Yes, it IS expected.
"Full Duplex" is a term used in relation to the speed negotiation between LAN (local area network) clients and little to do with your modem speed. (It has an impact, but in your case seems to be working as expected)
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There is more than enough bandwidth in a 10 Mbps semi-duplex link to keep both download and upload fully loaded on a cable modem. So, yes, you can download and upload simultaneously with a half-duplex ethernet setting.
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Synchronous bandwidth levels are what you seem to be describing.
The "transaction" or ACK packets that you are correct in mentioning are the cause.
Have a read of this for some technical information on the behaviour you're getting:
http://homepage.ntlworld.com/robin.d...ps/downup.html
If you saturate your upload bandwidth, it's logical that there isn't therefore the capacity to download at full speed. (read the article above for why that is the case)
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To use a metaphor... how can you expect the postie to deliver new mail through your letterbox if you're pushing out as many as will fit in the letterbox at the same time. (yes, I know it's a bit of a simplification, but bear with me)
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Technically, the metaphor is more accurate if we say this:
The postie is delivering lots of parcels (your download) you're sending lots of parcels too (your upload).. the postie can send his parcels through the open door, but you can only send yours through the letterbox.
The postie however needs a confirmation slip through the letterbox for each of his parcels, so even though the door is open and able to take several parcels at a time, he is sat there waiting for you to push your parcel out and get another confirmation slip out after it.