Question: isdn uses b & d channels. what is d channel use for
[{'answer': 'control and signaling information', 'provenance': [{'wikipedia_id': '41578', 'title': 'Primary Rate Interface', 'start_paragraph_id': 7, 'start_character': 78, 'end_paragraph_id': 7, 'end_character': 111, 'bleu_score': 1.0, 'section': 'Section::::ISDN service types.\n'}]}, {'answer': 'data', 'provenance': [{'wikipedia_id': '41578', 'title': 'Primary Rate Interface', 'start_paragraph_id': 1, 'start_character': 166, 'end_paragraph_id': 1, 'end_character': 170, 'bleu_score': 1.0, 'section': 'Section::::Abstract.'}]}, {'answer': 'carries control and signaling information', 'provenance': [{'wikipedia_id': '41578', 'title': 'Primary Rate Interface', 'start_paragraph_id': 7, 'start_character': 70, 'end_paragraph_id': 7, 'end_character': 111, 'bleu_score': 1.0, 'section': 'Section::::ISDN service types.\n'}]}, {'provenance': [{'wikipedia_id': '41578', 'title': 'Primary Rate Interface', 'start_paragraph_id': 7, 'start_character': 0, 'end_paragraph_id': 7, 'end_character': 622, 'bleu_score': 0.926482513265501, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 4034217389567586220}, 'section': 'Section::::ISDN service types.\n'}]}, {'provenance': [{'wikipedia_id': '41578', 'title': 'Primary Rate Interface', 'start_paragraph_id': 2, 'start_character': 0, 'end_paragraph_id': 2, 'end_character': 755, 'bleu_score': 0.9115641024409191, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 4034217389567586220}, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '41578', 'title': 'Primary Rate Interface', 'start_paragraph_id': 0, 'start_character': 0, 'end_paragraph_id': 0, 'end_character': 0, 'bleu_score': 1.0, 'meta': {'yes_no_answer': 'NONE', 'annotation_id': 4034217389567586220}, 'section': 'Section::::Abstract.'}]}, {'provenance': [{'wikipedia_id': '41578', 'title': 'Primary Rate Interface', 'section': 'Section::::ISDN service types.\n', 'start_paragraph_id': 7, 'end_paragraph_id': 7, 'meta': {'evidence_span': ['The D-channel carries control and signaling information', '+ 23 = 47 B-channels and 1 D-channel (often called "47B + D"), but more commonly has 46 B-channels and 2 D-channels thus providing a backup signaling channel. The concept applies to E1s as well and both can include more than 2 PRIs. When configuring multiple T1\'s as ISDN-PRI\'s, it\'s possible to use NFAS (non-facility associated signalling) to enable one or two D-channels to sup']}}]}, {'provenance': [{'wikipedia_id': '15231', 'title': 'Integrated Services Digital Network', 'section': 'Section::::Bearer channel.\n', 'start_paragraph_id': 22, 'start_character': 109, 'end_paragraph_id': 22, 'end_character': 201, 'bleu_score': 1.0}]}]