🔗 Data Link Protocols (HDLC, PPP) & Error Control | 🚀 For Engineer Exam

💡 Review questions for the engineer certification exam are at the end of this post. Make sure you understand the concepts before trying them.

❗️It is important to memorize the characteristics of each protocol and error control method.

🚀 Data Link Layer at a Glance

The Data Link Layer is the second layer of the OSI 7-layer model, responsible for reliably transmitting data over a physical link. Its main functions are Framing, Flow Control, and Error Control.

• Data Link Protocol: A communication standard that manages data exchange between node-to-node.
• Error Control: A technique to detect and correct data errors that may occur during transmission.

---

1. 🔗 Data Link Protocols

Data link protocols manage data transmission between two adjacent devices.

1.1. HDLC (High-level Data Link Control)

• A bit-oriented protocol that supports both point-to-point and multi-point links.
• Frame Structure: Consists of Flag - Address - Control - Information - FCS - Flag.
• Features:
  • High transmission efficiency and reliability.
  • Uses Bit Stuffing to ensure data transparency (inserts a 0 after five consecutive 1s in the data).
  • Primarily used for synchronous transmission.

1.2. PPP (Point-to-Point Protocol)

• A character-oriented protocol mainly used for communication over serial interfaces between two computers.
• Features:
  • Unlike HDLC, it is character-oriented and has a more flexible frame structure.
  • Performs error detection only, leaving error recovery to upper layers.
  • Supports various Network Control Protocols (NCPs) like IP address assignment and authentication (PAP, CHAP).
  • Primarily used for asynchronous transmission but also supports synchronous.

1.3. Frame Relay

• A high-speed data transmission protocol developed to improve upon the shortcomings of X.25.
• Features:
  • Increases transmission speed by simplifying error and flow control (discards frames with errors and lets upper layers handle it).
  • Uses a Virtual Circuit method to establish a logical connection.
  • Mainly used for WAN services connecting LANs.

1.4. ATM (Asynchronous Transfer Mode)

• A high-speed switching technology that transmits data in small, fixed-size cells of 53 bytes (5-byte header, 48-byte payload).
• Features:
  • Can transmit various types of information like voice, video, and data over a single network.
  • Provides very high speeds as the fixed-length cells allow for hardware-based switching.
  • Used as the core technology for B-ISDN.

---

2. 🔧 Data Link Error Control

This refers to the methods for detecting and correcting errors that occur during transmission.

🤔 Why Add Redundant Bits?

If only the original data is sent, the receiver has no way to determine if the data was corrupted during transmission or arrived intact. Redundant bits provide the receiver with a basis to determine if an error has occurred.

• In Backward Error Correction (BEC), minimal information (like a 'fingerprint') is added to detect an error.
• In Forward Error Correction (FEC), enough information (like an 'answer key') is added to both detect and correct the error directly.

Both methods use redundant bits, but they differ in their purpose and the amount of information they carry.

2.1. Forward Error Correction (FEC)

• A method where the receiver autonomously detects and corrects errors.
• Features:
  • Sends redundant bits required for error correction along with the data (e.g., Hamming code, CRC).
  • Suitable for real-time or one-way communication as it doesn't require a reverse channel for retransmission requests.
  • Can be complex to implement and may reduce transmission efficiency due to the extra bits.

2.2. Backward Error Correction (BEC)

• A method where the receiver requests the sender to retransmit data upon detecting an error. Also known as ARQ (Automatic Repeat reQuest).
• BEC consists of an error detection mechanism and the ARQ protocol that requests retransmission for detected errors.

Error Detection Techniques

Redundant bits are added to the data for transmission to detect errors.

1. Parity Check

   • The simplest method, where a single parity bit is added to a block of data to make the number of 1s either even (even parity) or odd (odd parity).
   • Limitation: It cannot detect an even number of bit errors (e.g., if two bits flip from 0 to 1, the parity remains even).

2. Cyclic Redundancy Check (CRC)

   • A powerful method that uses polynomial codes to detect errors. The sender divides the data by a predetermined Generator Polynomial to calculate a remainder, which is appended to the data and transmitted.
   • The receiver divides the received data by the same generator polynomial. If the remainder is 0, the data is considered error-free.
   • It is highly effective at detecting burst errors. The Frame Check Sequence (FCS) in HDLC uses CRC.

3. Block Sum Check

   • The data block is divided into several rows of a fixed size. The sum (or XOR operation) of each column is calculated to create a check block, which is sent with the data.
   • The receiver performs the same calculation to detect errors.

ARQ (Automatic Repeat reQuest) Protocols

This is the procedure for managing retransmissions when an error is detected.

• Types:
  1. Stop-and-Wait ARQ: Sends one frame and waits for an acknowledgment (ACK/NAK) before sending the next.
  2. Go-Back-N ARQ: Retransmits all frames starting from the one where the error occurred.
  3. Selective Repeat ARQ: Retransmits only the specific frame where the error occurred.

---

📝 Practice Questions for the Engineer Exam