BF3: Why do i get killed when i knife someone?

I'm new to BF3 i've only played Call of Duty...
So im in campaign and i go to knife someone and they just turn around and kill me?
Call me a noob but is there a way to counter knife or is knfing different?
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Answer:
Knifing is different from CoD.
You can only 1-hit kill with knife, by knifing someone in the back.
Don't try rushing into people to knife, you'll need about 3 hits to kill.
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