Properties, Methods and Events

Figure 1.15. Project Properties dialog box

1.7.8. Create EXE file

After setting up the startup form for the Project, to run a program, we use the Start button of Visual Basic or press the F5 key. However, when the program is completed, we need an executable file, or EXE file. Follow these steps: Step 1. From the File menu, select Make EXE file... The Make Project dialog box appears asking for a file name. Enter the file name (say Clipboard), Visual Basic will automatically add the .EXE extension and generate an executable file named Clipboard.EXE.

Figure 1.16. Make Project dialog box

Step 2. Click the Option button to open the Project Properties dialog box and enter the application name in the Title box. This name will be displayed in the list of applications when the program is executed. It is not necessary to name the application the same as the .EXE file name. Suppose we name the application: First Application.

Figure 1.17. Project Properties dialog box

B3. If you want to change the version of the application, you can change the values ​​in the Major, Minor and Revision boxes. Check Auto Increment and Visual Basic will automatically increase the Revision number each time the EXE file for the project is recreated. In addition, you can add notes for each version in the Version Information section. These notes include: the name of the agency or company, trademark, copyright information, etc. corresponding to the components in the list.

Step 4. Click OK to return to the Make Project dialog box.

Step 5. Click the OK button in the Make Project dialog box to create the EXE file.

Step 6. Run the program from My Computer or Explorer window, find the correct path to the file name and double-click on it.

1.8. Multiple choice questions

1) Choose the correct way to resize a control on a form:

a. Change the width and height attribute values.

b. Click on the 8 control boxes and drag and drop.

c. Hold down the Shift key and use the arrow keys.

d. All three of the above.

2) Choose the correct way to move the position of the control on the form:

a. Use the arrow keys.

b. Hold down the Ctrl key and use the arrow keys.

c. Hold down the Shift key and use the arrow keys.

d. Double-click the object.

3) Choose the correct way to select a control on a form:

a. Click on the object to select.

b. Hold down the Ctrl key and click on the objects you want to select.

c. Hold down the Shift key and click on the objects you want to select.

d. All three of the above.

4) Choose the correct way to place a control on a form:

a. Double-click the control.

b. Click the control and drag the mouse on the form.

c. Click on the same type of control on the form-> copy-> paste.

d. All three of the above.

CHAPTER 2

PROGRAMMING FUNDAMENTALS IN VISUAL BASIC 6.0

2.1. Properties, methods and events

Visual Basic 6.0 supports Object-Oriented Programming. With Object-Oriented Programming, programmers divide problems into objects. In Visual Basic, controls are objects. Each object has characteristics called properties and separate functions called methods. Programmers must provide properties and methods that the object should display.

2.1.1. Subject

An object is a concept that refers to an entity that exists in the real world that we can recognize, as well as distinguish it from other objects. Each object has characteristics that we call its own properties, based on these characteristics we can recognize, distinguish the object from other objects. In addition to properties, each object also has actions, reactions that are specific to each type and to each specific object that we call methods or events. For example: A TV is an object, it has the following properties: Brand, size, color, weight, number of channels, ... and methods: changing channels, scanning channels, adjusting colors, ...

Visual Basic provides many objects, in which controls are also objects, each object has its own set of properties, methods and events.

2.1.2. Properties

Attributes are characteristics of an object that describe the object. Although each object has a different set of attributes, there are some attributes that are common to most objects. For example, attributes define the name, size, location, color, etc. Here are some common attributes:

Properties

Explain
Left	The left edge position of the control relative to its container
Top	The position of the top edge of the control relative to its container
Height	Height of the control
Width	Width of the control

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• Qos Assurance Methods for Multimedia Communications zt2i3t4l5ee zt2a3gs zt2a3ge zc2o3n4t5e6n7ts low. The EF PHB requires a sufficiently large number of output ports to provide low delay, low loss, and low jitter. EF PHBs can be implemented if the output port's bandwidth is sufficiently large, combined with small buffer sizes and other network resources dedicated to EF packets, to allow the router's service rate for EF packets on an output port to exceed the arrival rate λ of packets at that port. This means that packets with PHB EF are considered with a pre-allocated amount of output bandwidth and a priority that ensures minimum loss, minimum delay and minimum jitter before being put into operation. PHB EF is suitable for channel simulation, leased line simulation, and real-time services such as voice, video without compromising on high loss, delay and jitter values. Figure 2.10 Example of EF installation Figure 2.10 shows an example of an EF PHB implementation. This is a simple priority queue scheduling technique. At the edges of the DS domain, EF packet traffic is prioritized according to the values ​​agreed upon by the SLA. The EF queue in the figure needs to output packets at a rate higher than the packet arrival rate λ. To provide an EF PHB over an end-to-end DS domain, bandwidth at the output ports of the core routers needs to be allocated in advance to ensure the requirement μ > λ. This can be done by a pre-configured provisioning process. In the figure, EF packets are placed in the priority queue (the upper queue). With such a length, the queue can operate with μ > λ. Since EF was primarily used for real-time services such as voice and video, and since real-time services use UDP instead of TCP, RED is generally not suitable for EF queues because applications using UDP will not respond to random packet drop and RED will strip unnecessary packets. 2.2.4.2 Assured Forwarding (AF) PHB PHB AF is defined by RFC 2597. The purpose of PHB AF is to deliver packets reliably and therefore delay and jitter are considered less important than packet loss. PHB AF is suitable for non-real-time services such as applications using TCP. PHB AF first defines four classes: AF1, AF2, AF3, AF4. For each of these AF classes, packets are then classified into three subclasses with three distinct priority levels. Table 2.8 shows the four AF classes and 12 AF subclasses and the DSCP values ​​for the 12 AF subclasses defined by RFC 2597. RFC 2597 also allows for more than three separate priority levels to be added for internal use. However, these separate priority levels will only have internal significance. PHB Class PHB Subclass Package type DSCP AF4 AF41 Short 100010 AF42 Medium 100100 AF43 High 100110 AF3 AF31 Short 011010 AF32 Medium 011100 AF33 High 011110 AF2 AF21 Short 010010 AF22 Medium 010100 AF23 High 010110 AF1 AF11 Short 001010 AF12 Medium 001100 AF13 High 001110 Table 2.8 AF DSCPs The AF PHB ensures that packets are forwarded with a high probability of delivery to the destination within the bounds of the rate agreed upon in an SLA. If AF traffic at an ingress port exceeds the pre-priority rate, which is considered non-compliant or “out of profile”, the excess packets will not be delivered to the destination with the same probability as the packets belonging to the defined traffic or “in profile” packets. When there is network congestion, the out of profile packets are dropped before the in profile packets are dropped. When service levels are defined using AF classes, different quantity and quality between AF classes can be realized by allocating different amounts of bandwidth and buffer space to the four AF classes. Unlike EF, most AF traffic is non-real-time traffic using TCP, and the RED queue management strategy is an AQM (Adaptive Queue Management) strategy suitable for use in AF PHBs. The four AF PHB layers can be implemented as four separate queues. The output port bandwidth is divided into four AF queues. For each AF queue, packets are marked with three “colors” corresponding to three separate priority levels. In addition to the 32 DSCP 1 groups defined in Table 2.8, 21 DSCPs have been standardized as follows: one for PHB EF, 12 for PHB AF, and 8 for CSCP. There are 11 DSCP 1 groups still available for other standards. 2.2.5.Example of Differentiated Services We will look at an example of the Differentiated Service model and mechanism of operation. The architecture of Differentiated Service consists of two basic sets of functions: Edge functions: include packet classification and traffic conditioning. At the inbound edge of the network, incoming packets are marked. In particular, the DS field in the packet header is set to a certain value. For example, in Figure 2.12, packets sent from H1 to H3 are marked at R1, while packets from H2 to H4 are marked at R2. The labels on the received packets identify the service class to which they belong. Different traffic classes receive different services in the core network. The RFC definition uses the term behavior aggregate rather than the term traffic class. After being marked, a packet can be forwarded immediately into the network, delayed for a period of time before being forwarded, or dropped. We will see that there are many factors that affect how a packet is marked, and whether it is forwarded immediately, delayed, or dropped. Figure 2.12 DiffServ Example Core functionality: When a DS-marked packet arrives at a Diffservcapable router, the packet is forwarded to the next router based on Per-hop behavior is associated with packet classes. Per-hop behavior affects router buffers and the bandwidth shared between competing classes. An important principle of the Differentiated Service architecture is that a router's per-hop behavior is based only on the packet's marking or the class to which it belongs. Therefore, if packets sent from H1 to H3 as shown in the figure receive the same marking as packets from H2 to H4, then the network routers treat the packets exactly the same, regardless of whether the packet originated from H1 or H2. For example, R3 does not distinguish between packets from h1 and H2 when forwarding packets to R4. Therefore, the Differentiated Service architecture avoids the need to maintain router state about separate source-destination pairs, which is important for network scalability. Chapter Conclusion Chapter 2 has presented and clarified two main models of deploying and installing quality of service in IP networks. While the traditional best-effort model has many disadvantages, later models such as IntServ and DiffServ have partly solved the problems that best-effort could not solve. IntServ follows the direction of ensuring quality of service for each separate flow, it is built similar to the circuit switching model with the use of the RSVP resource reservation protocol. IntSer is suitable for services that require fixed bandwidth that is not shared such as VoIP services, multicast TV services. However, IntSer has disadvantages such as using a lot of network resources, low scalability and lack of flexibility. DiffServ was born with the idea of ​​solving the disadvantages of the IntServ model. DiffServ follows the direction of ensuring quality based on the principle of hop-by-hop behavior based on the priority of marked packets. The policy for different types of traffic is decided by the administrator and can be changed according to reality, so it is very flexible. DiffServ makes better use of network resources, avoiding idle bandwidth and processing capacity on routers. In addition, the DifServ model can be deployed on many independent domains, so the ability to expand the network becomes easy. Chapter 3: METHODS TO ENSURE QoS FOR MULTIMEDIA COMMUNICATIONS In packet-switched networks, different packet flows often have to share the transmission medium all the way to the destination station. To ensure the fair and efficient allocation of bandwidth to flows, appropriate serving mechanisms are required at network nodes, especially at gateways or routers, where many different data flows often pass through. The scheduler is responsible for serving packets of the selected flow and deciding which packet will be served next. Here, a flow is understood as a set of packets belonging to the same priority class, or originating from the same source, or having the same source and destination addresses, etc. In normal state when there is no congestion, packets will be sent as soon as they are delivered. In case of congestion, if QoS assurance methods are not applied, prolonged congestion can cause packet drops, affecting service quality. In some cases, congestion is prolonged and widespread in the network, which can easily lead to the network being "frozen", or many packets being dropped, seriously affecting service quality. Therefore, in this chapter, in sections 3.2 and 3.3, we introduce some typical network traffic load monitoring techniques to predict and prevent congestion before it occurs through the measure of dropping (removing) packets early when there are signs of impending congestion. 3.1. DropTail method DropTail is a simple, traditional queue management method based on FIFO mechanism. All incoming packets are placed in the queue, when the queue is full, the later packets are dropped. Due to its simplicity and ease of implementation, DropTail has been used for many years on Internet router systems. However, this algorithm has the following disadvantages: − Cannot avoid the phenomenon of “Lock out”: Occurs when 1 or several traffic streams monopolize the queue, making packets of other connections unable to pass through the router. This phenomenon greatly affects reliable transmission protocols such as TCP. According to the anti-congestion algorithm, when locked out, the TCP connection stream will reduce the window size and reduce the packet transmission speed exponentially. − Can cause Global Synchronization: This is the result of a severe “Lock out” phenomenon. Some neighboring routers have their queues monopolized by a number of connections, causing a series of other TCP connections to be unable to pass through and simultaneously reducing the transmission speed. After those monopolized connections are temporarily suspended, Once the queue is cleared, it takes a considerable amount of time for TCP connections to return to their original speed. − Full Queue phenomenon: Data transmitted on the Internet often has an explosion, packets arriving at the router are often in clusters rather than in turn. Therefore, the operating mechanism of DropTail makes the queue easily full for a long period of time, leading to the average delay time of large packets. To avoid this phenomenon, with DropTail, the only way is to increase the router's buffer, this method is very expensive and ineffective. − No QoS guarantee: With the DropTail mechanism, there is no way to prioritize important packets to be transmitted through the router earlier when all are in the queue. Meanwhile, with multimedia communication, ensuring connection and stable speed is extremely important and the DropTail algorithm cannot satisfy. The problem of choosing the buffer size of the routers in the network is to “absorb” short bursts of traffic without causing too much queuing delay. This is necessary in bursty data transmission. The queue size determines the size of the packet bursts (traffic spikes) that we want to be able to transmit without being dropped at the routers. In IP-based application networks, packet dropping is an important mechanism for indirectly reporting congestion to end stations. A solution that prevents router queues from filling up while reducing the packet drop rate is called dynamic queue management. 3.2. Random elimination method – RED 3.2.1 Overview RED (Random Early Detection of congestion; Random Early Drop) is one of the first AQM algorithms proposed in 1993 by Sally Floyd and Van Jacobson, two scientists at the Lawrence Berkeley Laboratory of the University of California, USA. Due to its outstanding advantages compared to previous queue management algorithms, RED has been widely installed and deployed on the Internet. The most fundamental point of their work is that the most effective place to detect congestion and react to it is at the gateway or router. Source entities (senders) can also do this by estimating end-to-end delay, throughput variability, or the rate of packet retransmissions due to drop. However, the sender and receiver view of a particular connection cannot tell which gateways on the network are congested, and cannot distinguish between propagation delay and queuing delay. Only the gateway has a true view of the state of the queue, the link share of the connections passing through it at any given time, and the quality of service requirements of the traffic flows. The RED gateway monitors the average queue length, which detects early signs of impending congestion (average queue length exceeding a predetermined threshold) and reacts appropriately in one of two ways: − Drop incoming packets with a certain probability, to indirectly inform the source of congestion, the source needs to reduce the transmission rate to keep the queue from filling up, maintaining the ability to absorb incoming traffic spikes. − Mark “congestion” with a certain probability in the ECN field in the header of TCP packets to notify the source (the receiving entity will copy this bit into the acknowledgement packet). Figure 3. 1 RED algorithm The main goal of RED is to avoid congestion by keeping the average queue size within a sufficiently small and stable region, which also means keeping the queuing delay sufficiently small and stable. Achieving this goal also helps: avoid global synchronization, not resist bursty traffic flows (i.e. flows with low average throughput but high volatility), and maintain an upper bound on the average queue size even in the absence of cooperation from transport layer protocols. To achieve the above goals, RED gateways must do the following: − The first is to detect congestion early and react appropriately to keep the average queue size small enough to keep the network operating in the low latency, high throughput region, while still allowing the queue size to fluctuate within a certain range to absorb short-term fluctuations. As discussed above, the gateway is the most appropriate place to detect congestion and is also the most appropriate place to decide which specific connection to report congestion to. − The second thing is to notify the source of congestion. This is done by marking and notifying the source to reduce traffic. Normally the RED gateway will randomly drop packets. However, if congestion If congestion is detected before the queue is full, it should be combined with packet marking to signal congestion. The RED gateway has two options: drop or mark; where marking is done by marking the ECN field of the packet with a certain probability, to signal the source to reduce the traffic entering the network. − An important goal that RED gateways need to achieve is to avoid global synchronization and not to resist traffic flows that have a sudden characteristic. Global synchronization occurs when all connections simultaneously reduce their transmission window size, leading to a severe drop in throughput at the same time. On the other hand, Drop Tail or Random Drop strategies are very sensitive to sudden flows; that is, the gateway queue will often overflow when packets from these flows arrive. To avoid these two phenomena, gateways can use special algorithms to detect congestion and decide which connections will be notified of congestion at the gateway. The RED gateway randomly selects incoming packets to mark; with this method, the probability of marking a packet from a particular connection is proportional to the connection's shared bandwidth at the gateway. − Another goal is to control the average queue size even without cooperation from the source entities. This can be done by dropping packets when the average size exceeds an upper threshold (instead of marking it). This approach is necessary in cases where most connections have transmission times that are less than the round-trip time, or where the source entities are not able to reduce traffic in response to marking or dropping packets (such as UDP flows). 3.2.2 Algorithm This section describes the algorithm for RED gateways. RED gateways calculate the average queue size using a low-pass filter. This average queue size is compared with two thresholds: minth and maxth. When the average queue size is less than the lower threshold, no incoming packets are marked or dropped; when the average queue size is greater than the upper threshold, all incoming packets are dropped. When the average queue size is between minth and maxth, each incoming packet is marked or dropped with a probability pa, where pa is a function of the average queue size avg; the probability of marking or dropping a packet for a particular connection is proportional to the bandwidth share of that connection at the gateway. The general algorithm for a RED gateway is described as follows: [5] For each packet arrival Caculate the average queue size avg If minth ≤ avg < maxth div.maincontent .s1 { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: normal; text-decoration: none; font-size: 15pt; } div.maincontent .s2 { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: bold; text-decoration: none; font-size: 15pt; } div.maincontent .p { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: normal; text-decoration: none; font-size: 14pt; margin:0pt; } div.maincontent p { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: normal; text-decoration: none; font-size: 14pt; margin:0pt; } div.maincontent .s3 { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: bold; text-decoration: none; font-size: 14pt; } div.maincontent .s4 { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: normal; text-decoration: none; font-size: 14pt; } div.maincontent .s5 { color: black; font-family:"Times New Roman", serif; font-style: italic; font-weight: normal; text-decoration: none; font-size: 14pt; } div.maincontent .s6 { color: black; font-family:"Times New Roman", serif; font-style: italic; font-weight: bold; text-decoration: none; font-size: 14pt; } div.maincontent .s7 { color: black; font-family:Wingdings; font-style: normal; font-weight: normal; text-decoration: none; font-size: 14pt; } div.maincontent .s8 { color: black; font-family:Arial, sans-serif; font-style: italic; font-weight: bold; text-decoration: none; font-size: 15pt; } div.maincontent .s9 { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: bold; text-decoration: none; font-size: 14pt; } div.maincontent .s10 { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: normal; text-decoration: none; font-size: 9pt; vertical-align: 6pt; } div.maincontent .s11 { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: normal; text-decoration: none; font-size: 13pt; } div.maincontent .s12 { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: normal; text-decoration: none; font-size: 10pt; } div.maincontent .s13 { color: black; font-family:"Times New Roman", serif; font-style: normal; font-weight: normal; text-d
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Gets a string value (name) used to refer to the control.
Enable	Gets a logical value (True or False) that determines whether the user can work with the control or not.
Visible	Gets a logical value (True or False) that determines whether the user see control or not

Name

2.1.3. Method

It is a piece of program contained in an object that tells the object how to perform a certain task. Each object has its own methods, but there are also methods that are very common to most objects, such as:

Method

Explain
Move	Change the position of an object as required by the program
Drag	Execute user drag and drop operations
SetFocus	Provides sighting of the object specified in the call method
ZOrder	Specifies the order in which controls appear on the screen.

2.1.4. Event

While properties describe objects and methods indicate how objects behave, events are the reactions of objects. For example, when the user clicks the left mouse button on a command button, the events: MouseDown, MouseUp, Click, and SetFocus occur. Similar to properties and methods, each object has a different set of events, but the following events are common to many objects, especially controls.

Event

Occurs when
Change	The user modifies the character string in the combo box or the document
Click	The user uses the mouse button to click on the object.
Dblclick	The user uses the mouse button to double-click (click) on the object. statue
DragDrop	The user drags an object to another location.
DragOver	The user drags an object across a control. other
GotFocus	Bring an object into the user's view (focus)
KeyDown	The user presses a button on the keyboard while an object statue in sight

The user presses and releases a button on the keyboard while an object is in view.
KeyUp	The user releases a key on the keyboard while an object statue in sight
LostFocus	Take an object out of sight
MouseDown	The user presses any mouse button while the mouse pointer is is lying on an object
MouseMove	The user moves the mouse pointer across an object.
MouseUp	The user releases the mouse button while the mouse pointer is on an object

KeyPress

2.2. Introduce some common controls

2.2.1. Label Control

The Label control has an icon in the toolbox , the purpose of the Label is to display a paragraph of text on the Form. Some properties of the Label control:

Properties

Explain
Name	Used to name the Label control.
Alignment	Used to align the text content displayed on the Label: 0 : Left alignment 1 : Right Align 2: Center
Autosize	If the value is True, the size will automatically adjust to fit the content. If the value is False, the size will not be automatically adjusted. ruler
BackColor	Set the background color of the Label
Caption	Defines the text content displayed on the Label
Font	Specifies the font style of the text displayed on the Label
ForeColor	Specifies the color of the text displayed on the Label

2.2.2. TextBox Control

The TextBox control has an icon in the toolbox called . The TextBox control is used to enter a paragraph of text, or to display a paragraph of text. Some properties of the TextBox:

Properties

Explain
Name	Used to name the TextBox control.

Alignment	Used to align the text content displayed on the TextBox: 0 : Left alignment 1 : Right Align 2: Center
Appearance	Specifies the display of TextBox: 0 – Flat : flat 1 – 3D : floating
BackColor	Specifies the background color of the TextBox
Font	Specifies the font style of the text displayed on the TextBox
ForeColor	Specifies the color of the text displayed on the TextBox
MaxLength	Specifies the maximum number of characters that can be entered into the TextBox.
MultiLine	If the value is True, the text on the Textbox can be displayed on multiple rows. If the value is False, the text will only be displayed on one row. row.
ScrollBars	Determines whether the TextBox has scroll bars or not.
SelLength	Returns the length of the selected (highlighted) text in Textbox
SelStart	Returns the position of the first character in the selected text. in Textbox
SelText	Returns the selected text in the Textbox
Text	Defines the text content displayed on the TextBox
Visible	Specifies whether the TextBox control is visible or not. True: Visible False: Not visible

Some Events of TextBox control.

KeyPress Event

This event is fired when the user clicks on the TextBox. We can use the KeyPress event to control the data entered into the TextBox. The KeyPress event handler uses a parameter called KeyAscii, which is an integer representing the ASCII code of the character typed.

For example: We want the user to only enter numbers into the TextBox, then we use the following method: if the entered character is not in the number range, then assign the KeyAscii parameter to 0.

Private Sub Text1_KeyPress (KeyAscii As Integer)

If KeyAscii < Asc(“0”) Or KeyAscii > Asc(“9”) then KeyAscii = 0 „ cancel the key entered

Beep „ play error beep sound

End If End Sub

KeyDown, KeyUp Events

Each KeyPress event gives us a KeyDown/KeyUp event pair. The KeyDown/KeyUp event has 2 parameters: KeyCode and Shift. This event allows us to recognize special keys on the keyboard. In the example below, we display the names of the function keys that the user of the program presses:

Private Sub Text3_KeyDown(KeyCode As Integer, Shift As Integer) If (KeyCode >= 112) And (KeyCode <= 123) Then

MsgBox "You just entered the function key: F" & _ Trim(Str(KeyCode - 111))

EndIf End Sub

2.2.3. CommandButton Control

The CommandButton control has an icon in the toolbox . CommandButton is a command button that we can click on, when it will perform a certain action. Some common properties of CommandButton:

Properties

Explain
Name	Used to name the CommandButton control.
Caption	Defines the title of the command button
Font	Specifies the font style of the title displayed on the command button.
Visible	Specifies whether the command button is displayed on the Form or not. If True: the command button is visible. If False: it is hidden. go button

Note: Before a character of the title, if we put the "&" sign, when executing the program, the user only needs to press the Alt + character key combination, the computer will execute the event of pressing this command button.

2.2.4. Form

Application programs communicate with users through forms.

(also called a window, derived from the word Form or Windows); the controls placed on top help the form do its job. A form is a window programmed to display data and receive information from the user.