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Introduction to SA (StandAlone Call Flow)

The 5G SA call flow is used to establish a connection between two devices in order to communicate. A signaling connection is first established using the signaling channel, and then the data link connection is established via the data channel. The 5G SA Call is the first SA call delivered over a 5G network, and a significant milestone in the evolution of self-emerging networks. It represents a significant achievement in network evolution and paves the way for further enhancements in support of new capabilities and services on wireless networks. For 5G service assurance, an SA Call Flow can be accessed by a GTP-C EMM server through an S11 interface. The GTP-C EMM server is connected to multiple network nodes and can perform redundant functions even if some of these nodes are attacked by hackers or malfunctioning. SA call flows will be used to carry the 5G service data, that is, a session control protocol is created between the source and destination entities.

SA Call Flow procedure

The source entity creates a network entry point (NEP) that the destination entity can use to access the network services of the SAE based on the SA call flow. Typically, a SA call is triggered when a mobile device needs to inform the network of its location, or when it needs to be instructed by the network to perform some change in its behavior. The SA process involves three parties: the target mobile device, the SGW (serving gateway) and MME (mobility management entity). 5G SA Call flow is a standardized service architecture to enable easy integration of IP-based communications network, including Voice over LTE (VoLTE), media plane, and signaling plane. 5G SA Call flow is based on Service Attribute with Signaling Transport via IP addressed identified by a unique identifier. The SA call flow will be used to support the higher data rates of a 5G signaling scheme that is based on an IP-based network. The SA call flow supports multiple layers, including authentication and session setup. It also can send different messages and has additional logic for functions that are specific to 5G networks, such as hold time expiration notification, service request acceptance error notification, setup message confirmation request and completion notification.

SA Call flow Establishment

SA Call Flow is a service that enables VoLTE call establishment between 5G devices. The following figure illustrates an example for SA Call Flow with SIP User Agent, 2G/3G Function Gateways and VoLTE Application. The SA Call Flow is a key enabler of network intelligence and automation. It is based on the well-known 5G AIR interface and enables the creation of intelligent, context-aware services that can be used by a wide range of industry stakeholders. 5G SA is a service architecture based on the concept of "Service as a Platform" and allows applications to be independent of underlying communication technology, terminal capability and vendor lock-in. To achieve this goal, 5G SA aims to provide standard APIs and reference models that allow applications to be built with minimal dependence on underlying hardware, systems or standards. This call flow is the 5G SA Call flow. It includes initiating, updating, and deleting user profile information; identifying and acquiring spectrum resources to facilitate communications among eNBs over a shared air interface; supporting mobility management by providing information regarding neighboring cells located in a same or different RAT; providing service continuity during eNB power off periods through periodic polling of dormant UE devices for paging messages; and dynamic connectivity management for seamless data transfer between networks. The 5G System Architecture SA (SA) Call Flow specification provides the signal flow interfaces between the hosts, network endpoints and transport domains in which a 5G RAN is deployed. This interface defines how interactions take place between different parts of the 5G ecosystem through transports and protocols such as 3GPP2 IP Multimedia Subsystem (IMS). 

The service access call flow is the 5G SA call flow. The INITIAL CONTEXT SETUP message is sent by the Serving GW to the Initiating GW, at this stage PGW sends a setup request to HSS and sends an INITIAL CONTEXT SETUP ACK message to SGW. The SGW forwards this INITIAL CONTEXT SETUP ACK message to HSS through the Gx session and receives an INITIAL CONTEXT SETUP RESPONSE with S-TMSI. Then SGW sends a ROBUST SECURITY ASSOCIATION REQUEST message to MME, MME forwards this RAB RESPONSE message to PGW and receives a RAB RESPONSE from PGW. SA Call Flow is a 5G SA call flow used by UEs to establish a shared data session with an eNB in 3GPP Release-15 and beyond. This call flow specifies an LTE evolved NodeB (eNB) configuration supporting 3GPP Rel-15 and beyond.

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