Key technical provisions of the Protocol

Due to the fact that wireless LAN transmission media (microwave and infrared) are not "Limited" wired, there are some new technical difficulties objectively. For this reason, the Protocol sets out some crucial technical mechanisms, for example, CSM/CA and RTS/CTS.

1. CSMA/CA protocol we know that the standard protocol for a bus LAN on the MAC layer is CSMA/CD, that is, the carrier listens for Multi-Point Access/conflict detection (carriersensemultipleaccesswithcollisiondetection ). However, wireless product adapters are not easy to detect channel conflicts. Therefore, 802.11 defines a new protocol, that is, the carrier listens for Multi-Point Access/avoids collision with CSMA/CA (withcollisionavoidance ). On the one hand, carrier listening-check whether the media is idle; on the other hand, avoid collision-minimize the probability of a Signal conflict by waiting for a random time. When the media is listened to idle, send messages first. In addition, to make the system more stable, 802.11 also provides CSMA/CA with a validation frame ack. In the event of other noise interference or failure in listening, the signal conflict may occur, and the ACK working on the MAC layer can provide quick recovery. 2. the RTS/CTS protocol is the request sending/allowed sending protocol. It is equivalent to a handshake protocol and is mainly used to solve the problem of "Hiding terminals. "Hidden terminals" (hiddenstations) means that base station a sends information to Base Station B, and base station C does not detect that a also sends information to B, so both A and C send signals to B at the same time, as a result, all signals sent to B are lost. "Hidden terminals" mostly occur in large units (usually in outdoor environments), which results in efficiency loss and requires an error recovery mechanism. When you need to transfer large-capacity files, especially to prevent the occurrence of "hidden terminals. 802.11 provides the following solutions. In parameter configuration, if you use the RTS/CTS protocol and set the maximum number of bytes to be transferred-once the data to be transferred exceeds the upper limit, start the RTS/CTS handshake protocol: first, A sends an RTS signal to B, indicating that a wants to send some data to B. B sends a CTS signal to all the base stations after receiving the RTS, indicating that a is ready and a can send, the other base stations are "not moving" Temporarily. Then, a sends data to B. Finally, B receives the data and broadcasts ack confirmation frames to all base stations, all the base stations are now available for equal listening and competitive channels. 3. re-transmission of packets when frames are seriously disturbed. Therefore, if a packet is larger, the retransmission cost (time, control signal, and recovery mechanism) will increase, if the frame size is reduced, the large packet is divided into several small packets. Even if the packet is re-transmitted, it only re-transmits a small packet, which consumes a relatively small amount. In this way, the anti-interference capability of Wireless LAN products in noise interference areas can be greatly improved. Of course, as an option, you can disable this function if you are in a "clean" area. 4. Multi-Channel roaming human beings are infinitely pursuing freedom. With the increasing popularity of mobile computing devices, we hope to have a truly pervasive network access device. The 802.11 product is such a device. The transmission frequency band is set on the access device AP (accesspoint), and the base station does not need to set a fixed frequency band, and the base station has an automatic identification function. The base station dynamically adjusts the frequency to the frequency band set by the AP, this process is called scan ). Defines two modes: passive scan and active scan. Passive scan refers to the process where the base station listens on the indication signal sent by the AP and switches to the given frequency band. active scan refers to the process where the base station initiates a visit request and the AP sends a response containing the frequency band information, the Base Station switches to the given frequency band. Wavelan802.11 adopts active scanning and can be determined as the current transmission channel based on the antenna receiving sensitivity. In this way, when the base station originally located in the AP (a) coverage area roaming to the AP (B), the base station can adapt to the current access point again with AP (B. 5. Reliable security performance at present, the launch power of Wireless LAN products on the market is very small, less than 35 MW, and is also extended to 22 MHz bandwidth. On the one hand, the average energy is very low (15dbm ), on the other hand, there is no single-frequency carrier, so it is difficult to be scanned and tracked, which is why this technology has been used for military purposes. These are physical security mechanisms. In software, multiple security mechanisms such as domain name control, access permission control, and protocol filtering are also adopted. In terms of Wired Equivalent confidentiality (WEP, for special users, the following accessories are available: RC4-based encryption (1988rsa algorithm) and password (40-or 128-bit encryption key ).