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I recently downloaded XCode. The file size is 1.7 GB. In Activity Monitor, it indicated while downloading, that my upload speed was basically 50% of my download speed for the entire duration of the download. After the download was complete, the 'Data received' in Activity Monitor read 1.7 GB and the 'Date sent' read just under 1 GB (numbers are approximate). I wasn't doing anything else that would involve network usage. This isn't the first time that I've seen this pattern.

It seems to apply when using ethernet or Wi-Fi. If it makes a difference, I'm running Lion on a MBP, connecting through ethernet, and downloading with App Store. Below is a screen shot from Activity Monitor taken during a previous download.

What is being uploaded and why is it always approximately 50% of the download? Does this pattern apply to all data transmission?

Activity Monitor screen shot

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What app are you using to download? – Paul Nov 21 '11 at 23:57
I'm using App Store to download it. But the same pattern applies when I use a web browser. – mockman Nov 21 '11 at 23:58
The only packets sent in return for an http GET would be ACKs, which of a file that size would be less than 1% compared to downloaded. If you are reading the numbers right, then I would get wireshark out and take a look. – Paul Nov 22 '11 at 0:03
Do both lines (sent/sec and recieved/sec) drop to zero once the download has completed? – Paul Nov 22 '11 at 0:18
Yes. They move in lock-step. – mockman Nov 24 '11 at 19:20

Bad connection quality and re-transfer requests and re-transfers for TCP-packets probably.

Checked rather easy: try any other download from nearest (in net, not geo, sense) location

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I just downloaded iTunes using a web browser (Omniweb). The same pattern occurred. How can I find a nearby download to test? And by the way, if the connection were bad, would I be able to get 20 MB/s speeds? – mockman Nov 24 '11 at 19:43
While a bad connection would result in retransmission of data, the downloaded quantity would increase at a far greater rate than the uploads. A TCP ACK is tiny, as is an entire HTTP request for the 1.7GB to be sent again - less than 1KB. As the download took 1.7GB for a 1.7GB file and the speeds are healthy, this does not account at all for the upload. – Paul Nov 24 '11 at 20:17

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