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How to Address the Last Few Problems with Wi-Fi Roaming

With the emergence of mobile office, fixed work stations, offices, and other forms of conventional workplaces have been rendered redundant. People can roam and work anytime and in any location: at home and offices, and even during journeys. The binds that tie people and network infrastructure together have become increasingly weak as people can access the Internet through 4G, wired, or Wi-Fi networks. Mobile office focuses on flexible switching of mobility policies between different areas, and Wi-Fi roaming is the cornerstone of wireless mobility.

Wi-Fi roaming is commonly applicable to mobile office scenarios for the purposes of receiving and sending office data (for example, copying files and receiving or sending emails). It is also typically applicable to warehousing scenarios where voice and video data is synchronously transmitted during mobile inventory checking. In the former scenario, resumable data transfer or data retransmission is supported, so customers focus more on the bandwidth requirement during roaming. In the latter scenario, customers focus more on real-time data transmission for ensuring production efficiency. Users are unaware of roaming in both scenarios. However, having benefited from mature WLAN deployment planning and more cost-effective APs, users may have connected to four or five APs within a distance of only 100 m.

A qualified Wi-Fi roaming network can be built by deploying more APs and conducting accurate network planning. However, exceptions always arise. There are still several problems with Wi-Fi roaming. The following describes how to address these problems in a few specific scenarios.

Why Is the Wi-Fi Network Disconnected?

Network Speeds of Some STAs Are Slow although They Are Very Close to APs

In a canteen, Jim uses his mobile phone to connect to a newly deployed Wi-Fi network and watch videos while waiting in a queue. When seated, he continues to watch videos, but notices that the video has begun to stall and buffer. He notices that an AP is located on the ceiling just overhead. Then why is the network speed so slow? After waiting a while, Jim finds that the video is still buffering. He has to disable the wireless network and enable it again. After reassociation, the video can once again play properly.

Wi-Fi Signals of Handheld STAs Are Interrupted Intermittently as Environments Change

Today is a very busy day for Jason as a shipment of goods is arriving. Jason has worked in this warehouse for three years. He is very familiar with the in and out processes for goods. He is required to place goods onto shelves, scan labels on them using a handheld scanner, and then upload data about the goods to the inventory management system. Everything is generally done in order. With the help of the Wi-Fi network, Jason’s work efficiency has been greatly improved. He moves between shelves to scan labels and upload data. However, the Wi-Fi signal of the handheld scanner is cutting out intermittently today. By 3 o’clock, he has only uploaded 2/3 of the data that should have been uploaded. As goods stack up higher and higher on shelves, Wi-Fi signals are interrupted more and more frequently and even become wholly disconnected in some areas. Jason’s work efficiency is therefore greatly affected.

Performing Tests to Find the Root Causes

In the preceding scenarios, the problems are not caused by signal coverage or the quantities of deployed APs. Then what causes low network speeds and signal interruptions during movements of STAs? To identify the root causes, engineers built two test environments.

Test in an Environment Where STAs Move Irregularly

Engineers then allowed robots carrying different types of STAs to move irregularly. Engineers transmitted videos between these STAs. Real-time video transmission is sensitive to bandwidth changes, so this test was able to accurately reflect problems with mobile roaming.

This test showed that videos froze or even stopped playing for a long time when some STAs moved from one roaming domain to another. To check whether this problem occurred only occasionally, engineers continued this test for half an hour and tested signal coverage and strength. The test results showed that signal coverage and strength remained normal, while videos still froze or stopped playing.

Figure 1. Switching between Different APs during Irregular Movements of STAs

After checking STA roaming tracks on the Network Management System (NMS), engineers found that some STAs were always associated with APs far away from them. This indicated that these STAs were sticky and did not actively roam when entering a new roaming domain. When a sticky STA moves to a new roaming domain, it sticks to the initially associated AP with poor signals and does not roam to an AP with better signals. As the STA gets farther away from the associated AP, the signals received by the STA from the AP become weaker, and the bandwidth is also lower.

Figure 2. Relationship between a STA’s Network Speed and the STA’s Distance from the Associated AP

Test in an Environment with Obstacles

Engineers deployed a Wi-Fi network in an equipment room full of racks, simulating warehousing scenarios in order to test the impacts of obstacles on the Wi-Fi network. In warehousing scenarios, Wi-Fi network traffic is not massive but roaming speeds are high. Scanners and tablets are used in such scenarios. These STAs have high signal receive sensitivity and low transmit power (no more than 100 mW, which is even lower than the transmit power of mobile phones). Therefore, warehousing scenarios impose high demands on Wi-Fi signal coverage and strength. To simulate a real warehousing scenario, engineers used a tablet for video transmission to test bandwidth changes and signal continuity during the tablet’s movement. Engineers came across the following problem: When warehouses were full of obstacles on racks and an engineer moved quickly while holding the tablet, the Wi-Fi signal became weak and videos froze or even stopped playing. The test results showed conclusively that obstacles have negative impacts on Wi-Fi signals.

Figure 3. Wi-Fi Roaming Test in a Scenario with Obstacles

The test results explain the problems that Jim and Jason are encountering — some STAs are sticky and obstacles block radio signals in warehousing scenarios. These problems are not encountered or given much consideration during Wi-Fi network deployment. Although these problems occur only in partial areas, they deteriorate user experience, reduce efficiency, and may lead to unexpected consequences.

How to Improve STAs’ Roaming Behavior?

The ‘stickiness of STAs’ problem that Jim is encountering is not an absolute issue. Not all STAs on the market are sticky. However, the stickiness of certain STAs can have significantly negative effects. Roaming on Wi-Fi networks is different from doing so on 3G/4G networks. On Wi-Fi networks, AP selection and roaming decision-making are determined by STAs. When a sticky STA does not actively roam, a common solution in the industry is to log out the STA and enable it to go online again. This solution is easy to implement but does not consider STA capabilities, user experience, or whether the STA can associate with other suitable APs.

Now that roaming on Wi-Fi networks is driven by STAs, we can attempt to modify STA parameter settings to optimize their roaming behavior. We can also optimize Wi-Fi networks to enable neighboring APs to detect STAs’ signal strength and steer STAs to APs with better signals.

Modifying Driver Parameter Settings of STAs to Optimize Their Roaming Behavior

Some STAs’ drivers provide parameters that can be configured by users as required. Users can modify the settings of roaming-related parameters to optimize STAs’ roaming behavior. The roaming sensitivity of some STAs can be increased or decreased to adjust their roaming aggressiveness. For example, Intel’s Network Interface Card (NIC) drivers provide a roaming aggressiveness parameter, so users can adjust roaming sensitivity as required.

Enabling Huawei’s Smart Roaming Feature to Trigger Active Roaming of STAs

Huawei smart roaming is enabled to change the strength of signals received by STAs from neighboring APs to trigger active roaming of the STAs. After smart roaming is enabled on an AP, the AP can use the 802.11k or 802.11v protocol to help associated STAs quickly identify surrounding APs to which the STAs can roam. In addition, the AP perceives the strength of STAs’ signals, checks whether STA roaming is allowed based on specified thresholds, and steers STAs to APs with better signals. In this way, the active roaming of STAs is realized.

Improving the Adaptability of Wi-Fi Networks

These roaming problems can be addressed by modifying the driver parameter settings of STAs or enabling smart roaming. However, these solutions must be implemented on the premise that Wi-Fi signals and signal strength are ensured. Scenarios such as warehouses, which contain tall shelves and workshops full of dust, are complicated. Network planning is very important to ensure stable signals in such scenarios. To ensure optimal network status, Wi-Fi networks need to automatically adjust radio parameters to adapt to changing wireless network environments.

Professional Network Designs Provides Full Wireless Coverage without Coverage Holes

When planning a Wi-Fi network for a warehouse, engineers need to consider signal blocking by unfixed obstacles on shelves and the capabilities of low-power handheld STAs to receive and transmit signals. Generally, shelf locations are fixed, but goods on shelves changes every day. Different types of goods cause different types of signal attenuation. Therefore, it is recommended that APs be installed on the ceilings of passageways between shelves. AP signals should be overlapped to ensure no coverage holes. Furthermore, channels must be properly planned to prevent interference. Antennas can be installed indoors, or Huawei Agile Distributed Wi-Fi Solution can be deployed.

Enabling Automatic Radio Calibration to Strengthen the Adaptability of Wi-Fi Networks

Wireless environments are changing every day. The movements of obstacles and interference from other devices will cause unexpected affects on transmission of radio signals. In such scenarios, Wi-Fi networks require automatic radio calibration. If Wi-Fi signals are interfered, APs can automatically collect information on surrounding radio resources for analysis, automatically adjust radio transmit power, and then select radio channels to reduce interference. In this way, Wi-Fi networks can automatically adapt to changing wireless radio environments and maintain the optimal radio resource utilization.

Providing Optimal User Experience

With rapid development of WLAN technologies and application of 802.11ac Wave 2 technology, Wi-Fi networks are deployed in more and more scenarios. High density, rate, and bandwidth are no longer bottlenecks for Wi-Fi network deployment. Customers focus more on user experience during Wi-Fi network deployment. Although many users are unaware of mobile roaming, it is nonetheless significantly important. It directly affects user experience. For Wi-Fi device vendors, focusing on every detail to improve user experience should not only be considered a desirable attitude but as a responsibility.

By Lin Jian and Ji Yongjun