5.2. SIP ComponentsSIP reports are transmitted between the following components:• UAC (User Agent Client) – a terminal Client that initiates SIPsignalling;• UAS (User Agent Server) – a terminal Server that responses toSIP signalling from a UAC;• UA (User Agent) – a SIP network terminal (SIP telephone, orgateway to other networks), contains a UAC and UAS;• Proxy server – receives connection requests from a UA andtransfers them to another Proxy server if the particular stationis beyond its control;• Redirect server – receives connection requests and sends themback to the requester including destination data instead ofsending them to the calling party;• Location server – receives registration requests from the UAand updates the terminal database with them.All server sections (Proxy, Redirect, Location) are typicallyavailable on a single physical machine called Proxy server, whichis responsible for client database maintenance, connectionestablishing, maintenance and termination, and call directing.The 2N - VoiceBlue Lite gateway is always a UA (has the samefunctions as a VoIP telephone), i.e. receives call requests anddirects calls to GSM networks according to the internal LCR.There are no SIP-defined server sections in the 2N - VoiceBlueLite gateway.5.3. SIP ReportsHere is a list of main signalling reports sent in the SIPenvironment:• INVITE – connection establishing request;• ACK – acknowledgement of INVITE by the final messagereceiver;• BYE – connection termination;34
