In general, to use questions effectively, for each lesson or even each teaching activity, the teacher needs to plan how many questions there will be, the specific content of each question, what the function of each question is, when it will be asked, etc. In terms of pedagogical value and role, two types of questions need to be prepared:
Closing questions: cover the basic learning content , related to the main ideas of the lesson. They correspond to the central concept or the main skills and methods that the learner must acquire.
Extended questions, or supplementary questions, are prepared in the form of expected, hypothetical situations because this type of question only really arises depending on the specific situation at that time. Although this type of question cannot be prepared exactly, it must have a clear direction, because it plays a decisive role in interaction, discussion and keeping thoughts continuous, situations lively and attractive.
In addition, in this step, it is necessary to analyze the characteristics and properties of the questions to be used so as to best meet the knowledge, skills and experience levels of learners about the topic or lesson, considered in groups and individuals depending on the level of understanding of the teacher . From here, there is a relatively clear orientation about the types and categories of questions, the language form for each type and category. In this step, it is necessary to consider the following characteristics of the questions:
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Qos Assurance Methods for Multimedia Communications
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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
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Illustration of Teaching Organization Model Using M-Learning -
Applying Project-Based Learning to Organize Study Tours to the Hoa Lu Ancient Capital Cultural Heritage
- Clarity and lucidity of meaning: the question must be clear in meaning, clear in meaning, and focus on an issue related to the main topic or concept .
- Intellectual challenge: the challenge here is not simply to stimulate purely rational thinking but also includes challenges to arouse emotions , encourage positive attitudes... In general, questions used in teaching for students - adult learners are not simply to reproduce knowledge because learning requires more than just memorization. While people often forget specific events quickly, high-level thinking skills have the ability to last a long time, because these skills are often practical and therefore they are often used . An author commented that: "Education is what remains when we have forgotten everything taught in school" [17, p. 171]. Therefore, in teaching, teachers need to increase questions that develop high-level thinking ability in learners.
- Group or mass orientation: The question in this case is not
not intended for individual learners, but rather for a group or a whole class. If
It is a terrible mistake to prepare questions for individual answers, because then the questioning technique is no longer valuable in supporting the teacher to have a positive influence on the learners . Because of the group or majority orientation, the question will affect many people and that impact has the effect of encouraging participation, thinking together, empathy, connection and cooperation in action as well as friendly relationships and a sense of shared responsibility of many learners. To clarify the group or majority orientation, one must choose questions and the way of expression so that the learners feel that it is not a question or interrogation, but feel as if it is the students asking or it is their own question, or it is the teacher expressing this problem or idea on their behalf. The question must cause a reaction in many students, everyone feels responsible for answering, everyone is concerned and thinks, thanks to that the group or class atmosphere is not scattered. The pronouns referring to people in the questions should be used in the plural, avoid calling an individual by name, avoid using the pronoun I but should use We, Our problems, Our thoughts...
- Appropriate for the age and ability of the learner: Questions must be within the limits of the learner's ability to perceive linguistic information, understand semantics, meaning in sentences and speech etiquette, experience in perceiving communication situations, thinking ability , imagination , etc. Moreover, it is necessary to use question types in a harmonious and balanced way. Not only should we avoid using questions that are too difficult, too complex, highly problematic, too multi-valued for weak learners or vice versa, but we also need to avoid applying only one-sided questions that are too easy, too difficult, too narrow for a group of good, average or weak students. It is also absolutely necessary to avoid using questions that create conditions for students to follow suit, agree or show off their knowledge .
- Variability or situationality: Questions need to be presented in a flexible and adaptable way to control the learning activities of learners, not mechanically or in a formulaic way that reduces interaction or curiosity in learners. The set of questions needs to be mixed in terms of type, form, difficulty, tendency , and target audience. It is necessary to take advantage of different questions and clearly visualize when and how to use which questions to teach effectively.
After having a fairly clear orientation on the content as well as the question type based on the learner's basic ability and experience, this is the time to
The teacher seeks to reflect the ideas and content that will be included in the question-and-answer interaction in the form of conversational or dialogic language, defining the questions in physical form rather than in ideas. The general question words and phrases used are as follows:
- Who, What, When, Where, Which, When?... are often used in convergent, single-valued, simple, low-level questions, aimed at events, questions of reproduction , association, review, systematization, and learning about the learner's experience.
- Why, Why, How, Wherefore, How, By what means, What will happen?... are often suitable for expressing high-level, difficult, divergent, multi-valued, complex, problematic, deductive, generalizing, conceptual, and evaluative questions.
With the same content and ideas, the same purpose, the shorter the question, the fewer words, clauses, structures, and new terms, the better. In the question, avoid rhetorical forms, reduplication, repetition, figurative meanings, and avoid using homonyms.
2- Organize questions and answers in class
Good questioning techniques must encourage all members of the class to think, have the ability and desire to answer the question. To do this, when asking questions, teachers need to flexibly apply some of the following techniques: 1/ Ask questions that students are able to answer. This includes both the appropriateness and sensitivity of the question. That is, the content of the question is within the cognitive ability of the student, by mobilizing experience, potential reasoning, association... they can answer the question. At the same time, the question does not touch on sensitive, secret issues that make students not want to answer. 2/ Give enough time for students to think and answer. Knowing how to wait for the necessary moment is a trick to encourage students to answer. If the time between asking a question and when the student answers is 1-2 seconds, then that is not enough waiting time to create efficiency. The teacher 's impatient attitude when urging students to answer immediately will actually make them feel that they do not need to answer, because answering is useless, and the teacher does not need their answer but only asks for formality. Marry Rowe (1990) has carefully researched and determined that the most reasonable waiting time in small classes is 5-6 seconds, in large classes such as university students is 3-5 seconds. That has a good effect on the students' reactions because: the length of the reaction time increases; increases the opportunity for voluntary reactions; reduces the possibility of shyness and hesitation; improves
persuasive because learners have time to think; enhance reasoning reactions because learners still have time to choose different directions of thinking; enhance reactions between learners; create conditions for collecting more evidence when reasoning and answering ; increase opportunities for learners to ask questions; create conditions for slow-responding learners to participate in time . 3/ Use body language (eye contact, smiling, nodding, ...) to encourage learners to answer. 4/ Praise or acknowledge the correct answers of learners. 5/ Avoid making learners " embarrassed " with their answers . 6/ Questions are given briefly, clearly, using easy-to-understand language. 7/ Distribute questions evenly throughout the class.
Often we have to use probing, diagnostic and guiding types of questions to help learners overcome impasses, redirect their thinking and reasoning, and feel secure and safe while participating in answering questions from the teacher and from their peers. If we can express our questions in the questions of the learners, all the better.
When you find that your questioning method is not getting a response from the learners, or that they are not enthusiastic about the content or implications of the question, you need to go back to combining questioning with visual demonstration of the material or skills, then the question must be based on visual and vivid events. To maintain this process, the teacher must promptly form and use supplementary and extended questions to both guide and consolidate the results that the learners have achieved.
After the question-and-answer process, the teacher needs to ask questions that are evaluative and collect feedback on the results and learning process that the learner performs . Evaluation has at least two aspects: appraisal and diagnosis, from which the necessary information is drawn. To grasp the current learning situation at that time, it is impossible to measure each learner, it is impossible to test each student according to each event, each idea, each topic, each concept and each skill , but it is necessary to use divergent, problematic questions , with a difficult option and an easy option combined together.
This question affects many learners at the same time, and only a few students need to answer , but the teacher can still grasp the general situation by observing the reactions of groups or the whole class. It allows to "scan" different levels from easy to difficult, so it can evaluate the average level. The question should be focused on a few key points of the lesson, especially the conceptual and application factors. If the learner
To grasp the theory and understand the practical application of things, of course, is not
There is nothing to worry about regarding the individual events in the article.
2) Discussion organization techniques
The key principles of discussion are: 1/ The meeting, direct contact between teachers and students , between students with each other organized by the teacher . 2/ The free exchange of ideas and feelings about a learning topic. If considering the nature, it can be divided into 3 types of discussion: controlled, semi-controlled; uncontrolled - showing the decreasing level of control from the teacher over the teaching and learning process and the increasing level of initiative of the student in the discussion. It is also possible to divide discussion models into two types: formal and informal. Formal discussion is a model that has been carefully planned and clearly conceived before implementation, while informal discussion is not planned and is carried out spontaneously. To use the teaching process, it is necessary to organize it in a formal discussion style. There are
The main discussion techniques commonly used are : 1/ Small group discussion; 2 /
Whole class discussion; 3/ Discussion and answers. Below is a general description of each organization technique.
discussion function above .
1- Small group discussion techniques
Organizing small group discussions for learners often aims at the following key objectives: 1/ Help learners better understand the content or topic of study; 2/ Develop each individual's critical thinking based on the common spirit and intelligence of the whole group; 3/ Enhance each person's sensitivity to the topic of study and to their fellow learners; 4/ Develop communication etiquette for giving and receiving ; 5/ Develop the ability to screen problems as well as problem-solving methods. The most typical requirements of this technique include:
- Small group size from 4 to 6 people. Group members demonstrate unity and positive interdependence , all acknowledge that there is a common problem that requires group effort and the solution depends on the efforts of each individual.
- The interaction between learners is high and continuous: no one has a monopoly on imposing ideas, no one is allowed to hide their thoughts. The main responsibility of the discussion leader is to maintain the discussion, explain the key points more clearly, clarify the places where learners are easily confused or distract the discussion, and prevent opinions.
extreme and encourage shy people to participate, not giving answers or many
ideas are directive and imposed.
- Free exchange of ideas : friendly, open exchange , continuous flow of ideas
unrestricted and natural way.
- There is a clear division of roles , the discussion leader must maintain working order , encourage maximum participation of members; the reporter must take notes and summarize the ideas of his group and other groups, and restructure the content to be more complete.
- Friendly team atmosphere, mutual concern and tolerant attitude.
- There is multi-directional communication between group members. 2- Large group discussion technique (whole class)
Here, the main idea is to exchange ideas between the whole class and the teacher. Under the organization and encouragement of the teacher, students actively participate in the exchange and discussion . In this process, the teacher only plays the role of a collaborator and supporter without imposing his way of thinking and viewpoints on the whole class. This technique has the following specific requirements:
- Create a sense of unity and positive interdependence between learners and
teacher
- Requires changes in classroom layout, to create an intimate atmosphere between students and teachers , between students with each other ; creates opportunities for students to have direct contact with teachers and classmates, stimulates excitement in exchanging ideas and personal opinions .
- Highly promote the teacher's efforts: not simply presenting or displaying knowledge, but requiring understanding of common rules of action, knowing clearly the steps that will follow each other in the discussion; ready to support behind the scenes so that the discussion does not get stuck; respecting the opinions of learners, creating equal opportunities in presenting ideas; being polite , impartial, friendly and supportive to all learners.
- Focus on developing intellectual dynamism and exploiting emotions and values
of each individual learner.
3- Discussion and answer techniques
This discussion technique is also called social discussion or panel discussion, it requires that the direct participants in the discussion be carefully selected.
The person tasked with answering the questions should be very knowledgeable about the topic, preferably an expert in the field. Sometimes excellent students can be chosen as answerers, but they need to be carefully guided and prepared . Characteristics of a question-and-answer discussion include:
- Speakers exchange and respond directly to each other, this process is a conversation, discussion is less strictly controlled in terms of time.
- The continuous interaction and continuity of ideas between speakers who are experts or knowledgeable figures (usually from 3 to 6 people) and a moderator who has deep knowledge of the topic of discussion, good communication skills, and sharp thinking.
- This is a discussion technique for special types of content, only topics that need to be debated and are relatively difficult for learners.
This discussion technique is usually conducted in the following main stages:
+ Planning the discussion: Selecting a topic or issue for discussion; then selecting and assigning tasks to the symposium members and the audience; holding preparatory meetings between speakers and the speakers; preparing situations and planning the conference based on general requirements; synthesizing and staging the whole.
+ Conducting a discussion : Presenting the topic content using different techniques;
encourage panelists' participation; guide learners through the process of asking questions of the presenters; discuss the speakers' ideas; summarize or provide final comments from each presenter.
+ Open discussion : Raise necessary questions for the audience to answer; clarify important points, contents, and details; stimulate reactions and opposing opinions ; complete the ideas that have been discussed.
2.2.5. Teaching model based on situational interaction - research
The situational teaching method - research is typical of the learning style by rational thinking (by intellectual activities or logical consciousness). This learning style is mainly based on the cognitive experience and rational thinking of the learner, by the intellectual activities of the individual. The core principle here is the relatively clear and high problem nature of the teaching and learning process. The problems in the learning content are organized so that they become the core elements of the teaching and learning situations. Such situations are essentially the direct learning environment of the learner created on the basis of the learning problem and contain
contains certain connections with the learner's experience and values , that is , with the learner's psychology. When these connections are established between the individual and the teaching situation , from that moment on, the problem situation appears in the learner , the beginning of thinking activities. Thus, in this teaching model, the main task and activity of the teacher is most clearly to create a direct teaching environment through designing teaching situations (didactic situations). Then, skillfully organize the transfer from this teaching situation (the situation designed by the teacher) to the learner's problem situation (the problem that appears in the learner's psychology). Next, step by step help, encourage, and motivate the learner to solve the problem situation and acquire the knowledge that needs to be acquired. Below will describe in detail the steps according to the organizational process of this teaching method model.
- Design teaching situations:
Modern teaching theory affirms that any teaching must be based on the problem nature within the teaching process and content. The problem nature conceived here is not limited to the category of thinking, but it can originate from emotions, specific states of needs, from aesthetic and ethical vibrations ... Of course, the most important objective basis of the problem nature is the learning problems in the learning content of the subject, lesson , topic. When the problem nature is subjectified by the learner, it becomes the problem of the individual learner (or the learner in a problematic situation), becoming the goal and motivation for the learner to act to solve or master them.
In teaching, teachers can create problem situations, or problems in learners through the design and organization of teaching situations. Here, in essence, teachers create a favorable learning environment , so that factors from the environment can most likely activate the learner's experience, creating some connection between experiences , especially the connection between those experiences and the learning problem. These connections act as a stimulant to the learner's need to explore and discover, so that they can solve their concerns and worries that can only be achieved through their own active physical and intellectual activities.
It should be clarified that the environment mentioned here is considered a micro-teaching environment, which corresponds to the teaching situation and is a combination of material elements (documents, visual aids, relationships, space, time, micro-landscape,



![Qos Assurance Methods for Multimedia Communications
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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 ports bandwidth is sufficiently large, combined with small buffer sizes and other network resources dedicated to EF packets, to allow the routers 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 routers per-hop behavior is based only on the packets 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 routers 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 connections 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
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