Let us take a quick look at the common header. The Address field identifies
which of the eight active devices the frame is intended for. The Type field identifies the frame type (ACL, SCO, poll, or null), the type of error correction used in
the data field, and how many slots long the frame is. The Flow bit is asserted by a
slave when its buffer is full and cannot receive any more data. This bit enables a
primitive form of flow control. The Acknowledgement bit is used to piggyback an
ACK onto a frame. The Sequence bit is used to number the frames to detect retransmissions. The protocol is stop-and-wait, so 1 bit is enough. Then comes the
8-bit header Checksum. The entire 18-bit header is repeated three times to form
the 54-bit header shown in Fig. 4-36. On the receiving side, a simple circuit examines all three copies of each bit. If all three are the same, the bit is accepted. If
not, the majority opinion wins. Thus, 54 bits of transmission capacity are used to
send 10 bits of header. The reason is that to reliably send data in a noisy environment using cheap, low-powered (2.5 mW) devices with little computing capacity,
a great deal of redundancy is needed.