Physical layer – Defines how bits are encoded for transmission over a physical medium. Example: ITU-T V.90.
Network layer – Defines how data is routed between devices that may be connected to different link layers. Example: IP (RFC 791).
Social layer – Defines how the information exchanged by applications refers to human beings, their activities and relationships. Example: A telephone number, a bank account, a Facebook profile and an email address are distinct identifiers that may refer to the same human being.