In order to deploy any type of wireless LAN network, organisations need to determine the number and placement of access points. Providing a reliable wireless medium is important to any deployment, but some environments are more sensitive than others. In a recent post 5 Reasons to do a Wireless Site Survey Dr. Greg Daley, Solutions Architect at Logicalis, explained the importance of these surveys.
For this post, we asked Dr. Daley to outline approaches to wireless surveys and analysis so that your organisation can identify the best approach for its environment.
In order to achieve a high quality cell layout, there are four basic approaches. These can be applied in combination or alone (with varying success):
- Cell Size Calculation
- Predictive Cell Placement
- On-Site Wireless Survey
- Operational Monitoring
1. Cell Size Calculation
Cell Size Calculation makes use of wireless channel models and uses factors such as frequency, transmission power and cell thresholds to provide a rough calculation of the cell size. By taking into account published propagation characteristics of typical working environments and information about the target site, this gives an approximate number of APs required to cover an area.
2. Predictive Cell Placement
In Predictive Cell Placement, modelling software uses similar wireless channel models, but incorporates maps, AP Characteristics, deployment constraints, wall and boundary information to build AP layouts. These calculations can provide improved accuracy in AP cell quantities and locations, but may require operator time and expertise to provide high quality predictive placements.
3. On-Site Wireless Survey
Traditional On-Site Wireless LAN surveys undertake a wireless LAN analysis using actual access points and a survey computer to measure the received signal strength around the floors. Where there is an existing wireless LAN, this may be measured for performance. For new deployments, and environments where wireless requirements are changing significantly, a test AP rig is placed and relocated around the survey zone. As considerations of cell size and placement are required in test AP placement, skilled technicians are necessary. Additional site-specific tests like spectrum analysis can be performed, which provide confidence that cell plans and wireless performance targets are feasible in the environment tested.
4. Operational Monitoring
Online Operational Monitoring measures the achieved data rates and received signal strength during operation, either before or after go-live. The wireless platform itself collects and correlates information from APs and clients, before sending them to a wireless monitoring and mapping tool such as Cisco Prime Infrastructure. The level of visualisation available in such platforms makes it possible for operator level staff to readily identify coverage holes. This may permit the addition of access points to provide in-fill coverage. This is an approach that can be combined with prior approaches.
The correct approach to wireless surveys can vary from organisation to organisation, and may even depend on the application or site. Each of the approaches has strengths, weaknesses, costs or expertise that need to be taken into consideration when planning a deployment.
The facts
The accuracy of Cell Size Calculation is limited. It provides basic sizing/AP quantity requirements, which could be out by +/- 30%. This approach has an advantage of being very cheap and fast to undertake. In initial budgetary analysis for new sites, this method can be effective to rapidly identify cost impacts, or to provide indications of how much effort is required for (on-site or predictive) wireless site surveys. Variability in the deployed cells’ size requires that caveats about accuracy of AP quantity noted to the stakeholders.
Predictive Cell Placement requires some expertise and a specialised tool (i.e. Cisco Prime Infrastructure, in planning mode). Normally cell plans and AP quantities are accurate within approximately 10%. Because the modelling requires walls to be drawn in and skill to be taken (in order to be accurate), the costs of these surveys can be 50-80% of On-Site Wireless Survey, except for travel and accommodation costs. When a building has not yet been constructed, this is typically the best analysis available.
Inaccuracies in the first two approaches are largely because they rely upon maps and diagrams, rather than site visits and measurement. Additionally, invisible local limitations such as existing radio interference or site workflow factors may not be apparent unless a site visit is made. Since predictive surveys and calculations can be inaccurate, there is the possibility of further cost and disruption if new APs or additional cable runs are required. In certain circumstances it is cheaper to run additional UTP cabling to in-fill locations even if the Access Points are not initially purchased.
Traditional On-Site Wireless LAN Survey (Approach #3) is relatively accurate, although it can be affected by factors such as changes to the layout (e.g. if undertaken before furnishings are installed in a site). On-site surveys have the advantage of being able to test for interference sources, which are at the site; this assists in channel planning and validation. In many circumstances (except for business or safety critical applications) a prior on-site survey is sufficient to be the only required survey. In higher reliability environments, a pre-placement survey is followed up with either a post installation survey, or ongoing wireless monitoring (Approach #4).
Operational Monitoring requires integration of the deployed APs into a mapping tool, which collates up-to-date receiver reports and signal strength information. This provides powerful analysis of the received service, and allows black spots to be identified and remediated by addition of access points. This would require licensing, integration of the APs under the management tool, training of staff, and ongoing time to review wireless performance. An online wireless medium management tool can save lots of IT team members' time in troubleshooting, and can help to resolve wireless issues more quickly, which increases staff productivity at affected sites.
Dr. Daley states he would usually recommend at least a predictive analysis (Approach #2) with ongoing monitoring (Approach #4). “If the customer will not have ongoing monitoring, then an on-site survey would be recommended prior to deployment. Higher reliability environments may employ all of these approaches, with work done in initial planning and assessment reused in operational management tools such as Cisco’s Prime Infrastructure.”
Investing in a wireless LAN survey is about delivering productivity, service delivery and workflow gains for an organisation. In order to effectively achieve these gains, the wireless medium has to meet criteria that support the business activities delivered wirelessly. In determining the best approach to a wireless survey for your own organisation, please weigh the costs of providing surveys against the costs to troubleshoot or remediate an ineffectively deployed wireless LAN.
Logicalis has experience in delivering networks from kiosk-style branch environments to life-critical systems in complex and high density environments. From our broad experience, we are able to recommend efficient survey mechanisms which map to your business’ requirements.
For more information about Wireless Site Surveys head over to our blog.
