February 11, 2026
Monitor Logistics: Decoding the Flow
It isn't just about cables and screens; it’s about traffic control. We dive into the invisible logistics, from the 'private charters' of SST to the 'carpools' of MST that turn a single laptop port into a panorama of productivity.
In our previous discussion, “Decoding the USB-C Chaos,” we covered the Physical Media (Capacity), the Infrastructure (Quality), and the Cargo (Protocols). But having a wide road and fast trucks isn’t enough; you need a Traffic Control System.
This layer—Logistics Management—dictates how your data cargo is packed, routed, and delivered. This is why a $50 USB Hub might work perfectly on Windows but fail miserably on a $3,000 MacBook, while a $200 Thunderbolt 4 dock might work perfectly for both.
SST (Single-Stream Transport)#
- Role: The “Express Charter” Truck
- Definition: The Operating System dedicates one entire data “vehicle” (DisplayPort Lane) to a single destination (Monitor).
- The Analogy: You book a truck exclusively for Client A. Even if the truck is half-empty, the driver is forbidden from picking up cargo for Client B. It is inefficient but guarantees quality.
- Windows Behavior: Windows uses this by default for single monitor connections. It creates a direct, dedicated link for maximum stability.
- macOS Behavior: Apple enforces this strictly. To ensure maximum signal integrity, macOS insists on sending one dedicated stream per monitor. If you try to split a stream (via a cheap hub), macOS refuses to cooperate and sends the same “charter” to everyone, resulting in mirrored screens.
MST (Multi-Stream Transport)#
- Role: The “Carpool” Delivery
- Definition: A protocol that bundles multiple video signals into one DisplayPort stream and splits them at a hub. Crucially, it is a “First-Come, First-Served” scramble. It ignores the host’s total limit; Monitor A grabs what it needs, and Monitor B fights for the remaining scraps. It activates when the number of physical DisplayPort lanes is less than the number of monitors.
- The Analogy: A single truck carries crates for both Client A and Client B. The driver stops at A first. If A takes up 80% of the truck space, B gets whatever is left, regardless of what B actually ordered.
- Windows Behavior: Fully supports “Carpooling.” It allows extended mode on almost any USB-C hub, automatically negotiating (and sometimes downgrading) resolution to fit everyone on the truck.
- macOS Behavior: Does NOT support MST over USB-C. macOS refuses to sign the “Carpool Manifest.” It treats the MST hub as a single destination and delivers one block of cargo, forcing all screens to display the exact same image (Mirroring).
Daisy Chaining (via USB-C DP Alt)#
- Role: The “Multi-Stop Route” (Shared Truck)
- Definition: A topology where monitors are linked in series (PC → A → B) using the MST Protocol. It activates because a single USB-C cable (1 DisplayPort stream) is trying to feed 2 monitors. Monitor A siphons off its share of bandwidth and passes the rest to Monitor B.
- The Analogy: One shared truck drives down a standard paved road. It stops at House A to drop off packages, then drives through House A’s driveway to reach House B. If the truck is empty by the time it leaves A, House B gets nothing.
- Windows Behavior: Works seamlessly. Windows recognizes the “Route” and assigns unique desktops to Monitor A and Monitor B, provided there is enough total bandwidth in the cable.
- macOS Behavior: Fails to extend. Because this relies on the MST “Carpool” protocol, macOS treats the entire route as one stop. Monitor A and Monitor B will show the exact same mirrored desktop.
Daisy Chaining (via TB 3/4)#
- Role: The “Dual-Convoy” (Express Trucks)
- Definition: A topology where monitors are linked in series (PC → A → B), but utilizing Thunderbolt’s ability to carry two independent DisplayPort streams. Since the number of DisplayPort streams (2) equals the number of monitors (2), MST (splitting) is not required or activated.
- The Analogy: Two separate express trucks travel down a massive Superhighway (Thunderbolt). Both arrive at Station A. Truck #1 unloads for A. Truck #2 enters a “tunnel” inside Station A and passes straight through, untouched, to Station B. No sharing, no fighting for space.
- Windows Behavior: Works perfectly. Windows sees two distinct, high-bandwidth connections and drives them independently.
- macOS Behavior: Works perfectly. Because this uses two distinct streams (Dual-SST) instead of splitting one (MST), macOS happily supports extended desktop mode for both monitors.
DSC (Display Stream Compression)#
- Role: The “Vacuum Packer”
- Definition: A visually lossless compression standard that reduces the bandwidth required for high-resolution video. It activates when the video signal exceeds the physical bandwidth limits of the cable.
- The Analogy: The cargo (4K 144Hz video) is too fluffy and bulky for the truck. The GPU uses an industrial vacuum sealer to suck the “air” (redundant pixel data) out of the cargo, shrinking it to 1/3rd of its size. It re-inflates instantly upon delivery.
- Windows Behavior: Widely supported on modern GPUs (NVIDIA/AMD/Intel). It enables high-refresh-rate monitors to run over older cables or allows 3-monitor setups on a single dock.
- macOS Behavior: Supported on M1/M2/M3 Pro & Max chips. It enables these Macs to drive high-spec displays (like Pro Display XDR) that would otherwise choke the connection.
DisplayLink (USB Graphics)#
- Role: The “Furniture Disassembly” Service
- Definition: A technology that transmits video over standard USB Data protocols (not Video protocols) using software encoding. It completely bypasses the GPU’s video lane limits but relies on the CPU to process the image.
- The Analogy: The main highway is closed to heavy trucks (No Video support on the port). So, you hire a team to disassemble the furniture (video frame) into sawdust and screws. You ship the parts via a standard mail courier (USB Data), and a chip at the destination glues them back together.
- Windows Behavior: The “Universal Fix.” It allows users to connect 4+ monitors to basic laptops that technically only support one. Great for spreadsheets, but the “reassembled furniture” might look laggy during gaming.
- macOS Behavior: The “Cheat Code.” This is the only way to get dual extended screens on base-model M1/M2/M3 MacBooks (which natively support only one external screen). It bypasses Apple’s artificial restriction by pretending the second screen is just a USB data device.
Bottleneck: Ports ≠ Screen#
Don’t let the number of Thunderbolt ports fool you. Here is the hidden logistics logic:
- Ports are just “Roads.” The Chip is the “Factory.” Even if you have 10 empty lanes (Thunderbolt 5 Ports), if the Factory Manager (Display Engine) can only process 2 orders at a time, the other 8 lanes stay empty.
- Mac mini M4 Pro: It has 3x Thunderbolt 4 ports + HDMI. Theoretically, these ports could drive 7 screens via daisy-chaining. However, the chip strictly cuts off the signal after the 3rd monitor.
- M3 (The “Clamshell” Compromise): Facing “toothpaste squeezing” criticism (since M3 still only had 2 engines), Apple finally unlocked a logic switch. You must close the laptop lid to disable the internal screen, freeing up its engine for a second external monitor. It wasn’t new hardware, just a permission slip to reroute existing traffic.
| Chip | MacBook Air | MacBook Pro | Mac mini | iMac | Mac Studio | Mac Pro |
|---|---|---|---|---|---|---|
| M1 | 1 | 1 | 2 | 1 | – | – |
| M1 Pro | – | 2 | – | – | – | – |
| M1 Max | – | 4 | – | – | 5 | – |
| M1 Ultra | – | – | – | – | 5 | – |
| M2 | 1 | 1 | 2 | Skipped | – | – |
| M2 Pro | – | 2 | 3 | Skipped | – | – |
| M2 Max | – | 4 | – | Skipped | 5 | – |
| M2 Ultra | – | – | – | Skipped | 8 | 8 |
| M3 | 1/2 (Clamshell) | 1/2 (Clamshell) | Skipped | 1 | – | Skipped |
| M3 Pro | – | 2 | Skipped | – | – | Skipped |
| M3 Max | – | 4 | Skipped | – | Skipped | |
| M3 Ultra | – | – | Skipped | – | 8 | Skipped |
| M4 | 2 | 2 | 3 | 2 | – | Skipped |
| M4 Pro | – | 2 | 3 | – | – | Skipped |
| M4 Max | – | 4 | – | – | 5 | Skipped |
| M5 | 2 | 2 | Pending | Pending | Pending | Pending |
Summary#
| Tech | Role | Win | mac | Display Engine | Image Quality | Note |
|---|---|---|---|---|---|---|
| SST | Standard Truck | Extend | Extend | 1 | Native | Standard connection |
| MST | Cargo Splitter | Extend | Mirror Only | 1 | Bandwidth Shared | Mac mirrors external screens (A-B-B). |
| Daisy Chaining (via USB-C DP Alt) | 1-Lane-Tunnel | Extend | Mirror Only | 1 | Bandwidth Shared | Mac mirrors external screens (A-B-B). |
| Daisy Chaining (via TB 3/4) | 2-Lane-Tunnel | Extend | Extend | 2 | Native | Supports up to 2 chained monitors |
| Daisy Chaining (via TB 5) | 3-Lane-Tunnel | Extend | Extend | 3 | Native | Supports up to 3 chained monitors |
| DSC | Vacuum Packer | Extend | Extend | N/A | Visually Lossless | Does not take additional DP |
| USB Graphics | Furniture Smasher | Extend | Extend | 0 | Latency | Bypasses DP limits |
FAQ#
Q: What is the difference between a $50 USB-C Hub and a $200 Thunderbolt 4 Dock?
A: The difference lies in how they manage the data traffic.
- The $50 USB-C Hub (The “Splitter”): If a seller doesn’t explicitly state “Thunderbolt,” the device uses the MST Protocol. It only pulls one DisplayPort stream from your computer (even if your port could provide two). It then tries to slice that single stream into pieces for multiple monitors. However, macOS strictly forbids “slicing” a single stream. It treats the hub as one destination, so every monitor connected to it shows the exact same image (Mirroring).
- The $200 Thunderbolt 4 Dock (The “Dual-Lane Highway”): This dock accesses the full 40Gbps bandwidth and extracts two independent DisplayPort streams directly from the host. It routes Stream #1 to the first video port and Stream #2 to the second. Since each monitor gets its own dedicated, unsplit Displayport stream, macOS fully supports extended desktop mode.
Q: Does a Thunderbolt 4 Dock use MST technology? If so, will macOS only mirror?
A: Not for the first two monitors. Unlike a regular USB-C hub, a Thunderbolt 4 Dock creates a high-bandwidth “Tunnel” that carries two independent DisplayPort streams directly from your Mac.
- Regular USB-C Hub (The “1-Lane” Tunnel): It extracts only 1 DisplayPort Stream from the host, even if the host is capable of more.
- Connecting 1 Monitor: The math is balanced (1 Stream = 1 Monitor). No splitting is required. macOS sees one connection and allows Extended Mode.
- Connecting 2 Monitors: The limit is exceeded (2 > 1). The second monitor forces the hub to use MST to split the single lane. Since macOS rejects MST, the 2nd monitor will Mirror the first.
- Thunderbolt 4 Dock (The “Two-Lane” Tunnel): It natively receives 2 DisplayPort Streams from the host’s Thunderbolt controller.
- Connecting 2 Monitors: The math is balanced (2 Streams = 2 Monitors). No splitting (MST) is required. macOS sees two separate, independent connections and allows Extended Mode.
- Connecting 3 Monitors: The limit is exceeded (3 > 2). The 3rd monitor would require MST to share a lane. Since macOS rejects MST, the 3rd monitor will typically Mirror one of the others (or remain black, depending on the dock’s internal wiring).
- Thunderbolt 5 Dock (The “Three-Lane” Tunnel): Thanks to massive bandwidth increases (80Gbps+), TB5 increases the tunnel capacity to 3 DisplayPort Streams.
- Connecting 3 Monitors: The math is balanced (3 Streams = 3 Monitors). No splitting is required. macOS allows Extended Mode for all three.
- Connecting 4 Monitors: The limit is exceeded (4 > 3). The 4th monitor would require MST to share a lane. Since macOS rejects MST, the 4th monitor will Mirror one of the others.
