Electromagnetic interference (EMI) is unwanted noise or interference in an electrical path or circuit caused by an outside source. Hence shielded Cat 6 is better If a technician determines that the root cause of sporadic network connectivity issues is EMI, the most cost-effective cable medium that can be used without sacrificing system performance is shielded Cat 6 cable. Shielded cables protect against electromagnetic interference (EMI) and radio frequency interference (RFI). References: CompTIA A+ Certification All-in-One Exam Guide, Tenth Edition by Mike Meyers (Page 548)