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5G Attach procedure

5G Attach is a solution that enables mobile backhaul operators to leverage best in class hardware and software to deliver secure, high-performance data services to subscribers while lowering overall costs. With 5G Attach you can use your existing LTE network infrastructure to deliver end to end 5G services. 5G Attach is a new module designed to integrate a fifth-generation cell phone via the SFP port. The module supports future innovation and features such as Rich Communication Services (RCS), Multimedia Broadcast Multicast Service (MBMS) and Narrow Band Internet of Things (NB-IoT) services. 5G Attach is a new way for customers to purchase and attach insurance to their mobile devices - from the moment they first use their phone, via automatic underwriting. Smart Cities drive smart services, and 5G Attach is an important enabler for customers. It adapts to varying traffic conditions, such as a high demand for cloud storage or video streaming on a crowded 4G network”. In this way users will enjoy high-quality experiences at all times, even when accessing high data volumes. 

5G Attach call flow 

In the case of 5G Attach, the UE initiates a request for user context retrieval to the MME in order to retrieve initial NAS and other context elements (e.g., security parameters) that were used during an earlier attach. This activity results in the establishment of a non-access stratum (NAS) session between UE and MME. Previously, the 5G Attach process began with a TAU message from a UE, which was transparently redirected to MME/S-GW due to the change of PDN. In the new process, TAU can also be triggered by MME/S-GW as part of the attach procedure. This includes 3GPP network elements and non-3GPP network elements such as WiMAX and VoLTE. In 5G Attach, an RRC Connection Setup message is sent to the UE, then it responds with RRC Connection Setup Complete message. After that, a new VAP be formed, the UE will do necessary signaling between CN and MME in order to setup NAS Traversal tunnel. After all the connection is established, the UE will send RRC Connection Reconfiguration Complete message to MME and then UE may send Paging messages to request paging. 

Upon receiving a 5G ATTACH request, the UE broadcasts an ATTACH request on all available RATs. When a network node receives the request and accepts it, it replies with an ATTACH ACCEPT message in the same RAT as where the UE’s broadcast was sent. The 5G Attach call flow comprises a attach procedure and an attach success event, which are shown in Figure 1. To begin the attach procedure, both a UE and an E-UTRAN need to perform an RRC connection establishment. The UE initiates the RRC connection request to the E-UTRAN by sending an RRCConnectionRequest message including an RRC message sequence number (RSN) that is initialized with the most significant 4 bits of the Timing Advancement Count value. 5G Attach is a framework for data transmission. It enables the mobile network to initiate an RRC connection and attach to the access network on its own, without going through an intervening gateway or PDN-GW. 5G Attach provides a direct request for connection of a mobile user device with the nearby wireless access point. The request can be initiated e.g., upon admission by an access network node upon detection of incoming traffic requests from a particular UE.

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