aal5.java

一个小型网络仿真器的实现

/*
   JaNetSim  ---  Java Network Simulator
   -------------------------------------

   This software was developed at the Network Research Lab, Faculty of
   Computer Science and Information Technology (FCSIT), University of Malaya.
   This software may be used and distributed freely. FCSIT assumes no responsibility
   whatsoever for its use by other parties, and makes no guarantees, expressed or
   implied, about its quality, reliability, or any other characteristic.

   We would appreciate acknowledgement if the software is used.

   FCSIT ALLOWS FREE USE OF THIS SOFTWARE IN ITS "AS IS" CONDITION AND
   DISCLAIM ANY LIABILITY OF ANY KIND FOR ANY DAMAGES WHATSOEVER RESULTING
   FROM THE USE OF THIS SOFTWARE.
*/

package janetsim.component;

import janetsim.*;

/* A generic class to handle AAL5 */

public class AAL5 implements java.io.Serializable {
  private static int easy_crc=1;

  private java.util.List cell_queue;

  public AAL5() {
    cell_queue=new java.util.ArrayList();
  }

  public void reset() {
    cell_queue.clear();
  }

  public java.util.List sendData(Object pdu,int length,int vpi,int vci) {
    length+=8; //aal5 trailer size
    int pad=length%48;
    if(pad>0) pad=48-pad;
    int num_cells=(length+pad)/48;
    int crc=easy_crc++;

    java.util.List q=new java.util.ArrayList();

    for(int i=1;i<num_cells;i++) { //send all but the last cell
      Cell cell=new Cell();
      cell.vpi=vpi;
      cell.vci=vci;
      cell.pti=0;
      cell.crc=crc;
      q.add(cell);
    }

    //the last cell
    Cell cell=new Cell();
    cell.vpi=vpi;
    cell.vci=vci;
    cell.pti=0x01;
    cell.payload=pdu;
    cell.len=num_cells;
    cell.crc=crc;
    q.add(cell);

    return q;
  }

  //return 1: pdu ready (caller get payload)
  //       0: pdu not ready
  //      -1: crc error
  public int receiveCell(Cell cell) {
    cell_queue.add(cell);

    //check pti
    if((cell.pti & 0x01) == 0x01) { //the last cell
      if((cell_queue.size() != cell.len) ||
          (((Cell)cell_queue.get(0)).crc != cell.crc)) {
        cell_queue.clear();
        return -1; //bad crc
      }
      cell_queue.clear();
      return 1; //good & ready
    }
    return 0;
  }
}