In data communication, Data Link Control (DLC) refers to the services and protocols that ensure reliable and efficient communication between two adjacent nodes connected by a link. Flow control and error control are two critical aspects of Data Link Control to ensure that data is transmitted accurately and efficiently.
### Flow Control:
**Flow control** is the management of the rate of data transmission between two connected nodes to prevent a fast sender from overwhelming a slow receiver. The main goals of flow control are to prevent data loss due to buffer overflows and to optimize the use of network resources.
1. **Stop-and-Wait Flow Control:**
- Simplest form of flow control.
- Sender sends one frame and waits for an acknowledgment before sending the next frame.
- Efficient for short-distance communication but can be inefficient for long-distance or high-latency connections.
2. **Sliding Window Flow Control:**
- Allows the sender to send multiple frames without waiting for individual acknowledgments.
- The receiver acknowledges frames and indicates its readiness to accept more frames.
- Provides better utilization of the link and higher throughput.
### Error Control:
**Error control** is the process of detecting and correcting errors that may occur during data transmission. The key mechanisms for error control include error detection codes, acknowledgment and retransmission, and automatic repeat request (ARQ) protocols.
1. **Error Detection Codes:**
- Parity bits, checksums, and cyclic redundancy checks (CRC) are commonly used for error detection.
- Detect errors but do not correct them.
2. **Acknowledgment and Retransmission:**
- After receiving a frame, the receiver sends an acknowledgment (ACK) to the sender.
- If the sender doesn't receive an ACK within a specified time, it assumes an error and retransmits the frame.
- This process is part of Automatic Repeat reQuest (ARQ) protocols.
3. **Automatic Repeat reQuest (ARQ) Protocols:**
- Stop-and-Wait ARQ, Go-Back-N ARQ, and Selective Repeat ARQ are common ARQ protocols.
- These protocols define how the sender retransmits data upon detecting an error.
### Combined Flow and Error Control:
- **Selective Repeat ARQ with Sliding Window Flow Control:**
- Allows multiple frames to be outstanding (sent but not yet acknowledged) without waiting for an acknowledgment for each frame.
- If an error is detected, the sender retransmits only the frames that were not successfully received.
In summary, Data Link Control employs flow control to manage the rate of data transmission and error control to detect and correct errors during the transmission process. The specific mechanisms used depend on the protocols and techniques implemented in the communication system.
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