LTE: What are all KDF used in LTE ?

Question

In 3GPP 33.401, KDF is shown.
What is KDF and what does exactly mean ?

Answer 1

KDF are the key derivation function. These are the algorithms used to generate the keystream, these keystream is used as the security keys for the ciphering and integrity.
As in LTE no security keys are exchanged on radio interface.
These KDF uses underlying algorithm to generate the keys. (e.g to generate KeNB key underlying algorithm HMAC SHA 256 is used for KDF.)