Old data may be worse than no data. This sits somewhere between the other two in that the sender keeps connection state, but may not necessarily retransmit unacknowledged frames. Likewise, the receiver may hand received packets to higher layers in the order in which the arrive, regardless of the original sending order. Typically, each frame is assigned a unique sequence number , which the receiver returns in an acknowledgment frame to indicate which frame the ACK refers to. That way the receiver knows how big the current frame is and can determine where the next frame ends.
Receiver loses synchronization when bits become garbled.
Although checksum will detect the incorrect frames, the receiver will have difficulty resynchronizing to the start of a new frame. This technique is not used anymore, since better techniques are available. For instance, use the 4-bit sequence of 0111 to delimit consecutive frames. A frame consists of everything between two delimiters. Within the frame, replace every occurrence of two consecutive 1's with 110.