IEEE 802.11, 802.11a, and 802.11b, Continued
DSSS works in a different manner altogether. DSSS combines the data stream with a higher speed digital code. Each data bit is mapped into a common pattern of bits known only to the transmitter and the intended receiver. This bit pattern is called a chipping code. This code is a random sequence of high and low signals that signify the actual bit. This chipping code is inverted to represent the opposite bit in the data sequence. This frequency modulation, if the transmission is properly synchronized, offers it's own error correction, and thusly has a higher tolerance for interference.
The MAC layer defines a way of accessing the physical layer and also controls the services related to the mobility management and the radio resource. It is similar to the wired Ethernet standard for data transmission. The differences arise in the way data collisions are handled. In the wired standard, data packets are sent out to the network indiscriminately. Only when two packets in a sense "collide" does the system use additional measures to ensure packets get to their destination. In the 802.11 standards, collision avoidance is implemented. In these, the receiving wireless host sends an acknowledgement (ACK) packet to the receiver once it has received the data successfully. If the sender does not receive an ACK packet, the sender then waits a period of time before it attempts to resend the data.
Unfortunately, there are some unresolved issues about the 802.11 standard that need to be addressed. Standardization and interoperability are the goals of the standard, yet there are some key issues necessary to achieve multiple-vendor interoperability are absent in the standard. These include access point coordination for roaming - there is no hand-off mechanism in the standard as one moves out of range of one AP and into another. Also, there is no test suite designed to test whether or not a device actually conforms to the standard.
