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.

GTP是负责将SGSN的用户数据上传到GGSN的协议。

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.

GGSN是会对通过SGSN连入它的设备进行是否ACTIVE的监督的,如果有从CORE去往某inactive设备的流量,就会被DROP,这样可以节省带宽和资源。

GGSN负责GPRS数据包到IP的转换,Authentication和charging。另外还是所有EU的默认网关,它会用作EU的DHCP服务器。它同时还有QoS功能。

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.

用户的IMSI等信息存在SGSN上。SGSN只管理自己领域内的用户。SGSN的作用是,对于用户和Internet,都代表GGSN。在Internet看来,因为不论用户连入哪个SGSN,那个SGSN都会用GTP将用户信息上传到GGSN上,所以Internet总会认为是同一个点发来的数据,回应的时候也只回给那个GGSN,然后再由GGSN去判断要去往用户应该选哪个SGSN。这样保证了不论用户地理位置如何变化,都不会影响数据传送,因为总是用同一个GGSN和Internet通信。

HLR负责Authentication,GGSN负责IP分配。

Traffic Flow:

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

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