Strange errors in Apple's Reachability files

I'm trying to work with reachability in my project. I added Reachability.h and Reachability.m files. But after building project xCode shows me strange errors:

Undefined symbols for architecture i386:
"_SCNetworkReachabilityCreateWithName", referenced from:
  +[Reachability reachabilityWithHostname:] in Reachability.o
"_SCNetworkReachabilityCreateWithAddress", referenced from:
  +[Reachability reachabilityWithAddress:] in Reachability.o
"_SCNetworkReachabilitySetCallback", referenced from:
  -[Reachability startNotifier] in Reachability.o
  -[Reachability stopNotifier] in Reachability.o
"_SCError", referenced from:
  -[Reachability startNotifier] in Reachability.o
"_SCErrorString", referenced from:
  -[Reachability startNotifier] in Reachability.o
"_SCNetworkReachabilitySetDispatchQueue", referenced from:
  -[Reachability startNotifier] in Reachability.o
  -[Reachability stopNotifier] in Reachability.o
"_SCNetworkReachabilityGetFlags", referenced from:
  -[Reachability isReachable] in Reachability.o
  -[Reachability isReachableViaWWAN] in Reachability.o
  -[Reachability isReachableViaWiFi] in Reachability.o
  -[Reachability connectionRequired] in Reachability.o
  -[Reachability isConnectionOnDemand] in Reachability.o
  -[Reachability isInterventionRequired] in Reachability.o
  -[Reachability reachabilityFlags] in Reachability.o

 Probably you forgot to add the SystemConfiguration.framework to your project. If you don't know how to add a framework have a look at this SO question.