HSS

The HSS is the master record of who's allowed on the network. Every subscriber's permanent identity (IMSI), their authentication keys, their subscribed QoS and APNs, roaming permissions — it all lives here. When you power on and attach, the MME pulls your profile and authentication vectors from the HSS to verify you and decide what service you get.

Two things make it more than a passive database. First, it's evolved from the older HLR/AuC of 2G/3G, and in many networks the same platform serves all generations at once, which matters for fallback and roaming. Second, it's a control-plane single source of truth, so it has to be highly available and consistent — a sluggish or unreachable HSS means attaches and authentications stall network-wide. In the 5G core the equivalent function is the UDM (with the UDR holding the data and the AUSF handling authentication), and a converged HSS/UDM platform is common in operators running both 4G and 5G.