| 摘要 |
In a network data packets are transferred between stations through communications lines. The data packets consist of a sequence of bytes, some of which are flags and other bytes contain the true information of the data packet. The bytes, which contain flags, have a unique configuration whereby it is always possible to distinguish them from other bytes comprised in the message. The flags signal the beginning of various information fields and they may also contain information which is associated with the local transmission on the communication line where the data packet at a special moment is being transferred. A flag byte may thus have a one as its most significant bit and other bytes may have a zero in this position. Flagbytes containing status information for the communication line, whereon the data packet is being transferred at the considered moment, are normally always transmitted on the communication lines when there is no data packet to be sent. They can also be transmitted within data packets in order to quickly inform the other station, which is connected to a communication line, of a change of the state of the transmitting station. These sending patterns are also used to determine whether a communication line is operative or not and whether a data packet has been transferred correctly from one attached station to the next station. |
