|
Virtual Cells: The Only Scalable Multi-Channel Deployment
As wireless LAN deployments and usage has increased, coupled with the near ubiquity of embedded Wi-Fi in laptops and smart phones, many enterprises are experiencing complaints from their users of slow or even dropped connections. Many times this is due to a high density of users all accessing the wireless LAN (WLAN) simultaneously. Unfortunately, today's wireless LAN solutions are not designed to deal with high densities of users that comes along with a pervasive1 WLAN in a corporation and what was formerly liberating is now frustrating.
While some vendors have created WLAN switch architectures to help large deployments, they focus primarily on security management and central management of access points (APs) configuration parameters. Solving these problems is both important and necessary, but it is not enough. The key challenge is managing contention, interference and Quality of Service (QoS) in a pervasive Wi-Fi deployment with both data and voice clients. This means minimizing the problems of co-channel interference, inter-AP handoff, as well as wireless network fidelity (for reliable communication) even with the limitation of few available channels. Meru Networks has created a Wi-Fi certified system designed from the ground up to account for co-channel interference and allow single channel or Virtual Cell operation creating the only truly scalable deployment architecture available today.
Why Traditional WLANs Fail in High Density Situations
Traditional wireless LANs are analogous to an Ethernet hub. As the 802.11 protocol is a shared medium, all users contend for the same bandwidth delivered by a single Access Point (AP). As higher numbers of users access the wireless LAN, the collision avoidance protocol used by 802.11 to manage contention increases each individual client's time to access the network. As the number of users increases, more and more delay occurs resulting in slower speeds for each user and even application timeout.
To solve these issues, conventional wisdom is to deploy more access points at a closer spacing. The reasoning behind this practice is, in summary, that if one access point provides 54 Mbps of bandwidth (802.11g or 802.11a), then additional APs will increase the capacity. This is true only to a limited extent. A number of reasons including only three non-overlapping channels2 and co-channel interference as more than three APs are deployed limit the effectiveness of this approach. Reducing cell size by reducing the output power of the AP from the default 100mW to somewhere on the order of 30mW (a practice also known as creating "Microcells") is often mentioned as a way to mitigate this, but when it comes right down to it, the solution is still inadequate for pervasive deployments. The more APs deployed in close proximity, the more contention there is, resulting in even more problems with throughput and connectivity for high numbers of users. And with a limited number of channels it is impossible to avoid having some APs on the same channel (particularly in a multi-story building).
In fact, because of the nature of the 802.11 protocol, with only three to five simultaneous contenders 802.11 throughput dramatically decreases because of the increased collisions and the subsequent backoff caused by contention among the users.
With the power reduced on the AP you also artificially increase the signal to noise ratio (the interference level doesn't change but the AP signal is lower). This further increases the chances of corrupt packets and retransmissions and lowers the quality and fidelity of the wireless network Significantly.
Voice Applications Aggravate the Problem
The challenges described above are even worse as voice clients enter the WLAN environment. Voice traffic requires low latency, low jitter, and fast handoffs. The traditional method for increasing client density - using more APs - has exactly the opposite intended effect. More access points will increase latency, increase jitter and create more handoffs. All of these combine to make most pervasive WLAN deployments unusable for voice at all. The significant contention in such an environment will cause such a high packet loss that voice calls will be unintelligible.
Technology to Scale WLANs
Managing Contention
With the foresight that Wi-Fi will become the primary enterprise network connection in the years to come, the Meru WLAN System uses advanced techniques within the 802.11 protocol to schedule client access to the network.
|
