The link manager protocol sets up logical channels, called links, to carry
frames between the master and a slave device that have discovered each other. A
pairing procedure is followed to make sure that the two devices are allowed to
communicate before the link is used. The old pairing method is that both devices
must be configured with the same four-digit PIN (Personal Identification Number). The matching PIN is how each device would know that it was connecting to
the right remote device. However, unimaginative users and devices default to
PINs such as ‘‘0000’’ and ‘‘1234’’ meant that this method provided very little security in practice.