Since the linespeed function use a mathematical function to define the connection per user, you have the closest "software" solution that i know.
The other solution is material.
You can use another computer to be a proxy between your injector and the network.
On this computer (with two network cards), you can install a proxy software (like 3proxy, if i remember).
With this type of software, you will be able to limit the bandwidth or simulate packet loss and collision.
Some companies rent a box that do the same.
Just plug it in, configure it via a administration website like some switches and play your load test.
We prototyped this solution for some projects.
Hope it helps,
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And life is an learning adventure.