800 m Point-to-Point Link Across a Public Road
Two manufacturing buildings are 800 m apart across a public road. The company needs a 300 Mbps point-to-point link for a new MES system. Fiber is quoted at $180,000 with a nine-month lead time. Rooftops are available at both ends with clear line of sight and 12 m mounting height. Each building has a 24-port access switch with an 802.3af PoE budget of 180 W, of which 110 W is already used by phones and APs. The candidate outdoor radio draws 22 W and needs a 30 dBi dish at each end. Local rules cap EIRP at 36 dBm in the 5 GHz band and permit 60 GHz without a licence. Summer rain is heavy. Security requires AES encryption and 802.1X authentication on the link, and the network team wants it managed from the existing WLAN controller.
1. Choose between 5 GHz and 60 GHz for this link, and justify the choice with link-budget and environmental reasoning.
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Choose 5 GHz. The decisive factors are rain and fade margin, not raw throughput.
60 GHz (V-band) has high free-space loss and strong atmospheric attenuation from oxygen absorption, and rain fade at 60 GHz is severe; an 800 m path in a heavy-rain climate would need a large fade margin that a modest 30 dBi antenna cannot reliably provide. V-band links are viable over these distances only with very high-gain antennas and short paths, and availability would still dip in storms. Because this link carries MES traffic, availability matters more than the extra bandwidth.
5 GHz with 30 dBi dishes at both ends has ample margin. With EIRP capped at 36 dBm and 30 dBi of antenna gain, conducted power must stay near 6 dBm before cable loss. Even so, the two-way gain is huge: free-space loss at 800 m and 5.8 GHz is roughly 106 dB, so receive level lands near −40 dBm, tens of dB above the noise floor. That margin absorbs rain fade and misalignment. Practical points: verify whether the chosen 5 GHz sub-band is subject to DFS and radar detection; if so, pick a non-DFS channel or accept the channel-availability rules, and confirm the antenna gain plus conducted power still respects the 36 dBm EIRP cap after cable and connector losses.
2. Can the existing switches power the radios? Show the arithmetic and the options.
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No. Two constraints fail at once.
Per-port power class. The radio draws 22 W. 802.3af delivers a maximum of 15.4 W at the PSE and 12.95 W at the powered device, so an af port cannot power a 22 W load. The link requires 802.3at (30 W at the PSE, 25.5 W at the PD) at minimum.
Switch budget. The switch has 180 W total and 110 W already committed, leaving 70 W. One 22 W radio fits within 70 W, but only if the port is at or above 802.3at - which this switch is not. Adding the second building's radio does not change the per-switch math, but it does not fix the port class either.
Options, in order of preference:
- Add a single-port 802.3at midspan injector at each end between the switch and the radio. Cheapest, no switch replacement, and it keeps the 22 W load off the switch's PoE budget entirely because the injector has its own supply.
- Replace the access switch with a PoE+ model with adequate budget.
- Use a local power supply at the radio if an outdoor-rated outlet and surge protection exist on the roof - usually the worst option for lightning and maintenance access.
Document the actual draw with a PoE tester after install, since inrush and cable length affect delivered power.
3. Security and management requirements: what must the chosen hardware and licensing support, and what would you verify before purchase?
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Verify, in writing, before purchase:
- Link encryption: AES (prefer AES-256) on the wireless bridge itself, with no fallback to WEP or TKIP. Confirm whether encryption is applied to management traffic as well.
- Authentication: 802.1X on the wireless link with RADIUS, or a documented alternative - many point-to-point bridges only support PSK. If 802.1X is not supported on the link, the compensating design is a firewall or router at each end terminating 802.1X for wired hosts, plus link-level AES with a strong, rotated key. Get the security team to accept that in writing, because the stated requirement is 802.1X.
- Controller management: confirm the outdoor unit is supported by the existing controller model and firmware, that the required licence SKU (AP licence plus any outdoor or mesh licence) is in stock, and that the controller's regulatory domain matches the country of installation. A unit ordered for the wrong regulatory domain cannot legally transmit at the planned EIRP.
- Physical and electrical: outdoor-rated enclosure and connectors, surge arrestors on both Ethernet and antenna feeds, grounding to building earth, and a mounting kit rated for the local wind load at 12 m.
Also confirm the throughput claim is for the configured channel width and modulation, not a marketing aggregate, and ask for the dish's actual gain pattern to verify the EIRP calculation after cable loss.