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Configuration Guide - WLAN-AC

S5700 and S6720 V200R012C00

This document describes the configurations of WLAN, including WLAN Service Configuration, Radio Resource Management, Roaming, WLAN QoS, WLAN Security, WDS, Mesh, Location, Hotspot 2.0, Dual-Link Cold Backup, N+1 Backup.
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AP-based Load Balancing

AP-based Load Balancing


Load balancing can evenly distribute AP traffic loads to ensure high bandwidth for each STA. The load balancing function applies to wireless networks with high user densities to ensure access of STAs. Two load balancing technologies are available: AP-based load balancing and radio-based load balancing.

In Figure 6-8, AP_1 and AP_2 associate with an AC. Four users (STA_1 to STA_4) associate with AP_1, and one user (STA_5) associates with AP_2. If too many users connect to the Internet through AP_1, AP_1 will be overloaded, whereas resources on AP_2 are not used.

After load balancing is configured on an AC, the AC uses a load balancing algorithm to determine whether a new STA (STA_6 in Figure 6-8) can associate with an AP. The load balancing algorithm prevents new STAs from associating with heavily-loaded APs to reduce loads on these APs.

Load balancing can be implemented among APs only when the APs are connected to the same AC and all these APs can be discovered by a STA.

Currently, the load balancing function is implemented in the STA access phase. In scenarios with complex user service types and unstable traffic, the expected load balancing effect cannot be achieved. In this case, you are not advised to enable load balancing based on the channel usage.

Figure 6-8  WLAN load balancing


Depending on whether a load balancing group needs to be manually created, load balancing is classified as either static or dynamic load balancing:
  • Static load balancing: APs providing the same services are manually added to a load balancing group. Each AP periodically reports STA association information to the AC, and the AC distributes user traffic among APs based on received STA association information. When a STA sends an association request, the AP uses a load balancing algorithm to determine whether to accept the association request. Static load balancing can be implemented when the following conditions are met:

    • APs in Figure 6-8 are single-band APs that support only one frequency band. If dual-band APs are used, traffic is load balanced among APs working on the same frequency band.
    • Each load balancing group supports a maximum of 16 APs.
  • Dynamic load balancing: A STA sends broadcast Probe Request frames to scan available APs. The APs that receive the Probe Request frames all report the received STA information to the AC. The AC adds these APs to a load balancing group and then uses a load balancing algorithm to determine whether to permit access from the STA. Static load balancing supports a limited number of group members, and all members must be manually added to the group and work on the same frequency band. Dynamic load balancing overcomes these limitations.

When a STA sends an association request to an AP, the AC checks whether the number of STAs connected to the AP exceeds the load balancing start threshold. If no, the STA is allowed to go online. If yes, the AC uses the load balancing algorithm to determine whether to permit or deny the access of the STA.

When implementing load balancing, the AC calculates the load percentage of each radio in a load balancing group using the formula: Load percentage of a radio = (Number of associated users on the radio/Maximum number of users allowed on the radio) x 100%. The AC compares load percentages of all radios in the load balancing group and obtains the smallest load percentage value. When a STA requests to associate with an AP radio, the AC calculates the difference between the radio's load percentage and the smallest load percentage value, and compares the load difference with the threshold (configured using a command). If the load difference is smaller than the threshold, the AC allows the STA to associate with the radio. If not, the AC rejects the association request of the STA. If the STA continues to send association requests to this AP, the AC allows the STA to associate with the AP when the number of association requests sent by the STA exceeds the maximum number of association rejections configured on the AC.

In the formula, the value of Maximum number of STAs allowed on the radio depends on AP types, which can be obtained using the display ap-type { id type-id | type ap-type } command. Maximum number of STAs allowed on the radio refers to the value of the field Maximum station number in the command output.

The following example explains the implementation of static load balancing.

As shown in Figure 6-8, four STAs (STA_1 to STA_4) are online on AP_1, and only STA_5 is online on AP_2. Assume that AP_1 and AP_2 each allow a maximum of 10 users, the start threshold for load balancing is set to 5, and the load difference threshold is set to 5%. Now, STA_6 requests to associate with AP_1.

STA_6 is the sixth STA requesting to associate, and the number exceeds the start threshold; therefore, the AC uses a load balancing algorithm to determine whether to allow STA_6 to associate.

According to the load percentage calculation formula, the load percentage of AP_1's radio is 40% (4/10 x 100% = 40%), and the load percentage of AP_2's radio is 10% (1/10 x 100% = 10%). Therefore, the smallest load percentage value is 10%. When STA_6 associates with AP_1, the load percentage of AP_1 will reach 50% (5/10 x 100% = 50%). The difference between this load percentage and the smallest value is 40% (50% - 10% = 40%), larger than the load difference threshold (5%). Therefore, the AC determines that traffic is not evenly distributed between the two APs and prevents STA_6 from associating with AP_1.

Updated: 2018-12-24

Document ID: EDOC1100038361

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