Domains and Strati | Area and Subscriber Identities

For the EPC a complete new NAS was designed including a new NAS protocol layer described in 3GPP 24.301.

In contrast to the core network of 3GPP Release 99 to Release 6 where a CS and PS domain were defined as subdomains of the serving network domain, the EPC will not host any CS domain due to its all-IP character. However, it still distinguishes between AS and NAS signaling and functions as shown in Figure 1.

 

Figure 1: Domains and strati in E-UTRAN and EPC

The AS comprises the radio chipset of the UE including the RRC protocol entity and all underlying transport layer entities. Here all parameters that more or less frequently change during radio access can be found, including transport formats and radio-specific identities of serving cell and possible handover candidates (neighbor cells).

The NAS covers all signaling exchanged between the USIM (UMTS Subscriber Identity Module) and the core network node, in case of LTE radio access: the MME. This is the home of all parameters that allow unambiguous identification of a subscriber or the handset hardware such as International Mobile Subscriber Identity (IMSI) and International Mobile Equipment Identity (IMEI). There are also temporary identities stored on the USIM card like Temporary Mobile Subscriber Identity (TMSI) and Globally Unique Temporary UE Identity (GUTI). From a protocol point of view the NAS is the home of network access, initial subscriber registration, and mobility management procedures. Due to the all-IP concept of LTE/EPC, a new NAS protocol was defined, namely 3GPP 24.301, while similar functions for 2G/3G networks are defined in the standard 3GPP 24.008. The E-UTRAN NAS protocol 3GPP 24.301 does not contain any functions for CS call control and SMS. In the early planning stages of the E-UTRAN it was assumed that all speech services via the E-UTRAN would use VoIP and the IMS architecture. As an alternative the CS fallback option (implemented in the S1AP protocol) was designed, but obviously this did not satisfy the need for reliable and cost-efficient CS services in the E-UTRAN. Hence, an initiative formed of operators and Network Equipment Manufacturers (NEMs) started to work on the Voice over LTE via Generic Access standards (VoLGA). VoLGA is beyond the scope of 3GPP. Its principle is to establish an IP connection between the UE and E-UTRAN and use the radio bearer for transparent forwarding of 3GPP 24.008 NAS signaling message and AMR (Adaptive Multirate) voice packets across the logical Z1 interface. Instead, in the S-GW the RAB used for VoLGA is terminated in a special protocol converter and media gateway device, the VoLGA Access Network Controller (VANC), that is, the interconnecting point between the E-UTRAN/EPC and UTRAN/GERAN/Legacy Core Network.