Data-Link Control Protocols

 

Two Data-Link Control Protocols

The two important Data Link Control (DLC) protocols are:

1. HDLC – High-Level Data Link Control
2. PPP – Point-to-Point Protocol

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1. HDLC — High-Level Data Link Control

HDLC is a bit-oriented data-link protocol used for communication over point-to-point and multipoint links. It implements the Stop-and-Wait protocol for flow and error control.

HDLC configurations

HDLC supports two important modes:

• NRM (Normal Response Mode)
  • One primary station and one or more secondary stations.
  • Primary sends commands; secondary responds.
  • Can be used for point-to-point and multipoint links.
• ABM (Asynchronous Balanced Mode)
  • Used for point-to-point communication.
  • Both stations are equal and can act as primary or secondary.
  • This is the commonly used mode today.

HDLC frame

+-------+---------+---------+-----------+---------+-------+
| Flag  | Address | Control | Information|  FCS   | Flag  |
+-------+---------+---------+-----------+---------+-------+

The Flag is:

01111110

It identifies the beginning and end of a frame.

Three types of HDLC frames

Frame	Purpose
I-frame (Information)	Carries user data and can carry control information
S-frame (Supervisory)	Used for flow and error control
U-frame (Unnumbered)	Used for link/session management

S-frame types

• RR – Receive Ready: Acknowledges successfully received frames.
• RNR – Receive Not Ready: Receiver is busy and asks sender to slow down.
• REJ – Reject: Used as a negative acknowledgment, particularly with Go-Back-N.
• SREJ – Selective Reject: Used with Selective Repeat.

So, HDLC provides framing, flow control, error control, and link management.

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2. PPP — Point-to-Point Protocol

PPP is one of the most common data-link protocols for point-to-point communication. It was designed particularly for connections between a user's computer and an Internet service provider.

PPP services

PPP provides:

• Frame formatting
• Link establishment and negotiation
• Support for multiple network-layer protocols
• Optional authentication
• Network address configuration

However, PPP does not provide flow control and has only simple error control.

PPP frame

Important fields:

Field	Purpose
Flag	01111110; marks frame boundaries
Address	Constant 11111111
Control	Constant 00000011
Protocol	Identifies the type of data carried
Payload	Carries user/network-layer data
FCS	CRC-based error detection

PPP operation

PPP goes through several phases:

Dead
  ↓
Establish
  ↓
Authenticate
  ↓
Network
  ↓
Open (Data Transfer)
  ↓
Terminate
  ↓
Dead

During Establish, the two endpoints negotiate options. Authentication may then be performed, followed by network-layer configuration and data transfer.

PPP protocols

PPP uses three groups of protocols:

• LCP – Link Control Protocol: Establishes, configures, maintains and terminates the link.
• Authentication Protocols: PAP and CHAP.
• NCP – Network Control Protocols: Configure the link for different network-layer protocols.

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HDLC vs PPP

Feature	HDLC	PPP
Full form	High-Level Data Link Control	Point-to-Point Protocol
Orientation	Bit-oriented	Byte-oriented
Link type	Point-to-point and multipoint	Point-to-point
Flow control	Supported	Not provided
Error control	Detection + retransmission mechanisms	Mainly error detection using CRC
Frame types	I, S, U	Single general frame format
Authentication	Not a main feature	PAP/CHAP supported
Main purpose	Reliable data-link communication	Simple point-to-point Internet access

Easy way to remember

HDLC = bit-oriented + I/S/U frames + flow/error control

PPP = point-to-point + simple framing + authentication + network-layer configuration