What is OTDR - Optical Time-domain Reflectometry?

Optical Time-domain Reflectometry (OTDR) is a technology that allows you to determine the length of a fibre optic cable. It can also be used to identify any imperfections or faults in a fibre cable. A pulse of light is sent into the fibre from a light source at a known time. The pulse is reflected back, ideally from the far end of the cable, then the amount of time this has taken is used to calculate the distance the pulse has travelled.

How does OTDR help me?

That is a good question, however the following questions might help clarify.

  • How much time could you save by knowing where the physical fault is located?
  • How much money could the saved time save you?

We all know that if you have all your expensive network test equipment (assuming you have some) in the right place together with the specialist support team to operate it then physical fault locations can be identified in reasonable time frames. What if you don't?

Let's suppose you do have a failure and let's suppose all your expensive equipment and resources are not just sitting around waiting for the fault to happen but doing something else at the time.

  • How long to get the team to the right locations?
  • How long to get the right test equipment in the right places?
  • How long to run the tests to find the fault?
  • How long to contact the people and get a fix underway?

How useful would it be to get an alert that you had an SLA failure and to get standard networks operations staff identify where physically that fault was?

Read on...

Ethernet Demarcation Devices (EDDs) and other netrwork equipement is commonly used for Layer-2 and, to a certain extent layer-3 performance baselining and monitoring, what about the underlying physical, or Layer-1 network? In the case of Wide-Area Ethernet networks, such infrastructure is often fibre-based and network devices have offered little by way of assessment or monitoring of the status of the physical fibre infrastructure.

Metrodata's EDDs and NetTESTER Embedded Ethernet Testers offer the facility to report the 'Digital Diagnostics Monitoring' parameters captured by suitably equipped Optical Transceivers. By far the most common type of Transceivers used for services are the 'Small Form Pluggable' (SFP) type, and these incorporate facilities for reporting on the following characteristics:

  • Optical signal transmission power, measured in dBm
  • Optical received signal power, measured in dBm
  • Bias current applied at the SFP, measured in mA
  • SFP temperature, measured in oC
  • SFP power supply, measured in V

Since the transmission distance parameters of optical transceivers are normally 'rated' based on optical transmission power. received power sensitivity, then the ability of such transceivers to report on power reception levels providing a useful tool to determine whether the fibre link between two points is well within tolerance or maybe 'borderline' and likely to cause some problems affecting the integrity of the link. Some network devices can link measurements through to 'threshold level alarms' to alert the Carrier to potential problems associated with the fibre link as seen directly from fibre transceiver interfaces.

But, what of the situation of fibre breakages and/or disconnections? EDDs have not brought any value in this respect. Metrodata has this situation addressed.

Alerting and localising fibre breakages or disconnects
General principle of this OTDR in operation: EDD/iSFC combination provides a basis for alerting and localising fibre breakages or disconnects

Metrodata has worked to incorporate software drivers for Optical Zonu's iSFC® Fast Fiber Fault Finder transceivers, such that whenever a disconnect or breakage occurs in the fibre connected to one of Metrodata's EDD family or NetTESTER, it can automatically alert both the loss of signal and, more critically, the distance to the fault, which becomes immediately available to the Carrier's Network Operations Centre.

Typical 'single-ended' deployment scenario
Typical 'single-ended' deployment scenario

The image above shows a typical 'single-ended' deployment scenario, however we [Metrodata] recommend that the OTDR facility is used in pairs for best results. There is of course a clear disadvantage here that if you lose the link completely you will not be able to communicate with the remote end of the link. However, if the link is impaired having both ends active will assist greatly in fully resolving and accurately pin-pointing where the issue is located. The scenario above illustrates deployment of this capability in a single-ended manner.

Each unit maintains a degree of 'historic event' information and that they respond rapidly in the event of a fibre discontinuity. This facilitates detection and localisation of even intermittent problems. It can be the case that when using external OTDR test equipment, by the time this has been brought to site, connected and testing conducted, then a temporary but potentially service effecting fault may not be evident at the time of a test. In contrast we can record the locality of a discontinuity even of sub-millisecond duration.

Fault awareness at the Network Centre

Alarm generation whereby, in the event of a fibre failure, a pro-active standards-based SNMP 'Trap' alarm message is generated by the EDD or NetTESTER, containing both distance and time-stamp information, which can be transmitted directly to the user's Network Operation Centre (NOC), so it is absolutely not necessary to be 'logged in' the regular Management system of any product in order to be alerted to the incidence of a problem.

Below is a screen shot for a management system indicating the type of alert the you could expect which indicates the geographic location of the fault and with the circuit number in the icon it is possible to identify the effected customer circuit. The red circle draws attention to the automatic reporting of the distance from the device where the fault condition has been identified.

Management system indicating fault
Management system indicating fault

Benefits of OTDR enabled device

Network links do fail so if you have planned for this possibility by deploying devices with OTDR you will get the following benefits:

  • Automatic alert in your NOC at the time a failure occurs
  • Automatic alert giving you the physical location (by distance) of the fault location
  • Ability to automatically have the contact details of the circuit supplier
  • Ability to invoke SLA clock on your supplier with documented time stamped report
What does all this mean to your business?
  • Reduced time to fault fix
  • No immediate need to deploy expensive specialist resources
  • No immediate need to deploy expensive specialist equipment
  • Reduced business disruption due to faster fault fix time
  • Above equal potential costs savings
What does this mean to you?
  • You have planned for a predictable network outage
  • You have a process in place to handle the event
  • You have minimized the cost to the business
  • You have reduced business stress levels

The MetroCONNECT EDD Family

Metrodata's family of MetroCONNECT EDD products currently include the FCM9003, the FCM9005 and the FCM10G05. The MetroCONNECT EDD products offer advanced custom-hardware based facilities for comprehensive services for both Layer-2 (Ethernet Switched) and Layer-3 (IP Routed) network infrastructures.

The NetTESTER Family

Metrodata's family of NetTESTER products currently include the NT1003, the NT1005 and the NT10G05. The NetTESTER products offer advanced custom-hardware based facilities for comprehensive test and monitoring for both Layer-2 (Ethernet Switched) and Layer-3 (IP Routed) network infrastructures.

Networking The World Since 1989