l'autoroute du ccna

l'autoroute du ccna.3. 2.4GHzThe 2.4GHz range is probably the most widely used frequency range in WLANs. 802.11.802. lib. 802.llg and 802.11n IEEE standards all use this range. The 2.4GHz frequency range used by WLANs can be subdivided into subchannels in the range 2.4000GHz. 2.4835GHz. The United States uses 11 channels with a bandwidth of 22 MHz each. some channels can overlap with other channels and cause interference. For this reason, channels 1, 6, and 11 are typically used because they do not overlap. In fact, many residential grade wireless devices are hard set (hardset), so one of these three channels can be selected. Figure 1.2 shows11 channels including the overlap. Note that there is no overlap between channels 1, 6, and H.The energy of both 802.11b and 802.11g is distributed over the bandwidth range. The channel bandwidth for 802.11b or 802.llg products is 22 MHz, which allows for 3 non-overlapping and interference-free channels in the same range.The 2.4GHz range uses Direct Sequence Spread Spectrum (DSSS) modulation. DSSS will be discussed later in this chapter. IMbit/s. 2MbiVs, 5.5Mbit/s and HMbit/s data rates are defined for this band.Figure 1.21.2 Basic Topics 9 4. 5GHzBoth the 802.11a standard and the 802.1 hi draft standard use the 5GHz range. In the 802.11a standard, the data rate can vary from 6Mbit/s to 54Mbit/s. It was not until 2001 that 802.11a devices were available on the market, so these were not as widely used as 802.11b devices in the 2.4GHz range. The 5GHz range is also subdivided into smaller channels, each 20MHz wide. there are a total of 23 non-overlapping channels in the 5GHz range.The 5GHz range uses Orthogonal Frequency Division Multiplexing (OFDM) technology. OFDM will be discussed later in this chapter. The channel range defines data rates of 6Mbit/s, 9Mbit/s, 12MbiVs,18Mbit/s, 24MbiVs, 36Mbit/s, 48Mbit/s and 54Mbit/s.1.2.3 Modulation techniques and how they work In simple terms, the modulation process is a change in the signal or tone of a signal known as the carriersignal. The process of loading data into the carrier signal is called encoding. Imagine you are singing a song and the lyrics are written on a sheet of music. If you're just singing to yourself, your tone is soft and won't carry very far. In order to sing to a large group of people, you have to raise your vocal cords and modulate them so that the sound travels farther. When you sing, you encode the lyrics in the form of sound waves and modulate them with your vocal cords. People hear you sing and decode the lyrics to understand the song. Modulation is a way that wireless networks use to send data. It makes it possible to send encoded data using radio signals. Wireless networks use a modulated signal as the carrier signal, which means that the modulated tones carry the data. The modulated waveform contains the following 3 components. Amplitude: the volume of the signal. Phase: the timing between the peaks of the signal. Frequency: the tone of the signal. Wireless networks use several different modulation techniques, as shown below.