The core network SAE: Envolved Packet Core

When it began the standardization of LTE RAN, began the work involved for the CN under LTE System Architecture Evolution (SAE). The core network SAE defined in the system is a radical evolution of the core GSM / GPRS, and it is therefore she has a new name, or EPC Evolved Packet Core (Evolved Packet Core). The SAE system only covers the field of packet switching, not circuit switched. Looking back, the philosophy of minimizing the number of nodes also reigns in the core network standards. Consequently, the EPC network architecture began as a single node with all functions in the same, except the Home Subscriber Server (HSS), which remains outside the node. The HSS is a database node HLR corresponding to the core GSM / WCDMA.

Figure 4.8 is shown as the Evolved Packet Core fits into the overall architecture. The EPC is connected to the LTE RAN through the S1 interface to the Internet through the IMS and the HSS interface via interface S6. S1 is the interface between eNBs and EPC, which is very similar to the interface Iu_ps. The S1 and user plane tunnel Iu_ps are IP-based transport, they do not know the contents of the packet. The end-user IP packets are placed in the S1 IP tunnel by the EPC or the ENB and recovered at the other end (ENB or EPC).

Figure 4.8 Simplified description of the Core Network for LTE SAE.

To control the level of the difference between S1 and Iu is not much, in fact, only in the details of establishing carrier is visible. The difference lies in the way of indicating quality of service assigned on a user's specific flow. For WCDMA / HSPA is through the parameters of Radio Access Bearer (RAB) while LTE is done by means of which points to a specific priority class.

S6 interface is shown connecting the EPC to the HSS. This evolution is a Gr interface used by WCDMA / HSPA to connect to the HLR. Therefore, a combination of HLR / HSS for the Evolved Packet Core, may be the same as that shown in the core GSM network to WCDMA.

Perhaps the biggest difference between WCDMA / HSPA and LTE is the mobility management. In LTE, the EPC serves as an anchor in the SAE core network for mobility, being an EPC node that handles the user plane is not changed during a connection. The EPC takes here the role of a GGSN for GSM / GPRS and WCDMA / HSPA. Due to the flat architecture, the node must be able to connect to each ENB essentially on the network, and updated within it, which should guide the user's packets. This is the big difference compared to the RAN WCDMA / HSPA, where the RNC hides this kind of mobility from the core network. Tres son las entidades básicas para soportar la movilidad: la MME (Mobility Management Entity), el S-GW (Serving-Gateway) y el PDN-GW (Packet Data Network-Gateway). Por medio de la interfaz S1, éstos se interconectan con la RAN. Dicha interfaz consta del plano de control S1-mme, entre el eNB y el MME, y del plano de usuario S1-u, entre el eNB y el S-GW (ver Figura 4.9).

Figure 4.9 Functional elements and interfaces of the EPC. 4.4.2.1 SAE Gateway.

It consists of two logical entities of the user plane, the S-GW and the PDN-GW, and provides an interface between the access network and the various packet networks. In practice both gateways can be deployed as a single network element. The S-GW is responsible for

the following functions:

- Actively involved in the mobility process when a transfer (handover) between eNBs.

- Through the interface S4, based on the protocol GTP (GPRS Tunneling Protocol) is the entity involved in user traffic in case of mobility between 3GPP LTE and other technologies.

- In case of need for user traffic information to a legislative requirement, is responsible for replicating the information.

The PDN-GW is seen as the input / output traffic to / from the user, providing connectivity to the rest of external networks, highlighting the following tasks:

– Through the interface S7 is performed transferring the QoS and pricing policies that apply to user traffic between the PCRF (Policy and Charging Rule Function) and the PDN-GW.

− Facilitate the seamless mobility and continuity in the sessions when the user moves between access networks technologically heterogeneous, i.e, from a network 3GPP (GSM, WCDMA, HSPA) to another network that is not 3GPP (WiMAX or Wi-Fi).

Networks that are not part of the 3GPP, are distinguished in two types of access: Confident and distrustful, and the operator is to decide the type of each network who would allow their connection. The interconnection network considered suspicious, is performed using a ePDG (evolved Packet Data Gateway), which implements IP mobility protocols, being necessary to access the services offered by the operator. The user terminal establishes an IPsec tunnel through the interface ePDG Wn. The interconnection of confidence does not use the ePDG, so PMIP protocols are used directly with PDN-GW by S2a interface.

4.4.2.2 MME.

Is a controlling entity, solely responsible for signaling, and that she did not pass data packets from users. By S3 interface based on the GTP protocol, the control is made for mobility signaling 3GGP networks and interacts with the HSS (Home Subscriber Server), based on the Diameter protocol, which performs the authentication of users. Gives operators the advantage of increasing the capacity of signaling independently of user traffic, and which is a network dedicated to functionally separate signaling and gateways.

4.4.2.3 S1 Flex.

Similar to Iu flex, flex S1 enables more robust core network, with more flexibility in interconnect access nodes and the central system, breaking the usual hierarchical network. If one node EPC is unavailable, another node can take over the loss of traffic. Furthermore, the expansion of the network is easier because the EPC nodes can be added as needed by the traffic demand and not by an increase in coverage.

Figure 4.10 S1 Flexibility function.

As shown in Figure 4.10, S1 flex provides redundancy and load sharing, enabling ENB be connected to more than one node or Serving Gateway MME. The application is for both S1 flex independently.

4.4.2.4 Roaming.

Similar to the scenarios considered in today's networks, a distinction depending on whether the user traffic is routed to the home network or not. This latter scenario is called local breakout.

In the case that the user traffic is directed to the Home network, the solution adopted by SAE would make a separation between the S-GW and the PDN-GW. The first, along with the eutrophi and MMES, would be located in the visited network while the PDN GW and the PCRF HHS would be in the home network.

point between the SGSNs Gp and GGSNs. However there are still define some unresolved issues such as the location of the PCRF in the visited network and a possible interaction between the PCRF in the home network and visited network.

The local breakout scenario is applicable in certain situations in which the service can be directly offered by the visited operator, eg is delegated to the visited network operator's Internet access service. Regarding the above scenario, the PDN-GW would be located in the visited network, leaving unresolved the interaction between the PCRF in the home network and visited.

4.4.2.5 Control Policies and Pricing.

In Release 5 and 6 of the 3GPP defines the first architectures for pricing and flow control for IP QoS. The unification of these architectures is finalized in Release 7 on behalf of PCC (Policy Control and Charging). This architecture defines a node, the PCRF, which is responsible for authorizing the services or IP flows that are accessed by a user as well as performing the provision of pricing policies and QoS at the node in charge of run, the PCEF (GGSN in GPRS / UMTS).

Another feature of PCC is the improvement in the standard form of the QoS for "PDP Context", taking a very important step which is to control the QoS, the new data networks, rests with the operator and not the EU.

A key issue for operators is to reuse the long run all the deployed architecture for policy management in the network and the terminal, which is why, in order to ensure a smooth migration to SAE, is taken as a criterion design, reuse of the PCRF interfaces defined for the Release 7.