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Fat AP and Cloud AP V200R008C00 CLI-based Configuration Guide

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Huawei uses machine translation combined with human proofreading to translate this document to different languages in order to help you better understand the content of this document. Note: Even the most advanced machine translation cannot match the quality of professional translators. Huawei shall not bear any responsibility for translation accuracy and it is recommended that you refer to the English document (a link for which has been provided).
Principles

Principles

WMM

Background

Before learning WMM, you must understand 802.11 link layer transport mechanism.

802.11 MAC layer uses the coordination function to determine the data transmitting and receiving methods used between STAs in a BSS. 802.11 MAC layer consists of two sub-layers:
  • Distributed Coordination Function (DCF): uses the CSMA/CA mechanism. STAs compete channels to obtain the authority to transmit data frames.
  • Point Coordination Function (PCF): uses centralized control to authorize STAs to transmit data frames in turn. This method prevents conflict.
NOTE:

In 802.11 protocol, DCF is mandatory, and PCF is optional.

Figure 27-21 shows how CSMA/CA is implemented.
Figure 27-21  CSMA/CA working mechanism

  1. Before sending data to STA B, STA A detects channel status. When detecting an idle channel, STA A sends a data frame after Distributed Inter-Frame Space (DIFS) times out and waits for a response from STA B. The data frame contains NAV information. After receiving the data frame, STA B updates the NAV information, indicating that the channel is busy and data transmission will be delayed.
    NOTE:

    According to 802.11 protocol, the receiver must return an ACK frame each time it receives a data frame.

  2. STA B receives the data frame, waits until Short Interframe Space (SIFS) times out, and sends an ACK frame to STA A. After the ACK frame is transmitted, the channel becomes idle. After the DIFS times out, the STAs use the exponential backoff algorithm to compete channels. The STA of which the backoff counter is first reduced to 0 starts to send data frame.

Concepts

  • InterFrame Space (IFS): According to 802.11 protocol, after sending a data frame, the STA must wait until the IFS times out, and then sends the next data frame. The IFS length depends on the data frame type. The high-priority data frames are sent earlier than the low-priority data frames. There are three IFS types:
    • Short IFS (SIFS): It is the time interval between a data frame and its ACK frame. SIFS is used for high priority transmissions, for example, transmissions of ACK and CTS frames.
    • PCF IFS (PIFS): PCF-enabled access points wait for PIFS duration rather than DIFS to occupy the wireless medium. PIFS length is SIFS plus slot time. If a STA accesses a channel when the slot time starts, the other STAs in the BSS detect that the channel is busy when the next slot time starts.
    • DCF IFS (DIFS): Data frames and management frames are transmitted at the DIFS interval. DIFS length is PIFS plus slot time.
  • Contention window: backoff time. When multiple STAs need to transmit data and detect that all channels are busy, the STAs use the backoff algorithm. That is, the STAs wait for a random number of slot times, and then transmit data. Backoff time is a multiple of slot time, and its length depends on the physical layer technology. An STA detect channel status at the interval of slot time. When detecting an idle channel, the STA starts the backoff timer. If all channels become busy, the STA freezes the remaining time in the backoff timer. When a channel becomes idle, the STA waits until DIFS times out, and continues the backoff timer. When the backoff timer is reduced to 0, the STA starts to send data frames. Figure 27-22 shows the data frame transmission process.

    Figure 27-22  Backoff algorithm diagram

    1. STA C is occupying a channel to send data frames. STA D, STA E, and STA F also need to send data frames. They detect that the channel is busy, so they wait.
    2. After STA C finishes data frame transmission, the other STAs wait until DIFS times out. When DIFS times out, the STAs generate random backoff time and start their backoff timers. For example, the backoff time of STA D is t1, the backoff time of STA E is t1+t3, and the backoff time of STA F is t1+t2.
    3. When t1 times out, the backoff timer of STA D is reduced to 0. STA D starts to send data frames.
    4. STA E and STA F detect that the channel is busy, so they freeze their backoff timers and wait. After STA D completes data transmission, STA E and STA F wait until DIFS times out, and continue their backoff timers.
    5. When t2 times out, the backoff timer of STA F is reduced to 0. STA F starts to send data frames.
Principles

Channel competition is based on DCF. To all STAs, the DIFS is fixed and backoff time is random; therefore, all the STAs fairly compete channels. WMM is an enhancement to 802.11 protocol. It makes channel competition unfair.

  • EDCA parameters

    WMM defines a set of Enhanced Distributed Channel Access (EDCA) parameters, which distinguish high priority packets and enables the high priority packets to preempt channels.

    WMM classifies data packets into four access categories (ACs). Table 27-18 shows the mappings between ACs and 802.11 user preferences (UPs). A large UP value indicates a high priority.
    Table 27-18  Mappings between ACs and UPs
    UP AC
    7 AC_VO (Voice)
    6
    5 AC_VI (Video)
    4
    3 AC_BE (Best Effort)
    0
    2 AC_BK (Background)
    1

    Each AC queue defines a set of EDCA parameters, which determine the capability of occupying channels. These parameters ensure that the high priority ACs have higher probability to preempt channels than low priority ACs.

    Table 27-19 describes the EDCA parameters.
    Table 27-19  EDCA parameter description

    Parameter

    Meaning

    Arbitration Inter Frame Spacing Number (AIFSN)

    The DIFS has a fixed value. WMM provides different DIFS values for different ACs. A large AIFSN value means that the STA must wait for a long time and has a low priority.

    Exponent form of CWmin (ECWmin) and Exponent form of CWmax (ECWmax)

    ECWmin specifies the minimum backoff time, and ECWmax specifies the maximum backoff time. They determine the average backoff time. Large ECWmin and ECWmax values mean that the average backoff time for the STA is long and the STA priority is low.

    Transmission Opportunity Limit (TXOPLimit)

    After preempting a channel, the STA can occupy the channel within the period of TXOPLimit. A large TXOPLimit value means that the STA can occupy the channel for a long time. If the TXOPLimit value is 0, the STA can send only one data frame every time it preempts a channel.

    As shown in Figure 27-23, the AIFSN (AIFSN[6]) and backoff time of voice packets are shorter than those of Best Effort packets. When both voice packets and Best Effort packets need to be sent, voice packets can preempt the channel.

    Figure 27-23  WMM working mechanism

  • ACK policy

    WMM defines two ACK policies: normal ACK and no ACK.

    • Normal ACK: The receiver must return an ACK frame each time it receives a unicast packet.

    • No ACK: The receiver does not need to return ACK frames after receiving packets. This mode is applicable to the environment that has high communication quality and little interference.

      NOTE:
      • The ACK policy is only valid to APs.
      • If communication quality is poor, the no ACK policy may cause more packets to be lost.

Priority Mapping

Packets of different types have different priorities. For example, the 802.11 packets sent by STAs carry user priorities or DSCP priorities, VLAN packets on the wired networks carry 802.1p priorities, and IP packets carry DSCP priorities. Priority mapping must be configured on network devices to retain priorities of packets when the packets traverse different networks.

Figure 27-24  Priority mapping diagram
As shown in Figure 27-24:
  1. In the upstream direction: The RU maps the user or DSCP priority of 802.11 packets received from STAs to the DSCP priority of tunnel packets.
  2. In the downstream direction: The central AP forwards 802.3 packets received from the Internet to the RU through a tunnel. The RU maps the DSCP priority of the 802.3 packets to the user priority of 802.11 packets.
Precedence field

As defined in RFC 791, the 8-bit ToS field in an IP packet header contains a 3-bit IP precedence field. Figure 27-25 shows the Precedence field in an IP packet.

Figure 27-25  IP Precedence field

Bits 0 to 2 constitute the Precedence field, representing precedence values 7, 6, 5, 4, 3, 2, 1 and 0 in descending order of priority.

Apart from the Precedence field, a ToS field also contains the following sub-fields:

  • Bit D indicates the delay. The value 0 represents a normal delay and the value 1 represents a short delay.

  • Bit T indicates the throughput. The value 0 represents normal throughput and the value 1 represents high throughput.

  • Bit R indicates the reliability. The value 0 represents normal reliability and the value 1 represents high reliability.

DSCP Field

RFC 1349 initially defined the ToS field in IP packets and added bit C. Bit C indicates the monetary cost. Later, the IETF DiffServ Working Group redefined bits 0 to 5 of a ToS field as the DSCP field in RFC 2474. In RFC 2474, the field name is changed from ToS to differentiated service (DS). Figure 27-25 shows the DSCP field in packets.

In the DS field, the first six bits (bits 0 to 5) are the DS Code Point (DSCP) and the last two bits (bits 6 and 7) are reserved. The first three bits (bits 0 to 2) are the Class Selector Code Point (CSCP), which represents the DSCP type. A DS node selects a Per-Hop Behavior (PHB) based on the DSCP value.

802.1p Field

Layer 2 devices exchange Ethernet frames. As defined in IEEE 802.1Q, the PRI field (802.1p field) in the Ethernet frame header identifies the Class of Service (CoS) requirement. Figure 27-26 shows the PRI field in Ethernet frames.

Figure 27-26  802.1p field in the Ethernet frame with VLAN tags

The 802.1Q header contains a 3-bit PRI field, representing eight service priorities 7, 6, 5, 4, 3, 2, 1 and 0 in descending order of priority.

Traffic Policing

Traffic policing discards excess traffic to limit the traffic within a specified range and to protect network resources as well as the enterprise benefits.

Traffic policing is implemented using the token bucket.

A token bucket has specified capacity to store tokens. The system places tokens into a token bucket at the configured rate. If the token bucket is full, excess tokens overflow and no token is added.

When assessing traffic, a token bucket forwards packets based on the number of tokens in the token bucket. If there are enough tokens in the token bucket for forwarding packets, the traffic rate is within the rate limit. Otherwise, the traffic rate is not within the rate limit.

The working mechanisms of token buckets include single rate single bucket, single rate dual bucket, and dual rate dual bucket.

Single Bucket at a Single Rate

If burst traffic is not allowed, that is, one token bucket is used.

Figure 27-27  Single bucket at a single rate

As shown in Figure 27-27, the bucket is called bucket C. Tc indicates the number of tokens in bucket C. A single bucket at a single rate uses the following parameters:
  • Committed Information Rate (CIR): indicates the rate at which tokens are put into bucket C, that is, the average traffic rate permitted by bucket C.
  • Committed burst size (CBS): indicates the capacity of bucket C, that is, maximum volume of burst traffic allowed by bucket C each time.

The system places tokens into the bucket at the CIR. If Tc is smaller than the CBS, Tc increases. If Tc is greater than or equal to the CBS, Tc remains unchanged.

B indicates the size of an arriving packet:
  • If B is smaller than or equal to Tc, the packet is colored green, and Tc decreases by B.
  • If B is greater than Tc, the packet is colored red, and Tc remains unchanged.
Dual Buckets at a Single Rate

Dual buckets at a single rate use A Single Rate Three Color Marker (srTCM) defined in RFC 2697 to assess traffic and mark packets in green, yellow, and red based on the assessment result.

Figure 27-28  Dual buckets at a single rate

As shown in Figure 27-28, the two buckets are called bucket C and bucket E. Tc indicates the number of tokens in bucket C, and Te indicates the number of tokens in bucket E. Dual buckets at a single rate use the following parameters:
  • CIR: indicates the rate at which tokens are put into bucket C, that is, average traffic rate permitted by bucket C.
  • CBS: indicates the capacity of bucket C, that is, maximum volume of burst traffic allowed by bucket C each time.
  • Excess burst size (EBS): indicates the capacity of bucket E, that is, maximum volume of excess burst traffic allowed by bucket E each time.
The system places tokens into the bucket at the CIR:
  • If Tc is smaller than the CBS, Tc increases.
  • If Tc is equal to the CBS and Te is smaller than the EBS, Te increases.
  • If Tc is equal to the CBS and Te is equal to the EBS, Tc and Te do not increase.
B indicates the size of an arriving packet:
  • If B is smaller than or equal to Tc, the packet is colored green, and Tc decreases by B.
  • If B is larger than Tc and smaller than or equal to Te, the packet is colored yellow and Te decreases by B.
  • If B is larger than Te, the packet is colored red, and Tc and Te remain unchanged.
Dual Buckets at Dual Rates

Dual buckets at dual rates use A Two Rate Three Color Marker (trTCM) defined in RFC 2698 to assess traffic and mark packets in green, yellow, and red based on the assessment result.

Figure 27-29  Dual buckets at dual rates

As shown in Figure 27-29, the two buckets are called bucket P and bucket C. Tp indicates the number of tokens in bucket P, and Tc indicates the number of tokens in bucket C. Dual buckets at dual rates use the following parameters:
  • Peak information rate (PIR): indicates the rate at which tokens are put into bucket P, that is, maximum traffic rate permitted by bucket P. The PIR must be greater than the CIR.
  • CIR: indicates the rate at which tokens are put into bucket C, that is, average traffic rate permitted by bucket C.
  • Peak burst size (PBS): indicates the capacity of bucket P, that is, maximum volume of burst traffic allowed by bucket P each time.
  • CBS: indicates the capacity of bucket C, that is, maximum volume of burst traffic allowed by bucket C each time.
The system places tokens into bucket P at the PIR and places tokens into bucket C at the CIR:
  • If Tp is smaller than the PBS, Tp increases. If Tp is larger than or equal to the PBS, Tp remains unchanged.
  • If Tc is smaller than the CBS, Tc increases. If Tc is larger than or equal to the CBS, Tp remains unchanged.
B indicates the size of an arriving packet:
  • If B is larger than Tp, the packet is colored red.
  • If B is larger than Tc and smaller than or equal to Tp, the packet is colored yellow and Tp decreases by B.
  • If B is smaller than or equal to Tc, the packet is colored green, and Tp and Tc decrease by B.
Implementation of Traffic Policing
Figure 27-30  Traffic policing components

As shown in Figure 27-30, traffic policing involves the following components:

  • Meter: measures the network traffic using the token bucket mechanism and sends the measurement result to the marker.

  • Marker: colors packets in green, yellow, or red based on the measurement result received from the meter.

  • Action: performs actions based on packet coloring results received from the marker. The following actions are defined:

    • Pass: forwards the packets that meet network requirements.

    • Remark + pass: changes the local priorities of packets and forwards them.

    • Discard: drops the packets that do not meet network requirements.

    By default, green and yellow packets are forwarded, and red packets are discarded.

If the rate of a type of traffic exceeds the threshold, the device reduces the packet priority and then forwards the packets or directly discards the packets based on traffic policing configuration. By default, the packets are discarded.

Airtime Scheduling

Overview

Airtime scheduling schedules channel resources based on channel occupation time of users connected to the same radio. In this way, each user is assigned equal time to occupy the channel, ensuring fairness in channel usage.

On a WLAN, the physical layer rates of users have great differences due to different radio modes supported by the terminals or radio environment where the terminals reside. If the users with lower physical layer rates occupy wireless channels for a long period, user experience of the entire WLAN is affected. When airtime scheduling is enabled, users on the WLAN equally occupy the wireless channel. This improves the overall user experience when high- and low-speed users are connected concurrently.

Principles

After airtime scheduling is enabled, the device collects statistics on the time within which each user occupies a wireless channel for sending packets on the same radio, calculates the total sum of time that each user occupies the wireless channel, and sequences the STAs in ascending order of channel occupation time.

Compared with traditional scheduling modes, airtime scheduling provides the following additional functions:
  • Inserts new users to specified positions according to the users' wireless channel occupation time. In traditional scheduling modes, the new users are placed to the end of the user queue.
  • Checks whether a user continues to send data after the user finishes sending the first queue of data. If no, the device directly schedules channel resources for the second user. If yes, the user is inserted into the queue according to the user's wireless channel occupation time and the device preferentially schedules channel resources for the user with the shortest channel occupation time.
Figure 27-31 shows the airtime scheduling process.
Figure 27-31  Airtime scheduling process
There are four users on a radio waiting to transmit data: User1, User2, User3, and User4. The four users have occupied the channel for a time of 3, 4, 6, and 7 respectively, and require a corresponding time of 2, 4, 6, and 7 for a round of data transmission.
  1. After airtime scheduling is enabled, the device collects channel occupation time of the four users. Channel occupation time of User1, User2, User3, and User4 becomes 3, 4, 6, and 7 respectively. User1 occupies the channel for the shortest time; therefore, the device allocates channel resources to User1 first.
  2. It takes a time of 2 for User1 to finish a round of data transmission. The channel occupation time of User1 increases to 5. Channel occupation time of User1, User2, User3, and User4 becomes 5, 4, 6, and 7 respectively. User2 occupies the channel for the shortest time; therefore, data of User2 is preferentially transmitted.
  3. It takes a time of 4 for User2 to finish a round of data transmission. The channel occupation time of User2 increases to 8. Channel occupation time of User1, User2, User3, and User4 becomes 5, 8, 6, and 7 respectively. User1 occupies the channel for the shortest time; therefore, the device preferentially schedules channel resources for User1.
  4. If User1 finishes all data transmissions, the device collects only the channel occupation time of the remaining users. Channel occupation time of User2, User3, and User 4 is 8, 6, and 7 respectively. The channel occupation time of User3 is the smallest; therefore, data of User3 is preferentially transmitted.
  5. It takes a time of 6 for User3 to finish a round of data transmission. The channel occupation time of User3 increases to 12. Channel occupation time of User2, User3, and User4 becomes 8, 12, and 7 respectively. User4 occupies the channel for the shortest time; therefore, channel resources are preferentially scheduled for User4.
The device preferentially schedules channel resources for the user that occupies the channel for the shortest time. In this way, each user is assigned equal time to occupy the channel, ensuring fairness in channel usage.

To prevent that the first access users fail to occupy the wireless channels for transmitting data, the device periodically clears all users' wireless channel occupation time. In this way, all access users have the same occupation weight.

After WMM is enabled on the device and terminals, user packets are scheduled based on different types (service types include VI, VO, BE, and BK). For example, voice packets are scheduled only with other voice packets, and video packets are scheduled only with other video packets.
NOTE:
If the packets of multiple users are of different types, airtime scheduling does not take effect. For example, two users perform packet transmission: one transmits voice packets and the other transmits video packets. In this case, airtime scheduling is not performed for the two users.

ACL-based Simplified Traffic Policy Configuration

The device to which an ACL-based simplified traffic policy is applied matches packet characteristics with ACLs and provides the same QoS for packets matching ACL rules, implementing differentiated services.

To control traffic entering a network, configure an ACL to match information such as the source IP address, fragment flag, destination IP address, source port number, and source MAC address and then configure an ACL-based simplified traffic policy so that the device can filter packets or priority remarking matching ACL rules.

Compared with a traffic policy based on traffic classifiers, an ACL-based simplified traffic policy is easy to configure because you do not need to configure a traffic classifier, traffic behavior, or traffic policy independently. However, an ACL-based simplified traffic policy defines less matching rules than a traffic policy based on traffic classifiers.

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Updated: 2019-01-11

Document ID: EDOC1000176006

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