Evolved Packet Core (LTE EPC)

What is EPC in LTE ?

The EPC represents the Core of an LTE network. It is formed by multiple nodes, the main ones being MME, SGW, PGW and HSS. This nodes offer multiple functionality like mobility management, authentication, session management, setting up bearers and application of different Quality of Services.

To summarize the point, the LTE EPC:

  • Makes the LTE core network function;
  • Authenticates subscribers;
  • Determines the subscribers’ access to the network;
  • Helps to locate the UE;
  • Essentially aids mobility management.

To better understand the concept of Evolved Packet Core, you need to have a clear image of its nodes and how each of them work. 

The sections below describes the LTE EPC architecture and the role of each component. 

LTE EPC function: HSS, PC
The LTE EPC made of YateUCN and YateHSS/HLR

The Evolved Packet Core (LTE EPC) has the following nodes:

  • MME (Mobility Management Entity)
    The MME is located at the edge of LTE EPC, between the core network and the radio network. The MME is responsible for handling signals between active UEs and the network within the LTE EPC architecture. It is also responsible for signaling between eNodeBs and the core network. For continuous functionality, MME authenticates UEs by communicating with the HSS, and the mobility function allows the UE to access the network and keeps track of its location and state.
  • HSS (Home Subscriber Server)
    The HSS is the central database that contains all relevant details about a subscriber's information and user authentication. Home Subscriber Server also provides information for calls and IP session Set up. This server makes it easier for service providers to manage the information of their subscribers in real time. A regular mobile network needs only one HSS, but there can exist more than one. If there are two or more, they need to communicate with each other and have their databases updated because the servers need to have a synchronized database to function properly.
  • SGW (Serving Gateway)
    A serving gateway node handles the user data traffic, but isn’t responsible for the signaling data used. It transports IP data from UE's to the LTE Core Network. The SGW also routes incoming and outgoing IP packets for better system collaboration and serves as an anchor for the UE when it moves from one eNodeB to another.
  • PGW (PDN Gateway)
    PGW is the network node that connects the EPC to external IP networks. What the PGW does is that it routes packets to and from external IP networks. Beyond that, it also allocates an IP address to all UEs and enforces different policies regarding IP user traffic such as packet filtering.
  • PCRF (Policy and Charging Rules Function)
    The PCRF node that specifies the service policy and the Quality of Service information. This node guarantees that the users will receive services according to the contract, meaning that services are charged accordingly. It also makes sure the charging system is defined.
  • PCEF (Policy and Charging Enforcement Function)
    What this node does is to simply make sure that PCRF rules are enforced thus deciding whether data should go through the PGW or not.

Note as well that all the nodes mentioned above communicate with each other through Diameter and GTP, as a part of the LTE EPC architecture.

LTE Core Network Background - the evolution of Circuit-switching to Packet-Switching

You need to learn the context that leads to such technology in order to fully understand the concept of LTE EPC.

In the 2G network (GSM), the architecture was built based on circuit-switching (CS), meaning circuits were settled between the caller and the called device throughout the network. It is circuit-switched because the core network uses SS7-based protocols that in certain configuration requires allocation of dedicated channels for said signaling.

Telecommunications worked this way until the GPRS network appeared. It came with a new technology called packed-switching that was added to the circuit-switching. Along with this upgrade of the core network, data is being carried in packets without the formation of dedicated circuits. Some of the most important benefits of this development are flexibility and efficiency, although the circuits still carried voice and short text messages (SMS).
When the third-generation of mobile telephony was created the concept remains the same, but some of the elements evolved. In 3G, the core network had two main components: circuit and packet.

It was then decided that the LTE core network would not keep the circuit-switched domain technology anymore and that the LTE EPC would be an evolution of the packet-switched architecture used in GPRS and 3G. The EPC has a “flat” IP architecture that allows the network to handle a great amount of data traffic in an efficient and cost-effective manner using Diameter protocols.

The relevance of Quality of Service (QoS) to operators and users

There are many benefits of using the LTE core network, such as Quality of Service.

  • The Quality of service is a set of parameters that specifies minimum and maximum performance limits on the type of service a UE receives.
  • The Quality of Service (QoS) can be used to ensure that users are afforded a certain minimum amount of speed or do not exceed a certain maximum amount.
  • For operators, QoS helps them control how resources are allocated in the network.
  • As far as billing is concerned, it is important for operators to bill users differently according to the services they are enjoying.


CSFB (Circuit Switch Fallback) is a technology that supports voice and SMS services in 4G networks using the 2G/3G systems.

VoLTE (Voice over LTE), on the other hand, means that a call is made through a 4G network (Making calls over IP).

Ideally, you would want to make use of VoLTE because IP calls do not require a 2G/3G coverage in that particular area, but the downside to this is that it requires the phone to support VoLTE, in order to make it possible. There are very few phone models in circulation today that support VoLTE out of the box. So depending on the situation, activating it requires the manufacturer’s support, and you may be ignored if you are not a pretty big mobile operator.

CSFB allows access to calls and SMS services for phones that work on a 4G network but doesn’t support VoLTE. When a phone is on 4G, and you want to make a call with it, the network will simply move to 3G to make the call possible through circuit-switching. This requires the presence of a 2G/3G coverage in the same area as the 4G network due to the cooperation between these networks.

To finalize the dispute about which technology is preferable you will have to look at things within the context of limited VoLTE support. CSFB ensures that operators provide users with the service they pay for. The only downside to this is that the operator will have to keep older generation networks operating within the same coverage area. Managing a single type of network might be simpler than managing two or more types.

Circuit Switched Fallback (CSFB)

Yate-based LTE EPC solutions for operators - highly scalable and suitable for any operator size and structure, private networks included.

We provide the following Evolved Packet Core products:

YateHSS/HLR, HSS and HLR in a LTE and GSM mobile network

YateHSS/HLR as a solution for HSS/HLR:

Our solution doesn’t require the use of any special hardware since our YateHSS/HLR is software on Linux machine. It is scalable because you are able to add more nodes as your needs increase and the nodes are kept in sync through clustering. Our solution also provides functionality for HLR and AUC functionality for 2G, 3G, and even WiFi networks, not only for LTE Core Network (HSS, Diameter Based)

YateUCN acting as Core Network for a LTE and GSM mobile network


YateUCN (Unified Core Network) is our solution that provides maximum functionality for all the other Evolved Packet Core nodes in a single server: Our solution comprises the MME, SGW, PGW, and PCEF nodes. It is very flexible and can communicate with HSS via 2G/3G protocols or 4G protocols. This is the most complex product we developed. Having all of these functions in one machine decreases the need for setting up extra machines for each node. Our YateUCN is a software-based solution and it requires only an installation procedure on a Linux machine. System configuration is simple through the Mobile Management Interface and it can be accessed remotely. Our system also provides 2G core network functionality.

YateUCN is a high capacity unified core network solution intended for new LTE networks, for upgrading GSM/GPRS networks to LTE EPC or for extending existing LTE networks.

LTE Core Network for Test - The Minicore

The Minicore is a small computer that comes with YateHSS and YateUCN fully installed. It also contains YateSMSC (for SMS services), YateSTP (for SS7 routing), YateDRA (for Diameter routing) and YateMMI (the configuration interface for all the other equipment). Having all these technologies on a single machine provides a simple solution for setting up and testing how mobile networks work. Our LTE core netwotk test equipment helps in capturing traffic between nodes to help you understand mobile procedures as well as experimenting with both 2G and 4G networks – Setup, and Routing.

Evolved Packet Core (EPC) – LTE CORE Network | YateBTS 1

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