Page 134 - From GMS to LTE

120 From GSM to LTE-Advanced Pro and 5G

For other services, like web browsing, such behavior is beneficial as delay times are
shorter if lost packets are only retransmitted in the radio network instead of end to end.

3.2.2 The Access Stratum and Non‐Access Stratum

UMTS aims to separate functionalities of the core network from the access network
as much as possible, in order to be able to independently evolve the two parts of the
network in the future. Therefore, UMTS strictly differentiates between functionali-
ties of the Access Stratum (AS) and the Non‐Access Stratum (NAS) as shown in
Figure 3.3.
The AS contains all functionalities associated with the radio network (‘the access’)
and the control of active connections between a user and the radio network.
Handover control, for example, for which the RNC is responsible in the UTRAN, is
part of the AS.
The NAS contains all functionalities and protocols that are used directly between the
mobile device (UE) and the core network. These have no direct influence on the properties
of the established RAB and its maintenance. Furthermore, NAS protocols are transparent
to the access network. Functionalities like call control, mobility and session manage-
ment, as well as supplementary services (e.g. SMS), which are controlled via the MSC
and SGSN, are considered NAS functionalities.
While the NAS protocols have no direct influence on an existing RAB, it is neverthe-
less necessary for NAS protocols like call control or session management to request the
establishment, modification or termination of a bearer. To enable this, three different
service access points (SAPs) have been defined between the NAS and the AS:

notification SAP (Nt, e.g. for paging);
●
dedicated control SAP (DC, e.g. for RAB setup);
●
general control SAP (GC, e.g. for modification of broadcast messages, optional).
●

Non-access Stratum (NAS)
(Mobility Management, Session Management, GMM/SM)

SAP SAP

Access Stratum (AS) Access Stratum (AS)

Protocols for the establishment Protocols for the establishment
of a radio channel of a radio channel

Network Radio network (UTRAN) Terminal

Figure 3.3 Separation of protocols between the core and radio network into Access Stratum (AS) and
Non‐Access Stratum (NAS).