Chapter 11 Data Link Control

Chapter 11 of Data Communications and Networking by Behrouz A. Forouzan, titled "Data Link Control", explores the mechanisms that enable reliable and efficient communication between adjacent nodes in a network. Here are detailed notes from the chapter:

1. Introduction to Data Link Control

• The data link layer is responsible for node-to-node communication. Its functions include framing, addressing, flow control, and error control. This layer ensures that data frames are transmitted between nodes without error and in the correct sequence.

2. Framing

• Framing: The process of dividing data streams into frames for easier management. It separates messages from different sources and destinations, adding sender and receiver addresses to each frame.

• Fixed-Size Framing: No need to define the frame's boundaries because the frame size is known (used in technologies like ATM).

• Variable-Size Framing: Uses specific delimiters (flags) to define the start and end of frames. Two methods are used:

  1. Character-Oriented Protocols: Use 8-bit characters as delimiters, popular when only text data were transmitted. Byte stuffing is used to prevent misinterpretation of flag patterns within the data.

  2. Bit-Oriented Protocols: Use a special bit pattern (01111110) as the flag to separate frames. Bit stuffing is used to ensure that the pattern is only recognized as a delimiter.

3. Flow Control

• Flow Control ensures that the sender does not overwhelm the receiver by sending data faster than it can be processed. The main techniques include:

  1. Stop-and-Wait: The sender sends one frame, then waits for an acknowledgment (ACK) before sending the next frame.

  2. Sliding Window: Allows multiple frames to be sent before an acknowledgment is required, improving efficiency.

4. Error Control

• Error Control manages the detection and correction of errors during transmission. The basic methods include:

  • Error Detection: Using techniques like parity checks and checksums to detect errors.

  • Error Correction: Either through retransmission (Automatic Repeat reQuest or ARQ) or using Forward Error Correction (FEC).

• Automatic Repeat reQuest (ARQ) is the common method for error correction:

  1. Stop-and-Wait ARQ: Retransmits a frame when an acknowledgment is not received.

  2. Go-Back-N ARQ: Acknowledges multiple frames at once, but retransmits all frames after a lost frame.

  3. Selective Repeat ARQ: Only retransmits erroneous frames, increasing efficiency compared to Go-Back-N.

5. Protocols

• Several protocols manage data link control:

  1. Simplest Protocol: No flow or error control, with the sender continuously transmitting frames.

  2. Stop-and-Wait Protocol: Adds basic flow control, with the sender halting transmission until an acknowledgment is received.

  3. Stop-and-Wait ARQ: Enhances error control, retransmitting frames if an acknowledgment is not received within a time-out period.

  4. Go-Back-N ARQ: Allows multiple frames to be in transit but requires retransmission of all frames after a lost frame.

  5. Selective Repeat ARQ: Only retransmits frames that were lost or corrupted, increasing efficiency.

6. High-level Data Link Control (HDLC)

• HDLC is a widely used bit-oriented protocol for data link control. It supports both connection-oriented and connectionless services and has three frame types:

  1. I-frames (Information frames): Carry user data and control information.

  2. S-frames (Supervisory frames): Provide acknowledgment and flow control.

  3. U-frames (Unnumbered frames): Manage control tasks like link setup and disconnection.

7. Point-to-Point Protocol (PPP)

• PPP is a byte-oriented protocol used primarily in point-to-point links. It provides framing, error detection, and the negotiation of options for data transmission. It supports multiple network layer protocols through the use of Network Control Protocols (NCPs).

• PPP operates in phases:

  1. Link Establishment: Negotiation of options between sender and receiver.

  2. Authentication: Optional step using protocols like PAP or CHAP.

  3. Network Layer Protocol Configuration: Agreement on network layer protocol options.

  4. Data Transmission: Normal data transfer between nodes.

  5. Link Termination: The connection is terminated.

8. Summary

• Data link control involves framing, flow control, and error control to ensure smooth and reliable data transmission between nodes. Protocols like HDLC and PPP implement these controls in real-world systems, allowing for efficient communication.

These notes cover the essentials of Chapter 11 and explain the key functions and protocols involved in data link control【7:8†source】【7:14†source】【7:19†source】.