Cisco ASR 5000 Series, Обзор продукта

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GGSN Support in GPRS/UMTS Wireless Data Services
How GGSN Works ▀
Cisco ASR 5000 Series Product Overview ▄
OL-22938-02
1.
The Mobile Station (MS) goes through the process of attaching itself to the GPRS/UMTS network.
2.
The Terminal Equipment (TE) aspect of the MS sends AT commands to the Mobile Terminal (MT) aspect of the
MS to place it into PPP mode.
The Link Control Protocol (LCP is then used to configure the Maximum-Receive Unit size and the
authentication protocol (Challenge-Handshake Authentication Protocol (CHAP), Password Authentication
Protocol (PAP), or none). If CHAP or PAP is used, the TE will authenticate itself to the MT, which, in turn,
stores the authentication information.
Upon successful authentication, the TE sends an Internet Protocol Control Protocol (IPCP) Configure-Request
message to the MT. The message will either contain a static IP home address to use or request that one be
dynamically assigned.
3. The MS sends an Activate PDP Context Request message that is received by an SGSN. The message contains
information about the subscriber such as the Network layer Service Access Point Identifier (NSAPI), PDP
Type, Access Point Name (APN), Quality of Service (QoS) requested, and PDP configuration options.
Note that regardless of whether or not the MS has a static address or is requesting a dynamic address, the
―Requested PDP Address‖ field is omitted from the request when using Mobile IP.
4.
The SGSN authenticates the request message and sends a Create PDP Context Request message to a GGSN
using the GPRS Tunneling Protocol (GTPC, ―C‖ indicates the control signaling aspect of the protocol). The
recipient GGSN is selected based on either the request of the MS or is automatically selected by the SGSN.
The message consists of various information elements including: PDP Type, Requested PDP con, APN,
charging characteristics, and Tunnel Endpoint Identifier (TEID).
5.
The GGSN determines if it can facilitate the session (in terms of memory or CPU resources, configuration, etc.)
and creates a new entry in its PDP context list and provides a Charging ID for the session.
From the APN specified in the message, the GGSN determines how to handle the PDP context including
whether or not Mobile IP should be used.
If authentication is required, the GGSN attempts to authenticate the subscriber locally against profiles stored in
memory or send a RADIUS Access-Request message to an AAA server.
6. If the GGSN authenticated the subscriber to an AAA server, the AAA server responds with a RADIUS Access-
Accept message indicating successful authentication.
7.
The GGSN replies to the SGSN with a PDP Context Response using GTPC. The response will contain
information elements such as the PDP Address, and PDP configuration options specified by the GGSN. Note
that for Mobile IP, the GGSN returns a PDP Address of 0.0.0.0 indicating that it will be reset with a Home
address after the PDP context activation procedure.
8. The SGSN returns an Activate PDP Context Accept message to the MS. The message includes response to the
configuration parameters sent in the initial request.
9.
The MT, will respond to the TE‘s IPCP Config-request with an IPCP Config-Ack message. This ends the PPP
mode between the MT and TE components of the MS.
Data can now be transmitted between the MS and the GGSN.
10.
The FA component of the GGSN sends an Agent Advertisement message to the MS. The message contains the
FA parameters needed by the mobile such as one or more card-of addresses. The message is sent as an IP
limited broadcast message (i.e. destination address 255.255.255.255), however only on the requesting MS‘s
TEID to avoid broadcast over the radio interface.
11. The MS sends a Mobile IP Registration request to the GGSN/FA. This message includes either the MS‘s static
home address or it can request a temporary address by sending 0.0.0.0 as its home address. Additionally, the
request must always include the Network Access Identifier (NAI) in a Mobile-Node-NAI Extension.