ers, acknowledgements, sends an acknowledgement carrying the sequence number of that retransmission requests and other control information. first frame then adds one to the sequence number, which is the Next comes the information field, containing the actual user data, anticipated number of the next frame. The sender receives the followed by a frame check sequence (FCS) of bits by which the acknowledgement, adds one to the sequence number, attaches it to receiver determines, by reference to a checksum, whether or not the next frame and repeats the process, which is known as a Modulo- frames have been received correctly. 8 or Modulo-N frame level window. A final or trailing flag marks the end of the frame and indicates to If, therefore, a receiver's reply is corrupted, the transmitter can the receiver that the preceding 16 bits should be interpreted as the ignore the frame and wait for the acknowledgement time to expire. FCS field. The recipient processes the data stream, compiles its own It then sends the original frame again: as the receiver has success- FCS and compares the result with the FCS received in the frame. If fully received the frame on the sender's first attempt, it will be they match, the data is correct, otherwise the frame is discarded and expecting a frame marked with the next sequence number along. It the receiver issues a retransmission request. can conclude that the sender did not receive a correct acknow- HDLC and LAPB contain three types of frame, each built up from ledgement, discard the repeated frame and resend the original ac- the building blocks outlined above: knowledgement. Sequence numbers range from zero to seven, avoiding the prob- 1. Unnumbered frames carry no send or receive sequence num- lem of large numbers taking up valuable frame space. It could work bers and simply offer additional data link control functions such with just two numbers: in the example above, the initial frame could as circuit initialisation and disconnection, helping to set up and be labelled 0 and the fault