Link encryption is an approach to communications security that encrypts and decrypts all traffic at each end of a communications line (e.g. a teleprinter circuit or the line between two network switches). It contrasts with end-to-end encryption where messages are encrypted by the sender at the point of origin and only decrypted by the intended receiver. Link encryption offers a couple of advantages:
On the other hand, end-to-end encryption ensures only the recipient sees the plaintext.
Link encryption can be used with end-to-end systems by superencrypting the messages.
Bulk encryption refers to encrypting a large number of circuits at once, after they have been multiplexed.
References
Link encryption Wikipedia(Text) CC BY-SA