EPS Protocols and Planes

Access and Non-Access Stratum
Over the interfaces between UE and EPS, protocols are split into AS and NAS. Figure 1.28
describes the LTE entities involved for both NAS and AS procedures. The NAS and AS layers
exist equally in the UE and EPS to handle the related control and user plane procedures.
The AS resides between the UE and E-UTRAN and consists of multiple protocol layers:
RRC, PDCP, RLC (radio link control), MAC, and the PHY layers. The AS signaling provides
a mechanism to deliver NAS signaling messages intended for control plane procedures, as
well as the lower layer signaling and parameters required to set up, maintain, and manage the
connections with the UE.
The NAS layer between the UE and EPC is responsible for handling control plane messaging
related to the CN. NAS includes two main protocols: evolved mobility management (EMM)
and evolved session management (ESM) [28]. Tables 1.5 and 1.6 summarize the functions of
each of these NAS entities.
 
Control Plane
The protocol stack of an EPS system is designed to handle both control and user planes,
as shown previously in Figure 1.2. The control plane is responsible for signaling message
exchange between the UE and the EPC or E-UTRAN.
When the UE is in LTE coverage, there are two control planes set up to carry the signaling
messages between the EPS and the UE. The first is provided by RRC and carries signaling
between the UE and the eNB. The second carries NAS signaling messages between the UE
and the MME.





The main functions of the control plane are
• To facilitate the NAS and AS signaling messages between the concerned interfaces.
• To define the NAS and AS system parameters and protocol layer mapping. The parameters
are defined for the UE to be able to connect with the EPS and control all subsequent
procedures. The NAS parameters define the EPS bearer-related procedures. The AS parameters
define the mechanisms to maintain and manage the connection and the user plane data
transfer on the uplink and downlink.

User Plane
The user plane is used for forwarding any uplink or downlink data between the UE and the
EPS. In particular, it is used for the delivery of IP packets to and from the S-GW and PDN-GW.



The user plane is established when the UE is in connected mode where the data can flow
across the protocol layers. The user plane primarily utilizes the AS of the protocol. The NAS
layer only provides the information of mapping of upper layer channels needed for the data to
flow. Additionally, NAS provides the user plane with the required parameters including QoS.
The UE and eNB then utilize these NAS configurations to exchange the user plane data.



sumber : Book "DESIGN, DEPLOYMENT AND PERFORMANCE OF 4G-LTE NETWORKS"
 A PRACTICAL APPROACH -Ayman Elnashar dkk WILEY