The Home Subscriber Server is the authoritative repository for every piece of subscriber information in an LTE network. It stores permanent identities such as the IMSI and MSISDN, subscription profiles defining allowed APNs and maximum bitrates, roaming restrictions, and the address of the MME where the subscriber is currently registered. Whenever a user powers on and attempts to attach, the MME must query the HSS before it can proceed with authentication or session establishment. Without a successful HSS lookup no subscriber can access any network service whatsoever.