Wi-Fi Network: Single-Channel Architecture vs Multi-Channel Architecture

    The purpose of this article is to provide a brief, high-level comparison of the two main architectures to date that have interpreted the set of 802.11 standards. Based on these architectures, the vast majority of enterprise and carrier-class Wi-Fi networks have been and are being built.

    We will use the following common terms:
    - Single Channel Architecture: Single Channel Architecture (SCA)
    - Multi-Channel Architecture: Multi Channel Architecture (MCA)

    In simple words, we can say that a single-channel architecture uses one frequency channel at all access points in the coverage area, while a multi-channel architecture uses a cellular structure with various non-overlapping frequency channels. When designing an MCA network, most often it is necessary to avoid neighboring cell locations with the same or adjacent frequency channels, the same applies to the vertical cell layout in a three-dimensional model (on different floors).

    Multichannel architecture has many obvious advantages. The main vendors of enterprise-level Wi-Fi solutions use just such an approach and most existing Wi-Fi networks are built on this architecture. Statistics show that ISA substantially prevails as the basis for most Wi-Fi networks in the world. Much has been written about the MCA networks, but for those who want to quickly brush up on the basic principles of planning and deploying a multi-channel architecture, you can contact here .

    The main players in the field of single-channel architecture is the company Meru and Extricom, between which there was aThere are many disagreements regarding patent superiority. Larger players, such as Aruba, also did similar research. Nevertheless, this technology is interesting and has its advantages and disadvantages, which we dwell on in more detail.

    So, MCA uses various frequency channels on geographically dispersed access points to minimize the effects of interference and related problems of the client device with decoding signals from various access points. Manual engineers (and / or the WLAN solution itself automatically) control the size of cells, frequency plan, and interference levels in their network. This happens by changing the radiation power of access points, using various antennas and using a static, dynamic or semi-automatic way to change the frequency channels at specific points.

    Once high-quality coverage is achieved, the next most important task of a Wi-Fi network is to provide mobility. In the case of MCA, the client always chooses which access point to associate with, and the manufacturers of mobile devices have different algorithms (in general, the available signal levels from access points on different channels, frame loss levels, etc., are taken into account, but how the algorithm works - this is a secret sauce of manufacturers and this sauce is not always tasty, remember how iPhones stuck to access points until recent iOS releases). Therefore, we often observe completely different results when roaming various smartphones on the same Wi-Fi network. Hence the significantly more serious requirements for designing a WLAN and conducting radio surveys for roaming tasks.

    In contrast to the MCA, single-channel architecture SCA networks generally use access points configured on a single frequency channel. As in the MCA, SCA uses overlapping network cells when planning. Given the practical impossibility of performing territorial diversity of transmitters on the same frequency in order to avoid interference, the main approach in SCA is to temporarily transmit diversity. The entire network must be very precisely synchronized and coordinated from the central device in order to avoid sending frames simultaneously on the same frequency. This is one of the main disadvantages of the SCA network, as the growth in the number of customers on the network who want to actively transmit or receive traffic quickly reduces the overall network performance. Especially if there is a mix of client devices, including obsolete and slow.

    In SCA networks, the client device cannot distinguish between single-channel architecture access points. In this case, the Wi-Fi network (controller) makes a decision when each specific access point must transmit and receive data from each specific client. Thus, the client device itself is not involved in the decision-making process on the execution of the handover, for it the Wi-Fi network performs it. The user moves in the coverage area, and the network routes traffic to this user through the access point closest to him, taking into account the availability of available resources at the access points. In fact, the main advantage of SCA networks is here - providing a handover without the need for client hardware and software, which makes the solution to practical mobility much more predictable compared to MCA networks.

    SCA networks use the modified IEEE 802.11 model, which forms the vision of client devices so that they “think” that they always interact with only one access point. In reality, there can be many access points in such a network. Such a client “vision of the world” is achieved due to the fact that all points transmit in bicons identical values ​​of their BSSID and MAC. Such a model has become possible due to the "non-analog" nature of 802.11 standards. All information is transmitted using frames. Because of this, with very precise control of the transmission and reception of frames, interference problems between access points operating on the same frequency channel can be avoided. The control task usually lies with the central brain of the WLAN, the Wi-Fi network controller. In some cases, LAN switches with integrated WLAN controller functionality perform this task.

    The problem of the lack of frequency channels in SCA has led to the emergence of such solutions as Channel Blanket, where more than one disjoint frequency channel can be used in parallel at access points (extension of the SCA model). This forms layers where each is represented by a single frequency channel (blanket / blanket) at all access points. But each of these parallel layers will have the same capacity problems, and plus the problems of inter-channel interference will sharply increase due to the minimal separation of emitters and receivers, which automatically leads to an increase in frame losses and an additional drop in performance. Even sophisticated art relating tocoordination of sending frames, taking into account all available layers, will quickly approach the limit of their effectiveness with an increase in the number of users. Of course, the more layers, the greater the total network capacity, but, obviously, the growth will be far from linear when each new layer is added. Moreover, there are real limitations expressed in the fact that in 2.4GHz there are only 3 disjoint channels, which does not allow using more than 3 layers in this band effectively. Switching to 5GHz is unlikely to be an option in the foreseeable future, given that most mobile devices even with 11n do not yet support 5GHz. At the same time, on the MCA network, by simply adding an additional number of cells and reducing the radius of existing cells, we can quickly increase capacity as on the entire network,

    Interference affects the SCA network significantly more than the MCA network due to the use of one frequency channel in the entire coverage area. The most suitable for Wi-Fi is the 2.4GHz spectral band, but it is also the most “dirty” in terms of the presence of interference sources . In the MCA, the frequency plan can be dynamically and quickly rebuilt in the entire network or in any local part of it, but in SCA, any significant interference will immediately affect the entire coverage area.

    Thus, it can be expected that in tasks with the main focus on providing real mobility with support for a media session, primarily applications such as VoIP on Wi-Fi networks or video streaming in motion (but in this case with small requirements for video capacity), SCA networks can be a good choice. Although if you think about it - how often do we build networks specifically for voice? At the same time, if it is necessary to ensure high network capacity with a high user density, with serious bandwidth requirements, it is worthwhile first of all to think about a solution based on MCA. Perhaps you should stop at the MCA in the case where it is supposed to use a Wi-Fi network for various tasks at the same time.

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