FireNet Design: Multi-Site Coverage

Coverage Design in Hurricane Alley

One RF luxury we have in south Florida is extreme flatness, and this can make coverage analysis “Spreadsheet-Easy.”? This Shared Solution shows how we design and manage UHF coverage in our wide-area multisite environment for FireNet, a critical fire dispatch system in Broward County at the business end of Hurricane Alley.

We needed to design and maintain multiple layers of RF redundancy, and this took a geographically diverse approach for dispatch data operations supported with adroit coverage modeling.? ?

About FireNet

FireNet provides fire dispatch communications on a wide-area licensed 4.9 GHz network to 107 Fire Stations throughout Broward County, Florida. ? Multiple UHF sites provide overlapping layered backup coverage. FireNet is essentially a multiband, multilayered, Public Safety WISP with OCD-grade backup reliability.

A traditional 15-hop 6/11 GHz microwave ring is enhanced by overlaying hub-and-spoke ancillary MPLS/L2 connectivity at 7 key sites. The network supports 44 high profile 4.9 GHz quadrant panels on 11 County and co-located tower sites.

Read more about FireNet: Critical Tropical Communications?

Geodiverse UHF Layer

We designed UHF coverage with geodiversity at the core. It was important to optimize dispatch data delivery to a sequence of best-to-least RF quality sites, to ensure optimum delivery in normal conditions, but also provide incremental near-relentless retries across multiple sites in times of trouble. To make it play, come what may. This was readily supported using the excellent USDD Honeywell Gateway controller which allows delivery grooming uniquely for each Fire Station.

Each possible delivery path is specified in delivery order preference, and each station is designed with at least two and many with three or four available RF paths.

The RF resources and?order of delivery attempts and retries for the data paths are specified uniquely for each Station. ? This provides robust data communications with a simple, cheap and effective approach to fault tolerance.

Designing and Managing Coverage

With 107 fire stations built out over several years, we needed an agile method to predict and maintain hundreds of individual RF coverage designs. ? Coverage design is usually performed in isolation at the start of a project, and it is often static throughout the service life. For FireNet we needed to know, on a moving build-out basis, the optimum set of sites for each station, and in what preferred order data should be sent.? With such a large and growing footprint to support on a 24/7/365/15-year basis, coverage design becomes a long term maintenance and migration requirement, and we did not leave that to chance.??

We needed a simple way to design and manage a large number of RF paths for this vast RF footprint.

This was a perfect job for a Shared Solutions Spreadsheet!

Coverage Made Simple

In this sheet we specify our four Base Station site locations (by Longitudes and Latitudes)? in a table at the upper left.? We designate colors for coverage, based on conservative mileage thresholds for our locale. (Please note that this works well in our flat-as-swamp/pancake environment, but may not be adequate for yours). We color-code mileage distances for quick reference metrics of Great, OK and Poor coverage.

Finally, we automatically colorize each destination cell according to the metrics set in the Base Station configurations, providing at-a-glance color/distance reference to each candidate site. For example, Field Site 1 above has two Good UHF sites (Site 1 and Site 4) available.

Based on the coverage design sheet (and other site-specific considerations), we manage each destination Fire Station’s outbound RF configuration for optimum coverage.??Because we use directional/gain UHF antennas, we may optimize the azimuth between sites, and maintaining a single sheet as-designed, as-tested, and as-built provides a living reference document against which we assess and record Moves, Adds and Changes moving forward.

Station-specific build and change Notes are also?maintained in this Worksheet in ad hoc columns to the right. As a side benefit we have found that using the model as a recording document keeps technical management singularly-focused, an important bonus, especially in times of trouble.

This simple and effective tool has saved us untold hours of confusion, and provides a solid Good-Enough coverage design tool for the cost of a spreadsheet: Free.

How it Works

The first time I built a coverage spreadsheet was in 1989, before there was even an internet; it took a trip to the library and a good deal of?head scratching. This time it took all of an hour to find the trig and generalize it.

Here’s the secret sauce that calculates the distance between two points:

=ACOS(COS(RADIANS(90-C12)) COS(RADIANS(90-$C$7)) +SIN(RADIANS(90-C12)) SIN(RADIANS(90-$C$7)) COS(RADIANS(D12-$D$7))) * 3958.756

Change the following constant for distance-units:

Miles: 3958.756 (earth's average radius)

Nautical miles: 3440.065

Kilometers: 6371

Don’t fret much about the math, it just works. But do take note of the “anchored” cells, denoted like $C$7 above.? You probably already know this, but we use the '$' as an "anchor," such that when it is copied down a range it does not vary.? So those points are fixed (thus referencing the 4 source UHF transmitter sites) and the other cells vary by row as they are copied down a range.

To implement this for multiple sites, simply copy and paste a row at the bottom of the table, and presto-change-o, instant and right-priced coverage design on the fly.

If anyone would like a copy of this sheet, please contact me directly.

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