[PCI DSS 1.x] 11.4 Use network intrusion detection systems, host-based intrusion detection systems, and intrusion

11.4.a Observe the use of network intrusion detection systems and/or intrusion prevention systems on the network. Verify that all critical network traffic in the cardholder data environment is monitored
11.4.b Confirm IDS and/or IPS is in place to monitor and alert personnel of suspected compromises
11.4.c Examine IDS/IPS configurations and confirm IDS/IPS devices are configured, maintained, and updated per vendor instructions to ensure optimal protection