[citation needed] The concept of service layer is used in contexts such as Intelligent networks (IN), WAP, 3G and IP Multimedia Subsystem (IMS).
It is defined in the 3GPP Open Services Architecture (OSA) model, which reused the idea of the Parlay API for third-party servers.
In software design, for example Service-oriented architecture, the concept of service layer has a different meaning.
The network elements contained within the service layer are generically referred to as 'service platforms' however the 3GPP specification (3GPP TS 23.228 V8.7.0) defines several types of service platforms: The SIP Application Server (AS) performs the same function as a Telephony Application Server in a pre-IMS network, however it is specifically tailored to support the SIP signalling protocol for use in an IMS network.
An OSA Service Capability Server acts as a secure gateway between the IMS network and an application which runs upon the Open Services Architecture (this is typically a SIP to Parlay gateway) The IM-SSF (IP Multimedia Service Switching Function) acts as a gateway between the IMS network and application servers using other telecommunication signalling standards such as INAP and CAMEL.