The Data Link layer provides both flow control and error control to ensure reliable, well-paced delivery of frames across a single link.

Flow Control

Flow control prevents a fast sender from overwhelming a slower receiver's buffer capacity. The Data Link layer achieves this primarily through the sliding window mechanism:

• The receiver's available buffer space determines how many frames the sender is allowed to have outstanding (unacknowledged) at once — the window size.
• As the receiver processes and acknowledges frames, the window "slides" forward, permitting the sender to transmit more.
• Example: In Go-Back-N with window size N=4, the sender can have at most 4 frames in flight; it must pause once 4 are unacknowledged, resuming only as ACKs arrive.

Error Control

Error control detects (and where possible corrects) frames that were corrupted or lost during transmission, and arranges for recovery:

1. Error detection — redundant bits (parity, checksum, or CRC) let the receiver verify whether a received frame matches what was sent.
   • Example: CRC using polynomial division catches virtually all common burst errors.
2. Acknowledgement (ACK) — the receiver confirms successful, error-free receipt of a frame.
3. Timeout and retransmission (ARQ — Automatic Repeat reQuest) — if the sender doesn't receive an ACK within a set time, it assumes the frame was lost or corrupted and retransmits.
   • Go-Back-N: retransmits the lost frame and every frame sent after it.
   • Selective Repeat: retransmits only the specific lost/corrupted frame, using individual timers and receiver-side buffering for out-of-order frames.

Together: flow control governs how much data can be sent before pausing for acknowledgement, while error control governs what happens when a frame doesn't arrive correctly — both working through the same underlying sliding-window/ACK/timeout machinery to guarantee reliable link-level delivery.