GSM Circuit Switched Data (CSD)

Last updated: December 3, 2008

This section is only applicable to the lab applications.

This section discusses the GSM Circuit Switched (CSD) feature in the test set.

GSM CSD Capability in the Test Set

The circuit switched data (CSD) channel capability in the test set allows you to simulate a real-time GSM data transfer between your mobile station and a full network. The test set supports:

establishing a mobile originated circuit switched data call,
various bit rate, mode, and channel type combinations (See CSD Configurations .),
IP datagrams routing to and from the mobile station through the test set's LAN interface,
and terminating the PPP connection.

After establishing a CSD connection, the connection operates in full-duplex. IP datagrams originating from either the mobile station or the "network" may be transferred over this link.

The RF link is maintained even when there is no data being sent or received and is only terminated when you end the call from either the test set, mobile station, or from an attached PC. The only exception occurs if the test set determines that an error condition has been met. If this occurs, the test set drops the call.

For the duration of the CSD connection, the connection status displays "Connected + Data". For the location of the connection status on the front panel display, see Instrument Status Area .

NOTE

Only Mobile Originated CSD calls are currently supported.

Bit Rate and Mode Negotiation

Various modes and bit rates can be used in CSD connections. There are two possible modes:

Transparent - The raw rate-adapted bits from the over-the-air (OTA) frame are passed to PPP directly. A connection using transparent mode does not provide for re-transmissions and has no error detection mechanism beyond the CRC at Layer 1.
Non-Transparent - This mode uses the Radio Link Protocol (RLP) stack layer to provide a reliable transport mechanism. The RLP layer provides for the retransmission of corrupted data and uses a Frame Check Sequence (FCS) for error detection.

Bit rates can be 2400, 4800, 9600 or 14400 bps. The following table illustrates the possible configurations and the support provided by the test set.

CSD Configurations
Speed (bps)	Mode	Channel Type	Test Set Support
14400	Transparent and Non-Transparent	Full Rate	No
9600	Transparent and Non-Transparent	Full Rate	Yes
4800	Transparent and Non-Transparent	Full Rate	Yes
4800	Transparent and Non-Transparent	Half Rate	No
2400	Transparent	Full Rate	Yes
2400	Transparent	Half Rate	No

The test set provides the mode and bit rate requested by the mobile station, if they are supported. When the mobile station's request is not supported, a warning is generated and the call is dropped.

The test set does not have any control over the mode or bit rate requested by the mobile station. Control for this in the mobile station differs between manufacturers. Possible control methods include configuration options available via the mobile station's user interface or by establishing a dial-up connection with the mobile station and using your PC to send it AT commands. Refer to GSM 7.01 for more information.

Operating Considerations

Operating Mode

The Operating Mode must be set to Active Cell to use the CSD capability in the test set.

Connection Type

The Connection Type must be set to Auto to use the CSD capability in the test set.

NOTE

If the connection type is changed while a GSM CSD call is established, the call is dropped by the test set.

Measurements

You can make Bit Error Rate(BER) or Fast Bit Error Rate (FBER) measurements during CSD connections. See GSM Bit Error Measurement for more information.

It may be possible for you to make all the GSM transmitter measurements. If your phone goes into DTX mode and stops transmitting, it is not possible to make GSM transmitter measurements.