Cable Latency

From what I have learned spikes in latency making VoIP and other applications unusable in cable modem systems stems from the buffer bloat issues described a few years back. Specifically, cable modem front ends (UBRs) that may hang onto a packet for many seconds while waiting for an open upstream slot to send it on. I am sure there are many much more expert than I on this list but this explanation fits the problems I too have been struggling with over the past few years. One of the partial solutions is to make sure you never exceed the available upstream bandwidth. This can easily be done with a real router or any of the open source routers out there (I use vyatta but pfsense and others are fine as well as a used cisco off ebay). From the links found from the below link, the idea is to use traffic shaping on outgoing packets to match available upstream bandwidth so that the UBR never has to buffer an outgoing packet and hang onto it "forever". However, the reason I have found this to be only a partial solution is that the "available" bandwidth varies with time of day/number of users/etc... So even on my 20mbps link, the available bw sometimes drops to less than a megabit at times. To limit my upstream to 1mbps was just too bitter a pill to swallow when I normally have 20. So I just bought a 384kbps DSL line for exclusive use by my VoIP PBX and all the latency problems went away. Variation to my SIP provider over DSL is never more than 50ms (unlike the >8000ms I had over cable). This all being said, I would love to find a more elegant/cost-effective approach to this. Comcast clearly has a way to prioritize IP traffic for their own VoIP offerings. See http://forums.comcast.com/t5/Basic-Internet-Connectivity-And/Re-Crazy-Latency-Issues/td-p/2426214 for discussion and some things like backing off upload speeds that might improve things.

Changing from Cable to DSL 7 Dec 2014 (to VoIP provider in LA)


Cable Loss/Latency 26 Dec 2014 (to ICANN in LA)


DSL Loss/Latency 26 Dec 2014 (to VoIP provider in LA)


Update

Maybe complaints pay off - a bit?

Sometime early February 2015 I noticed a change in Comcast packet loss rates. As you can see from the graph below, there was an improvment in average latency and loss.

Possible Comcast-end Equipment Upgrade? Feb 2015 (to ICANN in LA)


This was good news, so I tried using VoIP over the improved connection. However, I was still told by others that latency and drops were still an issue although much less. Traffic shaping on out-going traffic was in effect in my router to avoid the UBR/packet bloat issue described above (set to my contracted upstream rate of 20mbps).

So I returned our PBX to using the ATT DSL line. As you can see the latency and drop rate for the DSL line continue to be much better than the cable connection (which I started experimenting with in early December).

DSL Loss/Latency Feb 2015 (to VoIP provider in LA)


Closer inspection of simple "mtr'ing and pinging" shows the following which do in fact show less latency for the DSL versus cable.

Mtr 5 Feb 2015 (to ICANN and VoIP)


Cable Loss/Latency Feb 2015 (to Google DNS)


DSL Loss/Latency Feb 2015 (to VoIP provider in LA)


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