Notes, summaries, assignments, exams, and problems for Computer science

Data Link Layer Frames

In the link layer, data is organized into units called frames. Each frame has a header, which includes an address and control information, and a tail that is used for error detection.

The header of a Local Area Network (LAN) contains the physical addresses of the source and destination. The header of a frame that is spread by a Wide Area Network (WAN) contains a circuit identifier in the direction field.

Remember that a link is a local area network, a peer-to-peer facility, or another wide-area facility by which systems can communicate using a data link layer protocol.

Core Functions of the Data Link Layer

The data link layer is responsible for the reliable transfer of information through a data transmission circuit. Data...

Criteria to organize files · quick-access necessary when a single record is accessed. -not needed if a file will be processed in batches. Easy to update-a file on CD-ROM updates and never ease storage is not considered economic. -minimum data redundancy. -redundancy can be used to speed up access. · a single-index · Maintenance cheaper. -fewer errors.· Reliability A directory can have subdirectories, thus forming a hierarchical structure in the shape of an inverted tree, is symbolized by a bar of division (/) Some directories on Linux: • / bin Contains executable programs that are part of the Linux operating system . Many Linux commands like cat, cp, ls, more and tar are located in this directory. • / bootIt contains the kernel (or...

Secure PHP Word Document Upload Script

This PHP script manages secure document uploads, specifically restricting files to the Microsoft Word format, validating file sizes, handling duplicate filenames, and saving the record to a MySQL database.

PHP Upload Code

<?php
// PROGRAMMING BEGINS //::::::::::::::::::::::::::::::: DOCUMENT
if ($success1) {
    // define() is a function that defines a constant. Equates to 300KB
    define('max_file_size', 3000000);
    define('UPLOAD_DIR2', 'documents/');

    $file = str_replace(' ', '_', $_FILES['file']['name']);
    
    // Calculate the maximum size of a file to upload
    $max = number_format(max_file_size / 2048, 1) . ' KB';
    
    // Define or calculate the file size
    $size = number_format(

Network Hardware Hubs, Switches, and Routers

• HUB: Functions like a power strip. It distributes network traffic to all connected devices without identifying specific recipients, which can limit individual performance.
• SWITCH: Contains internal memory to identify connected devices, ensuring data is sent only to the intended recipient. It supports individual device speeds, though total throughput is limited by the cable capacity.
• ROUTER: Acts as a gateway to external networks. It uses software to generate IP addresses and serves as a bridge between different networks. If data is intended for an external destination, the switch or hub forwards it to the router to find the recipient.

Network Cable Types

• Shielded Twisted Pair (STP): Each pair is coated

1. Functional Dependency (FD)

A functional dependency A → B means: if you know the value of attribute A, you can always determine the value of attribute B. In other words, no two rows can have the same A value but different B values.

Example: In a table Student(StudentID, Name, Email): StudentID → Name, Email (Given a StudentID, the Name and Email are always uniquely determined.)

2. Boyce-Codd Normal Form (BCNF)

A relation is in BCNF if, for every functional dependency X → Y, X is a superkey.

• Compliance: Every determinant in the table is a candidate key—no changes needed.
• Evaluation: Whenever there is a non-trivial FD X → Y where X is not a superkey. This means some attribute depends on only part of a key, or on a non-key attribute—causing

What is a Stream and its Classes in Java?

Introduction
A Stream is a sequence of data that flows between a program and an input/output device. Streams are used to perform input and output operations in Java. Java provides stream classes in the java.io package for reading and writing data. Streams simplify file handling and data communication.

Types of Streams

1. Input Stream

Used to read data from a source such as a file, keyboard, or network. Data flows from the source to the program.

2. Output Stream

Used to write data to a destination such as a file, screen, or network. Data flows from the program to the destination.

Various Stream Classes in Java

1. FileInputStream

Used to read data from a file. It reads data in byte format and is suitable for reading...

using System.Data;
using System.Windows;

namespace WpfApp1
{
public partial class LoginWindow : Window
{
private readonly DatabaseService db = new DatabaseService();

public LoginWindow()
{
InitializeComponent();
}

private void LoginButton_Click(object sender, RoutedEventArgs e)
{
string login = LoginTextBox.Text.Trim();
string password = PasswordBox.Password.Trim();

if (login == "" || password == "")
{
MessageBox.Show("Введите логин и пароль.", "Ошибка");
return;
}

DataTable user = db.Login(login, password);

if (user.Rows.Count == 0)
{
...

Админ панель

Инициализация страницы

public AdminPage()
{
    InitializeComponent();
    LoadUsers();
}

Загрузка пользователей

private void LoadUsers()
{
    using (SqlConnection conn = new SqlConnection(connStr))
    {
        conn.Open();
        string sql = "SELECT id_User, Login, Block, FailPassword FROM Users";
        SqlDataAdapter da = new SqlDataAdapter(sql, conn);
        System.Data.DataTable dt = new System.Data.DataTable();
        da.Fill(dt);
        dgUsers.ItemsSource = dt.DefaultView;
    }
}

private void btnRefresh_Click(object sender, RoutedEventArgs e) => LoadUsers();

Добавление пользователя

private