What is TAS

TAS (Telephony Application Server) refers to an entity in a telephone network that carries out functions that are not directly related to the routing of messages through the network. Such functions can include in-network answering machines, 800 (freephone) numbers, automatic call forwarding, conference bridges and many other types of application. In integrated telephone networks, these are mostly implemented as functions in the telephone exchange, but in more open networks such as IP telephony networks based on the SIP protocol, these are often separate computers. 

The application server provides call-termination or subscriber-independent applications. These include such capabilities as local number portability, free-call routing resolution, conference bridge services, and unified messaging. The PSTN versions of these applications are frequently known as 800 numbers in North America. The subscriber has to explicitly place a call to the application server. 

Application server applications are of two general types, those that are signaling only, and those that involve media manipulation. The former applications are often related to routing resolution — local number portability, free-call routing, and other services where the dialed number must be translated to a routable address. An example involving media manipulation would be conference bridge applications, something with which most business people are very familiar. The call steps include: 

• Each user calls in on a pre-published number. 

• The dialed number is translated into an IP address and named the endpoint of the application server, and the call is routed there. 

• The application server connects to the media server, instructing it to play a greeting and collect the conference number. 

• The media server returns and conference number, and the application server instructs the media server to play a prompt to collect the authorization number. 

• If the digits collected are correct, the application server tells the media server to move this call to a particular conference bridge. 

A mobile telephony application server (mobile TAS) is a component used in the core network of a mobile network operator to provide telephony applications and additional multimedia functions to mobile devices.  

A mobile TAS is typically deployed in mobile networks using 4G/LTE and IP Multimedia Subsystem (IMS) architectures, alongside components provided such as the Call Session Control Function (CSCF) and Home Subscriber Server (HSS).  

A mobile TAS can serve multiple functions, including:  

• Functioning as a multimedia telephony application server, providing call control for voice and video, including support for GSMA standards IR.92 and IR.94 for VoLTE, and IR.51 for Voice over Wi-Fi.  

• Emulating GSM supplementary services, such as call forwarding and conferencing, and maintaining feature parity between TDM and IP network infrastructures.  

• Interworking between circuit switched and packet switched networks.  

• Delivery of additional services and applications, such as anonymous call rejection and explicit call transfer.  

• Providing a framework for the creation of new services and applications, sometimes with an open development environment encouraging third-party development.  

A mobile TAS that supports both TDM and IP network infrastructures can be an essential element in IP network transformation.  

Some mobile TASs are purpose-built hardware appliances while some are virtualized network functions (VNFs) capable of running on general-purpose hardware or in a cloud environment as part of Network Functions Virtualization (NFV) deployment.