I have a Nortel 4548-GT ERS Switch. I have mirrored port 3/39 and port 3/14 and select monitoring mode as a [--->X<---] where port 3/14 is monitor port. After doing this the system connecting with port 3/14 is out of network. What is the problem

Are you monitoring the port only or a VLAN? What are the VLAN memberships of the ports? Are your monitor sessions the same?

Example Config:

Switch(config)#monitor session 1 source int g3/39
Switch(config)#monitor session 1 destination int g3/14
Switch(config)#do show monitor