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Understanding HDLC Packets

How HDLC packets work in SINTRAN

HDLC Frame Structure

Every HDLC frame follows this structure:

┌──────┬─────────┬─────────┬──────────┬────────┬──────┐
│ FLAG │ ADDRESS │ CONTROL │   DATA   │  FCS   │ FLAG │
│ 0x7E │ 1 byte  │ 1 byte  │ N bytes  │2 bytes │ 0x7E │
└──────┴─────────┴─────────┴──────────┴────────┴──────┘

Field Details

  1. Opening FLAG (0x7E): Marks the start of a frame
  2. ADDRESS: Identifies the destination/source
  3. CONTROL: Frame type and sequence numbers
  4. DATA: User payload (variable length)
  5. FCS (Frame Check Sequence): CRC-16-CCITT for error detection
  6. Closing FLAG (0x7E): Marks the end of a frame

Frame Types

I-Frames (Information)

  • Purpose: Carry user data
  • Control: 0x00-0x7F (bit 0 = 0)
  • Contains: Sequence numbers for reliable delivery

S-Frames (Supervisory)

  • Purpose: Flow control and acknowledgments
  • Control: 0x01, 0x05, 0x09, 0x0D
  • Types: RR (Receive Ready), RNR (Receive Not Ready), REJ (Reject)

U-Frames (Unnumbered)

  • Purpose: Control functions
  • Control: Various (bit pattern 11xxxxxx)
  • Examples: SABM (Set Async Balanced Mode), DISC (Disconnect), UA (Unnumbered Ack)

Packet Lifecycle

Transmission

sequenceDiagram
    participant App as Application
    participant SINTRAN as SINTRAN OS
    participant DMA as DMA Controller
    participant COM as COM5025
    participant Line as Physical Line

    App->>SINTRAN: Send Data
    SINTRAN->>SINTRAN: Validate & Setup
    SINTRAN->>DMA: Configure Descriptor
    DMA->>COM: Transfer Data
    COM->>COM: Add HDLC Framing
    COM->>Line: Transmit Frame
    COM-->>SINTRAN: Interrupt (Complete)
    SINTRAN-->>App: Success/Failure

Reception

sequenceDiagram
    participant Line as Physical Line
    participant COM as COM5025
    participant DMA as DMA Controller
    participant SINTRAN as SINTRAN OS
    participant App as Application

    Line->>COM: Receive Frame
    COM->>COM: Strip Framing & Check CRC
    COM->>DMA: Write Data to Memory
    DMA-->>SINTRAN: Interrupt (Data Ready)
    SINTRAN->>SINTRAN: Validate & Process
    SINTRAN->>App: Deliver Data

See Also

Previous: Getting Started
Next: Hardware Overview
Reference: Protocol Reference