Multiple HSS systems can be used by service providers to increase capacity within their network or to provide different classes of service for subscribers. A Subscription Locator Function (SLF) is required to route HSS traffic if a service provider's subscriber information is shared across multiple HSS systems. An HSS can have incomplete subscriber information where multiple HSS systems are deployed by the service provider. Protocols that require an HSS to resolve information about a subscriber must first use an SLF to find the correct HSS for that subscriber. The SLF is used by the Interrogating-Call Session Control Function (I-CSCF), the Serving-Call Session Control Function (S-CSCF), the AS, and the AAA servers to resolve HSS information about a subscriber (see the following figure). 

The DSC includes the SLF as defined by 3GPP.

SLF Network Overview

For more information about the SLF, see Subscription Locator Function.

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