||It has been suggested that Service Layers Pattern be merged into this article. (Discuss) Proposed since April 2010.|
||This article needs attention from an expert in Telecommunications. (April 2009)|
In intelligent networks (IN) and cellular networks, service layer is a conceptual layer within a network service provider architecture. It aims at providing middleware that serves third-party value-added services and applications at a higher application layer. The service layer provides capability servers owned by a telecommunication network service provider, accessed through open and secure Application Programming Interfaces (APIs) by application layer servers owned by third-party content providers. The service layer also provides an interface to core networks at a lower resource layer. The lower layers may also be named control layer and transport layer (the transport layer is also referred to as the access layer in some architectures).
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.
Service layer in IMS 
The service layer of an IMS architecture provides multimedia services to the overall IMS network. This layer contains network elements which connect to the Serving-CSCF (Call Session Control Function) using the IP multimedia Subsystem Service Control Interface (ISC). The ISC interface uses the SIP signalling protocol.
Elements of the IMS service layer 
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:
- SIP Application Server
- OSA Service Capability Server
SIP Application Server 
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.
OSA Service Capability Server 
Service layer in SOA 
In Service oriented architecture (SOA), the service layer is the third layer in a five abstraction layer model. The model consists of Object layer, Component layer, Service layer, process layer and Enterprise layer. The service layer can be considered as a bridge between the higher and lower layers, and is characterized by a number of services that are carrying out individual business functions.
- Kristofer Kimbler, Service Interaction in Next Generation Networks: Challenges and Opportunities. Feature Interactions in Telecommunications and Software Systems. Papers from the sixths International Workshop on Feature Interactions in Telecommunications and Software Systems May 2000. Edition: illustrated. Published by: IOS Press, 2000. ISBN 1-58603-065-5, ISBN 978-1-58603-065-0.
- Service-oriented Architecture Compass: Business Value, Planning, and Enterprise Roadmap. Author: Norbert Bieberstein, Sanjay Bose, Marc Fiammante, Keith Jones, Rawn Shah. Edition: illustrated. Published by: FT Press, 2005. ISBN 0-13-187002-5, ISBN 978-0-13-187002-4