LTE Network Architecture:
The Single E-UTRAN Node
Evolved Packet Core (EPC) and its Components
The Single E-UTRAN Node
The E-UTRAN OFDM-based structure is quite simple.
It is only composed of one network element: the eNodeB (for evolved Node B.). The 3G RNC (Radio Network Controller) inherited from the 2G BSC (Base Station Controller) has disappeared from E-UTRAN and the eNodeB is directly connected to the Core Network using the S1 interface. As a consequence, the features supported by the RNC have been distributed between the eNodeB or the Core Network MME or Serving Gateway entities.
The X2 Interface
A new interface (X2) has been defined between eNodeB, working in a meshed way (meaning that all Node Bs may possibly be linked together). The main purpose of this interface is to minimize packet loss due to user mobility. As the terminal moves across the access network, unsent or unacknowledged packets stored in the old eNodeB queues can be forwarded or tunnelled to the new eNodeB thanks to the X2 interface. From a high-level perspective, the new E-UTRAN architecture is actually moving towards WLAN network structures and Wifi or WiMAX Base Stations.
eNode B Functionalities
Functional definition eNodeB as WLAN access points – support all Layer 1 and Layer 2 features associated to the E-UTRAN OFDM physical interface, and they are directly connected to network routers. There is no more intermediate controlling node (as the 2G/BSC or 3G/ RNC was). This has the advantage of a simpler network architecture (fewer nodes of different types, which means simplified network operation) and allows better performance over the radio interface. As described in Chapter 4, the termination of Layer 2 protocols in eNodeB rather than in the RNC helps to decrease data-transmission latency by saving the delay incurred by the transmission of packet repetitions over the Iub interface. From a functional perspective, the eNodeB supports a set of legacy features, all related to physical layer procedures for transmission and reception over the radio interface:
· Modulation and de-modulation.
· Channel coding and de-coding.
Besides, the eNodeB includes additional features, coming from the fact that there are no more Base Station controllers in the E-UTRAN architecture. Those features, which are further described in Chapter 4, include the following:
· Radio Resource Control: this relates to the allocation, modification and release of resources for the transmission
over the radio interface between the user terminal and the eNodeB.
· Radio Mobility management: this refers to a measurement processing and handover decision.
· · Radio interface full Layer 2 protocol: in the OSI ‘Data Link’ way, the layer 2 purpose is to ensure transfer of data
between network entities. This implies detection and possibly correction of errors that may occur in the physical layer.
Evolved Packet Core (EPC) and its Components
The EPC (Evolved Packet Core) is composed of several functional entities:
· The MME (Mobility Management Entity)
· The HSS (Home Subscriber Server)
· The Serving Gateway.
· The PDN Gateway (Packet Data Network).
· The PCRF (Policy and Charging Rules Function) Server.
The following sub-sections discuss each of these in detail:
MME (Mobility Management Entity)
The MME is in charge of all the Control plane functions related to subscriber and session management. From that perspective, the MME supports the following:
· Security procedures – this relates to end-user authentication as well as initiation and negotiation of ciphering and integrity protection algorithms.
· Terminal-to-network session handling – this relates to all the signalling procedures used to set up Packet Data context and negotiate associated parameters like the Quality of Service.
· Idle terminal location management – this relates to the tracking area update process used in order for the network to be able to join terminals in case of incoming sessions.
The MME is linked through the S6 interface to the HSS which supports the database containing all the user subscription information.
HSS (Home Subscriber Server)
The HSS (Home Subscriber Server) is the concatenation of the HLR (Home Location Register) and the AuC (Authentication Center) – two functions being already present in pre-IMS 2G/GSM and 3G/UMTS networks. The HLR part of the HSS is in charge of storing and updating when necessary the database containing all the user subscription information, including (list is non exhaustive):
· User identification and addressing – this corresponds to the IMSI (International Mobile Subscriber Identity) and MSISDN (Mobile Subscriber ISDN Number) or mobile telephone number.
· User profile information – this includes service subscription states and user-subscribed Quality of Service information (such as maximum allowed bit rate or allowed traffic class).
The AuC part of the HSS is in charge of generating security information from user identity keys. This security information is provided to the HLR and further communicated to other entities in the network. Security information is mainly used for:
· Mutual network-terminal authentication.
· Radio path ciphering and integrity protection, to ensure data and signalling transmitted between the network and the terminal is neither eavesdropped nor altered.
The Serving GW (Serving Gateway)
From a functional perspective, the Serving GW is the termination point of the packet data interface towards E-UTRAN. When terminals move across eNodeB in E-UTRAN, the Serving GW serves as a local mobility anchor, meaning that packets are routed through this point for intra E-UTRAN mobility and mobility with other 3GPP technologies, such as 2G/GSM and 3G/UMTS.
The PDN GW (Packet Data Network Gateway)Similarly to the Serving GW, the PDN gateway is the termination point of the packet data interface towards the Packet Data Network. As an anchor point for sessions towards the external Packet Data Networks, the PDN GW also supports Policy Enforcement features (which apply operator-defined rules for resource allocation and usage) as well as packet filtering (like deep packet inspection for virus signature detection) and evolved charging support (like per URL charging).
The PCRF (Policy and Charging Rules Function) Server
The PCRF server manages the service policy and sends QoS setting information for each user session and accounting rule information. The PCRF Server combines functionalities for the following two UMTS nodes:
· The Policy Decision Function (PDF)
· The Charging Rules Function (CRF)
The PDF is the network entity where the policy decisions are made. As the IMS session is being set up, SIP signalling containing media requirements are exchanged between the terminal and the P-CSCF. At some time in the session establishment process, the PDF receives those requirements from the P-CSCF and makes decisions based on network operator rules, such as:
· Allowing or rejecting the media request.
· Using new or existing PDP context for an incoming media request.
· Checking the allocation of new resources against the maximum authorized
The CRFs role is to provide operator-defined charging rules applicable to each service data flow. The CRF selects the relevant charging rules based on information provided by the P-CSCF, such as Application Identifier, Type of Stream (audio, video, etc.), Application Data Rate, etc.