In an uncommon large-scale WLAN stress test using three vendors' devices, testers found that as WLAN scale and traffic increase, many WLANs may experience performance limits. The problem is not with vendor equipment, but with 802.11. The design of the agreement. Designed and operated this test was a consulting firm called Novarum. This test confirms two troublesome problems in high-density networks: First, the wireless interference of the same channel between access points can greatly reduce the total throughput of WLAN; secondly, the conventional controller + thin access point architecture cannot As the number of access points in a certain area increases, it expands very well. According to Novarum's report, the emergence of these issues is related to the design of the 802.11 Media Access Control (MAC) layer, the method of processing validation and retransmission, and the handling of problems under sustained high traffic loads. In many WLAN products and WLANs built using these products, the number of wireless clients in the network is relatively small, and their transport streams are mainly composed of a small number of bursts of data transmission streams, so the protocol is low. There is no problem with efficiency. 802.11n-based high-throughput WLANs (especially when operating on 5 GHz channels) will partially mitigate the effects of these problems, but they cannot be completely eliminated. 11n can only provide a larger "data pipeline", so that more network traffic is required to achieve network overload in high-density WLANs. However, due to the higher throughput of 11n, enterprises will seek to use it to do more things (such as transmitting voice and video), which is also likely to cause network overload. PhilBelanger, co-founder of Novarum, said: "If you don't address co-channel interference and the need for access point collaboration, it will put more pressure on protocols that support wireless voice and video." In the fall of 2007, Novarum's unusual (out of the actual deployment of access points and number of clients) was tested in a free two-story office (approximately 20,000 square feet) in Sunnyvale, USA, and the usual WLAN testing is just about deploying an access point and ten clients in a lab-like environment. In this test, Novarum used 72 laptops with wireless network cards and 54 wireless VoIP phones to connect to a typical office WLAN through 15 access points (then using 10 access points). Network devices were built using wireless controllers and access points from ArubaNetworks, Cisco, and MeruNetworks (Aruba800 controller + AP 70; Cisco 4402 controller + AP 1242; Meru MC3000 controller + AP 208). The test was performed a total of seven times. In most cases, 15 access points were used first, and then 10 access points were used. The access point is equipped with 802.11a/b/g devices, but Novarum only performs 11g testing on the 2.4GHz band. One test was pure data, using 72 laptops; the rest was voice transmission, with 24, 48, and 72 analog VoIP calls. There are also two tests for voice and data hybrid testing, one to test how many VoIP phones can support simultaneous calls. Belanger said the stress test was not intended to be a product evaluation of vendor equipment, although they have similar starting points. Regarding the test results, Belanger said: "The test results seem to have a threshold. After this threshold, the WLAN system does not perform well. The larger the load, the more errors there are, resulting in more retransmissions, which in turn adds Big load..." Different brands of WLAN devices handle this spiraling effect differently depending on the type of network architecture they use. Aruba and Cisco use what Belanger calls a "microcellular" architecture—a thin access point that connects to a central controller. Neighboring access points operate on separate channels with some overlay overlap to provide seamless coverage and roaming for mobile clients. This model is used by most WLAN vendors today. In contrast, Meru can set up neighboring access points to run on a single channel and have more control over its "over the air" behavior. According to Belanger, the Meru controller can see the transmit queue for each associated client on each access point, and can allocate the time that the access point uses for each client. Meru equipment did not try to squeeze into the transmission "gate", but patiently waited for its turn, so the throughput is relatively stable. Another vendor, Extricom, took a similar approach, placing four radios in one device, but running the entire 802.11 MAC layer in its controller. This type of access point has only radios and antennas, and even lacks a CPU. Extricom claims that this is to eliminate the same channel interference and better manage the client's wireless connectivity on a per-packet basis. In a pure data test using 72 laptops and 15 access points, Cisco and Aruba provided less than 50 Mbps of total throughput. In other words, in the entire network, each client only gets an average throughput of less than 1Mpbs, and people expect the result of 20Mbps throughput per access point or 300Mbps total throughput of 15 access points. . However, when the number of access points is reduced to 10, the throughput is greatly increased. In the case of Aruba, throughput increased by nearly 40%, from 47Mbps to 64Mbps. The report said: "More access points allow more simultaneous transmissions, which causes more interference and reduces the performance of these systems." In Meru's experiment, the office was covered by 5 access points (these access points all run on one of the 2.4 GHz bands), and two other access points (5 access points per group) were added to In the same location, each set of access points uses one of the remaining two channels, all of which operate at the maximum power setting. Test results show that Meru can provide 100Mbps system throughput, more than twice the throughput of Cisco and Aruba. Interestingly, when Novarum reduced the number of access points to 10 devices, Meru's total throughput dropped to 60 Mbps. The Novarum report concludes that the same channel interference from neighboring APs appears to have a significant impact on the microcellular system. The interference range of 802.11 radios far exceeds the effective communication distance - this interference becomes an influential factor under sustained high loads. At some point in these high-load networks, if Aruba and Cisco's access points are used, 30% to 40% of the packets are retransmitted, and the number of retransmissions using the Meru access point is small. many. In the voice test, Novarum found that the Meru architecture can provide better voice support in general - it can handle more simultaneous calls (Meru does not encounter the upper limit in voice testing) and provides voice-quality voice. When using 10 Cisco access points, the Cisco WLAN can handle approximately 24 VoIP calls. Cisco cannot provide quality charges when processing 48 or more analog calls. When testing real phones, the Cisco infrastructure seems to have reached its limit at 26 or 28 simultaneous calls. In the appendix to the report, Belanger wrote: "The problems I have seen are related to the 802.11 MAC layer protocol. These systems are very prone to unstable behavior when using many access points and sustained high loads. Aruba and Cisco There were no problems with the system. They just chose not to solve the problem." Currently, two 802.11 working groups 11k and 11t from the IEEE are working to solve this problem (at least partially) by providing more control over the access point radio (to some extent to the client radio). This work is still in progress.
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