Emmy Code

HDLC

Special Instructional Objectives:

· On completion, the student will be able to:

· Explain how High-Level Data Link Control (HDLC) works

· Explain how piggybacking is done in HDLC

· Explain how data transparency is maintained in HDLC

Introduction

HDLC is a bit-oriented protocol. It was developed by the International Organization for Standardization (ISO). It falls under the ISO standards ISO 3309 and ISO 4335. It specifies a packetization standard for serial links. It has found itself being used throughout the world. It has been so widely implemented because it supports both half-duplex and full-duplex communication lines, point-to-point (peer to peer) and multi-point networks, and switched or non-switched channels. HDLC supports several modes of operation, including a simple sliding-window mode for reliable delivery. Since Internet provides retransmission at higher levels (i.e., TCP), most Internet applications use HDLC's unreliable delivery mode, Unnumbered Information.

Other benefits of HDLC are that the control information is always in the same position, and specific bit patterns used for control differ dramatically from those in representing data, which reduces the chance of errors. It has also led to many subsets. Two subsets widely in use are Synchronous Data Link Control (SDLC) and Link Access Procedure-Balanced (LAP-B).

In this lesson we shall consider the following aspects of HDLC:

· Stations and Configurations

· Operational Modes

· Non-Operational Modes

· Frame Structure

· Commands and Responses

· HDLC Subsets (SDLC and LAPB)

HDLC Stations and Configurations

HDLC specifies the following three types of stations for data link control:

· Primary Station

· Secondary Station

· Combined Station

Primary Station

Within a network using HDLC as its data link protocol, if a configuration is used in which there is a primary station, it is

Programming Languages



The computer system is simply a machine and hence it cannot perform any work; therefore, in order to make it functional different languages are developed, which are known as programming languages or simply computer languages.

Over the last two decades, dozens of computer languages have been developed. Each of these languages comes with its own set of vocabulary and rules, better known as syntax. Furthermore, while writing the computer language, syntax has to be followed literally, as even a small mistake will result in an error and not generate the required output.

Following are the major categories of Programming Languages −

Machine Language

Assembly Language

High Level Language

System Language

Scripting Language

Let us discuss the programming languages in brief.

Machine Language or Code

This is the language that is written for the computer hardware. Such language is effected directly by the central processing unit (CPU) of a computer system.

Assembly Language

It is a language of an encoding of machine code that makes simpler and readable.

High Level Language

The high level language is simple and easy to understand and it is similar to English language. For example, COBOL, FORTRAN, BASIC, C, C+, Python, etc.

High-level languages are very important, as they help in developing complex software and they have the following advantages −

· Unlike assembly language or machine language, users do not need to learn the high-level language in order to work with it.

· High-level languages are similar to natural languages, therefore, easy to learn and understand.

· High-level language is designed in such a way that it detects the errors immediately.

· High-level language is easy to maintain and it can be easily modified.

· High-level language makes development faster.

· High-level language is comparatively cheaper to develop.

· High-level language is easier to document.

Although a

Fundamental Concept



A computer is basically a programmable machine capable to perform arithmetic and logical operations automatically and sequentially. It is also known as a data processor, as it can store, process, and retrieve data as per the wish of the user.



Data processing involves the following three activities −

Input of data

Manipulation/processing of data

Giving output (i.e. management of output result)

In computer system, data is arranged orderly and systematically.

The term “computer” is derived from a Latin term “compute,” which means ‘to calculate.’ Initially, the computer system had been designed to calculate; it was intended to be a computing device. However, over a period of time, this device technically advanced; at present, it can perform a wide range of desirable works apart from data processing.

Major Functions of Computer System

Following are the core functions of a computer system −

· A computer accepts the command and/or data as input given by the user.

· A computer follows the instructions and stores the data given by the user.

· A computer processes the data as per the instructions given by the user.

· A computer gives the desirable results in the form of output.

Salient Features of Computer System

Following are the salient features of a Computer System −

· Automation − The operating system of a computer system is automatic, as no human intervention is required; simply you need to give the command and then it will do the work automatically.

· Speed − Depending upon the power of the computer, it can perform, it can take Millions of instructions per second.

· Storage − A computer system can store enormous quantity of data in different format. The storage capacity of a computer system is normally expressed in terms of Kilobytes (KB), Megabytes (MB), Gigabytes (GB), or Terabytes (TB).

· Accuracy − The accuracy of a computer system is very high.

· Versatility