Re: [Discuss-gnuradio] How Can I support full-duplex using SBX board when running tunnel.py over OFDM?

[Alex Zhang]

Hi Josh,

Thanks for your attention.

I use the wireshark to monitor the ARP interaciton. 

Suppos we have node 1 (192.168.200.1) and node 2 (192.168.200.2). 

When Node1 initiates the ping request to node2, the ARP request is broadcast to query who is 192.168.200.2. 

At the node2, the ARP request is received by the tunnel device (gr0) and it does send back the ARP reply. 

Unfortunately, the node1 rarely receive the ARP reply from the node2. 

To avoid this ARP problem, I set the static MAC address for the tunnel devices on both node1 and node2. Just note, the MAC address is set as a fixed value when tunnel device is created.

Then node 2 can receive the ICMP request from node 1 and it does response with the ICMP reply.

However, similarly with the ARP case, the node 1 is very hard to receive the ICMP reply from the node2.

So, I dump the received payload at the RX thread of node 1.  Now I am surprised to find that most of the payloads at node1 are actually the content it send out to node 2. Only very small part of the replies from node 2 can be received by node 1.

Thus, I only observe the ping can be successful very rarely. 

On Mon, Apr 30, 2012 at 7:21 PM, Josh Blum <address@hidden> wrote:

On 04/30/2012 08:09 AM, Alex Zhang wrote:
> Hi,
>
> SBX is the daughterboard which supports full-duplex, but when I run the
> tunnel.py to do the Ping experiment over the OFDM communication link, I
> found that the transmitter can receive a lot of packets which are
> transmitted by itself.  This causes failure for ARP query and the Ping
> requests.  I see in the forum, many friends meet the failure of tunnel.py

I am a little surprised to find that there is a networking kind of
issue. I thought that any of these leakage packets would be discarded by
the network layer (because the IP source and destination would be
incorrect, etc...)

So, how do you think ARP is failing? Even if there is this "echo"
packet, a device should not be able to respond to an ARP request for an
IP address which is not its own. Are you setting different IP addresses?

 

I did set different IP addresses for the two nodes. 

 

> because they are using the XCVR2450 which is half-duplex.  But why I still
> have this tunnel.py failed even when I am using the full-duplex SBX?
>
> Should I use different frequencies for the TX and RX in the same USRP? Does
> the SBX support the TDD based full-duplex?
>

So if the TX and RX frequency are the same, the receiver (which is
always running) will hear the transmission leakage. That makes sense.
What I think is missing from the tunnel.py example is that RX is not
muted during a TX.

Yes, the receiver hears itself. I don't know why no other persons complained for SBX when using tunnel.py. But I did find many guys meeting the same situation when using XCVR2450 daughter board as it is half-duplex. SBX is promising to be full-duplex. And I am sure I am using the SBX.

 

Some recommendations:

1) Use different frequencies for each communication channel.

I will firstly try this option.

 

2) Or mute the RX stream when transmitting to avoid decoding leakage.

Could you give some indications on how to mute the RX when it is transmitting? Is it related to switching of the RX and TX of SBX? And do you think it is fast enough if I do it in host based software? 

 

-Josh

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Alex,

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