http://www.kamailio.org/docs/openser-performance-tests/ to load test my kamailio system.

I am a little unclear as to how many instances of sipp are running. For the first part i see the command

 ./sipp -sf uac_msg.xml -rsa 192.168.2.102:5060 192.168.2.102:5070 -m 200000 -r 10000 -d 1 -l 70

for generating the UAC part but is there another instance of sipp running on the kamailio computer acting as a UAS? I have seen come sites use for example: sipp 192.168.1.100:5060 -sn uas -p 5060 but is this not necessary?