ants to join a cell it must synchronize with the other stations in the cell and it must be allowed in. The synchronization can be gained by two means. The first is known as passive scanning where the station waits to receive what is termed a beacon frame from the access point or another station in an ad hoc network. This beacon frame is a periodic frame of data that is sent out with synchronization information on it. Apart from enabling new stations to synchronize, they also ensure that stations already in the cell maintain their synchronization. When entering a cell the station may alternatively send out a probe transmission and then wait for a response. Once the new station has the synchronization information it can then request access to the cell. It does this by going through an authentication process. Only when this has been completed can the station enter the cell and exchange data. The process includes exchanging passwords to ensure that the new station should be in the cell. When transmitting, the system uses a collision avoidance technique known as CSMA/CS and this is combined with a positive acknowl- edge. When a station wants to transmit, it checks the frequency to ensure no other stations are transmitting and if the channel is occupied the station delays sending its message for a random amount of time. When all is clear it sends its message. As there is a possibility another station could perform the same operation at the same time, an acknowledgement is sent to the station from whom the message is received. If the station sending the message does not receive its acknowledgement then it resends its message. The first message that is sent is what is termed a request to send (RTS). This is done in case there are other stations that cannot be heard. This RTS message defines the length of the message to be sent and the destination. If this is acceptable a clear to send (CTS) message is sent. Short-range wireless data communications 289 Source Destination Other stations Other stations quiet D = Delay Figur