Cisco Certified ExpertInformation Elements
The last field that is included in the Q.931 message format is the information or information element (IE) field. This field can consist of many different IEs that describe the features and capabilities of a given call. For instance, a single Setup message contains more than 15 different IEs including called party number, calling party number, bearer capability, transit network selection, and date and time. IEs are categorized as either single octet or multiple octet IEs. Figure 8-16 shows the format of the IE fields in the Q.931 messages.
The single octet IEs are used for functions, such as requesting more data, identifying that the call reception is complete, and identifying the congestion level. All the important IEs that you need to know to troubleshoot Q.931 are considered multiple octet IEs.
The multiple octet IEs serve most of the useful functions by exchanging the bearer capabilities of the circuits, sending calling and called numbers to the destination, sending called and calling party sub-addresses, sending the requested channel ID, and sending any cause codes if the connection is unsuccessful. Because there are so many of them, this section focuses on some of the more important IEs from a troubleshooting standpoint.
Figure 8-16 Information Element Formats
Type 1
1 Denotes a ■ Single Octet IE
0 Denotes a ' Multiple Octet IE
Type 1
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1 |
IE Identification |
Data of IE |
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Type 2 |
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1 |
IE Identification |
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0 |
IE Identifier |
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IE Length |
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Data of IE |
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