A physical layer protocol data unit transmission apparatus (100, 110), comprising: a processing unit (102, 112), configured to generate a physical layer protocol data unit, PPDU, wherein the PPDU comprises: a legacy preamble, a high efficiency preamble, a data field and a signal extension, SE, field, wherein the data field comprises one or more orthogonal frequency division multiplexing, OFDM, symbols that carries useful information, and the SE field is after the last OFDM symbol in the data field, the high efficiency preamble comprises a high efficiency field including an ambiguity indication, the ambiguity indication is used to indicate whether a receive end needs to adjust a calculated quantity of OFDM symbols, and; a sending unit, configured to send the PPDU when the following is satisfied, the ambiguity indication is a first value; T SE + 4 × ⌈ T L _ PREAMBLE + T HE _ PREAMBLE + T HE _ DATA + T SE 4 ⌉ − T L _ PREAMBLE + T HE _ PREAMBLE + T HE _ DATA + T SE 4 > 12.8 + T GI; or when the following is satisfied, the ambiguity indication is a second value; T SE + 4 × ⌈ T L _ PREAMBLE + T HE _ PREAMBLE + T HE _ DATA + T SE 4 ⌉ − T L _ PREAMBLE + T HE _ PREAMBLE + T HE _ DATA + T SE 4 < 12.8 + T GI; wherein T SE is a duration of the SE field; T GI is a cyclic prefix, CP, length; T HE_DATA = N ·(12. 8 + T GI), and N is a quantity of the one or more OFDM symbols; T L_PREAMBLE is transmission duration of the Legacy preamble; and T HE_PREAMBLE is transmission duration of the High efficiency preamble. a processing unit (102, 112), configured to generate a physical layer protocol data unit, PPDU, wherein the PPDU comprises: a legacy preamble, a high efficiency preamble, a data field and a signal extension, SE, field, wherein the data field comprises one or more orthogonal frequency division multiplexing, OFDM, symbols that carries useful information, and the SE field is after the last OFDM symbol in the data field, the high efficiency preamble comprises a high efficiency field including an ambiguity indication, the ambiguity indication is used to indicate whether a receive end needs to adjust a calculated quantity of OFDM symbols, and; a sending unit, configured to send the PPDU when the following is satisfied, the ambiguity indication is a first value; T SE + 4 × ⌈ T L _ PREAMBLE + T HE _ PREAMBLE + T HE _ DATA + T SE 4 ⌉ − T L _ PREAMBLE + T HE _ PREAMBLE + T HE _ DATA + T SE 4 > 12.8 + T GI; or when the following is satisfied, the ambiguity indication is a second value; T SE + 4 × ⌈ T L _ PREAMBLE + T HE _ PREAMBLE + T HE _ DATA + T SE 4 ⌉ − T L _ PREAMBLE + T HE _ PREAMBLE + T HE _ DATA + T SE 4 < 12.8 + T GI; wherein T SE is a duration of the SE field; T GI is a cyclic prefix, CP, length; T HE_DATA = N ·(12. 8 + T GI), and N is a quantity of the one or more OFDM symbols; T L_PREAMBLE is transmission duration of the Legacy preamble; and T HE_PREAMBLE is transmission duration of the High efficiency preamble. T SE is a duration of the SE field; T GI is a cyclic prefix, CP, length; T HE_DATA = N ·(12. 8 + T GI), and N is a quantity of the one or more OFDM symbols; T L_PREAMBLE is transmission duration of the Legacy preamble; and T HE_PREAMBLE is transmission duration of the High efficiency preamble.