Why Simplex Should Be Your Fireground Standard (Even With a P25 System)

Simplex vs. repeated vs. trunked - the actual difference

Most operational chiefs know these terms casually. Worth a tight refresher, because the SOP language has to be precise.

Simplex (sometimes called talkaround, direct, or DMO on digital systems) is radio to radio. No tower. No repeater. No infrastructure. You key up, your portable transmits on the same frequency it receives on, and any radio within RF range hears you. Power output is whatever your portable can push, typically 1 to 5 watts on a handheld VHF or UHF.

Repeated means your portable transmits on one frequency, the repeater on a tower receives it, and re-transmits on a different frequency at much higher power - 50 to 100+ watts from a tower site. Your radio receives that re-transmitted signal. Coverage extends dramatically because the tall tower has line of sight to a huge area.

Trunked is a coordinated network of repeaters that dynamically assigns talkgroups to available frequencies. P25 Phase I and Phase II are the digital trunked standards used by most large public safety systems. From the user's perspective it looks like channels, but under the hood every PTT is a request to a control channel that allocates a working channel for the duration of that transmission.

Trunked systems are great. They share spectrum efficiently, support hundreds of agencies on one infrastructure, and give you wide-area coverage from a portable. The problem is that all three of those benefits depend on the radio reaching the tower.

Why trunked systems fail on the fireground

A trunked radio that cannot hit the tower is a brick. It is worse than a brick, because the user does not always know the radio has lost the system. They key up, hear a bonk tone or nothing at all, and assume the message went through. It did not.

Common fireground scenarios where trunked falls over:

• Basements and below-grade structures. Concrete, soil, and rebar attenuate RF aggressively. A portable in a residential basement may be 40 to 60 dB down from where it was on the front lawn.
• Mid- and high-rise interiors. Steel-framed buildings act like a Faraday cage. Coverage on floor 1 may be fine. Coverage at the seat of a fire on floor 14 is often gone.
• Strip malls, big-box retail, and warehouses with metal roofs. Metal roofing and HVAC ductwork reflect and absorb RF.
• Collapse and confined-space rescue. A firefighter trapped under structural debris has the worst possible RF environment - close to ground, surrounded by mass, and likely with a damaged antenna.
• Tower or site outage. Microwave backhaul fails. Power fails. Generator runs out. The control channel goes down. Any of these and the trunked system is degraded or offline across a wide area.
• Channel busy / system loading. On a major incident with mutual aid, the talkgroup can hit a busy condition. The radio queues your transmission. You do not know it queued.

Simplex has none of those failure modes. If two radios are within RF range of each other, they talk. The signal does not have to climb a hill, hit a tower, get decoded, get assigned a working channel, and come back down. It just goes.

The RF physics nobody briefs you on

You do not need an RF engineering degree to write the SOP, but you need the physical intuition. Three concepts cover most of what matters operationally.

Path loss is dominated by what is between you and the other radio

Free-space path loss is well-behaved and predictable. Real-world path loss is dominated by obstructions. Drywall is roughly 2 to 4 dB per wall at VHF and UHF. Concrete block is 10 to 15 dB. Reinforced concrete is 20 to 30 dB. Metal is effectively a wall the signal has to go around, not through.

Stack four reinforced concrete walls between your trapped firefighter and the nearest tower (15 miles away) and your link budget is gone. Stack the same four walls between two firefighters 30 feet apart and you still have signal to spare, because the free-space loss over 30 feet is trivial.

VHF and UHF behave differently in buildings

Most fireground simplex in the U.S. is on VHF (around 154 to 159 MHz) or 700/800 MHz. VHF generally penetrates buildings better and reaches farther outdoors. 700/800 MHz penetrates wood-frame and drywall reasonably well but suffers more in heavy concrete and steel. The tradeoff: higher frequencies have shorter wavelengths, so they fit through small gaps (windows, doorways) better. There is no universally "best" band - it depends on building stock in your district.

If your county runs an 800 MHz P25 trunked system and you are in a rural district full of metal pole barns and old farmhouses, you may have better simplex performance on a legacy VHF channel than on the 800 MHz talkaround. Worth testing before you write the SOP.

Antenna position matters more than power

Doubling transmit power buys you 3 dB. Going from a flexible rubber duck antenna pressed against your turnout coat to a clear vertical antenna position can buy you 10 to 20 dB. Body absorption of RF at VHF is real and substantial. The single best thing a firefighter can do for radio reliability is get the antenna out of the armpit and clear of the body.

This is also why a portable on simplex from inside the structure to a tower 5 miles away may not work, but the same portable to another portable 100 feet down the hallway works fine.

What NIST and the federal research actually shows

The National Institute of Standards and Technology (NIST) has been studying public safety communications and fireground RF behavior for years. The Public Safety Communications Research (PSCR) program at NIST has published work on radio coverage in structures, propagation modeling for first responders, and the operational implications of building penetration loss. The Office for Interoperability and Compatibility under DHS has published similar guidance.

The consistent findings across this body of research:

• In-building loss is highly variable and not reliably predictable from outdoor coverage models.
• Trunked system coverage maps based on outdoor measurement consistently overstate in-building reliability.
• Simplex (direct mode / talkaround) provides materially better short-range communication inside structures than infrastructure-dependent modes.
• The single largest variable in radio performance for an interior crew is structural construction and depth into the building, not transmit power or system technology.

None of this is news to working firefighters. It is, however, news to a lot of city managers and county commissioners who approved trunked systems on the basis of vendor coverage maps. Your job as the chief or comm tech writing the SOP is to acknowledge the physics and write policy that handles it.

NFPA 1561 and the standard of care

NFPA 1561 (Standard on Emergency Services Incident Management System and Command Safety) is the document that most directly governs what your fireground communications plan must do. It does not mandate simplex specifically. It does require that the incident management system include a communications plan that supports accountability, allows the IC to maintain situational awareness, and supports emergency traffic and Mayday transmission.

Read closely, the requirements push you toward simplex as the operational default for tactical channels:

• The communications plan must support reliable PTT communication between the IC and operating crews.
• Mayday transmissions must be receivable by the IC and by other crews on the fireground.
• The IC must be able to issue emergency evacuation orders and confirm receipt by all crews.

If your tactical channel is a P25 talkgroup that may or may not work in the basement, you cannot meet those requirements with confidence. If your tactical channel is a simplex frequency that all crews on scene are on, and the IC is at the command post within RF range of the structure, you can.

NFPA 1500 (occupational safety) and NFPA 1710 / 1720 (career and volunteer deployment) reference the same incident management framework. NFPA 1221 covers public safety communications systems infrastructure. None of these standards require simplex by name, but the overall standard of care points there.

A working simplex SOP framework

Here is a framework you can adapt. It is deliberately short. The longer your SOP, the less likely a firefighter at 0300 on a working fire will execute it correctly.

Channel architecture

Define three classes of channels and use them consistently:

• Dispatch / hailing. Repeated or trunked. Used for dispatch, status updates to comm center, and resource requests. Wide-area coverage required.
• Command. Repeated or trunked. Used between the IC and command staff, between command and dispatch, and for inter-agency coordination at multi-agency incidents.
• Tactical (fireground). Simplex. Used by all crews operating on the fireground - interior, exterior, RIT, water supply. This is the channel where Mayday lives.

The default rule

Write one sentence in the SOP that everyone can quote: "All operating crews go to the assigned tactical simplex channel on arrival. Command and dispatch traffic stays on the assigned dispatch channel." That is the SOP in a nutshell. Everything else is detail.

Channel assignment by alarm level or by incident

Two reasonable approaches:

• Pre-assigned by alarm level. First-due structure fire goes to TAC-1. Second alarm goes to TAC-2 for divisions, TAC-1 stays as primary fireground. Simple, easy to remember.
• Assigned by dispatch on incident creation. Comm center picks an available tactical from a pool and announces it on dispatch. More flexible, requires dispatcher discipline.

Pick one and stick to it. Hybrid approaches confuse crews and create the dreaded "what channel are you on?" loop on a working fire.

Mayday on the tactical channel

Mayday goes on the tactical (simplex) channel. Period. Do not require the down firefighter to switch to a separate emergency talkgroup. They will not. They are upside down in a basement with an SCBA alarm chiming. The IC and RIT are already monitoring tactical. That is where the Mayday belongs.

The orange emergency button on the radio is a separate question. On most P25 systems it sends an emergency alert on a designated talkgroup. Set policy on whether crews are trained to use it on top of voice Mayday on tactical, or whether voice Mayday alone is the standard. Whichever you choose, train it consistently.

Backup plan if simplex fails

Simplex can fail too. Long incidents, large buildings, terrain that puts crews behind a hill from each other. Your SOP should name the backup:

• Move the IC closer to the structure to maintain RF range with interior crews.
• Position a portable repeater (vehicular repeater system on a chief vehicle, or a deployable cross-band repeater) if available.
• Stand up a designated relay - a member at a known position whose job is to relay traffic between IC and a distant crew.
• Fall back to the trunked tactical talkgroup if simplex range is exhausted, with explicit announcement to all crews.

Document the channel plan on the apparatus

Every apparatus should have a laminated or printed channel reference: which physical channel position is which logical channel, in plain English. "TAC-1" should not require translation. Officers should know without thinking which knob position is the simplex tactical channel for first-due.

Training, talkgroup hygiene, and the pitfalls

Writing the SOP is the easy part. Making it work in practice takes drilling. Common failure modes:

Crews never actually switch channels

The most common failure: dispatch sends a structure assignment. The first-due engine acknowledges on dispatch and arrives. Nobody changes channels. Command tries to assign a tactical and discovers half the apparatus is still on dispatch. Now command is repeating every transmission on two channels. Train channel changes as part of the response checklist. Officer's first action on arrival, after announcing on scene: "All crews, switch to TAC-1."

The IC stays on the wrong channel

Equally common: the IC sets up command and stays on dispatch because that is where they hear the resources rolling in. Meanwhile interior crews are on TAC-1 talking into the void. The IC's primary channel during active fireground operations is the tactical channel. Dispatch traffic is monitored or routed through a command aide, not heard directly by the IC.

Mutual aid arrives on the wrong channels

Mutual aid is where good SOPs go to die. The neighboring department's radios may not have your tactical simplex frequency, or it may be programmed in a different channel position. Solutions:

• Standardize on a regional set of NIFOG / interoperability simplex channels (VFIRE21-26, VTAC11-14, UTAC41-43, 8TAC91-94 on 700/800 MHz). These are nationally designated. Every public safety radio in the U.S. should have them. They are explicitly intended for incident-scene use without coordination.
• Pre-program a common regional tactical and rehearse it with mutual aid partners.
• Carry spare portables in the chief vehicle programmed with neighboring agencies' channels as a redundancy layer.

Training only happens with full crews on a clear day

Drill simplex transmission from inside the structures you actually run on. Have a probie with a portable in the basement of the local nursing home and check whether the IC at the front door hears them. If the answer is no, your SOP needs to specify that interior crews on that occupancy class use a relay or the IC repositions. Find out before the working fire, not during it.

Talkgroup proliferation on trunked systems

P25 trunked systems make it easy to create dozens of talkgroups. Departments end up with TAC-1 through TAC-12, plus FIRE-OPS, plus EMS-OPS, plus ADMIN, plus a half-dozen mutual aid groups. Crews cannot remember which talkgroup is which. Keep the active list short. Most departments need 4 to 6 talkgroups on the radio for daily use. Anything beyond that is a reference list, not a daily-use channel.

If you are a small or volunteer department

Most of this guide is written assuming you are inside a county or regional P25 system and need to work simplex into that environment. If you are a small career or volunteer department running a legacy VHF system, the picture is simpler and in some ways better.

VHF simplex on a single licensed channel, monitored by everyone, has been the working fireground communications model for decades. It works because everyone is on the same frequency, the radios are simple, and there is no infrastructure to fail. The downsides are that you have less spectrum to work with and inter-agency coordination requires explicit channel selection.

If your county is migrating to P25, the worst thing you can do is throw out the simplex VHF channel the day the new system goes live. Keep it as a backup. Program it into the new portables if they are dual-band, or keep enough VHF radios on apparatus to cover an interior crew. The new system will fail. Know what you fall back to.

For a small department, the SOP can be even simpler:

• Channel 1: Dispatch (repeated, talks to comm center).
• Channel 2: Fireground simplex. All operating crews go here on arrival.
• Channel 3: Mutual aid simplex (regional or NIFOG channel).

That is the entire SOP for a 25-member department. It will serve you better than a 14-page document copied from a metro agency.

The bottom line for the chief

Trunked P25 systems are good infrastructure. They are not a fireground tactical solution. The fireground is a short-range, RF-hostile environment where the radios that need to talk to each other are usually within a few hundred feet, separated by walls and floors instead of miles of clear air. Simplex is what that environment was designed for.

The SOP changes you can make this quarter:

1. Designate one or more simplex channels as the tactical fireground default.
2. Write a one-page SOP that says all operating crews go to tactical on arrival.
3. Train the channel switch on every drill until it is automatic.
4. Drill Mayday on simplex from realistic interior positions in your actual response district.
5. Coordinate with mutual aid partners on a common regional simplex channel.
6. Document the backup plan when simplex range is exhausted.

None of this requires a budget request. None of it requires the county to redo its system. It requires policy, training time, and the willingness to write something down that contradicts the impression a vendor coverage map gave to the elected officials who funded the radio system.

Further reading

• NFPA 1561 - Standard on Emergency Services Incident Management System and Command Safety
• NIST Public Safety Communications Research (PSCR) Program
• CISA SAFECOM - National Interoperability Field Operations Guide (NIFOG)
• U.S. Fire Administration - operational guidance and after-action reports