LAPB vs X.25 Protocol Handling in SINTRAN¶

Protocol Layer Analysis¶

LAPB (Link Access Procedure Balanced) - Layer 2¶

Pure HDLC data link protocol
No X.21 interface requirements
4-byte frames with HDLC flag/address/control/FCS
Direct hardware communication over synchronous lines

X.25 - Layer 3 Network Protocol¶

Runs over LAPB as the data link layer
Requires X.21 interface for call setup/clearing
Uses X.21 signaling for connection management
Packet-switched network protocol

SINTRAN HDLC Implementation Analysis¶

Looking at the code structure, SINTRAN appears to support both modes:

1. Pure HDLC/LAPB Mode (Your Current Use)¶

% For simple HDLC frames - bypass X.21 logic
IF A NBIT 0 OR A/\60000><0 THEN    % Only check DataAvailable + X.21 errors
   return; % Drop if no data or X.21 errors

For LAPB packets: - Bit 0 (DataAvailable) = 1 - Frame ready - Bits 13-14 (X21D/X21S) = 0 - No X.21 errors
- HX21S bits (1,2,3) = don't matter - not used in LAPB mode

2. X.25 Network Mode (Future Use)¶

% X.25 requires full X.21 interface handling
IF A/\ HX21M >< 0 THEN             % Check X.21 protocol status
   IF A BIT HX21S THEN             % Check receiver state for X.21
      % Handle X.21 connection management
   FI
FI

For X.25 packets: - X.21 signaling required for call setup/clearing - HX21S bits (1,2,3) become critical for protocol state - Full connection management needed

Current LAPB Processing Path¶

Your Working 4-byte LAPB Packets:¶

// Simple HDLC frame processing
ushort rrts = 0x0001;  // Only DataAvailable set

// Primary check (Line 51083):
if ((rrts & 0x0001) == 0 ||    // DataAvailable = 0? Drop
    (rrts & 0x6000) != 0)      // X.21 error? Drop  
{
    return; // Packet dropped
}

// For LAPB: This passes, packet gets processed
// HX21S logic is skipped for pure HDLC/LAPB frames

Why Your LAPB Works:¶

DataAvailable (bit 0) = 1 → Packet accepted
X.21 errors (bits 13-14) = 0 → No protocol errors
HX21S logic only triggers inside X.21 error handler → Skipped for LAPB
Packet processed normally through HDLC frame handling

X.25 Mode Requirements (Future)¶

When You Add X.25 Support:¶

// X.25 requires proper receiver state signaling
ReceiverStatusBits rrts = 
    ReceiverStatusBits.DataAvailable |      // Bit 0: Packet ready
    ReceiverStatusBits.StatusAvailable |    // Bit 1: Status info available
    ReceiverStatusBits.SyncFlagReceived |   // Bit 3: Frame boundaries
    // Bit 2 (ReceiverActive): Set during frame reception, clear when complete
    // Bit 14 (X21S): Set when DCE sends clear indication
    // Bits 13-14: Set for X.21 protocol errors

X.25 Call Clearing:¶

// When DCE wants to clear X.25 call:
ReceiverStatusBits rrts = 
    ReceiverStatusBits.DataAvailable |      // Data ready
    ReceiverStatusBits.X21S;                // Bit 14: X.21 clear indication

// This would trigger connection termination in X.25 mode

Protocol Detection Logic¶

The SINTRAN code appears to have mode detection:

Frame Type Check (Line 104631):¶

IF A /\ "LMASK" = 3 THEN           % Frame type = 3?
   A:=0; CALL SCRET; CALL SADTS    % Simple processing
ELSE
   IF A BIT HX21S THEN EX21 ELSE EINP FI  % Route based on receiver state

Interpretation: - LMASK = 3: Simple HDLC/LAPB frames (your current use) - Other types: Full X.21/X.25 processing with HX21S routing

Summary for Your Implementation¶

Current LAPB Mode (Working):¶

// Minimal RRTS for 4-byte LAPB packets:
ReceiverStatusBits rrts = ReceiverStatusBits.DataAvailable;  // 0x0001
// Result: Packet processed successfully, X.21 logic bypassed

Future X.25 Mode:¶

// Full RRTS for X.25 packet-switched calls:
ReceiverStatusBits rrts = 
    ReceiverStatusBits.DataAvailable |      // Packet data ready
    ReceiverStatusBits.StatusAvailable |    // Status available
    ReceiverStatusBits.SyncFlagReceived;    // Frame sync detected

// For connection clearing:
if (dceWantsToClearCall)
{
    rrts |= ReceiverStatusBits.X21S;  // Set bit 14 for clear indication
}

Your current LAPB packets work because they use the simple HDLC path that bypasses the complex X.21/X.25 connection management logic. The HX21S bits only become important when you implement full X.25 packet-switched networking.