Point to Point Wireless Bridges Between Buildings

When a wireless bridge beats trenching fiber#

Two buildings, one network, and something in between: a street, a parking lot, a rail spur, a creek. Three ways to close the gap. Lease a carrier circuit and pay every month. Trench fiber and pay once. Or put a radio on each roof.

A trench is priced by what sits on top of the ground, not by the distance. Three hundred feet across dirt you own is cheap. The same three hundred feet across a public street, with an encroachment permit, traffic control, saw cutting and asphalt restoration, is a different project on a different schedule. Where the path crosses land you do not control, fiber needs an easement and a bridge needs line of sight and roof rights. On an industrial site with a detached office or a yard building, the gap is often short, the ground is yours, and the answer flips back to conduit.

Factor	Wireless bridge	Trenched fiber
Cost driver	Radios, mounts and labor at two endpoints	Linear feet of trench, restoration and permits, not the fiber
Lead time	Path survey, equipment, two roof calls	Longer wherever a permit, easement or street cut is involved
Capacity	Set by band, channel width and path length	Set by the optics, upgradeable without touching the cable
Reliability	Depends on the path staying clear; weather affects the high bands	Unaffected by weather, fog, foliage or a new building
Permitting	Roof and structural work; FCC station license only on licensed microwave	Encroachment permit, easement, traffic control, restoration
Reversibility	Removable and relocatable	Permanent once it is in the ground

The third option is the one the table leaves out, because it is not an asset you own. A leased carrier circuit between the two addresses puts nothing on either roof and no capital in the budget, and the carrier holds the service level agreement and sends the truck when the circuit goes down. What the owner takes on instead is a monthly charge that continues for as long as the link does, a capacity fixed by the contract rather than by the hardware, and, at an address the carrier has not served before, a lead time measured in months while the circuit is built. It fits an organization that wants the link off its own maintenance list and can wait for it.

A bridge is not always the answer. Where the ground is yours, where it is already being opened for other utilities, where the link must be indifferent to weather and to whatever gets built next door, and where capacity keeps climbing, fiber wins on a twenty year view. At Touro University in Vallejo, the campus building renovation included a fiber backbone between buildings. On a campus with controlled ground, a long service life and rising uplink requirements, fiber was the right answer and a radio link would have been the wrong one. Our fiber optic cabling page covers that side of the decision. Telelink Business Services is a licensed C-7 Low Voltage Systems Contractor, CSLB License #472017.

Line of sight, Fresnel zone and the path survey#

Line of sight does not mean you can see the other roof. A radio link needs the ellipsoidal volume around the direct path kept clear, the first Fresnel zone, and the working rule is to keep at least sixty percent of it free of obstruction. That volume is widest at the midpoint of the path, and it grows as the path gets longer and as the frequency gets lower. A parapet that clears the sight line by a foot can still sit inside the zone and cost the link several decibels.

The survey covers four things.

• Endpoints. Exact mount locations, height above the roof, the azimuth and elevation between them, and whether the owner controls both roofs.
• Obstructions over time. Trees leaf out in spring. A tower crane on the neighboring lot blocks the path for a year. A new rooftop screen or mechanical unit blocks it permanently. We look at what is planned, not only at what is there.
• Spectrum. A scan at both ends in the band under consideration. In a dense area the band is already occupied, and the interference floor sets the rate the link will actually hold.
• Structure and roof condition. Whether the mount can be supported where it needs to go, and whether the roofing warranty allows a penetration.

We model the path first and confirm it on the roof second. Where the path is marginal we set a temporary link and measure it before anyone buys permanent equipment.

Choosing the band: 5, 6 and 60 GHz or licensed microwave#

Band choice sets the capacity, the usable distance and how much the weather matters.

Band	Licensing	Practical character
5 GHz	FCC Part 15, unlicensed	Longest reach of the unlicensed three, most crowded, radar avoidance obligations on some channels
6 GHz	FCC Part 15, unlicensed	Wider channels and less occupancy today; outdoor standard power operation is subject to automated frequency coordination
60 GHz	FCC Part 15, unlicensed	Multiple gigabits over short paths, heavily attenuated by the atmosphere and by rain
Licensed microwave	FCC Part 101, station license required	Exclusive coordinated frequency, protection from interference, the highest availability

Oxygen absorption sets the path length at 60 GHz. Atmospheric oxygen absorbs signal in that band at roughly 15 dB per kilometer, which is what makes 60 GHz a short path technology no matter how much power the radio has, and rain fade is additive on top of that loss, so the usable path length is set by the fade margin you want during a storm rather than by what works on a clear day. A 60 GHz link engineered to a fair weather distance will drop in a winter storm. We size the path to a target availability using regional rainfall statistics, not to the maximum range on the data sheet.

Licensing is the other split and it needs stating plainly. The unlicensed bands carry no station license at all. Licensed microwave is the exception, and it brings two obligations before a radio goes up: frequency coordination against the incumbents already on that path, and a station license held by the owner or operator of the building. We prepare the coordination and install the equipment, and the license stays with the owner.

Mounting, masts and roof penetration coordination#

A bridge is only as stable as what it is bolted to. Beamwidths are narrow, and at 60 GHz a mast flexing in an afternoon wind shows up as a link that degrades on a daily cycle. A short rigid mount beats a tall flexible one every time.

Non-penetrating ballasted mounts work on a flat roof where the structure carries the ballast and the wind loading has been checked. Penetrating mounts get used where the path needs the height, and the penetration is flashed by the roofing contractor so the warranty survives.

We provide the mount locations, the loading data from the manufacturer, the pathway and conduit requirement, and the power requirement at each end, and every handoff appears on our submittals. The steel, the mast, the roof membrane and the line voltage feed are another contractor’s licensed work, built to those numbers.

The cable between the building and the radio is the part that fails first. It gets outdoor rated shielded cable, a drip loop below the connector, a weatherproofed termination at the radio, and a run kept inside the same hundred meter limit as any other Ethernet channel. Where the radio takes power over that cable, the injector stays inside with the switch.

Grounding, surge protection and the building entrance#

An antenna on a roof is a lightning target and a surge path into the building. Antenna grounding and bonding are installed per California Electrical Code Article 810 as it applies to the antenna and the mast, and the communications circuit requirements for the Ethernet media entering the building.

In practice the mast and the radio are bonded to the building grounding electrode system, the bonding conductor is run as short and as straight as the building allows, and a listed surge protective device goes on the Ethernet run at the point of entrance, mounted on a bonded bracket with the cable shield landed on it. Sharp bends work against a bonding conductor, so the route matters as much as the size.

The bond to the building grounding electrode system is electrical, C-10 scope. We install the surge protection, the entrance hardware and the bonding within Division 27 where the spec assigns that work to us, and coordinate locations where it assigns it to Division 26. Either way the handoff is drawn, the same way it is on a network room buildout.

Alignment, commissioning and capacity planning#

Alignment is coarse and then fine. Coarse from surveyed coordinates, a compass bearing and an inclinometer. Fine from the radio’s received signal indicator, worked from both ends at once. The mistake to avoid is peaking on a side lobe: the link comes up, it looks acceptable, and it runs many decibels below where it should. If the received level does not match the calculated path loss, the antennas are not pointed at each other.

Commissioning is a set of numbers recorded on the day the link is accepted: received signal level at each end, the modulation rate the link settles at, the margin against the manufacturer’s recommended fade margin, throughput measured in both directions, and latency. Two winters later those numbers are the only way to tell whether a slow link has actually changed.

Capacity is planned against what crosses the link, not against the port speed of the switch it lands on. A remote building with cameras writing to a recorder in the main building generates continuous upload that sets a floor under the design; the security camera systems page covers how recording storage is sized. Interference shows up as retransmission and jitter long before it shows up as lost throughput. Where the network runs on Cisco Meraki, we install and configure the switching and wireless at both ends so the link is one hop inside a network with a single console, as described on our wireless networks and Wi-Fi page.

Bridges as a temporary link or a redundant path#

Not every bridge is permanent, and that is one of its advantages.

As a temporary link, a bridge brings a construction trailer, a swing space or a newly acquired building onto the network in days rather than at the pace of a permit cycle. When the fiber lands, the radios either come down and get reused or stay up as the second path.

As a redundant path, a bridge alongside trenched fiber gives real diversity, because the two fail for unrelated reasons. A backhoe takes the fiber. A storm degrades the radio. They rarely happen on the same afternoon. That value only exists if the failover is configured and tested, so we build the routing, force a failover at commissioning, and document what the network does when the primary path drops. A redundant path nobody has tested is a diagram, not a backup.

Next step#

Send the addresses of both buildings, the roof access situation and the bandwidth the link has to carry through request a bid. We run the path model, tell you whether the link is realistic before anyone buys equipment, and price the fiber alternative alongside it where that is a fair comparison. General contractors coordinating the roofing, structural and electrical scopes around a link can review our process on the for general contractors page.