Biterror when sending WSM synchronously through CCH

[3718...@qq.com]

Hi,

  

  I'm currently using Veins 4.4 and OMNET 4 .And I've met a problem which is that when I try to send two messages synchronously from two different nodes through CCH to all the other nodes (8 nodes in all and the two messages have the same priority ,actually they are completely the same messages ) without channel switching .It always failed due to biterror reported by phy layers of the receivers (It says the packet was not received correctly due to bit error ,lost and send control message to MAC etc.) .But when I use channel switching mode .They sent successfully . 

  So I wonder :

   1. Whether CCH does not allow two WSMs send at the same time ? If yes , what is the role of the queue in MAC layer ? When a message is about to send during channel busy time ,it will calculate a backoff time and wait in the queue until everything is ready to send .That's what I learned from the MAC Protocol 1609.4 .So does CCH follow the rules above ?  

   2.Furthermore, how CCH and SCH send a large number of WSMs under channel switch mode ? For example ,each node of 8 send messages synchronously.There are four SCHs ,I see that the first message is sent through the 172 SCH , which SCH number do the following messages choose and why they choose these channels ? 

    

   3.I'm not very clear about the detailed process while SCH and CCH alternately deliver messages , so if anyone can give me a description I would be really really appreciated !

   4.I made half of 50 nodes broadcast messages (have the same priority and content ) synchronously with channel switch previously and lots of packets were lost ,my teacher advise me to add random delay time when sending a message . That decreases packet loss ratio rapidly but the whole transmission time is still longer than required .Are there any other solutions to optimize transmission time ?

  Thanks in advance !

[Christoph Sommer]

3718...@qq.com wrote:
>
> when I try to send two messages synchronously from two different nodes [via WLAN] It always failed due to biterror

If you are sending both messages *perfectly* synchronously (in simulation you can do this with sub-picosecond precision) both WLAN radios will see an empty channel at the same time, switch from listening to sending at the same time, then send at the same time. Only then will the signal from one node's WLAN radio have propagated to the other node, but it's too late for the sending nodes to realize they are sending to a busy channel.

A third node at roughly the same distance from the two senders will very likely not be able to decode either signal from this mix.

This is expected behavior.

Best,

Christoph

--
Jun.-Prof. Dr.-Ing. Christoph Sommer
Cooperative Mobile Systems (CCS Labs)
Heinz Nixdorf Institute, Paderborn University, Germany
http://www.ccs-labs.org/~sommer/