1. First of all, the topic is "multiple access" technology, which should be understood as multiple access technology. Therefore, the problem here is easier to understand. CSMA/CA based on wifi is carrier sense multiple access with collision avoidance, which already embodies the elements of multiple access, so it is also based on multiple access technology. In Wikipedia (hmm. . In general, I think the definition in wiki is more credible, so I usually refer to wiki), "In telecommunications and computer networks, a channel access mode or multiple access mode allows several terminals connected to the same multicast medium to transmit through it and share its capacity. Examples of shared physical media are wireless networks, bus networks, ring networks and half-duplex point-to-point links. " (Refer to the channel access method in section 1). Therefore, in the definition of multiple access, I understand that "different nodes can send data to the same receiver", and this element is not emphasized here.
2. In the description of the topic, there are two problems that need to be divided.
One is "nodes in WLAN are based on contention transmission". There are actually three working modes in WLAN design: contention (DCF), non-contention (PCF) and hybrid (HCF). In practical application, all devices basically work in DCF mode (DCF is based on contention and PCF is based on polling). A large part of the reason lies in cost and application scenarios. Compared with DCF, PCF requires higher time synchronization, so it is necessary to ensure that everyone can make polling calls consistently. DCF is distributed and has good adaptability to different topologies and different environments. In the WLAN deployment environment, the nodes are still scattered and there will be some mobile situations, so it is more suitable for distributed algorithms. Moreover, in general WLAN scenarios, there are few nodes hanging from an AP (except for commercial scenarios), so there will be fewer competition conflicts, so the competition method will actually be more efficient. In polling, extra polling overhead is needed every time, so the efficiency will be reduced. (As for HCF, it is PCF+DCF for the same reason as PCF. )
Secondly, there can only be one transmission at a time. Then why not introduce a technology that can be transmitted to multiple users or to multiple users at the same time? "In fact, this sentence needs to be elaborated in two layers.
(1) I guess the original intention of the subject is to say, "Why is wifi not based on CDMA or FDMA, and can allow multiple users to access at the same time?" . First of all, CDMA is a technology with high synchronization requirements, which is easy to increase hardware requirements and may affect the cost. At the same time, CDMA also needs to allocate symbols and extra overhead. In fact, more importantly, in the WLAN environment (well, it is still non-commercial traffic), there are not many scenes in which multiple nodes surf the Internet at the same time, and CDMA technology also has many advantages. However, in the WLAN environment, there may be many devices hanging under an AP at the same time, such as 10 devices, but generally there may be 1~3 devices that need data transmission at a time, so it is not used like CDMA. If FDMA is used, the reason is similar to CDMA. Another point is that WIFI works in the ISM frequency band, and the frequency band resources are limited, and not all of them are used by WIFI. FDMA needs to protect the frequency band, and the frequency band is not particularly easy to allocate, which will also lead to lower efficiency.
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