The present invention provides a source line card. The source line card segments a data packet in a data stream into cells first, then inserts Time Stamps and Flow Identifications into cell headers, and sends the marked cell headers and cell payloads to the network card. The network card sends the cells to a destination line card or a lower-level network card in sequence according to the Time Stamps and the Flow Identifications, where the Time Stamps and the Flow Identifications are in the cell headers of the received cells. By inserting the Time Stamps and the Flow Identifications into the cell headers, it is ensured that an output sequence and an input sequence of cells that belong to a stream in the switching network are the same, so that the destination line card may reassembles a data packet easily according to a sequence in which the cells are received.