sending, by a terminal device, an authentication request to the access network device;

receiving, by the terminal device in a first transmission time interval, a first authentication request response in response to the authentication request, wherein the first authentication request response includes first time window information indicating a time range in which the terminal device receives the first authentication request response;

obtaining the first time window information in the first authentication request response, wherein a first time window indicated by the first time window information comprises at least one transmission time interval; and

determining, by the terminal device, that the access network device is a legal access network device when the terminal device determines that the first transmission time interval falls within the first time window.