Evolved Packet System
Evolved Packet System ( EPS ) describes the architecture of the LTE cellular standard . It comprises the core network ( Evolved Packet Core, EPC ), the radio networks ( E-UTRAN ), the devices of the end user (UE) and the services . EPS is based entirely on packet switching and thus differs fundamentally from the older UMTS and GSM technologies that still use circuit switching . Nevertheless, LTE is compatible with these and can be operated in parallel.
Core network: Evolved Packet Core
As Evolved Packet Core ( EPC ) architecture is the core network of LTE called. It enables the operation and coordination of different radio networks and thus guarantees mobility, handover and roaming between the participants .
Structure and components
In relation to the older UMTS and GSM networks, the EPC is characterized by a flat hierarchy. This results in short transmission times of a maximum of 5 milliseconds (ms) in the core network and 20 milliseconds for the entire route. The packet switching between two radio networks implemented by the core network only had to be routed via the so-called Service Architecture Evolution (SAE) gateway .
An SAE gateway consists of a serving gateway (S-GW) and a packet data network (PDN) gateway , which are logically separated from each other . Both are connected to each other by an open interface so that they can also be physically separated. The serving gateway takes on the role of a router and routes the packets from one network to the next. The PDN gateway forms the interface to the data network. It manages the communication when the end user is connected to several networks and assigns the IP addresses .
Each serving gateway is responsible for a specific area and routes all packet connections in the area of influence. The spatial extent of an area depends on the expected maximum capacity.
For a data connection with a terminal device, the device must be in connected mode , i.e. be able to establish communication connections. As this mode is very energy-intensive and thus greatly shortens the battery life, mobile phones are usually in idle mode . If the device is the transmitter, it switches the mode itself. In the case of incoming connections, however, there is no direct possibility for the S-GW to send the packets, since the potential recipient cannot be contacted. Therefore, the detour is taken via the Mobility Management Entity (MME) , which among other things has the function of " paging " and is thus able to activate the device. The data packets are temporarily stored in the S-GW for this period.
In the event of a handover , i.e. the user changing from one radio cell to the next, the call (or the data connection) may need to be rerouted to another gateway. A comparison with the Mobility Management Entity also takes place here.
The PDN gateway (P-GW / PGW) enables external data packets to access the cellular network. It serves as a gateway for services that were not originally used for mobile communications (e.g. web server). For this purpose, it assigns IP addresses to the end devices. It also monitors compliance with technical guidelines during communication, prepares billing information for any costs incurred by the external service, and checks or filters the data packets.
The P-GW also provides an interface that enables state authorities to tap or listen to data streams.
Policy and Charging Rules Function
The Policy and Charging Rules Function ( PCRF ) controls the various components, such as GGSN / P-GW, as the intelligence in the cellular data network. It is a pure signaling system that does not carry user plan traffic. A key function of the PCRF is
- Permission control for the use of cellular data network resource, e.g. B. Internet access allowed or not
- Control of the network resource, e.g. B. Internet access allowed at 2 Mbit / s or 150 Mbit / s
- Configuration of the GGSN / P-GW regarding the billing of network resources (time or volume based)
Network provider-specific rules are stored in the PCRF, which define rules from various input criteria, the output value of which controls data usage
- Data link information, e.g. B. used APN, country, device etc.
- Customer information, e.g. B. booked tariff
Home Subscriber Server
The Home Subscriber Server (HSS) is the database that stores user and subscription information needed to handle the calls. This includes, for example, the identification or access authorization of the user. It is connected to the Mobility Management Entity .
Radio network: Evolved UTRAN
The individual radio networks are called evolved UTRAN (eUTRAN) in the LTE architecture . The name is derived from the UMTS architecture, in which the radio networks are called UMTS Terrestrial Radio Access Network (UTRAN) . Similarly, the base stations are called eNodeB , based on the name NodeB from the UMTS network.
An eNodeB base station is the most complex assembly of the EPS and consists of the antennas , a radio module and a digital module .
As described above, LTE is a purely packet-switching, i.e. digital , network. For this reason, the digital module also serves as an interface to the core network. This takes over the actual signal processing.
The radio module, on the other hand, is responsible for converting the digital signal to the air interface, i.e. converting the signal into radio waves. Conversely, received radio waves are also converted into digital signals. The method used for this is modulation , as is the case with all mobile communications . For cost reasons, the digital and radio modules are placed close together and connected via optical conductors.
Compared to the UMTS architecture, the function of the radio module in particular is significantly expanded. While it is essentially a pure modem in the UTRAN , it now has its own logical components. As a result, all communication functions of the access network are shifted directly to the base stations that were previously performed by the radio network controller (RNC). The elimination of the RNC results in part of the required reduction in transmission times in the system. In particular, the base stations can now communicate directly with each other and organize the mobility management themselves within an access network. Other integrated functions are, for example, the distribution of resources between the participants ( user management ) or the reduction of their own transmission power when the neighboring stations are doing activities ( interference management ).
- R. Hofstetter, R. Tanner: The LTE core network - Part 3 of the series of articles about the new cellular standard. In: Bulletin SEV / VSE. 21, 2008, p. 22. (online at: www.htwchur.ch ; PDF; 143 kB) ( Memento from December 8, 2015 in the Internet Archive )
- R. Hofstetter, R. Tanner: The LTE core network - Part 3 of the series of articles about the new cellular standard. In: Bulletin SEV / VSE. 21, 2008, p. 23f.
- R. Hofstetter, R. Tanner: The LTE core network - Part 3 of the series of articles about the new cellular standard. In: Bulletin SEV / VSE. 21, 2008, p. 24.
- M. Sauter: Basic course in mobile communication systems - UMTS, HSDPA and LTE, GSM, GPRS and wireless LAN. 4th edition. 2011, ISBN 978-3-8348-1407-4 , pp. 285ff.