 If an error is detected in a packet then the receiver can request it to be resent. Error coding using a 16-bit CRC is also available. For the synchronous links packets cannot be resent as there is unlikely to be sufficient bandwidth available to resend data and 'catch up', although it is possible to include some forward error control. When communicating Bluetooth devices form small nets called 'piconets'. These comprise up to eight devices, of which one takes on the role of a master while all the others become slaves. If more than eight devices are within range, they may remain in an inactive standby state and may be requested at a later time to join the net. Still further devices may be in a standby state. When establishing a net, the master transmits an enquiry message every 1.28 seconds to discover whether there are any other devices within range. If replies are received then an invitation to join the net is transmitted to specific devices that might be in range. To set up the net the master allocates each device a member address and it then controls their transmissions. All Bluetooth devices have a clock that runs at twice the hopping speed and this provides synchronization to the whole net. The master transmits in the even numbered timeslots while the slaves transmit in the odd numbered slots once they have been given permission to transmit. Short-range wireless data communications 285 It is possible for Bluetooth devices to encrypt data. A key up to 128 bits is used and it is claimed that the level of security provided is sufficient for financial transactions. However, in some countries the length of the key is limited to enable the security agencies to gain access if required. IEEE 802.11 There is a family of '802' specifications relating to local area networks. For example, 802.3 is the Ethernet specification, although 802.11 is reserved for wireless local area networks (WLANs) and this is further subdivided. Of these 802.11b is becoming one of the most popular for computer and other similar applications