Becoming a good Wi-Fi design professional requires extensive knowledge in different aspects of networking and also some areas of project management. The 400+ pages of CWDP book from CWNP teaches you exactly that. From requirement gathering & analysis to post implementation validation, the CWDP curriculum is designed to make you a well rounded design professional. But are there nuggets to achieving good Wi-Fi design? In this blog post, I explain why nailing four fundamentals is the key to achieving this.
Choice of Access Points
Selecting the right access point drives the entire design process. This sounds a lot easier than done. So how can we get the first step of the process right. Access points are typically two types, the ones with internal omnidirectional antenna and the others with connections to external antenna. Choosing between the two types needs a thorough requirement analysis. Is the goal to provide coverage or capacity? The main intent of coverage design is to be able provide good signal without taking into consideration of how many clients connect. Coverage design works well for guest only or other networks where the WiFi performance is not critical. Access points with internal omnidirectional antennae are a great fit for this purpose. But WiFi performance is more critical in today’s world than ever. That is where capacity design comes into play. Capacity planning requires an understanding of the number of clients expected to be on the network, the applications that will be used and the throughput SLA requirements per user or device based on these applications. Ideally it should also contain room for growth in number of devices in the future. Capacity planner from Andrew is great resource to determine how many access points are needed to meet the capacity requirements. The type of access point for capacity design really depends on the number of required access points and the layout. High density spaces like auditorium, large conference rooms, lecture halls etc, where the number of client devices is high per square foot, using access points with directional external antennae will be highly beneficial. Office spaces with well spaced desks layout can work with APs with omnidirectional antennae. But as the scale increases (devices, additional floors), use of directional antennae may be required for creating smaller coverage cells. I have a blog post that explains the need for these antennae in modern enterprises. This should help in choosing the correct type of access point for your deployment.
Location of Access Points
Once the required types of access points are determined for the deployment, the next step would be determine the ideal placement of these access points on the floor plan. This can be done in a couple of ways. One design survey method is the AP on a stick method. This process involves placing AP in the actual environment, taking the readings from a site survey software and determining the correct placement for optimal signal. The clear to send blog has very good content on how to perform this type of surveys. It is important to note that this is not a scalable way of determining the AP locations. The second method called predictive design is a scalable solution. This requires use of site survey software like Ekahau Pro or iBWave Wi-Fi to identify the ideal AP placements. Most predictive site survey software comes with default attenuation values for walls and other obstacles in the environment. It is recommended to do a combination of both survey methodologies to make the design more accurate. AP on a stick method can be performed in parts of the environment to determine the attenuation values of different obstacles and input these into the site survey software to improve accuracy in the predictive models. As a rule of thumb, never place APs in the hallways and always try to leverage the walls to reduce cell size especially when omnidirectional antennae are in use. The AP placement should not solely depend on the coverage but also client density for capacity designs. High density spaces will require more number of access points in closer proximity than other areas. Designs that involve placing APs every ‘x’ feet might miss some obstacles that prevent signal penetration. Shotgun implementations like adding APs where people need it can result in over engineering and these are only few of the many reasons why determining ideal AP placements is the second fundamental one needs to get right to achieve required performance.
Channel Planning
With continuous improvements to the proprietary Radio Resource Management (RRM) protocols, many vendors today recommend using auto channel settings in any environment. This may not necessarily result in optimal performance. Coming up with a good channel plan that would reduce adjacent and co-channel interference is an important step in achieving better results. 20 MHz channels are widely recommended in enterprise environment. But each case is unique and needs to be evaluated accordingly. Perhaps, there is an area in the environment with clients performing file transfers frequently and can benefit from 40 MHz channels in the area. The environment might be closer to an airport that results in frequent channel switching when using default RRM settings. Such environments could benefit from disabling some of the DFS channels. 2.4 GHz range is better than 5 GHz considerably. So some 2.4 GHz radio might have to be turned off to reduce interference. Even on 5 GHZ channels, channel 36 will have better range than channel 165 although the difference is not too considerable. Some clients may not have support for all channels. All these factors need to be taken into account in the design phase to be able to deliver more predictable performance. Static channel assignment yields best results but performance when using RRM and device profiles with appropriate settings do not fall far off as well. More than anything, using RRM vs static channel assignment is a question of scalability. In any case, coming up with a channel plan manually or using auto channel assignment options on the predictive survey tools will give better insights into what the actual coverage and channel overlap is going to be post deployment.
Transmit Power Setting
One of the frequently overlooked setting is the transmit power on the access point radios. Using default RRM settings can be quite catastrophic in some cases. Especially when access points are transmitting at high power, the network can face multiple issues in the form of interference, asymmetric uplink/downlink connections, hidden node issues etc. Customizing Tx level in the RRM settings can yield best results without having the need to set static power levels on all access points. The ideal maximum Tx power at which APs transmit should be equal to the transmit power of least capable most important device in the environment and the minimum Tx power should be equal to the power at which all APs can provide required minimum coverage. Predictive site survey tools give you the ability to simulate coverage at different power levels and this will help in determining these values that need to be configured on RRM to make best use of it.
There a ton of other requirements for successful planning, implementation and validation of a good WiFi network but the design is always at the core of it. It is the foundation on which the entire process is built on and getting these four fundamentals right is the key to an optimal design.
Wireless Gnan 