code: public byte[] sendHeader(OutputStream os, int commandId, int commandStatus,
              int sequenceNumber) throws IOException {
          synchronized (os) {
              return pduSender.sendHeader(os, commandId, commandStatus,
                      sequenceNumber);
          }
      }
docs: /*
  (non-Javadoc)
  
  @see org.jsmpp.PDUSender#sendHeader(java.io.OutputStream, int, int, int)