[asterisk-users] concurrent channels limit

[Syco]

Hi,
I've some problem setting up Asterisk 1.8:
the configuration of the server is
8 core Intel(R) Xeon(R) CPU E5310 @ 1.60GHz
4GB of ram
CentOS release 6.2 (Final)
asterisk-1.8.10.1

I'm using sipp to do some tests, the command I use is "sipp -sf 
/root/uac_pcap_G729.xml -s 17000 192.168.200.64 -l 1000"
the file "uac_pcap_G729.xml" just establishes a call, wait 120 seconds 
and hangup.

Finally the problem is: I cannot manage more than 80 concurrent calls.
but I know that is a limit too low, the same machine in the production 
environment, with the same configurations and asterisk 1.4 manage up to 
400 concurrent calls.

How can I find my real upper call limit?

tnx