Conclusion
To summarize the benefits of 802.11n technology, it is simplest to say that there are two major areas of improvement over previous 802.11 devices. The first area of improvement is in the use of MIMO technology to achieve greater SNR on the radio link. The second area of improvement is in the greater efficiencies in both radio transmissions and the MAC protocol. These improvements translate into benefits in three areas: reliability, predictable coverage, and throughput.
- Reliability: Greater SNR on the radio link translates directly to more reliable communication, often at higher data rates. Higher SNR means that more interference is needed to corrupt a transmission. This means greater client densities can be supported. Predictable
- Coverage: The use of multiple spatial streams provided by MIMO technology means that there will be fewer dead spots in a coverage area. Areas that previously suffered from destructive multipath interference now make use of that same multipath effect to provide robust communication.
- Throughput: The efficiency improvement in 802.11n provides a greater transfer of the high bit rates of the 802.11n radio to effective throughput seen by actual applications, at least in greenfield deployments. Even in mixed-mode deployments with legacy 802.11 devices, 802.11n will provide greater effective throughput, although significantly less than the greenfield mode.
Questions
1. Why isn't interessant to use BA with A-MSDU?
BA is used to ack a group of frames in when single send. If an A-MSDU is received with an error bit on one of the aggregated frame, the receiver cannot tell on which one it happened so it will ask for the resend of the all A-MSDU. This way, a simple Ack can do the job.
2. On what physics principle relies Beamforming?
Beamforming relies on wave interference to raise the signal strength at the level of the receiver.
3. What is the main difference between A-MPDU and A-MSDU?
The main difference is the MAC encapsulation. An A-MPDU is the aggregation of encapsulated MSDUs while A-MSDU is the aggregation of MSDUs. The MAC encapsulations is done on the whole A-MSDU to form an PSDU.
4. What MRC does?
MRC (Maximal-Ration Combining) is an algorithm that mixes-up two or more signals (at most four) coming from the different antennas on the receiver. MRC produces a signal, extracting the best of the received signals for each frequencies.
5. Can Beamforming be used with an 802.11a receiver device? Why?
The transmitter tunes-up the phase of the sent waves, to optimize the signal strength at the level of the receiver. In order to do that, it needs to know the signal strength received by the targeted device. The problem is that only an 802.11n compatible device is able to transmit such an information to the transmitter. Therefore, an 802.11a can not take advantage of beamforming as defined by the 802.11n certification.
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