In addition to modulation schemes, The bit-rate for The MCS value related to the receiver sensitivity is listed on the product data sheet. Factors that contribute to Negotiation of the Modulation Scheme. The benefit to a modulation scheme that has more states is that each state can represent more bits. At the 5. OFDM is not considered a spread spectrum technology, but it is used for modulation in wireless networks.
Using OFDM, you can achieve the highest data rates with the maximum resistance to corruption of the data caused by interference. OFDM defines a number of channels in a frequency range. These channels are further divided into a larger number of small-bandwidth subcarriers.
The channels are 20 MHz, and the subcarriers are. You end up with 52 subcarriers per channel. Each of the subcarriers has a low data rate, but the data is sent simultaneously over the subcarriers in parallel. This is how you can achieve higher data rates. OFDM is not used in The way they are implemented is a little different because MIMO is a technology that is used in the new Although at press time, the Here is what you need to know about it, though. A device that uses MIMO technology uses multiple antennas for receiving signals usually two or three in addition to multiple antennas for sending signals.
MIMO technology can offer data rates higher than Mbps by multiplexing data streams simultaneously in one channel. In other words, if you want data rates higher than Mbps, then multiple streams are sent over a bonded channel, not just one. Using advanced signal processing, the data can be recovered after being sent on two or more spatial streams.
Now that you have an idea of how data is encoded and modulated, things will start to get a little easier. Another important aspect to understand, not only for the exam but for actual wireless deployments, is that the farther away you get from the access point, the lower the data rates are that you can achieve. This is true regardless of the technology.
Although you can achieve higher data rates with different standards, you still have this to deal with. All Cisco wireless products can perform a function called dynamic rate shifting DRS. It even happens without dropping your connection. Also, it is done on a transmission-by-transmission basis, so if you shift from 11 Mbps to 5.
This process also occurs with In all deployments, DRS supports multiple clients operating at multiple rates. Wireless networks have to deal with the possibility of collisions. This is because, in a wireless topology , the behavior of the AP is similar to that of a hub. Multiple client devices can send at the same time. When this happens, just like in a wired network where a hub exists, a collision can occur.
The problem with wireless networks is that they cannot tell when a collision has occurred. If you are in a wired network, a jam signal is heard by listening to the wire. To listen for a jam signal, wireless devices need two antennas.
They can send using one antenna while listening for a jam signal with the other. Although this sounds feasible, especially because MIMO technology defines the use of multiple antennas, the transmitting signal from one antenna would drown out the received signal on the other, so the jam signal would not be heard. Although the two are similar, collision avoidance means that when a device wishes to send, it must listen first.
If the channel is considered idle, the device sends a signal informing others that it is going to send data and that they should not send. The angle from the "I" axis to where the point is on the diagram shows the phase offset that the signal would have to represent that particular value. To demonstrate the modulation an 8PSK modulator is used in the following block diagram.
The example bit stream on the left hand side enters the modulator and has a bit slot of Being a sample of 60 seconds and a bit slot of 10 seconds, the sample bit stream is carrying 6 bits 1 1 1 0 1 0. The entire process is depicted below and can be applied to any modulation scheme and bitrates.
The example above is using full rate modulation. In many cases, a half code rate scheme will be used in conjunction with the modulation scheme such that only half of the bits would change every bit-slot. As the carrier is modulated, additional frequency components are created. Without these extra frequency components, the received signal would not resemble the original transmitted signal.
These side-bands define a definite signal physical spectrum bandwidth. The required physical bandwidth of a signal is determined by the maximum modulation frequency and how drastic the state change could be.
An example of how far the side bands would be in the worst case scenario is in the diagram below, where none of the spectral energy is in the carrier waveform.
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