State of Play: FPS Arch Explained

While playing a Call of Duty: Warzone match last night, I chucked a semtex grenade towards a window where I had seen a sniper glint. A second later, the hit marker confirmed the successful takedown. It felt incredible, but it also made me wonder.

How does the server, potentially hundreds of kilometers away, perfectly calculate the arc of my grenade, register its impact on a player sprinting inside a building miles away, and communicate that outcome to a hundred other players scattered across the region, all in the blink of an eye? How does it handle that, plus all the other bullets, vehicles, and explosions happening at the same time?

I read a number of articles available online, watched a few videos and did my own research. This is what I learned and got really fascinated about.

First-Person Shooter (FPS) games like Call of Duty, Valorant, etc. rely on a finely tuned network architecture designed for split-second responsiveness and fair play. Here's a look at the engine humming under the hood.

The Client-Server Model: Precision and Fairness

Fig.1

This diagram shows how multiple players, each on their own device (the clients), connect to a central dedicated server. This server acts as the single source of truth for the game, processing all player actions and sending back the updated game state to everyone. This prevents cheating and ensures a fair and consistent experience.

At the heart of any competitive FPS is the client-server model, with an extreme emphasis on server authority. This isn't a simple peer-to-peer connection between players; it's a centralized system built to ensure fairness.

• Client (Your Device): Your phone or PC is the client. Its job is to render the stunning graphics, play crucial audio cues (like enemy footsteps), and, most importantly, capture your inputs.

• Dedicated Server (The Ultimate Referee): This is a powerful, remote computer that runs the actual game simulation. These servers are often located in regional data centers to keep latency low for everyone in that area. The server receives inputs from all players, processes them, and determines the one true state of the game. It decides who shot first, if that shot was a hit, and where every player is at any given moment. This server-authoritative model is crucial for preventing cheating, as a player can't simply hack their own game to become invincible or get perfect aim. The server has the final say.

Ultra-Fast Networking: Why Your Ping Matters

In FPS games, even a tiny delay (latency or "ping") can be the difference between winning and losing a gunfight. This is why the choice of networking protocol is so important.

• UDP (User Datagram Protocol): UDP prioritizes speed over reliability. It fires off packets of information (like your position or that you've fired your weapon) without waiting for confirmation that they arrived. If a single packet of your opponent's movement is lost, the game can often predict where they were going anyway, and the delay of waiting for that lost packet would be far worse.

• TCP (Transmission Control Protocol): You'll find TCP used for less time-sensitive actions, like logging into the game, Browse the item shop, or sending a friend request. It's slow but reliable, ensuring the information gets there correctly, which is more important for these tasks.

Making Up for Lost Time: Lag Compensation

Because of network congestion, there will always be some latency. Servers use a clever trick called lag compensation. When you shoot at a player, the server essentially "rewinds" the game state back to the moment you clicked the button, based on your ping. It checks to see if your target was in your crosshairs at that exact moment in time on your screen. This is why you can sometimes get shot by an enemy even after you think you've made it safely behind a wall.

The Game Loop: High "Tick Rate" for High Stakes

Fig.2

This diagram illustrates the continuous cycle, known as the game loop, that powers the game on the server. It's a constant, high-speed process of Processing Inputs, Updating Game State and Broadcasting state.

The server's brain operates in a continuous, high-speed cycle called the game loop. The frequency of this loop is called the tick rate.

• Process Inputs: The server collects the latest batch of inputs from all connected players.

• Update Game State: It advances the game simulation. This is where it calculates bullet trajectories, collision detection (did the bullet hit a head or a wall?), player movement, and ability effects. Competitive games use high tick rate servers (e.g., 128-tick) which update the game state 128 times every second for maximum precision.

• Broadcast State: The server sends a snapshot of this new game state back out to every player's client.

Your client receives these snapshots and uses interpolation to smoothly animate the characters on your screen, creating the seamless illusion of a shared, real-time battleground.

So the next time you land that perfect shot, you'll know about the incredible, high-speed dance of data that made it happen.

Lastly, I would like to attach a link to a YouTube video below that beautifully captures the crux of this concept.