What is it?

Bandwidth saturation is a phenomena that occurs when all a circuits available bandwidth in a given direction is being utilized by a large upload or download; this results high latency and performance issues as subsequent packets start to buffer, eventually timing out and displaying symptoms such as slow speeds and packet loss.

Common Causes

Outside of simply uploading a large file via FTP or something like YouTube, commonly overlooked causes of bandwidth congestion are back-up software such as Dropbox or iCloud, seeding while torrenting, system updates (including mobile devices such as iPhones), large email attachments, and other similar circumstances.

Effects of Bandwidth Congestion on Latency

If you suspect upload saturation is occurring, the most direct method of identifying the source of the upload is to run a continuous ping test and remove devices from the network one at a time until ping responses go down to a reasonable level. Compare the following two ping test results:

Compare the following two ping tests:

Extremely high latency response times are one of the hallmarks of bandwidth congestion. Note that in addition to response times of over one second, that the above example is also losing packets due to time-outs.

In this second example, we see the same line after the upload has been stopped. Response times are back to normal, and we are no longer seeing packet loss.

Troubleshooting

File sharing program

File sharing programs are usually a fairly obvious generator of traffic on a DSL line. What many people are unaware of is that some of these programs do not fully stop when you close the program. Some of these programs keep their shared folder open and continue to allow others to have access, using up bandwidth on the DSL line. If you use any file sharing programs, take a look through the icons in the task bar. You should find the culprit there if it is causing the issue.

Virus or Worm

Computers infected with internet worms can generate an incredible amount of traffic. A virus does not usually cause a huge amount of traffic on its own, but many spammers use email viruses to create a zombie spam relay, and nothing chokes a connection quite like a hundred thousand emails sent every minute.

Open wireless connection

An open wireless connection can be an invitation for unwanted guests. If you have an unsecured wireless router, or any wireless router, try removing it from the picture and see how it affects the speed conditions on the line.

Useful Tools

Line Graphs

Sonic Fusion DSL customers can actually view their connectivity graphs via Member Tools -> Labs -> Fusion Line Profile -> View Graphs. As can be seen in the above example, this is what will show for periods where a line's upstream bandwidth is being intermittently saturated. In this instance, the circuit was hitting the maximum 1.2Mbps upload sync rate for extended periods of time, resulting in poor performance during these periods.

Network Resource Management Utilities

These resource management tools can also help diagnose if a specific program is the culprit behind upload saturation by checking Sent/Received Packets/Bytes.

The Windows Resource Monitor can be found by typing "perfmon /res" (no quotes) in Windows Search.

The Macintosh Activity Monitor is located in Applications > Utilities, or by searching in Finder for "Activity Monitor".

Further Reading:

• Network & DNS Troubleshooting Tools
• https://tools.ietf.org/html/rfc896
• https://en.wikipedia.org/wiki/Network_congestion
• https://en.wikipedia.org/wiki/Sliding_window_protocol