CSD Operation
1-2 AT Commands Reference Manual June 30, 2008
2. Upon being interrupted by the RI signal, drop the DTR line to switch to command mode
(depending on the previous DTR configuration: AT&D).
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:
•Use the MUX protocol for virtual channels support, with a unique channel for the GPRS
session (Data) and a unique channel for answering the voice call (command)
CSD Operation
Overview
GSM CSD bearer service, the most widely used data service, provides both a transparent and
non-transparent (error correction and flow control) data rate of 9.6 kbit/s.
Data transfer over Circuit Switched Data (CSD) is possible. 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.
Features and Benefits
CSD operation enables the terminal to perform a data transfer over a circuit switched link.
It enables the user to:
•Connect to a remote modem without any Internet network involvement.
•Own a real IP address and enable its access 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.
Technical Description
GSM network operators typically support the non-transparent CSD bearer service through a
modem interworking function. This means that a G24 initiates a data call and the network routes
the call to the modem interworking function, which is located at the Mobile Switching Center
(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