| The Care and Feeding of ISDN4BSD | ||
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| 2.3.1 D-Channel Layer 2 | 2.3 The D-Channel | 2.3.3 Interlayer Communication |
2.3.2 D-Channel Layer 3
At this point the major difference between countries implementing ISDN come into play: for the most parts of Europe (and a growing number of countries outside Europe) the telecommunication providers agreed on a common protocol, the so called ``Euro-ISDN'' protocol or DSS1 protocol specified in [4] and [5]. This is currently the only layer 3 protocol isdn4bsd supports. In general, the layer 3 protocol specifies messages which are used to control the establishment and closing of a connection to another member of the ISDN or telephone system. It contains descriptions and specifications of such commands as ``hook off the handset'', ``dial number 1234'', ``look, someone with number 5678 tries to call you'' and so on. Recommendation Q.931 specifies:
D-channel layer 3 messages are contained in layer 2 I-frames like this:
- message functional definitions and contents
- general message format and information element encoding and
- call control procedures
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