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I'd like to understand whether the link is really overloaded or this can be some carrier issue. On capacity graphs the link doesn't look overloaded, it get about 60 % sometimes, but not more
Likely link is overloaded, which isn't too hard to do with 128 Kbps.
As to capacity graphs, 60% doesn't sound that far off. If your running TCP traffic, TCP flows may probe for available bandwidth by overdriving the link and then backing off by 50%. So, bandwidth usage tend to be lower than available bandwidth. You can improve on that if you set the TCP receiver's RWIN and router's buffers to the bandwidth delay product. Then a TCP flow will cap itself at link capacity.
Also depending on you traffic, most monitoring is a multi-minute average.
Do you think I should ask carrier to check the real link throughput of this link? The issue is that users started concerning that some application works very slowly through this link
If you want to confirm you're able to get your full 128 Kbps, try a performance test tool.
As to user complaining about poor performance, not surprising if multiple users are sharing only 128 Kbps. If not already using, FQ might help a little.
