Hier mal was aus den FAQ`s der offiziellen CoD Black Ops Site:
Why is there lag when I play online
Playing online involves having your console or PC send packets of data through multiple servers, hubs and backbones to reach its destination server. Your Internet Service Provider may sometime have equipment fail somewhere, and sections of the network go entirely offline. Other times connections have to be re-routed due to power failures or natural disasters, and as a result your connection time (and game performance) suffers.
Why can't I connect to online games?
A common cause of problem with connecting to an online game is a router that needs configuration. While most of the time this isn't necessary, each game is different and it may be required even if you've never done so in the past. The most common type of configuration is port forwarding. Each router has a different menu setup so we can't give you exact instructions on how to do this on this site. You should refer to your router manufacturer or a site like
http://portforward.com/ for more assistance.
List of ports to forward:
Nintendo Wii - TCP [443, 3074] UDP [3074, 3075]
PC - TCP [3074] UDP [3074]
PlayStation 3 - TCP [80, 81, 443, 3074, 5223] UDP [3074, 3075, 3478]
Xbox 360 - TCP [80] UDP [88, 3074, 22728, 33233]
http://www.callofduty.com/blackops/help#