Product Features
10 98-08901C68-O
3. Answer the call (suspending the GPRS session).
4. At the end of the call, pull the DTR to resume the GPRS session.
Option 2:
1. Use the MUX protocol for virtual channels support, with a unique channel for the GPRS session (Data) and a unique chan-
nel for answering the voice call (command).
2.3 CSD OPERATION
2.3.1 Overview
GSM CSD bearer service, the most widely used data service, provides a non-transparent (error correction and flow control) data
rate of 9.6 kbit/s.
Data transfer is a Circuit Switched Data (CSD). Once the connection is established, data can be transferred to and from the
remote side.
The user should take the CSD call setup time into account.
Network operators charge the user for the call time regardless of data usage.
2.3.2 Features and Benefits
CSD operation enables the terminal to perform a data transfer over a circuit switched link.
It enables the user to:
• Connect a remote modem without any Internet network involvement.
• Own a real IPv4 adders by connecting to an external ISP.
The following are examples of standard CSD call uses:
• Connecting an Internet Service Provider (ISP).
• Remotely accessing corporate Intranet via Remote Access Server (RAS).
• User specific protocol, where the user defines both the remote and local sides.
2.3.3 Technical Description
GSM network operators typically support the non-transparent CSD bearer service through a modem interworking function. This
means that a g20 initiates a data call and the network routes the call to the modem interworking function, which is located at the
Mobile Switching Centre (MSC) of the GSM network. The modem interworking function then dials the number supplied by the
mobile station.
This is different from voice calls, where the GSM network itself routes the call, often to another mobile station on the same
network. The GSM network does not route data calls - it dials the requested number on behalf of the mobile station and leaves
the routing to the external wireline telephone network. The main reason for this is that the GSM network has information about
what the user wants to do with the data call. For example, the user may be contacting his or her Internet Service Provider (ISP)
to send email or dialing the corporate Intranet to set up a virtual private network (VPN) connection to retrieve confidential
customer information from a company database.
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