Читать книгу Mobile Communications Systems Development - Rajib Taid - Страница 20

Figure 2.4 shows the network architecture of the LTE system, refer to TS 36.300 [92], which consists of the different network elements: UE, Evolved (e)NodeB, and Evolved Packet Core (EPC) network to provide PS services only. LTE network, which provides higher data rates, was evolved from the previous UMTS system. The term called “Evolved”, denoted by “e” or “E” can be found in any descriptions related to the LTE system. The eNodeB performs the radio communication‐related functions and controls one or more cells, similar to a UMTS NodeB +RNC. Apart from this, eNodeB performs the radio resources allocation, UE scheduling, and forwarding of the user traffic/data functions to the S‐GW. An eNodeB is interconnected with other eNodeBs over the X2 interface, and together, they form the Evolved‐UMTS Terrestrial Radio Access Network (E‐UTRAN) of an LTE/EPS network.

The LTE EPC contains the network elements such as Mobility, Management Entity (MME), Serving Gateway (S‐GW), and Home Subscriber Server (HSS). An MME performs the signaling or controlling, e.g. mobility management, session management, and related functions between a UE and the EPC network. The HSS performs the similar functions of an HLR found in the GSM and UMTS system. The HLR/HSS is a database that stores the subscriber’s permanent information in a mobile communications network. Unlike the previous systems, i.e. GSM and UMTS, in the LTE system, the various management functions related to signaling and user data/call aspects are assigned separately to the MME and S‐GW. In an LTE network, the E‐UTRAN and the EPC are collectively known as the Evolved Packet System (EPS). An eNodeB is connected to the MME (for signaling) and S‐GW (for user traffic) over the S1 interface; refer to Figure 2.4.

Figure 2.4 Overall network architecture and elements of an LTE network.

Source: © 2015. 3GPP ™ TSs and TRs are the property of ARIB, ATIS, CCSA, ETSI, TSDSI, TTA and TTC who jointly own the copyright in them. © 2015, 3GPP.

The LTE system is an all IP‐based network, i.e. all the network elements communicate with the existing IP transport network only. This differentiates the LTE communication network from its predecessors, GSM, GPRS, and UMTS networks, which uses other transport networks, such as ATM and Frame Relay, apart from the IP transport network.

From the comparisons of Figures 2.1–2.4, it appears that the number of network elements in an LTE/EPS network has reduced. This has led to a smaller number of protocols and interfaces between network elements compared to the GSM and UMTS system. For an overall description of the functions performed by each of the network elements of the respective mobile communications network described above, refer to the TS 23.002 [29].