and predict failure possibilities and prevent such fault.

• Real-time digital diagnostic sensors—Via the two wire serial interface, the end user can access real-time internal measurements of the transceiver temperature, supply voltage, transmitter bias current, transmitter output power, and received optical power.

• Alarm and Warning thresholds —Corresponding high alarm, low alarm, high warning and low warning thresholds are defined in SFF-8472. The threshold values, which are typically defined by the vendor and factory preset, allow the user to determine whether or not the transceiver is operating outside target limits for desired performance.

An optional set of alarm and warning flags is also defined. When the alarm and warning feature is enabled, the transceiver, in addition to cyclically performing internal measurements of the diagnostic read-back parameters and digitally storing on memory, also checks how these read-back values compare with programmed alarm/warning threshold values.

By continually polling the alarm and warning flag bytes via its system software, the user has the power to better screen transceiver performance, so as to avoid the occurrence of link failures and potential system downtime.

Optical transceivers with DDM/DOM interfaces give the end user the ability to monitor parameters of the SFP, such as optical output power, optical input power, temperature, laser bias current, and transceiver supply voltage, in real time. This partly explains why Cisco GLC-SX-MMD replaced Cisco GLC-SX-MM. Although it is not designed with DDM function, GLC-SX-MM is still used by many users since it performs the similar functions as GLC-SX-MMD and costs less. Fiberstore supplies optical transceivers with DDM and without DDM to satisfy customers' different needs. Please feel free to contact us and visit Fiberstore for more information.

Article Resource:

https://www.fs.com/blog/digital-diagnostic-monitoring-introduction.html

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