Dark Fiber Testing and Debugging Process

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Isolating Dark Fiber Service Issues

Dark Fiber is where ACD provides physical fiber cable connectivity between two or more sites. ACD generally does not operate or maintain the equipment for the customer.


Point of Demarcation and Responsibility

ACD Responsibility:

  1. Fiber Cable that is outside of the building
  2. Fiber Cable coming into the building; and is prior to the fiber termination box.
  3. The fiber termination box, and the connectors directly attached to the fiber termination box.

Customer Responsibility

  1. Patch cables from the fiber termination box to their equipment.
  2. Network Switches/Routers
  3. Optics; typically these are pluggable modules, such as GBICs, SFPs, Xenpaks and other industry names for Optical Modules.

Problem Reporting by Customer

When a customer reports an issue with dark fiber we need to know the nature of the problem reported. The customer should be prepared to provide the nature of the issue at time of initiation of a service ticket with ACD.

  1. Outage: Where there is no connectivity between sites.
    1. Is the outage affecting all fibers between a site or only specific fibers?
    2. Need to know which fibers?
  2. Transmission Errors Transmission occuring, but CRC errors on equipment.
    1. Is this occuring on all fibers between a site or only specific fibers?
    2. Are the errors on both on the TX and RX sides of the circuit?
  3. Intermittent Trasmission Errors Transmission is occuring, but CRC errors on equipment is intermittent
    1. How often are the CRC errors occuring, i.e. is there a pattern or cycle to the errors?
    2. What time of day? Any pattern?
  4. Low Light Level The light emitted from the customer's laser is low on the RX end?
    1. What is the DB attenuation of the fiber strand that is having a problem?
    2. What is the loss ratio from the Launch Laser to the Terminating location?
    3. Is the low light level occurring on all fiber strands?

The more information we have the quicker we can isolate the issue and get the connection up and running. These other question that help ACD reduce the time of its own debugging process.

  1. Did the problem report start when the Customer was performing any maintenance?
  2. Did the problem report start when ACD was performing any maintenance that the customer is aware of (i.e. during a posted maintenance window relevant to the customers services)?
  3. What testing has the customer done on their equipment?
  4. What testing has the customer done on the fiber strands provided by ACD? Has the customer put a light meter on the RX end of the strand that is having a problem? Has the customer run an OTDR test?

Impact of ACD testing

In order to test dark fiber strands that are having issues, the customer equipment needs to be disconnected. There is no way to test fibers without disconnecting equipment, therefore any request should be with the understanding that it will be service affecting, at least on the fiber strands that the customer requests testing.

If there are spare adjacent fibers that are on the same cable, we can test these for continuity of the overall fiber cable. If continuity is good on the spare fibers, this can mean that the overall cable is not fully cut. If the spare strands are functional, equipment can be reloacted to the spare cable.

The customer should advise ACD:

  1. Invasive Testing: That they authorize or not authorize ACD to perform immediate intrusive testing.
    1. The fibers they authorize testing on means an existing connection will be down during the testing, if it is not already down.
    2. ACD can test adjacent spare fiber to determine overall cable integrity, the customer should advise if they authorize intrusive testing on spare fibers.
  2. Maintenance Window Testing: Customer may request that ACD can perform testing during a maintenance window.
    1. ACD's ability to initiate repairs will not start in most circumstances until testing with an OTDR can be performed.

Equipment and Documentation Required for Testing

  1. Light Meter: A light meter is a devices that is able to read the light strength from the emitting laser. The light meter tells whether the laser light being emitted is strong or weak, and within the range of the light sensor to read the light. As light travels down a fiber cable, the strength of light is slowly degraded.
    1. ACD typically sets the light meter to 1550 wavelengths, however the meter should always be set to the wavelegth of the transmitting laser. Usually this is 1310 or 1550 for most equipment nowadays.
  2. OTDR Meter: A Optical Time Domain Reflectometer (OTDR) is an expensive device that sends a series of signals down a fiber strand to determine a number of aspects about the fiber cable. A OTDR device can tell where a) A bad splice exists b) Potentially where there is damage that is decreasing the strength of the light signal, c) determine the length of the fiber strand. This is most relevant if the length is less than the actual length of a cable, this means that there is a break in the cable somewhere.
    1. ACD sets OTDR testing equipment to 1550 nanometers wavelengths. Variation in the the wavelength selection will substantially adjust the data results.
  3. OTDR Documentation: OTDR documentation are the documents that are created at the time of delivery of the fiber service to the customer. This documentation lists the characteristics of each individual fiber strand, just after it was built. By comparing the historical OTDR readings to the current OTDR reading it can be determined if there is now a difference in the cable. Only by knowing the difference between OTDR documentation at the time of delivery and current OTDR test results, will the problem likely be isolated.
  4. As-Built Drawings: As built drawings are documents that describe the construction performed by ACD. These drawings detail the overall route of the cable, including poles that the cable is attached to, handholes, splice cases, fiber termination panels, and whether the fiber is above ground or below ground.

Reasons for Outages


Outages that are on ACD's side of the demarcation:

  1. Storm/Weather damage to the cable
  2. Tree Trimming damage to our cable. Pole owners do not typically notify us of tree trimming activity.
  3. Utility or Contractor damage to the cable.
    1. Sometimes other utility contractors accidentally damage our cable when performing work on another cable or doing underground work. ACD typically does not know about this work beforehand.
  4. ACD caused a outage during a splicing. ACD mis-spliced the fibers on a cable.
    1. An ACD maintenance notice should have been posted on the same cable the customer is using.
    2. ACD employee mis-identified the fiber strands on the cable, and that is the cause of the error.
  5. Outage at the fiber panel.
    1. Some fiber panels are used to service multiple customers, and ACD or the end-user customer has inappropriately moved or damaged jumpers.
    2. ACD will typically not have a maintenance notice posted on this issue.

Outages that are determined to be on the customers Side of the demarcation:

  1. Router/Switch/SONET Equipment Issue
    1. Equipment failed or not powered on.
    2. GBIC/SFP/Xenpak is defective.
      1. We find that this the most common reason for outage. Lasers generally degrade (signal weakens) over time an this should be one of the first items to be looked at.
    3. Mis-configuration of equipment.
      1. Usually a maintenance notice will be posted by the customer.
    4. Jumper cables damaged

    1. Bad Jumper cable between ACD fiber panel and customer's equipment.
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