3.4.1 The AAL Protocol Suite

• AAL1: CBR traffic (Voice). Uses 1 byte of cell payload for Sequencing.
• AAL2: VBR traffic (Variable Voice).
• AAL3/4: Legacy data. 4 bytes of overhead per cell.
• AAL5 (SEAL): Modern data. 0 bytes of overhead per cell, 8-byte frame trailer.

3.4.2 The GCRA (Generic Cell Rate Algorithm) Logic

GCRA is the "Police Officer" of the ATM network.

Conformance Testing Pseudocode:

bool is_conforming(Connection *v, double t_arrival) {
    // T = Increment (1/Rate)
    // L = Limit (Tolerance)
    
    double tat = v->theoretical_arrival_time;
    
    if (t_arrival < tat - v->limit) {
        return false; // Cell too early (Non-conforming)
    }
    
    // Cell is conforming
    v->theoretical_arrival_time = max(t_arrival, tat) + v->increment;
    return true;
}

3.4.3 The AAL5 8-Byte Trailer Structure

3.4.4 Virtual Output Queuing (VOQ)

To avoid the 58% HOL Blocking limit, modern switches maintain a bank of queues at the input. - Input Port 1 has Queue [1$ ightarrow$1], Queue [1$ ightarrow$2], Queue [1$ ightarrow$3]...

• A central scheduler (iSLIP) matches inputs to outputs in a round-robin fabric.

iSLIP Cycle Logic:

1. Request: All inputs send requests to all outputs they have cells for.
2. Grant: Each output selects one input (RR).
3. Accept: Each input selects one output (RR).

• This achieves nearly 100% throughput for uniform traffic.