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1. The mobile station initiates a data call. In the call set-up message Bearer Capability (BC)
is included. The BC includes the bearer service type (data, fax) and the requested transmission
rate.
2. A connection between the mobile station and the network is set up, as in a normal call,
and authentication is performed.
3. The MSC analyses the BC, and then transfers it to the DTI/GIWU.
4. The DTI/GIWU is configured with the rate adaption and the fax/modem service.
5. The DTI/GIWU reroutes the call to the MSC.
6. The MSC routes the call to PSTN or ISDN.
For communication with a Packet Switched Public Data Network (PSPDN), a Packet Assembly
Disassembly (PAD) is needed. The PAD transforms the bitstream from an asynchronous terminal
to data packages.
12.4.2 Bearer services
The GIWU supports a wide range of bearer services. Synchronous and asynchronous data
transmission rates up to 9.6 kbps are supported. Here is a short description of some of the
services.
• Traffic to Public Switched Telephony Network (PSTN). Interworking with ISDN or directly
to PSTN, a suitable modem is selected in the GIWU.
• Traffic to Integrated Services Digital Network (ISDN). The entire set of data communication
services with ISDN terminals is available. Unrestricted digital information is transferred and
no modem is necessary.
• Traffic to Packet Switched Public Data Networks (PSPDN). The Packet services supports
synchronous data with rates of 1.2, 2.4, 4.8 and 9.6 kbps. Asynchronous data communication
between a mobile station and a packet switched network is possible via a PAD facility (see
above). Rates between 0.3 and 9.6 kbps are supported.
• Traffic to Circuit Switched Public Data Networks (CSPDN). A CSPDN is reachable via
PSTN or ISDN, but depends on the interface between CSPDN and the transit networks
12.4.3 Transparent Mode
In transparent mode, a non-error-corrected connection is established between the user and the
remote terminal. Data is then sent synchronously or asynchronously. Because no error correction is
taking place, there is little delay (latency) in transmitting data over the link.
12.4.4 Non Transparent Mode
In non-transparent mode, a secure error corrected link is established between the mobile terminal
and the remote modem. The connection between the mobile terminal and the GSM network uses
the Radio Link Protocol (RLP) for error correction. The GIWU then establishes a connection to
PSPD
MS
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