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It looks like there is an issue with some of the speed test servers in the Cox network in Oklahoma. For almost a month I've been working with Cox to fix my UPLOAD speed issue. I have the lowest level of service, 30 mpbs down / 3 mpbs up. I consistently get 33-35 mbps download speed when testing, but I was getting only 1.5-1.7 upload speeds. I've run 50+ tests in 3 weeks, almost all of them with a laptop (gigabit NIC) hard-wired directly to the Cox cable modem.
We are on the third new modem from Cox, but no change in upload speed. Three different Cox technicians came to the house to check equipment and lines and always reported, "the download speed is fine." But, it is the UPLOAD speed that was the problem. During one of the calls with Tech Support, the technician temporarily changed the service level to 150 down / 10 up. Immediately we started getting 5.0 - 5.5 mpbs UPLOAD speed. So, it appears that we are always getting about 50% of the provisioned speed. (Again, download speed was always great).
Finally, we discovered that the problem is with the UPLOAD part of the speed test when connecting to Cox servers in Oklahoma. I was testing my speed using the cox.com speed test service in their tech support tools section, and speedtest.net. Both were connecting to the closest server, which is located inside the Cox network. However, when the last Cox technician was at the apartment, he ran a speed test to a different server and was consistently getting 3.0-3.2 mbps UPLOAD.
So, I ran several more tests to a variety of servers. Every time I selected one of the cox.com servers in either OKC or Wichita, it always showed an UPLOAD speed of half what was expected. When I connected to the Cox.com server in Atlanta and Phoenix, it showed full upload speed. When I connected to other, non-Cox servers, I saw correct results of 3.0 - 3.2.
Summary, it appears that the cox.com server(s) have an issue where they will only provide 50% of the upload speed for testing.