Casino hotels: The largest service that brings the highest revenue for this type is gambling (Cassino). These hotels are very luxurious with many famous types to attract customers. The food, accommodation and drinking services at this hotel are mainly to serve the gamblers. Many famous varieties with casino hotel systems such as Las Vegas (USA), Monte-Carlo (Manaco), Macau (Special Administrative Region of China) ...
Resort hotels: This type in Vietnam is often called a resort. It is usually located far from the city center, in coastal areas, mountainous areas, areas with beautiful landscapes, with many types of rich and diverse entertainment services.
Special types of tourist hotels: Special types of hotels include hotels specializing in health care and fitness (Heslth spas), hotels specializing in serving sports activities (Sport hotels) such as skiing, mountain climbing, sea sports and diving...
City center hotels (City center hotels/ downtown hotels): This type of hotel is built in the centers of large cities, residential areas, and crowded urban areas to serve customers traveling for business, conferences, seminars, shopping, visiting relatives, cultural sightseeing, etc.
Maybe you are interested!
-
Applying Vietnam tourism occupational skills standards VTOS to research on security and safety in 4-star hotels in Hai Phong - 20 -
![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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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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Organize training activities for preschool teachers according to professional standards -
Improving the competitiveness of Vietnamese hotels in the coming time - 14 -
List of Universities, Colleges and Vocational Schools Training in Tourism, Restaurants and Hotels in the Northern Economic Zone Provinces.
Suburbs hotels: Usually built on the outskirts of the city or near a tourist attraction such as a temple, pagoda, shrine, located outside the city. The main target customers are weekend travelers, tourist groups traveling to suburban areas or tourists visiting tourist attractions.
Highway/interstate hotel: Highway hotel is a general term that includes highway hotels, motels, motor inns, and motor lodges... This type is built along national highways to serve customers traveling on national highways and interstates using cars and motorbikes as means of transportation.
Airport hotels: Built near major international airports. The main target of these types of hotels are flight crew members and passengers of airlines that stop over at airports.
1.2.5 Classification by link (Affiliation)
This classification is also called by many textbooks as the classification by "ownership". According to Bardi (2003:11), this classification includes the following types:
Independent hotels: Independent hotels are usually managed by an investor, an individual, a group of individuals, or a company. It is not related to the brand of any corporation.
Chain affiliations: These are hotels in a group of hotels that have the same (uniform) standard operating procedures such as branding, marketing, reservations, accounting, management methods, operating standards, layout, business processes, room service quality, restaurants and other services, etc. Currently, there are many famous hotel chains in the world such as InterContinental, Marriott, Sheraton, Sofitel, Hyatt, Hilton, Econc Lodge, etc. Chain affiliations have many specific forms:
Franchise: a hotel can sign a contract with a hotel corporation (also known as a franchise corporation) to receive a franchise to use the brand or receive support in management, business processes, development orientation, promotion, etc. In return, they must pay a fee for this franchise.
Management contract: a hotel can sign a management contract with a hotel group or a hotel management company. Depending on the terms of the contract, these hotels can receive support in terms of management, staff training, promotional activities, marketing, strategic orientation, etc. In return, they also have to pay for this management.
Company-owned hotels: These hotels are owned and operated by a hotel chain. These hotels are owned and operated by a hotel chain. These hotels are allowed to manage their operations as an independent hotel. Of course, these hotels enjoy the privileges of the chain such as the brand, management processes and technology, the chain's reservation system, etc.
Referral Hotel: A hotel that receives referrals from a hotel chain. These hotels are often contracted with the hotel chain to receive referral support.
1.2.6 Some other classification methods
Classification by basic terminology (Basic Terminology): With this classification, types are often distinguished such as: hotels, motels, motor inns, resorts, privately owned housing (Weissiger, 2000: 33) or hotels, motels, youth hotels, privately owned inns (Pension), bed and breakfast (B & Bs) etc. Currently, many people in the US still understand according to this classification (Foster, 1999).
Classification by size: As mentioned above, classification by room size of hotels was quite popular before 1970. Nowadays, this classification standard is only for reference. For the world, the division of hotels by room size (Weissinger, 2000) is as follows:
Large hotel has over 500 rooms.
A medium-large hotel has from 200 to 500 rooms. 
A medium-sized hotel has nearly 100 to 200 rooms.
Small hotels have about 100 rooms or less.
In addition to the above classifications, there are many other classifications such as depending on the business time of the hotel (hotels operate year-round or seasonally); depending on the way of serving food and drink in the hotel (hotels with and without food service); according to the purpose of the guest's visit such as: business hotels, holiday hotels, convention hotels, purely tourist hotels (tourist hotels)...
1.3 Hotel Rating
1.3.1 Ranking of hotels and restaurants in the world
According to Nguyen Quyet Thang (2013), in many countries in the past, people had different rating symbols. In the US, the American Automobile Association (AAA) ranked customers from 1 to 5 “diamonds”. In Bulgaria
Previously, people classified hotels into 5 categories: special, category I, II, III and IV. Some other countries used alphabetical symbols, such as Czechoslovakia, which used to have 5 hotel categories: A+ Deluxe, A+, B+, B, C; Hungary had 5 hotel categories: A1, A2, B, C1, C2…
Or combine letters and numbers like the former Soviet Union had 7 hotel classes: Special, A, B, class 1, 2, 3 and 4... In 1958, Forbes Travel Magazine (formerly known as Mobil Travel) first introduced the star rating system and this symbol has become popular today. In France, in the past, when evaluating and ranking hotels, people used the "4-star" system to rank (plus "L" for Luxus). Since 2009, the ranking system in France has been converted to the "5-star" system like other countries. However, not all countries rank hotels from 1 star to 5 stars, each country may have its own ranking method.
Facing the general trend and the requirement of international integration, more and more countries have used the "star" rating (usually from 1 to 5 stars). Since 1962, the World Tourism Organization (UNWTO) and other international organizations have been trying to unify hotel standards among member countries. In Europe, under the auspices of Hotels, Restaurants & Cafés In Europe (HOTREC), hotel associations of Austria, the Czech Republic, Germany, Hungary, the Netherlands, Sweden and Switzerland have created the "Hotelstars Alliance" to build a hotel rating and classification system. This system has been effective for member countries since January 1, 2010, except for Hungary, Switzerland and the Netherlands, which use different dates to apply this standard. First of all, talking about the basis for hotel rating, in most countries it is often based on the following 4 basic groups of requirements:
Architectural requirements, number of hotel rooms.
Requirements for equipment and facilities in the hotel.
Requirements on the number and qualifications of hotel service staff.
Requirements for quality and diversity of services provided at the hotel.
1.3.2 Standards for classifying and ranking hotels in Vietnam
According to Nguyen Quyet Thang (2013), although Vietnam's tourism industry was born in 1960, it was in the difficult circumstances of the prolonged war and the subsidy period that followed.
Hotel construction and business in Vietnam were also deeply affected. Tourism activities only really developed from 1990 onwards.
Along with the open-door policy of the state, the number of international tourists to Vietnam has increased rapidly. Faced with the rapid increase in the number of visitors to Vietnam, to ensure integration into the world and maintain and develop the system of accommodation facilities serving tourists. Since 1990, the Vietnam National Administration of Tourism has recognized the necessity and urgency of building a system of scientific standards and norms to serve the unified management of hotel facilities in the entire industry. The standard for ranking tourist hotels in Vietnam was first issued on June 22, 1994 under Decision No. 107/TCDL of the Vietnam National Administration of Tourism and this standard was amended and supplemented in Decision No. 02/2001/QD-TCDL dated April 27, 2001 of the General Director of the General Department of Tourism. This is a mandatory standard applied to all hotels. (NQThang, 2013)
According to Nguyen Quyet Thang (2013), the classification standards for Vietnamese tourist hotels are built on the basis of the minimum standards of the Hotel Classification Project in the Asia-Pacific region (PATA) of the World Tourism Organization (UNWTO) combined with references to many rules and regulations for hotel classification of a number of countries in Europe, Asia and the opinions of many foreign experts, with amendments and supplements to suit the reality of Vietnam. In general, the classification standards for Vietnamese hotels have met the basic criteria with the international standards, reflecting the appropriate quality and making a positive contribution to the development of the hotel industry in Vietnam.
The standards for classifying and ranking Vietnamese hotels are divided into 2 parts:
Hotel classification: The standard divides tourist hotels into 3 types: City hotels; Resort hotels; Transit hotels.
Hotel ranking: Regarding ranking, tourist hotels are ranked from 1 star to 5 stars. Vietnam's tourism industry applies hotel ranking standards equally to evaluate the quality of all hotels in Vietnam, without exception. In which, hotel quality is determined through 5 groups of requirements:
Requirements on planning and architectural location.
Requirements for equipment and facilities. 
Requirements for hotel services.
Service staff requirements. 
Hygiene requirements.
Depending on the proposed class and actual quality; based on the issued ranking standards, the General Department of Tourism and the Department of Tourism will assign scores appropriate to the quality, type and business efficiency of each hotel. Accordingly, the General Department of Tourism organizes the assessment and decides to recognize 3-star, 4-star, 5-star hotels; the Department of Culture, Sports and Tourism (the Department of Tourism or the Department of Trade - Tourism before) assesses and recognizes 1-star, 2-star hotels. Thus, in addition to the first 4 basic groups of requirements applied internationally, Vietnam has chosen a 5th group of requirements, which is hygiene requirements. (NQThang, 2013)
1.4 Hotel business operations
1.4.1 Hotel business concept
“Hotel business is a business activity based on providing accommodation, food and beverage services and additional services to guests to meet their needs at tourist destinations and bring economic benefits to the business establishment” (Nguyen Van Manh - National Economics University, Hanoi).
According to Dang Thanh Vu (2013), hotel business is an activity that includes two types of accommodation services and additional services. Additional service business activities are activities that provide services that are not in the form of serving people's daily essential needs. These are services that increase the needs of daily life such as entertainment; enjoying culture; art; playing sports. As life becomes more and more advanced, people's demand for services is increasing and is always of interest to hotel managers to exploit. In fact, any hotel that organizes the provision of services well will have a very large revenue from this activity, even larger than the provision of food and beverage services. Nowadays, when social life is increasingly
The higher the demand for additional services, the more diverse the additional services in the hotel.
1.4.2 Content and nature of hotel business activities
The main business of the hotel is accommodation services (main service). In addition, the need for food and drink is a very important content in the hotel business. However, of these two types, accommodation services are the most basic. In addition to the above two contents, the hotel also does business in many other types of services such as organizing entertainment activities, selling souvenirs (additional services) ... Here, the hotel not only does business in goods and services directly produced by the hotel but also sells some services and goods produced by other industries such as telephone services, car rental, buying airline tickets, train tickets, sightseeing tickets ... For guests, the hotel plays the role of a sales agent.
The nature of hotel business: The nature of the hotel business is the business of accommodation and food services. Currently, along with the development of the tourism industry from different perspectives and the competition in attracting guests, competition in hotels is constantly expanding and diversifying. In addition to the two basic services, businesses have organized other activities such as organizing conferences, seminars and exhibitions, providing entertainment services... And as we know, the demand for services is highly seasonal, so to reduce seasonality in the hotel tourism business, there must be appropriate methods in operation, it is necessary to build and apply a comprehensive program to limit the impact of seasonality. Among the above services, there are services produced by the hotel to provide to guests such as hotel services, food and beverage, entertainment, etc. There are services that the hotel acts as an agent to sell to other establishments such as: drinks, telephone, laundry, etc. Among the services that the hotel provides to guests, there are services and goods that guests have to pay for, and there are services and goods that guests do not have to pay for such as luggage storage services, luggage porter services, etc. The products of the hotel industry are mainly services and partly goods. In the hotel, technical facilities and services have a close relationship with each other. The products of the hotel industry are a combination of products and the participation of the service
Staff are two indispensable factors in hotel business operations. For guests, providing service is one of the important standards of the hotel.
1.4.3 Characteristics of hotel business
Hotel business is a service business, the products created mainly require contact between people, so it has its own unique characteristics:
Hotel business depends on tourism resources. Because hotels are temporary residences, stops for guests, the number of guests when they participate in tourism activities, sightseeing, resting, and entertainment in places with tourism resources.
Hotel business has a large direct labor force, and the main product of the hotel is service, so it requires a large labor force.
In hotels, working hours are extremely mentally stressful, employees work 24/7, and the work is highly specialized, so it also requires a large number of replacement workers to ensure product quality as well as the health of workers.
Hotel business requires a very high amount of initial investment and fixed investment, hotel business is cyclical. It operates according to the tourist season, because hotel business only exists and develops according to the needs of visitors, they need beautiful resources, stable weather and climate. We cannot change the laws of nature, the laws of physiology, so this system is cyclical.
1.4.4 The significance of hotel business
Economic
Is one of the main activities of the tourism industry and performs important tasks of the industry.
Through the hotel accommodation and catering business, a part of the people's consumption fund is used for the consumption of services and goods of hotel businesses at tourist destinations. Therefore, the hotel business also increases the GDP of the region and of the whole country - Hotel business