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SRVCC - Single Radio Voice Call Continuity

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SRVCC enables continuity of voice call anchored in IMS network, when UE changes its access network from packet based in eUTRAN to circuit based GERAN or UTRAN, without requiring the UE to have simultaneous access to both the radio network.   It accomplishes this by enhancing the MMEs, serving E-UTRAN and the 3GPP CS MSC serving GERAN/UTRAN such that when the UE is still anchored within the E-UTRAN network and triggers a measurement report indicating a need for handover (HO) to GERAN/UTRAN, the MSC can initiate IMS service continuity procedure towards the IMS domain in addition to preparing for a handover into its GERAN/UTRAN access, before the UE is directed to change its access network to GERAN/UTRAN.     The SRVCC feature requires the new Sv interface between MSC and MME. Sv Interface runs over UDP. Following messages are exchanged over Sv Interface:   - PS to CS Request - PS to CS Response - PS to CS Complete Notification - PS to CS Complete Acknowledge - PS t
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Diameter Messages

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Capability Exchange: When two peers established transport connection, they must exchange the capability exchange message. this message allows the discovery of peer's identity and its capabilities. Once transport layer is established, CER and CEA exchanged to determine the capability of peer endpoint followed by DWR/DWA. In below example DiameterNodeA start diameter connection with DiameterNodeB after transport layer connection established. In case of TCP (Syn/Syn-Ack) and  SCTP (Init/Init-Ack/Coockie Echo) exchanged. In case of SCTP Connection establishment(CER, CEA, DWR, and DWA) are exchanged with setting out-of-order delivery false to avoid race-condition and for connection termination (DPR/DPA) set the timer to receive out-of-order message before terminating the connection.               DiameterNodeA                     DiameterNodeB                                     <------Transport Layer Established--------->                ---------------------- CER -------
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Diameter Protocol

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    Diameter is AAA (authentication, Authorization, and Accounting protocol) such as RADIUS protocol. As name suggest, DIAMETER is double of RADIUS protocol. while there are many difference between two protocols but one of the main difference is diameter provides the security and reliability because diameter uses TCP/SCTP on transport level, it uses TLS and DTLS. whereas RADIUS runs over the UDP.   Diameter protocol is peer to peer protocol so that any node can initiates a request, all data delivered in form of AVPs and provides following facilities:   - Ability to exchange messages and deliver AVPs. - Capability Negotiation - Error Notification - Application specific services.     Diameter base protocol is run on port 3868 for both TCP and SCTP. TLS and Datagram Transport Layer Security (DTLS) uses port 5658. It is assumed TLS run on top of TCP and DTLS is run on top of SCTP. When no transport connection available with peer node, peer should try to connect it
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SIP Responses

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SIP responses generated by the UAS or SIP servers. SIP Response Classes: 1xx - Informational - Indicates the status of call prior to the completion. 2xx - Success -  Request has succeeded. 3xx - Redirection - The client should try to complete the request at another server. 4xx - Client Error - The request has failed due to error due to the client.  The client may retry. 5xx - Server failure - The request has failed due to server error. request may retry to another server. 6xx - Global failure - the request has failed and can not retry to any server. 1xx --Informational : 100 Trying : This response is used to indicate the next node receives the request and stop the retransmission. This response is sent if there is delay in sending the final response more the 200ms. 180 Ringing: The response is generated if UA receives the INVITE and started the ringing. It may used to initiate local ring back. 181 Call is being Forwarded: This response is indication of ca
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SIP Methods

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SIP Servers: Proxy Servers:  - A stateless proxy server processes each SIP request or response based solely on the message contents. Once the message has been parsed, processed, and forwarded or responded to,no information about the message is stored—no dialog information is stored. A stateless proxy never re-transmits a message, and does not use any SIP timers. - A stateful proxy server keeps track of requests and responses received in the past and uses that information in processing future requests and responses. For example, a stateful proxy server starts a timer when a request is forwarded. If no response to the request is received within the timer period, the proxy will re-transmit the request, relieving the user agent of this task. Also, a stateful proxy can require user agent authentication. Back-to-Back User Agents (B2BUA): An B2BUA is a type of SIP device that receives the SIP request, that reformulates the request and send it out as new request. Response to the
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