Two way or one-way transmission Standardized protocol dealing with electrical, mechanical and signaling interfaces. Groups bits into frames and ensures their correct delivery. Adds some bits at the beginning and end of each frame plus the checksum.
Receiver verifies the checksum.
If the checksum is not correct, it asks for retransmission. Two protocols are most widely used. Connectionless Part of Internet protocol suite. An IP packet can be sent without a connection being established. Each packet is routed to its destination independently.