How can I calculate bandwidth? Suppose I have 128b/s link - can I download 128 bytes per second? When 10 people are using that 128 link - how much is each person is using?
I suggest that you check back to one of my previous answers to the question "What is the bandwidth occupied by Telnet or a browser?" for reference. The answer to any available bandwidth/utilization question is "maybe" - the behavior of user applications depends on the type, frequency of transaction, and other usage questions.
The simplest aspect of your question is whether you should see 128 bps (are you sure you don't mean 128 Kbps?) download. The answer is no - but let me explain why:
First, your client (FTP or HTTP) is using a TCP stack. It is slowly increasing its rate of transfer until it maxes out. Whenever it sees any loss, it will slow down. So, thanks to congestion avoidance and slow start, you probably won't see the full datarate.
Second, 128 bps includes a number of headers at various Layers, not just data payload. So some significant fraction of the download time will NOT contribute to data transfer. This is referred to as the overhead.
So, even without any competing traffic, you will not see 128 bps but rather something less. And your mileage will definitely vary as the conditions vary. Maybe as high as 95% of the rated capacity but probably less due to TCP effects.
Now introduce 10 other people trying to use that link and it becomes a very ambiguous question. If they are all using the same client, at the same time, to effect a data transfer, then you can guess maybe 90% of your total capacity is available, divided by 10 people, so roughly estimate that each will get about 9-10% of the line rate.
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