LTE tutorial[9] LTE SAE system architecture evolution

Along with 3G LTE - Long Term Evolution that applies more to the radio access technology of the cellular telecommunications system, there is also an evolution of the core network. Known as SAE - System Architecture Evolution. This new architecture has been developed to provide a considerably higher level of performance that is in line with the requirements of LTE.

As a result it is anticipated that operators will commence introducing hardware conforming to the new System Architecture Evolution standards so that the anticipated data levels can be handled when 3G LTE is introduced.

The new SAE, System Architecture Evolution has also been developed so that it is fully compatible with LTE Advanced, the new 4G technology. Therefore when LTE Advanced is introduced, the network will be able to handle the further data increases with little change.

Reason for SAE System Architecture Evolution

The SAE System Architecture Evolution offers many advantages over previous topologies and systems used for cellular core networks. As a result it is anticipated that it will be wide adopted by the cellular operators.

SAE System Architecture Evolution will offer a number of key advantages:

1. Improved data capacity: With 3G LTE offering data download rates of 100 Mbps, and the focus of the system being on mobile broadband, it will be necessary for the network to be able to handle much greater levels of data. To achieve this it is necessary to adopt a system architecture that lends itself to much grater levels of data transfer.
2. All IP architecture: When 3G was first developed, voice was still carried as circuit switched data. Since then there has been a relentless move to IP data. Accordingly the new SAE, System Architecture Evolution schemes have adopted an all IP network configuration.
3. Reduced latency: With increased levels of interaction being required and much faster responses, the new SAE concepts have been evolved to ensure that the levels of latency have been reduced to around 10 ms. This will ensure that applications using 3G LTE will be sufficiently responsive.
4. Reduced OPEX and CAPEX: A key element for any operator is to reduce costs. It is therefore essential that any new design reduces both the capital expenditure (CAPEX)and the operational expenditure (OPEX). The new flat architecture used for SAE System Architecture Evolution means that only two node types are used. In addition to this a high level of automatic configuration is introduced and this reduces the set-up and commissioning time.

SAE System Architecture Evolution basics

The new SAE network is based upon the GSM / WCDMA core networks to enable simplified operations and easy deployment. Despite this, the SAE network brings in some major changes, and allows far more efficient and effect transfer of data.

There are several common principles used in the development of the LTE SAE network:

• a common gateway node and anchor point for all technologies.
• an optimised architecture for the user plane with only two node types.
• an all IP based system with IP based protocols used on all interfaces.
• a split in the control / user plane between the MME, mobility management entity and the gateway.
• a radio access network / core network functional split similar to that used on WCDMA / HSPA.
• integration of non-3GPP access technologies (e.g. cdma2000, WiMAX, etc) using client as well as network based mobile-IP.

The main element of the LTE SAE network is what is termed the Evolved Packet Core or EPC. This connects to the eNodeBs as shown in the diagram below.

LTE SAE Evolved Packet Core

As seen within the diagram, the LTE SAE Evolved Packet Core, EPC consists of four main elements as listed below:

• Mobility Management Entity, MME: The MME is the main control node for the LTE SAE access network, handling a number of features:
  • Idle mode UE tracking
  • Bearer activation / de-activation
  • Choice of SGW for a UE
  • Intra-LTE handover involving core network node location
  • Interacting with HSS to authenticate user on attachment and implements roaming restrictions
  • It acts as a termination for the Non-Access Stratum (NAS)
  • Provides temporary identities for UEs
  • The SAE MME acts the termination point for ciphering protection for NAS signaling. As part of this it also handles the security key management. Accordingly the MME is the point at which lawful interception of signalling may be made.
  • Paging procedure
  • The S3 interface terminates in the MME thereby providing the control plane function for mobility between LTE and 2G/3G access networks.
  • The SAE MME also terminates the S6a interface for the home HSS for roaming UEs.
  It can therefore be seen that the SAE MME provides a considerable level of overall control functionality.
• Serving Gateway, SGW: The Serving Gateway, SGW, is a data plane element within the LTE SAE. Its main purpose is to manage the user plane mobility and it also acts as the main border between the Radio Access Network, RAN and the core network. The SGW also maintains the data paths between the eNodeBs and the PDN Gateways. In this way the SGW forms a interface for the data packet network at the E-UTRAN.
  
  Also when UEs move across areas served by different eNodeBs, the SGW serves as a mobility anchor ensuring that the data path is maintained.
• PDN Gateway, PGW: The LTE SAE PDN gateway provides connectivity for the UE to external packet data networks, fulfilling the function of entry and exit point for UE data. The UE may have connectivity with more than one PGW for accessing multiple PDNs.
• Policy and Charging Rules Function, PCRF: This is the generic name for the entity within the LTE SAE EPC which detects the service flow, enforces charging policy. For applications that require dynamic policy or charging control, a network element entitled the Applications Function, AF is used.
  
  
  
  LTE SAE PCRF Interfaces

LTE SAE Distributed intelligence

In order that requirements for increased data capacity and reduced latency can be met, along with the move to an all-IP network, it is necessary to adopt a new approach to the network structure.

For 3G UMTS / WCDMA the UTRAN (UMTS Terrestrial Radio Access Network, comprising the Node B's or basestations and Radio Network Controllers) employed low levels of autonomy. The Node Bs were connected in a star formation to the Radio Network Controllers (RNCs) which carried out the majority of the management of the radio resource. In turn the RNCs connected to the core network and connect in turn to the Core Network.

To provide the required functionality within LTE SAE, the basic system architecture sees the removal of a layer of management. The RNC is removed and the radio resource management is devolved to the base-stations. The new style base-stations are called eNodeBs or eNBs.

The eNBs are connected directly to the core network gateway via a newly defined "S1 interface". In addition to this the new eNBs also connect to adjacent eNBs in a mesh via an "X2 interface". This provides a much greater level of direct interconnectivity. It also enables many calls to be routed very directly as a large number of calls and connections are to other mobiles in the same or adjacent cells. The new structure allows many calls to be routed far more directly and with only minimum interaction with the core network.

In addition to the new Layer 1 and Layer 2 functionality, eNBs handle several other functions. This includes the radio resource control including admission control, load balancing and radio mobility control including handover decisions for the mobile or user equipment (UE).

The additional levels of flexibility and functionality given to the new eNBs mean that they are more complex than the UMTS and previous generations of base-station. However the new 3G LTE SAE network structure enables far higher levels of performance. In addition to this their flexibility enables them to be updated to handle new upgrades to the system including the transition from �G LTE to 4G LTE Advanced.

LTE SAE summary

The new System Architecture Evolution, SAE for LTE provides a new approach for the core network, enabling far higher levels of data to be transported to enable it to support the much higher data rates that will be possible with LTE. In addition to this, other features that enable the CAPEX and OPEX to be reduced when compared to existing systems, thereby enabling higher levels of efficiency to be achieved.