What can be causing my tremendous amount of FEC and CRC errors via my DSL connection?

I get tons of FEC and CRC errors almost daily.

If you are seeing these errors in the modem logs then it is definitely an issue with the line from the local DSLAM to your house. Only the phone company (if they are also your ISP) can fix this. Them telling you it was an issue with your PC is plain lying.

What are CRC (Cyclic Redundancy Check) Errors?

• CRC is an error detection code used to verify packet transmission between the sender and receiving end. A CRC error indicates that part of the data packet is corrupt and requires retransmission.
• Many CRC errors in a short period of time will show a noticeable reduction in throughput speed.
• This can be an early indication that there is too much noise on the line and in extreme situations can lead to loss of sync (disconnection with the exchange).

What are FEC (Forward Error Correction) Errors?

• Count of errors that have been corrected due to error correction being applied to the line.
• Error correction is turned on at the same time as Interleaving.
• Its normal to see FEC errors on an Interleaved line and rather than anything to be too concerned about its more an indication that the Interleaving & Error Correction process is working and doing what it should.

For more information see xDSL Broadband Data - Error Correction

You might also want to read How to interpret your ADSL Line Stats

So what causes these errors?

• The length of your telephone line to your local telephone exchange/DSLAM

  • ADSL is capable of supporting download rates of up to 10 Mbps over telephone lines in good condition at line distances of up to 3.4 miles (5.4 kilometres). Bandwidth deteriorates rapidly with line length; which means that broadband-users located beyond 3.8 miles (6 kilometres) from their local telephone exchange are restricted to 1 Mbps or less.
  • ADSL 2 can provide download speeds of up to 12 Mbps over a single copper pair up to 1 mile (1.6 kilometres) from the telephone exchange, or 24Mbps up to 1 mile (1.6 kilometres) using 2 copper pairs bonded together.
  • ADSL 2+ can provide download connection speeds of 26 Mbps within 0.6 miles (1 kilometre) of the telephone exchange, falling to around 12.5 Mbps at a range of 1.1 miles (1.8 kilometres). Bandwidth deteriorates rapidly with line distance beyond 1.6 miles (2.5 kilometres) from the exchange, and at a range of 2.8 miles (4.5 kilometres) it does not offer any speed advantage over ADSL. The number and type of other digital services being used by other customers over copper pairs within the same cable sheath.

• The configuration and line quality of the copper wire pair between the exchange and your premises.

• Electrical interference from outside sources (underground phone lines that have to run under very large buildings and motorways can be affected by 'noise', older phone lines that run along telegraph poles also be affected).

• The configuration and quality of the copper wiring within your premises (use a central splitter for optimal performance).

• Weather conditions

  • Hot weather can cause a long line to expand.
  • Rain on a connection somewhere could cause dampness and deteriorate the signal.

My grandfather had this exact situation with his DSL years ago. It took a dozen visits (from myself and other local computer experts and multiple phone company technicians) and replacement of the modem twice, until someone at the ISP realized the problem three years into his subscription...

DSL has a maximum distance that the signal will travel. Different types of DSL have different maximum limits, but in general the maximum distances of all of them are pretty low. If you are outside the maximum distance for your type of service, it is possible that the sales department of the ISP doesn't know that and will happily sell you a subscription that you cannot physically use.

In my grandfather's case, they told him that his distance meant they could not provide the 6Mbps service he was paying for but they could switch his connection to 768kbps and it would work at longer distances.

If your connection previously worked well, it is possible that you were on the very edge of the service area and that construction or other maintenance between you and the DSLAM has lengthened the path of the wires. (I just saw our local phone company do this to make way for a road widening project and the new path of the wires is 50 feet longer than it used to be.)

July 17, 2020

I wanted to give everyone an update. The last I mentioned was that a team leader from the phone company (which provides my DSL service) told me that the problem was my computer and that they would be charging me for any more trips to address the matter. I connected a new computer (as they suggested), but still got tremendous FEC and CRC errors. I reported their company to the state attorney general's office and the Better Business Bureau, but decided to delay any action because they were maintaining contact with me about the matter.

I was still getting disconnections and speed issues over the past 2 months. Finally, they decided to connect my house "directly" to the local switch box across the road. Despite the many previous inspections of the equipment by their "best" people, the cable coming to my house was indeed bad as I had surmised! I've had no disconnections, no CRC errors and only 3000 FEC errors in the past 2 weeks!

Thanks to all of you who provided information, suggestions, or personal experiences to help inform and encourage me regarding this matter. If anything else transpires, I will let you know.