The Limits of WLAN Capacity

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We had a unique opportunity to test wireless LANs in a vacant, but fully built out office building. Most comparisons of  wireless LAN systems have been done with one access point and 10 to 20 clients in an RF chamber. While that is an  excellent controlled environment for repeatable tests with wireless clients, it doesn't reveal the subtlety of the  complete systems or demonstrate their ability to handle large scale deployments. We wanted to examine the behavior of wireless LAN systems in a more realistic environment - in this case in a 20,000 square foot office space with 10-15 APs and 72 data and 48 voice clients.


We tested wireless LAN systems from the leading enterprise vendors Cisco, Aruba Networks and Meru Networks. All of them are enterprise class wireless LAN systems with integrated security and management tools that are designed to handle very large deployments. All of the systems are 802.11 a/b/g and are Wi-Fi certified. All of them employ a wireless LAN controller that addresses the complexity of managing, securing and deploying these systems. 


There are two different system architectures represented. The Aruba and Cisco systems are examples of the micro- cellular architecture which has been the primary approach for deploying large scale enterprise wireless LANs. The Meru system uses a novel system architecture based on single channel spans and explicit AP coordination on packet transmission.    Aruba and Cisco assert the classic micro-cellular approach to scale - add more APs and decrease the transmission power of all the APs to minimize interference and maximize capacity.   Meru takes a very different approach - remembering that APs interfere at far greater distances than they can communicate and chooses to explicitly control interference.


The complete study examines what happens when we push these systems to their limits. We explore how much data capacity these systems deliver, how many voice calls are possible, and how the systems react with a mix of voice and data. The results are surprising, and illustrate some challenges for the 802.11 MAC protocol and highlight the differences between these two architectural approaches to large wireless LANs. 


The story that emerges from this enterprise wireless LAN scale testing is broader than the difference between products from 802.11 infrastructure vendors. It is really about the 802.11 protocol and how it responds when pushed to the limits in an enterprise environment.    And gives us a preview of what well crafter 802.11n versions of both these type of systems might deliver.


I would like to mention a few key results here.


Co-channel interference is a real factor in enterprise wireless LAN deployments whether they are hand tuned or configured with the vendors' automated RF tools. The interference range of Wi-Fi devices is greater than their useful communication range. There are not enough independent channels in the 2.4 GHz band to allow for deployments with continuous coverage that also have APs on the same channel spaced far enough apart to avoid self interference. This co-channel interference causes packet errors and retransmissions, limiting the overall performance of 802.11 systems under load. 


Cisco and Aruba are classic micro-cellular architecture systems. In a data only test, with 15 APs and 72 wireless clients they delivered less than 50 Mbps of system throughput. However, in a 10 AP configuration, both Cisco and Meru delivered more throughput than with 15 APs. Aruba's throughput increased from 46 Mbps to 64 Mbps - almost 40%. If the APs in the system were perfectly isolated from each other, we would have expected the 15 AP configuration to deliver 50% more throughput than the 10 AP configuration with the constant load in this test. But more APs allow more simultaneous transmissions which caused more interference and lowered performance for these systems. 


The Meru system has a very different architecture and deployment strategy that is designed to deal with these enterprise deployment issues. The recommended Meru deployment starts out with all APs on the same channel. In our test environment, we were able to cover the entire floor with 5 Meru APs operating at high power. On the surface, the Meru approach seems like it would be low capacity since it essentially groups APs together on the same channel and the same collision domain. However this deployment approach allows the Meru WLAN controller to coordinate the airtime access of the APs and (indirectly) wireless clients in the system. To increase capacity in the Meru system, a new set of APs is added in the same area, all tuned to a different channel and still operating at high power. The 15 AP Meru configuration we tested is three independent channel spans with 5 APs each on channels 1, 6 and 11. This approach runs contrary to the micro-cellular deployment exemplified by Cisco and Aruba, which adds more APs at lower power distributed throughout the coverage area in order to increase capacity. 



10 AP Throughput (Mbps)

15 AP Throughput (Mbps)

Aruba

64

47

Cisco

53

49

Meru

61

100


With 10 APs, the Meru, Aruba and Cisco APs delivered approximately the same aggregate capacity.   With the increase in APs to 15  (and the power and cell transmission frequency adjusted for Cisco and Aruba), the Cisco and Aruba capacities substantially decreased, while the Meru system increased its capacity to deliver twice the system throughput of the micro cellular systems in the 15 AP configuration and almost twice the capacity of the 10 AP configuration. 


This dramatic difference was surprising. Co-channel interference is worse that we expected at this scale, and AP coordination is a significant benefit for enterprise WLAN systems. The micro-cellular systems had a very high link level packet retry rate during the testing. (We saw retry rates greater than 40% during some of the tests.) The Meru system with AP coordination had a much lower retry rate. and the result is better system throughput. 


The micro-cellular systems did not scale well. We thought 15 APs would be reasonable for high capacity testing in our environment, but Cisco and Aruba did not perform well with that many APs in this space. They both delivered higher system throughput (and better voice performance) with 10 APs.   


Clearly, adding more APs did not increase the capacity of the micro- cellular systems and there is a limit to the number of APs (and the system capacity) that can be effectively deployed in these systems. The Meru system delivered better performance than the best micro-cellular system in the 15 AP tests. Coordinating access with neighboring APs is a promising area that should be pursued by 802.11 for both increased performance within the same system and better co-existence with other systems in the unlicensed bands. 


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