Managing the PPP Data Link Listing 720 ModemCommand in Java

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Managing the PPP Data Link Listing 720 ModemCommand
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import javaxcomm*; import javaio*; public class ModemCommand { private SerialPort sp; private InputStream in; private OutputStream out; public ModemCommand(SerialPort sp, InputStream in, OutputStream out) { thissp = sp; thisin = in; thisout = out; } public void sendCommand(String command, String response, int timeout) throws DataLinkException { try { // Transmit the command outwrite(commandgetBytes()); } catch (IOException ioe) { ioeprintStackTrace(); throw new DataLinkException( "Error sending command to modem"); } waitForMatch(response, timeout); } public void receiveMatch(String match, String response, int timeout) throws DataLinkException { try { waitForMatch(match, timeout); if ((response != null) && (responselength() > 0)) { outwrite(responsegetBytes()); } } catch (IOException ioe) { ioeprintStackTrace(); throw new DataLinkException( "IO Error receiving a match to:"+match); } } }
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The waitForMatch method, shown in Listing 721, takes a String used for the desired pattern match The pattern match is performed in a case insensitive manner It also takes an integer number of seconds used as a time-out value, where a value of 0 seconds is used to specify an in nite time-out It uses both serial port receive time-outs and thresholds to control the reading of data and manage a timer The receive time-out is set to 100 milliseconds and the threshold to the number of bytes equal to the length of the match String The overall time that has elapsed is tracked using SystemcurrentTimeMillis Listing 721 waitForMatch
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private void waitForMatch(String match, int timeout) throws DataLinkException { try { spenableReceiveTimeout(100); spenableReceiveThreshold(matchlength()); byte[] mb = new byte[matchlength()]; long timer = 0; if (timeout > 0) { // Time out when timer > currentTimeMillis timer = timeout*1000+SystemcurrentTimeMillis(); } StringBuffer modemSpew = new StringBuffer(); while ((timer == 0) || (SystemcurrentTimeMillis() < timer)) { int count = inread(mb); if (count > 0) { modemSpewappend((new String(mb,0,count))toUpperCase()); if (modemSpewtoString()indexOf( matchtoUpperCase()) >= 0) { return; } } } throw new DataLinkException("Timed out waiting for match:"+ match); } catch (Exception e) { eprintStackTrace(); throw new DataLinkException("IO Error receiving a match to:"+ match); } }
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The trick here is that the modem might send other unwanted bytes of information in the same stream of data that has the pattern that we re trying to match To deal with this problem, waitForMatch reads all serial bytes and stores them in a StringBuffer Each time data is available, the new bytes are appended
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to the end of the StringBuffer To check for a match, the StringBuffer is converted to a String, and the indexOf method is used to check to see if the desired response is contained anywhere within the resulting String If a match is found, waitForMatch returns normally Otherwise, it performs another blocking read until either the number of bytes equal to the length of the match String is available or until 100 milliseconds elapses If no match is found within the speci ed overall time-out, a DataLinkException is thrown The DataLinkException propagates up the call stack eventually notifying the PPP daemon of the modem s failure to respond
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Now that we have an implementation of a PPP daemon and the supporting data link classes, we can enhance the DataLogger class to accept network connections over both PPP and Ethernet interfaces Listing 722 shows the additions and modi cations made to the DataLogger class for the purpose of adding PPP daemon support The rst change to notice is that DataLogger creates and starts a new instance of PPPDaemon on construction The other change to DataLogger is that it now implements the PPPDaemonListener (Listing 711) interface and therefore provides implementations for the daemonError and isValidUser methods The daemonError method is invoked when a persistent error is preventing PPPDaemon from establishing PPP connections As implemented below, daemonError stops the PPP server However, DataLogger continues to run, allowing connections over the Ethernet network interface only Listing 722 DataLogger changes
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class DataLogger extends Thread implements PPPDaemonListener { PPPDaemon pppd; private String name; private String password; DataLogger(int samples, int delay, String name, String password) throws PPPException, LoggingException { // Set authentication information thisname = name; thispassword = password; // Create a server to manage PPP dial-up requests PPPDaemon pppd = new PPPDaemon(this, "serial0", 19200); pppdstartDaemon(); }
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