USB Data Transfer Protocols: UASP, BOT, and Mass Storage Compared

The Invisible Protocol That Affects Your Transfer Speed

When you plug a USB flash drive into your computer, data transfers at a certain speed. But did you know that the protocol used to manage that transfer can make a 20-30% speed difference? The protocol is invisible to the user — it’s negotiated automatically — but understanding it helps you choose the right hardware for maximum performance.

At USBToUSB, our USB 3.0 hubs and cables support the latest data transfer protocols. Here’s what UASP, BOT, and Mass Storage Class mean for your daily use.

USB Mass Storage Class (MSC)

USB Mass Storage Class is the base protocol that all USB storage devices use. It defines how the computer communicates with USB flash drives, external hard drives, and card readers. MSC is a set of commands that the computer sends to the storage device to read and write data.

Within MSC, there are two transport protocols that carry these commands over USB:

1. BOT (Bulk-Only Transport): The older, simpler protocol
2. UASP (USB Attached SCSI Protocol): The newer, faster protocol

BOT: The Legacy Protocol

BOT (Bulk-Only Transport) was introduced with USB 1.0 and remains the fallback protocol for all USB storage devices. It’s simple, universally compatible, and reliable — but it’s not efficient at high speeds.

How BOT works:

1. Computer sends a command to the USB device
2. Computer waits for acknowledgment
3. Computer sends or receives data in a single sequential stream
4. Computer waits for completion confirmation
5. Only then can the next command be sent

This sequential command processing creates a bottleneck: each command must complete before the next begins. At USB 2.0 speeds (480 Mbps), this overhead is negligible. But at USB 3.0+ speeds (5+ Gbps), the command overhead becomes a significant percentage of total transfer time.

UASP: The Modern Protocol

UASP (USB Attached SCSI Protocol) was introduced with USB 3.0 and is designed for high-speed data transfer. It uses SCSI (Small Computer System Interface) commands — the same protocol used by internal SATA and SAS drives — adapted for USB.

How UASP improves on BOT:

1. Command Queuing

UASP can send multiple commands simultaneously and process them in parallel. While one command’s data is transferring, the next command is already being prepared. This eliminates the “wait for completion” bottleneck of BOT.

2. Parallel Data Streams

BOT transfers data in a single stream. UASP can maintain multiple data streams simultaneously, using USB 3.0’s full bandwidth more efficiently. This is particularly beneficial for SSDs, which can handle multiple I/O operations in parallel.

3. Reduced CPU Overhead

UASP’s protocol design requires fewer CPU interrupts per transfer compared to BOT. This frees up CPU cycles for other tasks and reduces overall system load during large file transfers.

4. Better Error Recovery

When a transfer error occurs, BOT must restart the entire transfer from the beginning. UASP can retry individual packets without restarting the whole transfer, making error recovery faster and more efficient.

In practice Speed Comparison

We tested the same Samsung T7 SSD on the same computer with both protocols:

Test	BOT	UASP	Improvement
Sequential Read	480 MB/s	510 MB/s	+6%
Sequential Write	440 MB/s	480 MB/s	+9%
Random Read (4K)	28 MB/s	35 MB/s	+25%
Random Write (4K)	25 MB/s	32 MB/s	+28%
50GB File Transfer	1 min 52 sec	1 min 38 sec	-14 seconds
CPU Usage (during transfer)	12%	7%	-42%

The improvement is most noticeable with random I/O (small file transfers), where UASP’s command queuing makes the biggest difference. Sequential transfers see modest improvements because the bottleneck shifts to USB bandwidth rather than protocol overhead.

How to Check Which Protocol You’re Using

Windows

1. Open Device Manager
2. Expand “Universal Serial Bus controllers”
3. Look for “USB Attached SCSI (UAS) Mass Storage Device” — if present, UASP is active
4. If you see “USB Mass Storage Device” only, BOT is being used

macOS

1. Open System Information (Apple menu → About This Mac → System Report)
2. Under Hardware, select USB
3. Find your USB storage device in the device tree
4. Look for “Speed:” — if it shows “up to 5000 Mb/s” and the device is UASP-compatible, UASP is likely active
5. Check “Protocol:” field — it should show “USB Attached SCSI (UAS)” if UASP is active

Linux

Run: lsusb -t — if the driver shows “uas”, UASP is active. If it shows “usb-storage”, BOT is being used.

What Determines Which Protocol Is Used?

The protocol is negotiated automatically when you plug in a device. The selection depends on:

• Device support: The USB storage device must support UASP. Most modern SSDs and enclosures do; older HDDs may not.
• Computer/controller support: The computer’s USB controller and drivers must support UASP. Windows 8+, macOS 10.8+, and Linux 3.6+ support UASP.
• Hub support: If using a USB hub, the hub’s controller must support UASP pass-through. Our A-809 4-Port USB 3.0 Hub supports UASP.
• Cable quality: Poor quality cables may cause UASP negotiation to fail, falling back to BOT.

If any component in the chain doesn’t support UASP, the system falls back to BOT automatically. This is smooth — there’s no error message or indication that UASP failed to initialize.

USB Hubs and UASP Compatibility

Not all USB hubs support UASP pass-through. The hub’s controller chip determines UASP compatibility:

• VL817 (VIA Labs): Full UASP support, commonly found in premium hubs
• GL3523 (Genesys): Full UASP support
• FL1100 (Fresco Logic): Full UASP support
• Cheap unbranded chips: May not support UASP, forcing BOT fallback

Our A-809 Aluminum Hub uses a quality controller chip that supports UASP, ensuring maximum transfer speeds through the hub.

When UASP Doesn’t Matter

UASP provides negligible benefit in these scenarios:

• USB 2.0 devices: The 480 Mbps bandwidth limit dwarfs the protocol overhead
• Mechanical HDDs: The drive’s mechanical latency is the bottleneck, not the protocol
• Flash drives (basic): Cheap flash drives have slow internal controllers that don’t benefit from UASP
• Single large file transfer: Sequential transfer speed is limited by USB bandwidth, not protocol

When UASP Matters Most

• NVMe SSD enclosures: UASP is essential for achieving full SSD speed over USB
• Many small files: UASP’s command queuing dramatically speeds up transfers of many small files
• Multiple storage devices on one hub: UASP manages multiple I/O streams efficiently
• Low-latency applications: Audio recording, video editing, and real-time data acquisition benefit from UASP’s reduced latency