What Is a DisplayPort Branch Device in an MST Hub?

A DisplayPort branch device is an active hardware component defined in the VESA DisplayPort standard that enables a single video output to connect to multiple independent displays. This article explains what a branch device is, its role within Multi-Stream Transport (MST) technology, and the step-by-step process of how it manages, routes, and splits video signals inside an MST hub.

Defining the DisplayPort Branch Device

In the VESA DisplayPort specification, devices in a video transmission chain are categorized into two primary types: Source/Sink devices and Branch devices. While a source generates the signal (such as a GPU) and a sink consumes it (such as a monitor), a branch device acts as an intermediate routing node.

Introduced alongside the Multi-Stream Transport (MST) protocol in DisplayPort 1.2, a branch device accepts an incoming multi-stream signal from a single upstream port and distributes individual video streams to downstream ports. Physical implementations of branch devices include standalone MST hubs, daisy-chainable monitors with DP-out ports, and docking stations.

How a Branch Device Functions Inside an MST Hub

An MST hub relies on an internal branch device chip to manage communication, allocate bandwidth, and route data. Its operation involves four primary functions:

1. Topology Discovery and Communication

When an MST hub is plugged into a host PC, the branch device communicates with the GPU using the DisplayPort Auxiliary (AUX) channel. It reports its presence and queries any connected monitors downstream to gather their Extended Display Identification Data (EDID) and DisplayPort Configuration Data (DPCD). The branch device presents this layout to the host operating system as a tree topology, allowing the system to recognize each connected screen as an independent physical display rather than a mirrored clone.

2. Bandwidth Negotiation and Management

The total bandwidth of the single DisplayPort connection from the host is shared among all connected screens. The branch device negotiates link rates with the host (such as HBR2, HBR3, or UHBR rates) and verifies that the sum of the downstream displays' resolutions, refresh rates, and color depths does not exceed the link's maximum throughput. If bandwidth is constrained, the branch device and host coordinate fallback resolutions or utilize Display Stream Compression (DSC) if supported.

3. Stream Demultiplexing and Routing

In MST mode, the graphics card packets multiple independent video and audio streams into a single, time-multiplexed data stream across the main link lanes. The branch device's internal switching fabric inspects the transport packet headers, extracts the individual streams according to their Virtual Channel IDs, and routes each stream directly to its designated downstream physical port without adding perceptible latency.

4. Signal Conversion

Many branch devices inside MST hubs also incorporate protocol converters. In addition to routing native DisplayPort streams, they can convert downstream DisplayPort signals into other formats such as HDMI, DVI, or legacy VGA, allowing diverse display setups from a single source connection.