Evolved Packet System
Evolved Packet System (EPS ) refers to the architecture of LTE cellular standards. It includes the core network Evolved Packet Core ( EPC), the Wireless networks ( E-UTRAN ), the end user of the equipment (UE) and the services. EPS is completely based on the packet switching and thus differs fundamentally from the earlier UMTS and GSM technologies still using circuit switching. LTE is still compatible with them and can be operated in parallel.
Core network: Evolved Packet Core
As an Evolved Packet Core (EPC ), the architecture of the core network of LTE is called. It enables the operation and coordination of different wireless networks, thus ensuring mobility, handover and roaming between the participants.
Construction and components
The EPC is characterized by a flat hierarchy in relation to the earlier UMTS and GSM networks. This results in short transfer times of up to 5 milliseconds ( ms) in the core network as well as 20 milliseconds for the entire route. The realized through the core network packet switch between two radio networks had to be conducted only via the so-called Service Architecture Evolution ( SAE) gateway.
A SAE gateway is in each case a Serving Gateway (S -GW ) and a Packet Data Network ( PDN) gateway, which are separated from each other logically. Both of which are connected through an open port to each other so that they can be physically separated. The Serving Gateway assumes the role of a router and forwards packets from one network to the next. The PDN Gateway provides the interface to the data network. It manages the communication with the end user connections to multiple networks and assigns the IP addresses.
Each Serving Gateway is responsible for a specific area and routes all connections in the package of influence. The spatial extent of a region depends on the expected maximum capacity.
For a data connection to a terminal device, the device must be in connected mode, ie be able to establish communication links. Since this mode is very energy intensive and thus greatly reduces the battery life, mobile phones are usually in idle mode. If the device is the transmitter, it switches the mode to itself. For incoming connections is for the S- GW, however, no direct way to send the packets because of the potential recipient can not be contacted. Therefore, the detour via the Mobility Management Entity ( MME) is gone, the plays, among other things, the function of "Paging" and thus is able to activate the device. As long as the data packets in the S -GW to be cached.
In the case of handover, ie the change of the user from one radio cell to another, a redirection of the conversation (or data link ) to another gateway may be necessary. Again, there is a balance with the Mobility Management Entity.
The PDN gateway ( P- GW) enables external data packets access to the wireless network. It serves as the gateway for services that were not originally used for mobile communications (eg web server). It assigns for this purpose, IP addresses to the terminals. In addition, it monitors compliance with technical guidelines during communication, prepares billing information at any of the external service costs and checked and filters the data packets.
The P- GW also provides also an interface that allows the public authorities to tap or listen to streams.
Policy and Charging Rules Function
The Policy and Charging Rules Function ( PCRF ) meets the data- flow-based billing and policy control, so for example, the permission control for the use of a resource. The rights and costs of a service award, the operator can thereby define yourself and forgive if necessary free of charge. To ensure the functionality of the PCRF, they also collects information such as the identity of the user, the time of day or the roaming status.
Home Subscriber Server
The Home Subscriber Server (HSS) is the database that is stored in the user and subscription information that is needed for the treatment of calls. These include, for example, the identification or the access authorization of the user. It is connected to the Mobility Management Entity.
Radio network: Evolved UTRAN
The individual radio networks in the LTE architecture evolved UTRAN ( EUTRAN ) refers. The name is derived from the UMTS architecture in which the names of the radio networks UMTS Terrestrial Radio Access Network ( UTRAN). Be the eNodeB base stations called, in analogy to describe NodeB from the UMTS network analog.
An eNodeB base station is considered to be the most complex assembly of EPS and is composed of the antenna, a radio module and a digital module.
As described above are in LTE is a purely packet-switching, ie digital, network. For this reason, the digital module is used as an interface to the core network. This assumes the actual signal processing.
The radio module, however, is responsible for converting the digital signal on the air interface, the signal that is converted into radio waves. Conversely, received radio waves are converted into digital signals. The method used for this is the modulation as in the entire mobile. For cost reasons, digital and radio module are placed close to each other and connected via optical circuit.
Compared to the UMTS architecture, the function of the radio module will be expanded significantly in particular. While now it is a pure modem in the UTRAN, in essence, it has its own logic devices. Thus, all communication capabilities of the access network are transferred directly to the base stations that have previously been met by the Radio Network Controller (RNC). The elimination of the RNC causes a portion of the required reduction of the transmission times in the system. In particular, the base stations can now communicate directly with each other and organize the mobility management within an access network itself more integrated functions include the allocation of resources between the users ( User Management ) or the reduction of its own transmission power at the activities of the neighboring stations (interference management).