In USB 2.0, all communication between the host and a high-speed hub occurs at high speed (480 Mbps). However, low-speed and full-speed devices may still be connected downstream of the hub.

To communicate with these slower devices without blocking the high-speed bus, USB 2.0 uses the Split Transaction Protocol.

Why Split Transactions Are Needed

Without split transactions, a full-speed transaction (12 Mbps) behind a high-speed hub would occupy the high-speed bus for the entire duration of the slow transaction:

Split transactions solve this by decoupling the high-speed bus activity from the slow downstream transaction.

How Split Transactions Work

A split transaction divides communication into two separate high-speed transactions with a gap in between:

          High-Speed Bus                Hub TT              LS/FS Device
               │                          │                      │
   ┌───────────┤                          │                      │
   │Start Split│─── HS ──────────────────►│                      │
   │  (SS)     │  Token + optional data   │                      │
   └───────────┤                          │                      │
               │     ◄── ACK ─────────────┤                      │
               │                          │                      │
               │   ★ Bus is FREE here ★  │── FS/LS ────────────►│
               │    (other HS traffic     │  Actual transaction  │
               │     can use the bus)     │  at native speed     │
               │                          │◄─────────────────────│
   ┌───────────┤                          │  Response stored     │
   │Complete   │─── HS ──────────────────►│                      │
   │Split (CS) │  Request result          │                      │
   └───────────┤                          │                      │
               │     ◄── Data/Status ─────┤                      │
               │                          │                      │

The key insight: between Start Split and Complete Split, the high-speed bus is free for other traffic.

Split Transaction Phases

Phase 1: Start Split (SSPLIT)

The host sends a Start Split token at high speed, containing:

This is followed by the normal token (IN/OUT/SETUP) and optional data packet, all at high speed.

The Transaction Translator then:

1. Identifies the target downstream port
2. Buffers the transaction data
3. Responds to the host with ACK (accepted the split request)
4. Begins the LS/FS transaction on the downstream port at native speed

Phase 2: Complete Split (CSPLIT)

Later (typically in a subsequent microframe), the host sends a Complete Split token to retrieve the result:

The TT responds with:

• The data or status from the completed LS/FS transaction, or
• NYET if the transaction hasn't completed yet

Complete Split Response Codes

Example: Bulk IN Split Transaction

This example shows the packet sequence associated with a Bulk IN split transaction where the downstream hub ports have low- and full-speed devices attached. In this example, a single Transaction Translator sends a packet to the full-speed device.

Sequence

1. Start Split (SS) Transaction

Every split transaction begins with the host sending the Start Split (SS) token packet followed by either:

• An IN token packet, or
• An OUT token followed by a Data packet

2. Transaction Translator Operation

The Transaction Translator (TT) within the hub:

• Stores the token packets and data (if an OUT transaction)
• Starts the transaction at the native speed of the attached device (full-speed in this example)
• Stores the Data payload returned from the device
• Awaits the Split Completion transaction from the host

3. Complete Split (CS) Transaction

The host sends the Complete Split (CS) transaction consisting of the same token packets sent in the SS transaction to verify the entry being completed. In this example:

• The data payload is returned to the host via a Data packet
• This completes the Split transaction

Detailed Packet Sequence

Microframe N:
  Host ──── SSPLIT Token ──────────────────► Hub (TT)
  Host ──── IN Token ──────────────────────► Hub (TT)
  Host ◄──── ACK ──────────────────────────── Hub (TT)

       (TT stores tokens and begins FS transaction)
       (TT performs FS IN at native device speed)
       (TT stores returned data payload)
       (HS bus is FREE for other traffic)

Microframe N+1:
  Host ──── CSPLIT Token ──────────────────► Hub (TT)
  Host ──── IN Token ──────────────────────► Hub (TT)
  Host ◄──── DATA0 (64 bytes from device) ── Hub (TT)
  Host ──── ACK ───────────────────────────► Hub (TT)

Example: Interrupt IN Split Transaction

For periodic transfers (interrupt), the timing is tightly scheduled:

Major Components of a High-Speed Hub

1. Hub Repeater

Handles high-speed upstream/downstream traffic:

• Receives high-speed packets from the host
• Buffers and re-clocks packets
• Forwards packets that are not split transactions

2. Transaction Translator (TT)

The TT is the core component for split transactions — it acts as a miniature LS/FS host controller embedded in the hub:

• Receives Start Split data from the host
• Performs the actual LS/FS transaction at native device speed
• Stores returned data/status in internal buffers
• Returns results during Complete Split

Single-TT vs. Multi-TT Hubs

3. Hub Controller

Manages hub-level operations:

• Processes hub class requests (via EP0)
• Reports port status changes (connect, disconnect, overcurrent)
• Handles port reset and suspend

Benefits of Split Transactions

Applicability

Relationship to SuperSpeed

USB 3.0 SuperSpeed does not use split transactions. Instead:

• SuperSpeed hubs maintain a completely separate bus for SS traffic
• Legacy LS/FS/HS devices are handled by the hub's USB 2.0 section
• The two buses operate independently via the dual-bus architecture