Relation upload speed limit to download speed in Q-tree/adsl

RouterOS General
1

I have 3 x 10Mb/800kb adsl lines connected to one router. Traffic is distributed over the 3 WAN interfaces with PCC (in prerouting chains) and policy routing as result.
Router also mangles in forward chain traffic services (http, voip, pop3 etc)

In Queue tree 4 parents:

  1. Prioritize and speed limit for 4 groups of services by their package marks for upload to WAN1. Max. Limit is 700k and limit-at rates distributed over the four groups at need but not exceeding max limit for that parent (=incoming WAN1 port)
  2. Same as 1. but for WAN2 port
  3. Same as 1. and 2 but for WAN3 port
  4. Combined download traffic for all 3 lines. (3 x 10 = 30Mb) Parent is ´local´ port and max. limit set to 27Mb and limit-at values 512kb or 1Mb.

Now, in this setup I can only get about 6-7Mb download on each of the adsl lines when starting a download and after a split second it drops to about 5Mb and stays like that. Same result for all 3 lines.
The moment I remove the upload Queue’s download speed jumps to 8-9Mb for each line!

How can it be the upload limiting is having this effect on the download?
Is there any correlation between these? I’d notice on another 20mb link that when I download a lot of traffic from that same machine also an increase of upload is to be seen.
If there is a correlation, what shoulc the the calculation factor?

Upload max limit set at 700k with priority is needed to grant voip and ´real´live traffic going out with higher priority then browsing requests or (lowest priority) heavy_traffic upload or P2P.

So I need the settings for the upload to guarantee prioritization of the upload traffic. But I am not happy with the result on the download end!

Any suggestions?

2

Ok, just found one item that is causing the problem.
All queue types are pcq, with limit-at rate=0 so full bandwith available for all to share.
Only the “Low”, or lowest priority child had a pcq with a limit-at rate set to 128k.
I’ve done this to make sure P2P is not going to consume too much upload. Each stream would not get more then 128k upload traffic.

So I cured the fever but still don’t know what actually is the influence of the upload traffic flow on the download traffic flow.
I can understand that sending internet served needs tcp-ack back fm recieving PC (=my client) but is this influence that big?