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IMPORTANT NOTICE: This page contains information about a feature as it is specified in the relevant 3GPP/GSM/ITU/ETSI or other specifications. This does not mean that feature is already implemented in Osomocom, or that implementation is work in progress.

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Circuit Switched Data (CSD)¶

CSD is a way to transport user data over circuit-switched GSM calls. It is the cellular equivalent of the use of analog modems or ISDN TA over wired telephony networks: You dial a number, the usual call control protocol for signaling creates a dedicated, circuit switched connection to the destination. If they answer the call, a transparent communication channel is established between both sides.

GSM CSD implements both synchronous and asynchronous bearer services.

Facsimile / Telefax¶

CSD is also what is used to implement Facsimile (Telefax) services over GSM.

Interworking¶

When CSD services (sync/async/fax) are used between two GSM subscribers, no interworking is required. The data is passed transparently end-to-end.

However, when interworking with the wired network is required, an interworking function (IWF) is required.

Interworking with ISDN data calls¶

This is the situation where a GSM mobile subscriber is establishing a CSD call to a native ISDN subscriber.

In this case, the V.110 signal is rate-adapted at the IWF to the 64kBps of the ISDN bearer chanel.

Interworking with analog modems in POTS or ISDN¶

In this case, the IWF terminates the V.110 from the GSM RAN and implements a modem towards the external POTS/ISDN subscriber.

Interworking with analog Telefax in POTS or ISDN¶

This is similar to the analog modem situation above.