Custom authentication for WCF

Question

How to implement custom authentication for all .SVC available in application.

We want to implement WCF services (.svc) - employee.svc and department.svc.
We need to assure that for every operationcontract call in this services the user should be authenticated user.

We need to host this services in IIS 7

We have tried using "System.Web.ApplicationServices.AuthenticationService" in a new AuthenticationService.svc and registering event System.Web.ApplicationServices.AuthenticationService.Authenticating in Global.asax. But this event is called only for
AuthenticationService.svc but not for employee.svc and department.svc

Also, we donot want any CERTIFICATE needs to be installed on the server(or any other application like windows or mobile application which will use this service) where this service will be hosted.

Also, when the one of operationcontract of service is called it should not ask again for username/password.

Any suggesstion????