March 2009 Archives

Our testing of WLAN systems at scale dramatically demonstrated the effects of interference on limiting WLAN capacity and performance.   And that opens the question of what other means are available to control WLAN interference and hence increase performance.

We tested enterprise wireless LAN systems based on Cisco infrastructure deployed in two ways:

  • in a conventional microcellular architecture and 
  • deployed using the InnerWireless Horizon Distributed Antenna System (DAS). 

In both cases, the same Cisco access points and wireless LAN controller were used. The main difference is in the physical layer - how the wireless signals are distributed throughout the coverage area.

We discovered that controlling interference through carefully crafting antenna coverage doubled the WLAN capacity using the same APs.

Horizon is a Distributed Antenna System (DAS) that can accommodate a broad range of wireless services operating from 400 MHz to 6 GHz including paging, public safety, 2-way radio, cellular/PCS, WMTS and Wi-Fi.  Horizon implements Wi-Fi, 802.11 a, b, g, or n wireless LANs, as layers of independent wireless service. This layered architecture is known as a layered DAS or L-DAS. The Horizon L-DAS provides pervasive coverage, and layering increases capacity in the same area by using multiple Wi-Fi access points (APs) operating on different channels. 

The layering capability is unique when compared the conventional microcellular architecture for wireless LANs, where a single access point (AP) provides both coverage and capacity in an area. In the L-DAS approach the RF coverage is engineered ahead of time. The access points are located in a wiring closet (or other convenient location with other telecom gear) on each floor and multiple antennas are placed throughout the coverage area such that the wireless signal level for each channel used is spread consistently throughout the entire area.  

In the classic microcellular approach, the access points are placed out in the coverage area and each one has its own antennas. The channel and signal level of each AP are set independently by the Wireless LAN controller and can change dynamically. Usually, the resulting coverage pattern is that adjacent access points operate on different channels. In an L-DAS, all channels are available in all locations. We compared the difference in performance for a system deployed in the conventional manner and a system deployed using InnerWireless Horizon Distributed Antenna System (DAS).

Horizon is made up of multi-coverage diversity antennas that are engineered to provide uniform signal-level RF coverage throughout a facility.  Each coverage area uses a single coax cable with multiple RF radiating elements (antennas) to provide a typical coverage area of 6000 square feet, about three times the size of the coverage area expected from a discrete access point in a microcellular deployment. This coverage area is called a segment. 

An access point is connected to the antenna segment using an access point combiner. Access points for each segment are collocated in a common location (e.g. a telecommunication closet or recessed ceiling cabinet) so that they are secure and easy to maintain. Multiple segments can be combined to provide coverage throughout an entire facility. In this architecture, a single channel of Wi-Fi covers the entire area. If more capacity is needed, more APs can be added to the antenna segment and tuned to a different channel. Each new channel provides an independent layer of wireless LAN service. At any specific location, all channels that are in use are available. 

In the 2.4 GHz band there are 3 unique channels and the DAS distributes signals for each channel throughout the entire coverage area. More channels are available in the 5 GHz band depending on which version of 802.11 is being used. Horizon supports from one to six layers on a single DAS segment for wireless LAN coverage, - three channels of 2.4 GHz and up to three channels in the 5 GHz band.   

A fully loaded L-DAS with three layers operating in the 2.4 GHz band would have three access points per 6000 square foot segment, one for each channel.  A micro cellular deployment supporting the same applications and designed for the same capacity will usually require the same number of APs. 

Our testing configuration used 3 DAS segments with 3APs each for a total of nine APs. The Microcellular 

system we tested used 9 APs to deliver coverage and capacity to the same area. 

Key Findings

  • The L-DAS exhibited lower interference between APs in the same system than the classic microcellular system.   
  • The L-DAS delivered more than double the data capacity of the discrete microcellular system in our tests.   
  • The clients on the L-DAS exhibited more uniform and predictable performance.  
  • The DAS system did not compromise the basic functionality of the Cisco Wireless LAN system. All of the expected features worked well on the DAS system. 
  • Roaming, voice support, QoS mechanisms, 802.11a/b/g and 802.11n with 20 or 40 MHz channels worked on the L-DAS with the same clients and software as the discrete microcellular system. 

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)










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. 

Broadband Stimulus

There is a portion of the American Recovery And Reinvestment Act that will fund programs to accelerate the deployment and use of broadband in the United States. In particular, the NTIA's Broadband Technology Opportunities Program (BTOP) and the Department of Agriculture's Rural Utility Services (RUS) grants and loans will go to fund programs "in unserved, underserved, and rural areas and to strategic institutions that are likely to create jobs or provide significant public benefit". These programs are handing out billions of dollars in the next 18 months and will likely have thousands of applicants for the much needed stimulus money. How will these agencies do it? It is a daunting task and efforts are already underway. The first open public meeting was held earlier this week and two more will be held in the coming weeks. At this point there are more questions than answers. How are these agencies going to work together? Who is eligible to receive a grant? How will the success of the program be measured overall? What does "underserved" mean? For that matter - what does "broadband" mean? In the new era of transparency and accountability, grant applicants will have to explain how they are going to execute their program, deliver the claimed benefits, and measure the results. For the wireless broadband piece, this brings us right back to the basics. What are the technical requirements to support a given application - coverage, throughput, latency and overall system capacity? In order to support multiple different applications, what are the system level requirements? What performance is required to deliver an acceptable user experience? Perhaps it is time to dust off our Novarum lessons from the first round of Municipal Wireless. We can not afford to repeat those mistakes again.