Explain what FCS errors are and what they indicate.

svn path=/head/; revision=3474

FAQ/network.sgml

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-<!-- $Id: network.sgml,v 1.14 1998-08-26 09:52:11 brian Exp $ -->
+<!-- $Id: network.sgml,v 1.15 1998-09-06 13:55:10 brian Exp $ -->
 <!-- The FreeBSD Documentation Project -->
 
   <sect>
@@ -818,6 +818,44 @@ default            10.0.0.2           UGSc        0        0      tun0
         <p><bf>3)</bf> Redirect everything to the internal machine using
         ``alias addr''.  This is the sledge-hammer approach.
 
+      <sect2>
+        <heading>What are FCS errors ?</heading>
+
+        <p>FCS stands for <bf/F/rame <bf/C/heck <bf/S/equence.  Each
+        ppp packet has a checksum attached to ensure that the data
+        being received is the data being sent.  If the FCS of an
+        incoming packet is incorrect, the packet is dropped and the
+        HDLC FCS count is increased.  The HDLC error values can be
+        displayed using the <tt>show hdlc</tt> command.
+
+        <p>If your link is bad (or if your serial driver is dropping
+        packets), you will see the occasional FCS error.  This is not
+        usually worth worrying about although it does slow down the
+        compression protocols substantially.  If you have an external
+        modem, make sure your cable is properly shielded from
+        interference - this may erradicate the problem.
+
+        <p>If your link freezes as soon as you've connected and you see
+        a large number of FCS errors, this may be because your link is
+        not 8 bit clean.  Make sure your modem is not using software
+        flow control (XON/XOFF).  If your datalink <bf>must</bf> use
+        software flow control, use the command
+        <tt>set accmap 0x000a0000</tt> to tell <bf>ppp</bf> to escape
+        the ^Q and ^S characters.
+
+        <p>Another reason for seeing too many FCS errors may be that
+        the remote end has stopped talking <bf/PPP/.  You may want to
+        enable <tt/async/ logging at this point to determine if the
+        incoming data is actually a login or shell prompt.  If you
+        have a shell prompt at the remote end, it's possible to
+        terminate ppp without dropping the line by using the
+        <tt>close lcp</tt> command (a following <tt>term</tt> command
+        will reconnect you to the shell on the remote machine.
+
+        <p>If nothing in your log file indicates why the link might
+        have been terminated, you should ask the remote administrator
+        (your ISP?) why the session was terminated.
+
       <sect2>
         <heading>None of this helps - I'm desperate !</heading>