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Modulation Technology

Advantages to using DSSS:

● Error correction. With FHSS a single corrupted bit requires that specific bit to be retransmitted, as illustrated in Figure 5-4 above. DSSS, on the other hand, can recover the original data using advanced statistical techniques without the need for any retransmission. DSSS can actually recover from not just one corrupted bit but from
multiple corrupted bits in a transmission.

● Less interference on other systems. If the DSSS signal is picked up by an unintended device, the signal will appear as low-powered noise and will be ignored.

● Shared frequency bandwidth. Using DSSS makes it possible to share the frequency band with similar devices. Known as colocation, this is achieved by assigning each device a unique chipping code in order that all the transmissions can use the same frequency yet remain separate. The transmission of one network would only appear
as noise to another network and would be filtered out.

● Security. If an eavesdropper picked up the signal of the original data bit, it would be
a simple task to read the message. However, a DSSS transmission that has been
intercepted is far harder to decipher.

DSSS has a potential bandwidth of up to 11 Mbps whereas FHSS can only transmit at a maximum of 3 Mbps. Because of its higher throughput, DSSS systems are preferred over FHSS for 802.11b WLANs.

However, the dramatically increased throughput that can be achieved by OFDM has made it the leader today among the modulation schemes. OFDM is used in IEEE 802.11a/g/n networks (not in 802.11 or 802.11b). Because it can support speeds of up to 600 Mbps for IEEE 802.11n and 54 Mbps for 802.11a/g networks, OFDM is the preferred modulation technique for faster WLANs.

Two sublayer

The IEEE has also subdivided the Physical layer (PHY) for WLANs into two sublayers.

The Physical Medium Dependent (PMD) sublayer makes up the standards for both the characteristics of the wireless medium and defines the method for transmitting and receiving data through that medium.

The second sublayer of the PHY layer is the Physical Layer Convergence Procedure (PLCP) sublayer. The PLCP sublayer performs two basic functions: it reformats the data received from the MAC layer (when transmitting) into a frame that the PMD sublayer can transmit, and it “listens” to the medium to determine when the data can be sent.

  The frame is made up of three parts: the preamble, the header, and the data. The preamble prepares the receiving device for the rest of the frame, whereas the header provides information about the frame itself. The data portion of the PLCP frame is the information that is actually being transmitted. The size of the data or payload can be from 1 to 16,384 bits.

A description of the fields is as follows:

● Synchronization. The Synchronization field consists of alternating 0’s and 1’s. It alerts the receiving device that a message may be on its way so that the receiving device will then synchronize with the incoming signal.
● Start Frame Delimiter. The Start Frame Delimiter is always the same bit pattern (1111001110100000) and it defines the beginning of a frame.
● Signal Data Rate. The speed of the signal is designated by the Signal Data Rate field.
● Length. The value of the length of the frame is contained in the Length field.
● Header Error Check. The Header Error Check field contains a value that the receiving device can use to determine if the data was received correctly.
● Data. The data or payload can be from 1 to 16,384 bits, and that value is contained in this Data field.

The PLCP frame preamble and header are always transmitted at 1 Mbps. This was designed to allow a slower sending device (like an 802.11) to talk to a faster receiving device (like an 802.11b) by using the slowest speed.

802.11b: 3 nonoverlapping channels, 1, 6, 11

802.11a: standard has 12 nonoverlapping channels. IEEE 802.11a networks have 555 MHz spread across 23 nonoverlapping channels.