packet drops in 802.11p

[dev123...@gmail.com]

Hi,

while trying to extend the example "wave-simple-80211p.cc", to have multiple receive sinks and sources, I noticed that if packets generation is scheduled at same simulation time, all the packets are dropped. Any suggestions if this is an expected behavior, or any ideas as to how I can figure out where the packets are being dropped, would be much appreciated. I tried attaching packet drop tracers on PHY but that did not work.

I have verified the pcap files to ensures the packets are being sent but the same are not being received at any of the sinks. Just to be complete, the packets are received if scheduled at different simulation times.

Thanks

[D G]

Hey Dev,

I don't know your script, but is it possible, that you have some intereference models set up?
If you did so, then the packets may be "received" but not be decodable so the are being droped.

Best
D.G.

[dev123...@gmail.com]

Hi D.G,

Thanks for the reply and for pointing out that the reason could be interference.

I had not explicitly mentioned any interference, but the YansWifiPhy module has some inbuilt by default. That could be one of the reasons, but why would a node drop all packets? In other words, if multiple packets are sent to a node at a time instant, shouldn't it accept atleast one?

Attached is a small script to demonstrate my point, based on the "wave-simple-80211p.cc" example, but with two recv_sinks and two sources.
./waf --run "wave-simple-80211p-modified --choice=0"

Default configuration (choice=1) of the script sends packets one after the other and hence 12 packets are received (3seconds x 2 senders x 2 recv sinks). But with choice=0, if it were just interference the number packets received should be at least 6, but only 4 are received (node 1 drops all the packets at sim time 1s and 3s).

Best

[Konstantinos]

Hi,

The reason is due to ARP failure. When you start the two applications at the same time, it first sends ARP packets to resolve the addresses. 

These packets collide (expected), however the timeout is not random. It has been mentioned in the past several time that this behaviour is to be expected.

Solutions:

1) Give some time difference in the start of the applications. Subsequent transmissions could happen at the same time and the back-off of 802.11 would resolve it, but it's the first packet that needs to be different.

2) Populate the ARP cache beforehand manually. Then, there will be no need for ARP packets.

Regards

K

[dev123...@gmail.com]

Hi,

Thanks for the reply.

I have modified the script to allow the nodes to start at different times and then send packets simultaneously. But still all the packets do not get through. What could be the reason here?
 ./waf --run wave-simple-80211p-modified

Thanks

[dev123...@gmail.com]

Hi,

Here is a modified version of the script which  pre-populates the arp-cache. The output still remains the same. Any ideas?

Best

[Rediet]

Hello,

If you can see that packets are sent but not received, it may be that SNR at receiver's end is too low (i.e. below thresholds). Try checking that out.

BR,

Rediet