GPRS is just a network to forward IP packets between the terminal and an external IP network defined as an APN (Access Point Name) connected to the GPRS node GGSN. Unlike CSD, where the terminal dials a number and then starts the PPP negotiation, the GGSN expects the PPP negotiation to have been concluded prior to setting up a pdp-context.
The way this is solved is that the MT, holding the GSM and GPRS stack, negotiates the PPP connection with the TE (Terminal Equipment) before it sends an activate-pdp-context request message to the SGSN. All the PPP results are put in a field which is called 'Protocol Configuration Options' in the activate-pdp-context request message. The SGSN then forwards the request as an create-pdp-context request to the GGSN.
Dependent on the configuration the GGSN may now look into the Protocol Configuration Options information element and extract and optionally validate the IPCP fields to reject or grant the terminal access to the external IP network (the APN). The GGSN can also be set up to forward the authentication information to an external RADIUS server or set up an L2TP tunnel to make the external IP network take care of the authentication. Then the GGSN, RADIUS server, or potentially an external DHCP server will assign an IP address to the terminal, unless the HLR subscription of the subscriber allows static IP addresses to be used. The newly IP address to be used by the terminal is then returned in the create-pdp-context accept message, which in turn is translated by the SGSN to a activate-pdp-context accept message.
The MT then gets the accept message from the GPRS network, and signallises to the TE that the PPP negotiation is finished, ready to send payload.
There is only one IP address involved. When an IP packet is sent from the external IP network, the GGSN will then forward those packets to the correct mobile terminal via the GPRS network. The GSM stack in the MT will then decode all the GPRS protocol layers, and forward the IP packet to the TE on the PPP connection between the TE and the MT.
In the case where no PPP negotiation takes place between the MT and TE it is still possible to send IP packets to the TE. However, the TE will simply silently discard the packets as there is no receiver for them.
In essence, the main difference between CSD and GPRS is that the PPP session must be established PRIOR to contacting the GSM network, where the PPP parameters are forwarded to the GSM network in the protocol configuration options field, whereas CSD negotiates PPP after a connection between a dial in server and the terminal is established.
I hope this clarifies your issue.
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by: jmsousaPosted on 2003-10-31 at 07:15:25ID: 9658060
I'm afraid that when you do that you're only attaching the device to the GPRS network and receiving the mobile IP that id's your device to the same GPRS network (and it may change if you move around, but that's of no concern to you: it should be transparent)...
But now you need to log into some remote network USING the GPRS network: that's why you need to start the LCP, PAP/CHAP and IPCP negotiations with the remote server/network (just like a dial-up with your most current modem, forget about the GPRS network) and this should work.
To summarize, when you use a mobile GPRS enabled in fact you have two IP address's: the one from the GPRS network and your machine's IP over the remote network you need to access. The IP over the GPRS network it's just to overcome the TCP/IP drawback of rigid hierarchy (it assumes you don´t move or if you do, you reboot and you another IP address) and for subsequent routing between your mobile and the remote network: this GPRS IP should be of no concerne to you.
José Sousa