Analysis and discussion on various obstacles encountered in 802.11n planning

The field of wireless LAN is changing

With the advent of 802.11n, the wireless local area network (WLAN) field is undergoing a fundamental transformation, and this transformation is as dramatic as the birth of wireless LAN. The extremely high data rate of the 802.11n final standard provides a solid foundation for a fully wireless enterprise network. Rich multimedia applications will be seamlessly deployed across nodes in the network, with superior performance far beyond previous 802.11a/b/g technologies.

But the problem still exists, "How to plan a high-performance 802.11n network?" Although the advantages of 802.11n have been continuously valued by WLAN equipment manufacturers, and have caused a large-scale discussion in the industry, but no one can explain 802.11n yet. How does network planning work, or, more importantly, under what circumstances it fails! “How can I benefit from a fully cleaned 802.11n deployment implementation project?” “In order to migrate my existing network to the 802.11n platform, what if I just eliminated and replaced the access point (AP)?” Better end-to-phase migration to 802.11n?” This article explains the basics of 802.11n that are required to solve these problems and helps you choose the best strategy for your organization.

How does 802.11n affect the "3C" in network planning?

Network planning should pay attention to three factors: Context, Coverage and Capacity, collectively referred to as "3C" in network planning. Technological advances in the 802.11n standard have affected all three factors. In terms of background, network planners must consider the impact of new 40 MHz channel interference and multiple input multiple output (MIMO) techniques associated with specific on-site complexities on channel planning and access point settings. In terms of coverage, designers should understand the difference in coverage between 802.11n and legacy systems and correctly define coverage requirements based on network requirements. Finally, the data transmission speed and MAC layer efficiency of the 802.11n standard are improved, so the network capacity will also increase; however, only the correct planning of the network customer distribution can make full use of the increased capacity.

background

The background environment in which WLANs are deployed is very important. Neighboring access points or other wireless transmitters that use the same frequency band for propagation may cause interference to the network. This form of wireless congestion will result in packet loss, slower network speeds, and reduced network capacity. In addition to traditional co-frequency and near-frequency interference, 802.11n5 GHz band deployments should also consider potential interference from radar systems. The background environment also includes a wireless LAN environment related to the field structure, which will have a significant impact on the performance of 802.11n MIMO technology.

Interference and channel planning

Compared with the 20MHz frequency band used by the traditional 802.11a/b/g, the data speed of the 802.11n 40MHz channel is more than doubled, which is an essential technology for high-performance wireless networks. However, an increase in channel size also means an increase in potential interference and a narrowing of the spectrum of information planning.

In the United States, if a 40 MHz channel is used in the 2.4 GHz band, only one non-overlapping 20 MHz channel is available, resulting in an increased probability of adjacent channel interference for the 2.4 GHz channel. Since only three non-overlapping channels are available, planning for a 2.4 GHz channel has been very difficult, so it is not recommended to use a 40 MHz channel with an 802.11n 2.4 GHz deployment.

Fortunately, the 5GHz band frees 802.11n users from the strict 2.4GHz band limit. In the United States, if the access point is fully compatible with Dynamic Frequency Selection (DFS) (detailed in detail in the next section), the 5 GHz band allows the use of 11 non-overlapping 40 MHz channels. A large number of non-overlapping 40MHz channels in the 5GHz band enable 802.11n deployments to take full advantage of their performance advantages, so we recommend high-performance WLAN networks to use this deployment strategy. See the 802.11n channel overlap listed in Table 3.

Radar avoidance impact on channel planning in the 5 GHz band (DFS)

As mentioned above, to maximize the number of non-overlapping 40 MHz channels on the 5 GHz band, the access point must be fully compatible with DFS. According to the FCC (Federal Communications Commission) Rules and Regulations, Section 15 (47 CFR § 15), this means that if the equipment detects in-band interference from nearby radar systems, all transmission activities in that band must stop immediately. Up to thirty minutes and transfer to other non-interfering channels. Obviously, this federal regulation requires that the access point channel be dynamically changed, which will inevitably have an impact on the 5 GHz channel plan.

Although the inclusion of DFS in the 5GHz 802.11n deployment project may cause some problems, the optimal planning of the DFS bands (5.25-5.35 GHz and 5.47-5.725 GHz) has not changed much compared to the non-DFS band planning. The first step in the deployment process is to conduct a site survey to determine if there is radar interference in the deployment environment. Second, a network channel plan should be developed to avoid using channels that have already detected DFS. Finally, since the DFS standard requires that the operational channel be dynamically changed in the event of interference, an empty channel to be used after the detection of radar interference should be provided. As a rule of thumb, it is preferred to provide at least one idle channel in the non-DFS band.

Specific scene correlation effects of MIMO

MIMO essentially has specific site-specific characteristics

In the traditional system, the reflection caused by the reflection of the transmitted signal and the diffraction (called multipath) is regarded as one of the main factors that interfere with the performance of the system. In order to solve this problem, a large amount of attenuation margin (fademargins) is added to the system design. ), in order to improve signal quality in areas where multi-channel interference is more serious. Contrary to traditional systems, in MIMO systems, multipath can be used as a cornerstone to improve system performance! With complex signal processing, a MIMO system can simultaneously transmit multiple data streams. This means that system performance cannot be effectively predicted based solely on Received Signal Strength (RSSI). Considering the characteristics of MIMO systems related to feature scenarios, Motorola strongly recommends that users adopt planning and management tools related to specific scenarios for 802.11n networks.

Performance comparison of MIMO in dense office and longhallway environments

Under what circumstances can MIMO perform optimally? Described in technical terms, multi-channel rich environments (mulTIpathrichenvironments) are ideal for achieving optimal MIMO performance. In a multi-channel rich environment, the received signals are evenly distributed among a number of different paths from the transmitter to the receiver. The number of differences in individual paths is a basic metric for multi-channel richness. To better understand this concept, Motorola recommends considering two common deployment scenarios, a comprehensive office building and a long, straight corridor.

In an office building, access points are typically deployed centrally in planned coverage areas. Rooms with access points are usually surrounded by other rooms and are connected to each other by a shorter corridor. In general, this environment is full of obstructions (usually walls) that interfere with the signal path, and there are few or substantially no line-of-sight (LOS) receive paths in other areas than the room in which the access point is installed. The complexity of this environment produces many different signal transmission paths, so the performance of the MIMO system will be better, which is an ideal MIMO deployment scenario.

In the long straight corridor scene, the main path of receiving signals is mostly the line-of-sight (LOS) path, and the multi-path mainly comes from the reflection of information along the corridor wall. Network designers can foresee that in this environment, the performance of the MIMO system will drop significantly as the distance between the access point and the receiver continues along the corridor. The multi-path composition in this scenario is very similar, so it is not a multi-channel rich environment, and its MIMO performance gain (if it still exists) is also inferior to the comprehensive office scene. At this point, you can use traditional hardware to add an access point to the corridor to extend the coverage of the line of sight; however, this will weaken the performance of 802.11n, so this deployment scenario is not recommended.

Coverage changes in traditional networks and 802.11n networks

Table 1: Improvements in coverage of 802.11n networks over 802.11a/b/g networks

cover

There are still many misunderstandings about the difference in coverage between traditional 802.11a/b/g systems and new 802.11n deployments. In order to properly assess the difference in coverage between 802.11a/b/g and 802.11n hardware, the term coverage must be clearly defined. The definition of "coverage" in this document is "Communica TIonataspecified minimum data data at a given locaTIon". The term "Range" is defined as "CommunicaTIon at the minimum supported transmit data rate for an AP at a given location". The substantial differences in 802.11n coverage are discussed below and summarized in Table 1.

Fundamentally, 802.11n wireless communication is still subject to government regulations (effective isotropic radiated power, or EIRP) for the same power output as the 802.11a/b/g standard. This means that if a peer-to-peer comparison is made, the signal transmitted by the 802.11n access point is not transmitted farther than the signal transmitted by conventional hardware. Although the range has not expanded, 802.11n implementations can still use the antenna polarization added by 802.11n access points to expand the diversity gains. In this way, the access point can receive weak signals and effectively expand the "visible" coverage, thereby reducing hidden node problems.

The transmission range of the 802.11n access point is the same as that of the traditional access point transmission signal, so the data transmission rate of the given RSSI will become a key factor causing the difference between 802.11n and traditional network coverage. The transmission data rate represents the wireless transmission speed of individual data packets. When it comes to the coverage difference between 802.11n and traditional networks, it should be noted that the coverage area of ​​802.11n at 54Mpbs transmission rate is larger than the coverage area of ​​802.11a/g at 54Mbps transmission rate. Noting this difference, and considering that there is no substantial improvement in the scope of 802.11n, the basic difference between 802.11n and 802.11a/b/g coverage can be roughly summarized. It is important to note that the higher the data transfer rate, the greater the coverage improvement; the lower the transfer rate, the less improved. In addition, it should be noted that the use of a 40 MHz channel will further increase the coverage area of ​​802.11n access points at high data rates, but will still not increase the transmission range of 802.11n devices.

capacity

Due to the increased data transmission rate of the 802.11n standard, the capacity of an 802.11n access point may be larger than that of a conventional access point. However, this improvement can only be achieved when the 11n client is connected to the 11n access point and is within the coverage of the 802.11n standard data transmission rate. As traditional customers in the network have the potential to degrade overall system performance, customer distribution planning is a key factor in achieving high-performance 802.11n deployments.

Hybrid network

802.11n is fully backward compatible with 802.11a/b/g networks and devices, which is one of the most important features of 802.11n. With this feature, users can smoothly migrate existing wireless networks to the 802.11n platform, but this also means that when traditional devices transmit signals in the network, 802.11n networks must sacrifice some performance. Table 2 lists the customer distribution and its impact on network performance.

To reduce the impact of traditional customers on high-performance 802.11n networks, it is important to note the features of 802.11n that can operate in the 2.4 GHz and 5 GHz bands. Due to the low 802.11a network usage, the 5GHz band has been relatively idle for a long time and can be used as an ideal environment for the new 802.11n standard. Since there are only a few 802.11a users in the 5GHz band, it is easier to implement an "n-only" deployment environment in this space without worrying about the weakening of network performance due to the presence of traditional customers. Therefore, we recommend deploying "802.11n-only" high-performance wireless LANs in the 5GHz band.

802.11n network performance under various customer distribution scenarios

Table 2: Relative traffic performance of 802.11n networks under various customer distribution scenarios

Number of non-overlapping channels using 20 or 40 MHz channel bandwidth in 11n

Table 3: 802.11n channel overlap

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