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10.3 Q.931 CALL-CONTROL MESSAGES 251TABLE 10.3-2Information Element IdentifiersReference Number Information Element 8 7 6 5 4 3 2 1IE.1 Bearer capability 0 0 0 0 0 1 0 0IE.2 Called party number 0 1 1 1 0 0 0 0IE.3 Calling party number 0 1 1 0 1 1 0 0IE.4 Called party subaddress 0 1 1 1 0 0 0 1IE.5 Calling party subaddress 0 1 1 0 1 1 0 1IE.6 Cause 0 0 0 0 1 0 0 0IE.7 Channel ID 0 0 0 1 1 0 0 0IE.8 High-layer compatibility 0 1 1 1 1 1 0 1IE.9 Keypad 0 0 1 0 1 1 0 0IE.10 Low-layer compatibility 0 1 1 1 1 1 0 0IE.11 Progress indicator 0 0 0 1 1 1 1 0IE.12 Signal 0 0 1 1 0 1 0 0IE.13 Transit network selection 0 1 1 1 1 0 0 0IE.14 User–user 0 1 1 1 1 1 1 0Bits10.3.5 Information Element ContentsThis section describes the contents fields of the IEs listed in Table 10.3-2. At thispoint, it is sufficient to read quickly through these descriptions, which are primarilyintended as reference material for Sections 10.4 and 10.5.General Remarks. Q.931 coding uses the concept of “extended” octets. In someoctets of contents fields, bit 8 is an extension bit (ext). If ext ¼ 1, the octet consists ofone octet. An “extended” octet is an “octet” that is extended to a next octet. This isindicated by ext ¼ 0. For example, when the data item in octet N has a length of upto seven bits, octet N is not extended (ext ¼ 1). Next, suppose that the data item inoctet N has a length of 18 bits. The initial seven bits are then in octet N, which ismarked as extended (ext ¼ 0). Bits 8–14 are in the next octet, which is numberedN a (first extension of octet N) and is also marked as extended. Finally, bits 15–18are in an octet, which is marked as “not extended” (ext ¼ 1).The parameter coding standard appears in the contents field of some IEs (e.g., seeFig. 10.3-3). It indicates whether the field is coded according to ITU-T standards:0 0 ITU-T standard1 0 National standard1 1 Network specific standardThis allows other standards organizations to define country-specific parametercodings. The descriptions that follow assume the ITU-T standard.Some contents fields contain a fairly large number of data items. Only the mostessential ones are discussed here (for additional information, see [8] and [9]). In thefigures that follow, the blank fields represent data items that are not discussed.We now examine the IE contents.