This month’s Ask a Broadband Expert features Brady Volpe, a highly respected speaker and industry thought leader on broadband. Mr. Volpe is a frequent presenter at industry trade shows, conferences and regional seminars.  He has published numerous articles in worldwide trade journals and authored several white papers on DOCSIS protocol and VoIP test and analysis.  You can find him on Twitter at @bradyvolpe and on LinkedIn here.

Brady recently wrote a white paper for broadband providers titled, "DOCSIS Codeword Errors and Their Effect on RF Impairments".  In this first Ask a Broadband Expert blog, Brady answers three frequently asked questions about codeword errors with excerpts from the white paper.  And, once you realize the importance of tracking uncorrectable and correctable codeword errors, check out our TruVizion diagnostics application for a great way to do that.

Question: Are codeword errors really that important?

Brady Volpe: You may not be familiar with DOCSIS codeword errors, but it is important to know that monitoring codeword errors will enable you to improve your subscriber’s quality of experience (QoE). How is this possible?  DOCSIS codeword errors are the most effective metric to determine if a data issue, like slow web pages, slow gaming, poor voice quality, etc., is an RF plant problem or if it is a data network problem. If you work in the field you know how critical this information can be. DOCSIS codeword errors are the deciding factor if you have an RF impairment or if you do not have an RF impairment. Period.  If a high number of codewords go uncorrected then the subscriber may see their Internet slowdown.  If the subscriber is using voice-over-Internet Protocol (VoIP) offered by the cable operator, Skype, Vonage, etc. voice and video calls will be especially impacted.

Question: What is a codeword?

Brady: Data transmitted between the CMTS and the modem is split into codewords that are usually 16 to 256 bytes in length. Each codeword contains extra data, called Forward Error Correction (FEC) that allows the original codeword to be rebuilt if the data is in error. When a subscriber is using a device, such as a PC or iPad, and transmits data over a DOCSIS cable modem, the FEC kicks in. There are two functions to the FEC protocol, the encoder and the decoder. The cable modem and CMTS act as both interchangeably, depending on data direction.

Question: What are codeword errors?

Brady: Codeword errors are corrupted bits of data in the transmission or flow.

There are two types of codeword errors: correctable and uncorrectable.

Correctable codeword errors refer to damaged codewords that can be repaired using the FEC data explained above. If the decoder finds that any bits in the codeword were corrupted it will use the extra correction data to attempt to fix the corrupted bits. If the bits can be repaired, then the decoder reports back with a correctable codeword, because the codeword was saved thanks to the forward error correction. This means the subscriber never knew that any RF impairment occurred.  However, even with repairs, if a high number of correctable errors exist, this will also impede performance because the modem and CMTS are working to correct the data.

Uncorrectable codeword errors are those codewords that are damaged beyond the FEC’s ability to repair it. Uncorrectable errors require that the data be repeated and are usually the biggest concern. If the bits are not repairable the decoder reports an uncorrectable codeword. This means the subscriber’s device (PC, iPad, etc.) must re-transmit the data if possible and their QoE will start to diminish as uncorrectable codewords increase. This is especially true for real-time services such as gaming.
 
Want to know more about DOCSIS codeword errors?  Download a free white paper from Brady?