Technical Discussion on WLAN construction (1)

Wireless LAN (WLAN) has the advantages of convenient installation, good mobility, flexible use, and easy expansion. However, many new problems have emerged in the planning, design, testing, and security management of WLAN.

1. Features of Wireless LAN

The MAC protocol of WLAN is different from that of Ethernet. The transmission protocol of Ethernet is CSMA/CD ), the standard wireless network uses a MAC mode of CSMA/CA (with multi-channel access for anti-conflict carriers), which features that multiple access devices share a communication channel mechanism. Compared with wired networks, WLAN has the advantages of convenient installation, good mobility, flexible use, and easy expansion. It can provide powerful network support for local and remote data processing nodes that are not easy to route. Due to the irreplaceable advantages of WLAN, it can be quickly applied to mobile internet and inter-network roaming scenarios, at the same time, it brings many new problems to the planning, design, testing and Security Management of WLAN.

2. WLAN Planning and Design Principles

WLAN is different from traditional wired networks. Noise and interference, building structure, placement of wireless devices, and parameter settings have a great impact on the performance of WLAN, such as signal quality and transmission rate, therefore, the planning and design of WLAN are also different from those of wired networks. WLAN planning is designed to enable wireless access devices to cover all areas that are expected to be covered, and to have sufficient capacity to carry the expected load. Due to the complexity of the environment, WLAN planning and design must be measured to achieve the desired effect. The following factors must be taken into account during WLAN planning and design and follow relevant principles. The positioning and frequency allocation of access points (AP) are two important aspects of WLAN planning and design.

2.1 considering user mobility requirements

During AP planning, users must always maintain a high bandwidth and low latency on their mobile devices. Users' mobile requirements are divided into: a) Users should always maintain a good connection with WLAN when moving across the coverage area. B) The user accesses WLAN from time to time. For example, the person attending the meeting needs to check the email from time to time during the meeting. The first requirement requires Seamless Roaming Across the WLAN. WLAN to meet this requirement should increase the access point density. The second requirement is an intermittent wireless connection. The access point density can be relatively small.

2.2 AP coverage

The RF signal propagation principle of WLAN: the lower the frequency of the transmitted signal, the slower the transmission speed of the wireless network, and the wider the effective range. Because a large number of RF signals are transmitted at a low frequency, and the signal-to-noise ratio sensitivity increases with high-speed modulation, the propagation distance of the 2.4GHz802.11b signal with a speed of 11 Mbps is far greater than the 5GHz802 with a speed of 54Mbps. 11a signal.

In addition to the wave propagation characteristics caused by changes in the RF band and throughput, WLAN coverage is also limited by free space path loss and attenuation. The loss of free space paths is more caused by the open or outdoor environment. In fact, due to the spread caused by radio signal pre-wave expansion, the receiving antenna cannot receive these signals. Attenuation is common in indoor installation of WLAN. It is caused by amplitude reduction, or RF signal weakening when passing through walls, doors, or other obstacles, this is why WLAN has poor performance around Dense buildings. Even a 2-4 GHz signal with better elasticity than a 5 GHz signal may still encounter RF problems. The multi-path effect is also an important factor affecting coverage. The so-called multi-path effect is the phenomenon that the signal is reflected and sent back. In most cases, the multi-path effect can weaken or completely offset the received signal, resulting in almost or no RF signal coverage in some areas where the signal should be fully transmitted. Interference objects such as cabinets and network equipment racks can be removed or relocated to prevent the multi-path effect, and the Access Point density or output power should also be increased.

The AP coverage area should be determined based on the received signal strength. A) set a signal strength threshold. All areas whose signal strength is not lower than this threshold value are determined as AP coverage areas. B) conduct on-site measurement to form an AP coverage area map. C) adjust according to positioning principles until the requirements are met. Users in different regions have different user density. Generally, areas with high user density are more complex. Therefore, you should first deploy the AP in areas with high user density, and then deploy areas with low user density. Symmetric circular and spherical shapes can be used to define AP coverage areas outside the open world. Linear or rectangular shapes can be used to distribute aps in a narrow or rectangular building. However, due to the complexity of the indoor Building Structure (such as metal anti-theft doors, aluminum alloy doors and windows), the AP location should be carefully selected before careful measurement, to ensure that the configured AP can cover all regions.

2.3 load capacity considerations

The Planning and Design of WLAN should not only consider the coverage, but also consider its load capacity to ensure service quality. Before deploying a WLAN, you need to consider which communication is most commonly used by the WLAN, Which is email, Web communication, or enterprise resource planning (ERP) with high speed requirements) on-Demand Streaming Media or a computer-aided design (CAD) application, is it necessary to speed up to 54Mbps of 802.11a and 802.11g, or to speed up to 802.11 B of 11 Mbps is enough. The speed claimed by WLAN does not necessarily correspond to its actual speed. For example, if 802.11 B uses a relatively simple direct sequence spread spectrum (DSSS) technology, the theoretical speed can reach 11 Mbps, but considering the overhead of the physical layer (at least about 40%) and free-band interference, the rate is much lower than this. 802.11 a uses orthogonal frequency division multiplexing (OFDM) technology with a high transmission rate. The rate is as high as 54 Mbps in the 10 m range, but with the increase of distance, the rate decreases rapidly, when the distance reaches 70 m, the speed will drop to below 10 Mbps. Regardless of the communication method, when the distance between the user and the AP exceeds a certain distance, the network speed will be significantly reduced. Therefore, the installation of sufficient AP is not only to support connections of all users, it is also to achieve the user's required connection speed.

WLAN is different from switched Ethernet. It is a shared medium, more like an old-fashioned Ethernet Hub model. It splits available throughput into several portions, instead of providing independent leased line speeds for each access device, there will also be a 50% loss of data transmitted through the radio waves. These factors must be taken into account when planning and designing wireless network throughput, when calculating the number of APs, it is best to reserve more space. If you only calculate the number of APS based on the number of users and their minimum bandwidth requirements, although it can meet the capacity requirements within a period of time, it is more risky.

As user requirements change dynamically, the actual load of the AP may increase or decrease. These changes can be learned through testing and monitoring the WLAN. The network administrator should promptly adjust the number and distribution of APS based on actual changes.

2.4 frequency interference

After the AP is fixed and the coverage is determined, the frequency allocation should be considered. 802.11b and 802.11g occupy GHz free band. Because many countries use this band for cordless phones, Bluetooth devices, and microwave ovens, interference is more intense, the most serious interference to WLAN is the 2.4G cordless phone, followed by a microwave oven within 3 m, and a bluetooth device (such as a laptop and PDA ). These interference may cause unknown aps in the WLAN, so that other normal APS can transmit normally only a few seconds or several minutes after the data transmission on the WLAN is completed, it may also be several hours or several days (depending on the interference information). This will undoubtedly cause some inconvenience to normal users who have permission to use WLAN. In addition, it often encounters interference from other APs inside the network or even outside the network. Ideally, Channels 1, 6, and 11 in A GHz environment will never be adjacent to the same channel, so they will not interfere with each other, but this is unrealistic, in actual applications, a certain amount of benign cellular coverage is required to facilitate user roaming (the overlapping coverage rate is 20% ~ 30% best ). The 12 non-overlapping channels of 802.11a can alleviate channel allocation problems to a large extent. 802.11a occupies a 5 GHz free band. due to the small number of other applications in this band, it will hardly cause non-WLAN interference, and users are unlikely to encounter adjacent 802.11a access points, the reason is that this standard is not as popular as 802.11b and 802.11g.

2.5 planning and design through automated tools

All of the above aspects should begin with the wireless site survey, through the site survey and evaluation, planning the radio frequency environment of the wireless infrastructure and setting up the AP, to ensure that the WLAN works normally. From the portable WLAN hardware toolbox to the planning software that provides a detailed view of the site coverage area, the site survey can be completed to make the deployment of WLAN smooth. However, no matter what tools are used, you still need to perform site surveys manually, which is irreplaceable. The site survey and planning tools can be used to determine AP locations, channel distribution, power output settings, and other configuration attributes. They use user density, throughput, and other parameters as standards, however, you must still specify a preset attenuation level for buildings such as concrete exterior walls and metal doors in a CAD-based floor plan, unless the plan already contains this information. Planning tools have limitations. They are generally set up for the vendor's own wireless switches and APS, and lack versatility.

Good WLAN planning and design can not only ensure good service quality, but also reduce the number of APS used to save costs, provided that sufficient on-site surveys are performed first.