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diff --git a/gnuradio-examples/python/hier/digital/tunnel.py b/gnuradio-examples/python/hier/digital/tunnel.py
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+++ b/gnuradio-examples/python/hier/digital/tunnel.py
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+#!/usr/bin/env python
+#
+# Copyright 2005,2006 Free Software Foundation, Inc.
+# 
+# This file is part of GNU Radio
+# 
+# GNU Radio is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 2, or (at your option)
+# any later version.
+# 
+# GNU Radio is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+# 
+# You should have received a copy of the GNU General Public License
+# along with GNU Radio; see the file COPYING.  If not, write to
+# the Free Software Foundation, Inc., 51 Franklin Street,
+# Boston, MA 02110-1301, USA.
+# 
+
+
+# /////////////////////////////////////////////////////////////////////////////
+#
+#    This code sets up up a virtual ethernet interface (typically gr0),
+#    and relays packets between the interface and the GNU Radio PHY+MAC
+#
+#    What this means in plain language, is that if you've got a couple
+#    of USRPs on different machines, and if you run this code on those
+#    machines, you can talk between them using normal TCP/IP networking.
+#
+# /////////////////////////////////////////////////////////////////////////////
+
+
+from gnuradio import gr, gru, modulation_utils
+from gnuradio import usrp
+from gnuradio import eng_notation
+from gnuradio.eng_option import eng_option
+from optparse import OptionParser
+
+import random
+import time
+import struct
+import sys
+import os
+
+# from current dir
+from transmit_path import transmit_path
+from receive_path import receive_path
+import fusb_options
+
+#print os.getpid()
+#raw_input('Attach and press enter')
+
+
+# /////////////////////////////////////////////////////////////////////////////
+#
+#   Use the Universal TUN/TAP device driver to move packets to/from kernel
+#
+#   See /usr/src/linux/Documentation/networking/tuntap.txt
+#
+# /////////////////////////////////////////////////////////////////////////////
+
+# Linux specific...
+# TUNSETIFF ifr flags from <linux/tun_if.h>
+
+IFF_TUN		= 0x0001   # tunnel IP packets
+IFF_TAP		= 0x0002   # tunnel ethernet frames
+IFF_NO_PI	= 0x1000   # don't pass extra packet info
+IFF_ONE_QUEUE	= 0x2000   # beats me ;)
+
+def open_tun_interface(tun_device_filename):
+    from fcntl import ioctl
+    
+    mode = IFF_TAP | IFF_NO_PI
+    TUNSETIFF = 0x400454ca
+
+    tun = os.open(tun_device_filename, os.O_RDWR)
+    ifs = ioctl(tun, TUNSETIFF, struct.pack("16sH", "gr%d", mode))
+    ifname = ifs[:16].strip("\x00")
+    return (tun, ifname)
+    
+
+# /////////////////////////////////////////////////////////////////////////////
+#                             the flow graph
+# /////////////////////////////////////////////////////////////////////////////
+
+class my_graph(gr.flow_graph):
+
+    def __init__(self, mod_class, demod_class,
+                 rx_callback, options):
+
+        gr.flow_graph.__init__(self)
+        self.txpath = transmit_path(self, mod_class, options)
+        self.rxpath = receive_path(self, demod_class, rx_callback, options)
+
+    def send_pkt(self, payload='', eof=False):
+        return self.txpath.send_pkt(payload, eof)
+
+    def carrier_sensed(self):
+        """
+        Return True if the receive path thinks there's carrier
+        """
+        return self.rxpath.carrier_sensed()
+
+
+# /////////////////////////////////////////////////////////////////////////////
+#                           Carrier Sense MAC
+# /////////////////////////////////////////////////////////////////////////////
+
+class cs_mac(object):
+    """
+    Prototype carrier sense MAC
+
+    Reads packets from the TUN/TAP interface, and sends them to the PHY.
+    Receives packets from the PHY via phy_rx_callback, and sends them
+    into the TUN/TAP interface.
+
+    Of course, we're not restricted to getting packets via TUN/TAP, this
+    is just an example.
+    """
+    def __init__(self, tun_fd, verbose=False):
+        self.tun_fd = tun_fd       # file descriptor for TUN/TAP interface
+        self.verbose = verbose
+        self.fg = None             # flow graph (access to PHY)
+
+    def set_flow_graph(self, fg):
+        self.fg = fg
+
+    def phy_rx_callback(self, ok, payload):
+        """
+        Invoked by thread associated with PHY to pass received packet up.
+
+        @param ok: bool indicating whether payload CRC was OK
+        @param payload: contents of the packet (string)
+        """
+        if self.verbose:
+            print "Rx: ok = %r  len(payload) = %4d" % (ok, len(payload))
+        if ok:
+            os.write(self.tun_fd, payload)
+
+    def main_loop(self):
+        """
+        Main loop for MAC.
+        Only returns if we get an error reading from TUN.
+
+        FIXME: may want to check for EINTR and EAGAIN and reissue read
+        """
+        min_delay = 0.001               # seconds
+
+        while 1:
+            payload = os.read(self.tun_fd, 10*1024)
+            if not payload:
+                self.fg.send_pkt(eof=True)
+                break
+
+            if self.verbose:
+                print "Tx: len(payload) = %4d" % (len(payload),)
+
+            delay = min_delay
+            while self.fg.carrier_sensed():
+                sys.stderr.write('B')
+                time.sleep(delay)
+                if delay < 0.050:
+                    delay = delay * 2       # exponential back-off
+
+            self.fg.send_pkt(payload)
+
+
+# /////////////////////////////////////////////////////////////////////////////
+#                                   main
+# /////////////////////////////////////////////////////////////////////////////
+
+def main():
+
+    mods = modulation_utils.type_1_mods()
+    demods = modulation_utils.type_1_demods()
+
+    parser = OptionParser (option_class=eng_option, conflict_handler="resolve")
+    expert_grp = parser.add_option_group("Expert")
+
+    parser.add_option("-m", "--modulation", type="choice", choices=mods.keys(),
+                      default='gmsk',
+                      help="Select modulation from: %s [default=%%default]"
+                            % (', '.join(mods.keys()),))
+
+    parser.add_option("-v","--verbose", action="store_true", default=False)
+    expert_grp.add_option("-c", "--carrier-threshold", type="eng_float", default=30,
+                          help="set carrier detect threshold (dB) [default=%default]")
+    expert_grp.add_option("","--tun-device-filename", default="/dev/net/tun",
+                          help="path to tun device file [default=%default]")
+
+    transmit_path.add_options(parser, expert_grp)
+    receive_path.add_options(parser, expert_grp)
+
+    for mod in mods.values():
+        mod.add_options(expert_grp)
+
+    for demod in demods.values():
+        demod.add_options(expert_grp)
+
+    fusb_options.add_options(expert_grp)
+
+    (options, args) = parser.parse_args ()
+    if len(args) != 0:
+        parser.print_help(sys.stderr)
+        sys.exit(1)
+
+    if options.rx_freq is None or options.tx_freq is None:
+        sys.stderr.write("You must specify -f FREQ or --freq FREQ\n")
+        parser.print_help(sys.stderr)
+        sys.exit(1)
+
+    # open the TUN/TAP interface
+    (tun_fd, tun_ifname) = open_tun_interface(options.tun_device_filename)
+
+    # Attempt to enable realtime scheduling
+    r = gr.enable_realtime_scheduling()
+    if r == gr.RT_OK:
+        realtime = True
+    else:
+        realtime = False
+        print "Note: failed to enable realtime scheduling"
+
+
+    # If the user hasn't set the fusb_* parameters on the command line,
+    # pick some values that will reduce latency.
+
+    if options.fusb_block_size == 0 and options.fusb_nblocks == 0:
+        if realtime:                        # be more aggressive
+            options.fusb_block_size = gr.prefs().get_long('fusb', 'rt_block_size', 1024)
+            options.fusb_nblocks    = gr.prefs().get_long('fusb', 'rt_nblocks', 16)
+        else:
+            options.fusb_block_size = gr.prefs().get_long('fusb', 'block_size', 4096)
+            options.fusb_nblocks    = gr.prefs().get_long('fusb', 'nblocks', 16)
+    
+    #print "fusb_block_size =", options.fusb_block_size
+    #print "fusb_nblocks    =", options.fusb_nblocks
+
+    # instantiate the MAC
+    mac = cs_mac(tun_fd, verbose=True)
+
+
+    # build the graph (PHY)
+    fg = my_graph(mods[options.modulation],
+                  demods[options.modulation],
+                  mac.phy_rx_callback,
+                  options)
+
+    mac.set_flow_graph(fg)    # give the MAC a handle for the PHY
+
+    if fg.txpath.bitrate() != fg.rxpath.bitrate():
+        print "WARNING: Transmit bitrate = %sb/sec, Receive bitrate = %sb/sec" % (
+            eng_notation.num_to_str(fg.txpath.bitrate()),
+            eng_notation.num_to_str(fg.rxpath.bitrate()))
+             
+    print "modulation:     %s"   % (options.modulation,)
+    print "freq:           %s"      % (eng_notation.num_to_str(options.tx_freq))
+    print "bitrate:        %sb/sec" % (eng_notation.num_to_str(fg.txpath.bitrate()),)
+    print "samples/symbol: %3d" % (fg.txpath.samples_per_symbol(),)
+    #print "interp:         %3d" % (fg.txpath.interp(),)
+    #print "decim:          %3d" % (fg.rxpath.decim(),)
+
+    fg.rxpath.set_carrier_threshold(options.carrier_threshold)
+    print "Carrier sense threshold:", options.carrier_threshold, "dB"
+    
+    print
+    print "Allocated virtual ethernet interface: %s" % (tun_ifname,)
+    print "You must now use ifconfig to set its IP address. E.g.,"
+    print
+    print "  $ sudo ifconfig %s 192.168.200.1" % (tun_ifname,)
+    print
+    print "Be sure to use a different address in the same subnet for each machine."
+    print
+
+
+    fg.start()    # Start executing the flow graph (runs in separate threads)
+
+    mac.main_loop()    # don't expect this to return...
+
+    fg.stop()     # but if it does, tell flow graph to stop.
+    fg.wait()     # wait for it to finish
+                
+
+if __name__ == '__main__':
+    try:
+        main()
+    except KeyboardInterrupt:
+        pass