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The training process and the final factor (training outcome) will determine the training effectiveness.
In the context of competition between universities, each university must evaluate its own quality to make timely adjustments. In 1983, the US News and World Report newspaper, for the first time presented a list of
US university ranking
Maybe you are interested!
-
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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Directions for Improving State Management of Tourism Activities in Dak Nong Province -
State management of compensation, support and resettlement when the State acquires land in Buon Ho town, Dak Lak province - 12 -
Current Status of State Management Activities for Tourism Enterprises in Hanoi -
Factors Affecting the Training Activities to Improve State Administrative Management Capacity for Secondary School Principals of the Department of Education -
according to quality
high to low (America's Best

Colleges). Soon after, other media groups around the world such as Times Higher Education Supplement (THES in the UK), The Guardian University Guide (UK), Maclean's magazine with Maclean's University Ranking (Canada), etc. also began to develop criteria to evaluate local universities and compare them with international universities. In particular, in recent years, Shanghai Jiao Tong University (China) has also participated in the evaluation and ranking of universities around the world. The following are some of the criteria for evaluating the quality of foreign universities that we need to study and consider.
Table 1.6 Criteria used for university evaluation and ranking by groups
media implementation
Shanghai Jiao Tong University (China) Liu NC, Cheng, Y. Academic Ranking of World Universities – Methodologies and Problems. Higher Education in Europe 2005; 30(2), 14.
Number of alumni who won Nobel and Field Prizes: weight 10%
Number of professors winning Nobel and Field prizes: weight 20%
Number of scientific articles cited many times: weight 20%
Number of scientific articles in Nature and Science journals: weight 20%
Number of scientific articles in journals in SCIE, SSCI directory: weight 20%
Achievements of professors and academic staff: weight 10%
US News and World Report Stella A, Woodhouse D. Ranking of Higher Education Institutions (Vol. AUQA Occasional Publications Number 6): Australian Universities Quality Agency 2006.
Academic excellence through a survey of the principal and dean: weight 25%
Dropout rate and graduation rate: weight 20%
Facilities (class size, professor salary, professor qualifications, full-time professor ratio): weight 20%
Student selection score: weight 15%
School quota per student: weight 10%
Percentage of alumni contributing to the school budget: weight 5%
Graduation rate after adjusting for quota and admission score: weighted at 5% Times Higher Education Supplement (THES) Marginson S. Global university
52
comparisons: the second stage, International Trends in University Rankings and Classifications - Griffith University/ IRU Symposium 2007
Evaluation of academics from other schools: weight 40%
Number of graduates working in global companies: weight 10%
Percentage of foreign professors: weight 5%
Percentage of students who are foreign: weight 5%
Student / professor ratio: weight 20%
Number of citations per professor: weight 20%
Maclean University Ranking Macleans.ca. (2006, November 2006). Universities by the Number. Macleans.ca.
Student academic achievements: weight 23%
Class size and communication between professor and students: weight 17%
Qualification and reputation of the teaching staff and professors: weight 17%
Finance: weight 12%
Library: weight 12%
Alumni reputation: weight 19%
b) Experience in training program management
A major is a comprehensive programme of study. Meaningful programme design involves developing learning objectives for students, which are based on external inputs from stakeholders such as the job market and societal demands; and developing sub-specialisations that meet these objectives and provide students with the knowledge, skills and attitudes they need.
Most undergraduate programs in the United States are organized in a similar way, with slight variations from school to school. Typically, the first degree combines the university's broad requirements for communications, mathematics, natural sciences, general education, computers, and foreign languages, with the program's requirements for a true major, along with minors if applicable. These minors are also specialized courses but are not as in-depth as the major.
Recently, US universities have expressed concern about the lack of general analysis of the knowledge and skills of graduates, which should be the basis for the formation of specialized training programs at the departmental level. The fundamental difference between the US and European (Bologna) credit-based training systems is that in the European system, credits are considered a measure of the degree to which learning objectives are met (Heinze & Knill, 2008), so assessment of learning is an integral part. Recently, meaningful program design has been implemented in the Bologna system.
53
The US credit system as part of the adoption and application of the European credit system.
The African Conference on Education recommended that “Member States develop curricula that can meet the challenges of the widespread social changes and fundamental challenges that Africa is likely to face in the immediate future.” It also recommended that Member States believe that “investment in education is necessary provided that higher education institutions are oriented to meet the needs of society.”
The Dakar Conference also made a series of recommendations to guide higher education institutions in specific actions in the following areas:
- Integrated programmes need to be designed to seek strategies for creating a culture of peace and to address issues related to sustainable development (such as poverty reduction and environmental protection);
- Research must be linked to the needs of society;
- Training institutions need to affirm their mission, comprehensive orientations linked to national education programs and based on needs analysis;
- The educational program needs to clearly indicate the desired output and not simply be the content conveyed and reproduced or simply the names of the subjects;
- University training programs need to be organized with full participation.
enough of qualified lecturers and scientists working together in
A conducive environment ensures content relevant to Africa's development.
Curriculum development should be interdisciplinary, flexible, but within a coherent system of modules, credits and articulation, work experience recognition and the organisation of the academic year in semesters nationally and internationally. At the same time, this change points to self-managed learning, the role of coaching for lecturers, professional support services, investment in joint training and quality assurance mechanisms.
c) Managing financial activities
This is one of the most prominent issues in building a management mechanism.
54
Universities and colleges, in order to have the conditions to invest in expanding the scale and improving the quality of training, must build a transparent financial system, operate effectively , the current situation of countries in the world, the level of contribution of learners is increasing (Greater User-Pay). The increase in the number of learners and the actual cost per learner has forced most countries to come up with a solution to increase the level of contribution of learners through tuition fees. In addition to this increase, many countries have established a mechanism to allow learners to borrow and then pay it back gradually after graduation.
In addition, due to the increasingly large scale of State investment in universities,
with quality
(Growing Popularity of Performance Funding),
in many countries
Government investment in universities is no longer based on average or on the number of students entering the university, but is based primarily on the achievement of real indicators.
performance indicators show the ability of each
In the UK, for example, university funding is allocated based on results.
Audits of universities conducted by the Quality Assurance Agency for Education
Higher Education (QAA – Quality Assurance Agency for Higher Education).
In the UK there is a higher education system that enjoys institutional autonomy.
regime
very spacious, the state only
Managing schools through the allocation of resources
main. Universities and colleges have full rights to use the allocated funds without interference or inspection from the state. Students here are required to live in dormitories, forming a living and learning community under the management of the school. However, the limitation of education in the UK compared to other countries, the average cost of 15,000 USD per year for a student is too high, only well-off families can afford this huge amount.
The issue of studying the financial management of universities according to which model is reasonable, scientific, and suitable to the situation and characteristics of each country will bring positive effects to education and training. Currently, investment in higher education in the US is very focused, accordingly, the investment budget for education in the US is very high: in 1985 it was about 300 billion USD, in 1989 it was 353 billion USD, by 1999 it reached 653 billion USD, followed by an increase in the absolute amount of budget expenditure for higher education. Currently, the investment rate for education in the US accounts for about 7% of GDP, the total annual expenditure on education and training is approximately 1,000 billion USD, of which higher education accounts for more than 700 billion USD.
In China, the state budget is the main component of funding sources for higher education in China. Since 1994, the “three growth” requirement has been implemented, meaning that “the increase in the state budget for education must be higher than the increase in regular budget revenue to gradually increase the average education expenditure per student, ensure the increase in teachers’ salaries and increase the cost of using
55
Since then, China's education budget has been increasing continuously. However, the pressure of the need to expand the scale and improve the quality of higher education, combined with the need to invest in other areas, has forced the Chinese government to reduce the proportion of state budget investment in higher education and only increase the absolute amount of investment. Currently, China's education spending accounts for about 3.28% of GDP.
Mobilizing financial resources outside the state budget to invest in higher education is a common measure in countries around the world, from developed countries to underdeveloped countries, to help reduce the burden on the state budget and solve the problem of the need to expand the scale and improve the quality of higher education in the current period. Korea, Japan, and the US are countries with a higher rate of private contribution than the state budget contribution to higher education.
d) Management of enrollment work
Admissions to universities and colleges vary greatly from country to country in terms of procedures and priorities. In some countries, competition is based on simple entrance exams. In others, it is more complex, taking into account test scores, academic performance, aspirations, and even a prospective student's work outside of school. Some countries are particularly
also distinguishes the difference between public and private schools. Types
Current university and college admissions are in the following forms:
Type 1: Selection through high school graduation exam
Candidates for university and college must achieve a benchmark score in one or more final exams. These exams are usually conducted by country or region and are organized or oriented by the Government. Candidates can choose subjects to take either according to the general program or according to the university-college orientation program. The benchmark score for candidates can be based solely on exam scores or in some places, it can be combined with other factors such as high school grade point average. The organization of the exam can be by the government or other agencies. Universities can also organize this process according to their own selection criteria. Representative of the type 1 admission model are the education systems of Ireland and Tanzania.
Ireland: Before entering university, students need to take a national exam to
get certificate
high school graduate
Leaving Certificate is organized
by the Association
State Commission. Each university or college determines the number of students it will enroll in each program, but the admissions process is centrally coordinated by the Central Admissions Office. This is an agency independent of the schools. Candidates submit their applications to the Commission and
56
The computer automatically transfers the list of candidates to schools along with their wishes and exam scores.
Tanzania: Examinations and examinations for both public and private universities are coordinated by the Tanzania Commission on Universities and its member universities. Applicants apply directly to the universities of their choice (up to three). They must also submit their application to the Commission. The Commission will consider additional factors relevant to the applicant such as: gender, geographical location (remote areas), labor market requirements and other requirements of the national socio-economic situation.
Type 2: Selection through university entrance exam
Like type 1, entrance exams are usually organized by local authorities or the Government throughout the country. However, in some countries, entrance exams are organized by schools. They determine their own benchmarks and other standards. Like high school graduation exams, university entrance exams generally assess candidates' knowledge in high school subjects or can also be combined with some other factors. Representatives of this type 2 model are China and Serbia.
China: Candidates who want to enter university must take an entrance exam in one or two fields of engineering or social sciences. This is a national exam and is centrally administered by the Central Government. The government also determines the content.
Exam questions and admission quotas for each major and for each school. Candidates submit
wishes to the Examination Council. The examination results along with the candidates' wishes will be sent by the Government to the universities. Admissions are selected by the universities according to the wishes and on the basis of the candidates' exam scores.
Secbium: Number
amount of thread
Admission criteria for each school are determined by
National government. The government also determines the scholarship quotas. However, schools are free to organize and supervise their own exams in their own way. Schools also consider the average of the four years of high school along with the entrance exam scores.
Type 3: Selection through aptitude and cognitive testing
Aptitude tests are designed to measure a candidate's general cognitive ability rather than specific achievement. In the admissions process, schools often combine a variety of factors to measure the required knowledge. Representative of the third type of admissions model are Sweden and the United States.
Sweden: University applicants must take the Swedish Scholastic Aptitude Test. This test is administered by a
Government agency is the National Agency for Higher Education
57
Education). The admission process can be based on test scores or on high school grades. At least 1/3 of students are admitted based on test scores and at least 1/3 of students are admitted based on high school grades.
United States: Admission requirements and content at universities in the United States are different. Most schools consider the candidate's achievements through test scores such as SAT or ACT. SAT (standardized aptitude test), was formed in 1900 by the Eastern United States University Group to help candidates avoid having to take university entrance exams at many schools. The initial test method was essay, mainly assessing English and Math skills. The maximum score for 2 subjects is 800, the average score is
500. After 1926 the test was mainly multiple choice and changed to SAT
(Scholastic Achievement Test). Since 1994, the SAT has also increased the choice for candidates with the SAT II for students who want to choose the additional essay section. The ACT (American College Testing Program) is the second option for US candidates, organized in 1959 by schools in the Western United States and popularized throughout the United States since 1960. This is also a multiple-choice test, assessing 4 abilities of English, Mathematics, Reading and Science. The maximum score is 36, the average is
20. In addition, ACT also provides a set of questions to assess the candidate's aptitude and strengths, thereby advising the candidate on which school and major is suitable. After graduating from high school, students send applications to 5-6 universities, along with their transcripts and SAT or ACT test results. Based on these results, the schools can make selections. Some universities, especially elite schools such as (Havard, Yall) also require a few other procedures such as essays, recommendation letters, interviews and in some cases, also consider aptitude.
Type 4: Selection through organizing many exams
In this area, the primary consideration is the results of the high school graduation exam or the university entrance exam. In addition, one or more additional exams are organized. These exams can be organized by the government, universities or other independent organizations. Representatives of this group 4 are the educational models of Israel and India.
Israel: The government determines the minimum score on the Secondary School Leaving Examinations for admission to university. In addition, applicants are required to take a Psychometric Entrance Test and a Standardized Aptitude Exam, administered by the National Institute for Testing and Evaluation, a non-governmental, non-profit agency.
India: Candidates are admitted to Indian universities on the basis of their scores in the secondary school leaving and university entrance examinations. These examinations are conducted by
58
many muscles
various agencies including the Government
Central
Central, Provincial Government,
Universities and groups of universities. Each university sets its own requirements for the exams that candidates take, for example, although they all take the same foreign language exam, each university (or each major) has different weights and benchmarks.
Type 5: Selection but no exam
There are countries in the world that do not organize university entrance exams. They only consider the candidates' achievements in high school. Procedures that do not require exams when entering university have begun to appear in private schools in many different countries, especially the United States. Typical examples of this type 5 include Norway and some American universities.
Norway: The admission process at Norwegian universities is centrally coordinated by the
Government agency: Norwegian Universities and
Colleges Admission Service). Applicants may register up to 10 choices. They are selected based on their high school qualifications, plus bonus points for specific fields of study, geographic region, or military service experience.
Some US Universities: Since the mid-1980s, some US universities have adopted a “SAT optional” policy. This means that in actual admissions, in some cases, to ensure fairness, equality, reasonableness and many other values, the consideration of the SAT test results of applicants can be flexible, not rigidly following the standards.
For more details we can refer to the following table:

![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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