2G Network


Mobile switching center (MSC):The mobile switching center (MSC) is the primary service delivery node for GSM/CDMA, responsible for routing voice calls and SMS as well as other services (such as conference calls, FAX and circuit switched data).

The MSC sets up and releases the end-to-end connection, handles mobility and hand-over requirements during the call and takes care of charging and real time pre-paid account monitoring.


The home location register (HLR) for obtaining data about the SIM and mobile services ISDN number (MSISDN; i.e., the telephone number).


Examples of other data stored in the HLR against an IMSI record is:

  • GSM services that the subscriber has requested or been given.
  • GPRS settings to allow the subscriber to access packet services.
  • Current location of subscriber (VLR and serving GPRS support node/SGSN).
  • Call divert settings applicable for each associated MSISDN.

The base station subsystem (BSS) which handles the radio communication with 2G and 2.5G mobile phones.


The UMTS terrestrial radio access network (UTRAN) which handles the radio communication with 3G mobile phones.


The visitor location register (VLR) for determining where other mobile subscribers are located.


VLR Data stored include:

  • IMSI (the subscriber’s identity number).
  • Authentication data.
  • MSISDN (the subscriber’s phone number).
  • GSM services that the subscriber is allowed to access.
  • access point (GPRS) subscribed.
  • The HLR address of the subscriber.


CDMA使用1.25 MHz信道,W-CDMA使用5MHz信道,另外还具有一些密集度,性能,客户数和节能方面的优化。HSPA是W-CDMA基础上加入MIMO技术,优化调制技术后将速度由384 Kbps to 2 Mbps提升至下行14Mbit/s,上行5.76Mbit/s的技术。HSPA+使用更高的调制技术64QAM以及2×2 MIMO使速度进一步提升,uplink 22Mbit/s & downlink 168Mbit/s。Dual-Carrier HSPA 是允许同一个moblie device同时接入两个ACCESS的技术,达到速度加倍。LTE的目标是简化现有运营网络中复杂的各种协议转换与验证以及架构,全部使用IP/TCP网络,并使用1.4MHz、3MHz、5MHz、10MHz、15MHz和20MHz信道(这些信道都可以独立使用,所以比WCDMA的5MHz灵活)和4×4 MIMO,速度提升到uplink 75.4Mbit/s (SC-FDMA) & downlink 299.6Mbit/s (OFDM)。3G基站是femtocell和picocell的小半径基站,而LTE的macrocell半径超过100公里。





  • Gi – the reference point at which the GPRS core network connects to the internet. Alternatively, corporate customers may have a direct connection to this point for higher security. This reference point is normally just an IP network, though a tunnelling protocol such as GREIPinIP or IPsec may be used instead.
  • Gr – the reference point between the SGSN and the HLR used to allow the SGSN to retrieve subscriber information from the HLR. The communication here is based on SS7
  • Gb – the interface between the SGSN and the PCU which is the point of connection of the GPRS core network to the GSM radio network (BSS). This carries both signalling and user data. The communication here is based on Frame Relay.
  • IuPS – the interface between the SGSN and the RNC which is the point of connection of the GPRS core network to the UMTS Radio Access Network UTRAN. The communication here is abased on Broadband SS7 over ATM for signalling and GTP-U over IP for user data.
  • Gs – the connection between the SGSN and the MSC. This is optional and is used primarily in GSM networks to improve signalling efficiency, especially on the radio interface. In UMTS almost the same effect is achieved simply through the action of the RNC. This interface uses the BSSAP+ Protocol.

LTE 中以SGW+PGW(SPGW)替代umts中的SGSN和GGSN,MME控制eNodeB,DRA负责对ip和各种其他通信协议进行转换路由交换,PCRF进行Policy based QoS优化,计费等服务。

空中接口(连入Evolved Packet Core (EPC)前,手机和基站组成的网络):




System Architecture Evolution (aka SAE):系统架构演进,是LTE的核心架构,简化了以往GGSN和SGSN的UMTS复杂网络,以MME,SPGW取代。

MME (Mobility Management Entity): The MME is the key control-node for the LTE access-network. It is responsible for idle mode UE (User Equipment) tracking and paging procedure including retransmissions. It is involved in the bearer activation/deactivation process and is also responsible for choosing the SGW for a UE at the initial attach and at time of intra-LTE handover involving Core Network (CN) node relocation. It is responsible for authenticating the user (by interacting with the HSS). The Non Access Stratum (NAS) signaling terminates at the MME and it is also responsible for generation and allocation of temporary identities to UEs. It checks the authorization of the UE to camp on the service provider’s Public Land Mobile Network (PLMN) and enforces UE roaming restrictions. The MME is the termination point in the network for ciphering/integrity protection for NAS signaling and handles the security key management. Lawful interception of signaling is also supported by the MME. The MME also provides the control plane function for mobility between LTE and 2G/3G access networks with the S3 interface terminating at the MME from the SGSN. The MME also terminates the S6a interface towards the home HSS for roaming UEs.


SGW (Serving Gateway): The SGW routes and forwards user data packets, while also acting as the mobility anchor for the user plane during inter-eNodeB handovers and as the anchor for mobility between LTE and other 3GPP technologies (terminating S4 interface and relaying the traffic between 2G/3G systems and PGW). For idle state UEs, the SGW terminates the downlink data path and triggers paging when downlink data arrives for the UE. It manages and stores UE contexts, e.g. parameters of the IP bearer service, network internal routing information. It also performs replication of the user traffic in case of lawful interception.

SGW(ASR5000)负责路由和转发用户数据包,承载一部分UMTS中SGSN的功能。它还管控是否关闭空闲的UE SESSION。它管理用户的参数,比如用户连入网络后的路由表。它还会对用户数据包进行备份,以截获数据内容。

PGW (PDN Gateway): The PDN (Package Data Network) Gateway provides connectivity from the UE to external packet data networks by being the point of exit and entry of traffic for the UE. A UE may have simultaneous connectivity with more than one PGW for accessing multiple PDNs. The PGW performs policy enforcement, packet filtering for each user, charging support, lawful interception and packet screening. Another key role of the PGW is to act as the anchor for mobility between 3GPP and non-3GPP technologies such as WiMAX and 3GPP2 (CDMA 1X and EvDO).


HSS (Home Subscriber Server): The HSS is a central database that contains user-related and subscription-related information. The functions of the HSS include functionalities such as mobility management, call and session establishment support, user authentication and access authorization. The HSS is based on pre-Rel-4 Home Location Register (HLR) and Authentication Center (AuC).



GPRS部件名词与Core Network工作原理解读

GPRS tunnelling protocol (GTP):

GPRS Tunnelling Protocol is the defining IP-based protocol of the GPRS core network. Primarily it is the protocol which allows end users of a GSM or WCDMA network to move from place to place while continuing to connect to the Internet as if from one location at the Gateway GPRS support node (GGSN). It does this by carrying the subscriber’s data from the subscriber’s current serving GPRS support node (SGSN) to the GGSN which is handling the subscriber’s session. Three forms of GTP are used by the GPRS core network.


PDP Context作为GTP的组成部分,记录着用户数据,包括IMSI, IP, 以及SGSN和GGSN的连接ID号。

Gateway GPRS support node (GGSN):

From an external network’s (Internet) point of view, the GGSN is a router to a “sub-network”, because the GGSN ‘hides’ the GPRS infrastructure from the external network. When the GGSN receives data addressed to a specific user, it checks if the user is active. If it is, the GGSN forwards the data to the SGSN serving the mobile user, but if the mobile user is inactive, the data is discarded. On the other hand, mobile-originated packets are routed to the right network by the GGSN.



Serving GPRS support node (SGSN)

A serving GPRS support node (SGSN) is responsible for the delivery of data packets from and to the mobile stations within its geographical service area. Its tasks include packet routing and transfer, mobility management (attach/detach and location management), logical link management, and authentication and charging functions. The location register of the SGSN stores location information (e.g., current cell, current VLR) and user profiles (e.g., IMSI, address(es) used in the packet data network) of all GPRS users registered with it.



Traffic Flow:

EU->Tower->RNC->SGSN->HLR(Authentication)->SGSN->EU->SGSN->GGSN(DHCP)->SGSN->EU(IP Assigned)->SGSN->GGSN->Internet.