[hpsdr] Running Heterodyne through multiple wifi hops

[Mark Leone]

This is a follow-up to my previous post on trying to run Heterodyne from a
MacBook laptop remotely connected to Metis. It's working now (though
bandwidth limited), so I want to document what I did.

 

The Mac communicates via wifi link through a wireless router to an Ubuntu
system. The Ubuntu system has a wired Ethernet connection to Metis, on a
separate subnet from the one used on the wireless interface. I set up IP
forwarding in the Ubuntu system and the Mac (see  for details), to enable IP
connectivity in both directions from the Mac to Metis. This was confirmed
with ping, but something more is needed to run SDR software that
communicates with Metis.

 

Metis must be "discovered" by the SDR app using the pre-defined discovery
protocol. This protocol relies on the Link layer (e.g. Ethernet in this
case) in such a way that the SDR processor and Metis must be on the same LAN
segment. This does not mean just the same IP subnet; there must be no
routers between the two systems, as routers do not pass broadcast frames,
which are the frames that contain the discovery protocol packets.

 

Ubuntu supports "bridging" between two Ethernet interfaces, which means that
the software establishes a Link layer connection that makes the two
interfaces behave as if they are on the same LAN segment. This means they
should pass broadcast frames. I tried two documented approaches for doing
this, and in both cases, the broadcast frames were not transferred across
the bridge. I don't know if this is a bug in Ubuntu, or if I did it wrong,
but I gave up trying in favor of a hardware solution. My failed attempt
using Ubuntu is documented here:
http://ubuntuforums.org/showthread.php?t=1906359

 

For the hardware solution, I purchased a used wireless router for $10 that
is capable of running DD-WRT firmware. I used a Linksys WRT54G, but any
wireless router than runs DD-WRT should work. I connected the Linksys wired
ports (i.e. the LAN side) to the Ubuntu system and to Metis. I configured
the DD-WRT firmware to operate in "client bridge" mode. In this
configuration, the Linksys router connects wirelessly to the wireless router
that is connected to my ISP, and bridges its wired ports into that LAN. In
this way, the two routers are bridged together as if they were on the same
LAN segment. Instructions for configuring a DD-WRT device to run in "client
bridge' mode can be found here:
http://www.dd-wrt.com/wiki/index.php/Client_Bridged

 

With this configuration, the broadcast packets go all the way through from
the Mac laptop to Metis, and Heterodyne works, although only at the lowest
sample rate (48,000 sps). Both wireless routers are running the older
802.11g protocol, and one of them (the one connected to the ISP) has to
split its bandwidth, as it simultaneously receives packets from Metis and
forwards them to the MacBook. Nevertheless I'm surprised that the app is
bandwidth-limited in this configuration. I tried a separate wireless router
made by Rosewill with similar results. I will post a separate message with
some details about observed behavior and measured bandwidth, and related
questions.

 

Mark - K4XML

 

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