Today’s post has been contributed by our resident RF expert and wireless mesh guru Frank Jimenez. Frank has over 20 years of experience designing and troubleshooting mesh, point-to-point and point-to-multipoint systems. He is our go-to person on all things RF design, path analysis and overall system performance.
The series is aimed at those designing and deploying mesh networks (i.e. integrators), but will benefit consultants and end-users as well. Part 1 covers preliminary and final design, with more to come!
Successful Mesh Design and Deployment: Step by Step
The outline that follows will guide you through the elements of a proper mesh design. Consider these important steps as an investment that assures that the resulting mesh will satisfy the needs of your project, and minimizes the cost of deployment. Correcting design issues once a system has been deployed is significantly more costly (especially under pressure of a deployment deadline) than avoiding them up front at the design stage.
Experience has confirmed that a properly designed mesh will deploy on schedule, with minimal rework and labor overtime cost. I hope that you find the following information useful.
Preliminary Design and Topology Layout
- Identify the types of application traffic and protocols that your mesh will support, including any critical characteristics such as maximum latency or packet jitter (latency variation) requirements.
- Identify the locations requiring a mesh node for entry points of traffic, and determine the total amount of bandwidth that all devices connected to that node’s Ethernet ports will require or generate.
- Identify the location of the head node, and the location of any additional “repeater” nodes that will be required to establish line-of-sight connectivity.
- Lay out a traffic map documenting the aggregated bandwidth across every wireless link between nodes. Overlay the traffic map onto the RF topology to make sure that the topology will support traffic routing and bandwidth requirements. If necessary, reconfigure your wireless mesh topology to eliminate any bandwidth constriction points, to assure system performance.
- Document all node/antenna locations using Google Earth by creating a place mark for each node, naming them appropriately.
Preliminary link calculations and RF channel plan
- Using the Google Earth measuring tool and the previously created node layout, determine the link distances between nodes. Notice that the measuring tool will also provide the true degree heading (azimuth) from the reference node to the target neighbor node. Save the azimuth heading together with the distance between nodes for use in the next step when you perform link calculations
- Calculate target RSSI values for each link using one of the Excel calculators available for download from our FTP site: RSSI calculators. Enter frequency, link distance, antenna gains, and cable losses. Document the calculated target RSSI value along with the node’s true antenna heading onto a worksheet for all wireless links. Note that all of the antenna headings will need to be corrected for use with a magnetic compass in the next step.
- Magnetic north as indicated by a compass will vary to the left or right of true north depending on your location, and this deviation is referred to as magnetic declination or variation. Determine and document the magnetic variation for your deployment location by going to http://www.ngdc.noaa.gov/geomagmodels/struts/calcDeclination. Add or subtract the amount of magnetic declination to or from all of your antenna headings from the previous step. This will allow your installers to use a magnetic compass to determine proper antenna headings for each node without having to perform math and possibly introduce errors in the field.
- Based on the preliminary system design topology, determine an RF channel plan for your network that will avoid or minimize intra-system interference between mesh nodes.
Always keep the following rules in mind:
- Always use the narrowest possible beam-width antenna consistent with being able to properly “illuminate” the neighbor nodes you need to communicate with.
- Never use a single-radio mesh node as a repeater, since it will represent a network bandwidth and latency bottleneck.
- Never over-subscribe a mesh node with more than four neighbor nodes per radio.
- Never assign an adjacent frequency channel to dual radio or co-located mesh nodes, since a transmitter’s broadband noise extends into the adjacent channels.
- When assigning frequency channels adjacent to any frequency previously used within a mesh, always cross-polarize the antennas with respect to links with adjacent frequency channel assignments.
- When necessary to reuse a frequency channel within the same mesh, make sure that the second link reusing the frequency is cross-polarized with respect to the first, that LOS is blocked to the extent possible, and that path azimuths are as close to perpendicular as possible for maximum interference reduction.
Finalization of the system design
All of the previous steps performed so far represent preliminary design steps allowing budgetary proposals or quotations, but are not sufficient to assure a successful deployment. Proper finalization of any system design requires a field site, path, and RF survey for purposes of confirming variables affecting feasibility, performance, and cost of the system. With proper planning and coordination, these can all be performed during a single site visit.
In Part 2 we will cover site survey, path survey and RF survey. Stay tuned!
For more posts on mesh principles and mesh design, see: