The transmission mode involves the transmission medium adopted by the wireless network, the selected frequency band and the modulation mode.
At present, there are two main transmission media used in wireless networks, namely, radio waves and infrared rays. In wireless networks using radio waves as transmission media, there are different modulation modes, which can be divided into spread spectrum modulation and narrowband modulation.
1, spread spectrum mode
In the spread spectrum method, the frequency spectrum of data baseband signal is spread to several times-dozens of times before being moved to radio frequency for transmission. Although this method sacrifices bandwidth, it improves the anti-interference ability and security of communication system. Because the power in the unit frequency band is reduced, the interference to other electronic devices is also reduced.
Wireless local area networks using spread spectrum generally choose the so-called ISM frequency band, in which ISM is taken from the initials of industry, science and medicine respectively. The energy radiated by many industrial, scientific research and medical equipment is concentrated in this frequency band. For example, the ISM frequency band in the United States consists of three frequency bands: 902MHz-928MHz, 2.4GHz-2.48GHz and 5.725GHz-5.850GHz. If the emission power and bandwidth radiation meet the requirements of the Federal Communications Commission (FCC), the ISM frequency band can be used without special application to the FCC.
2. Narrow band modulation mode
In narrowband modulation mode, the spectrum of data baseband signal is directly transferred to radio frequency for transmission without any expansion.
Compared with spread spectrum method, narrowband modulation method occupies less frequency band and has high frequency band utilization rate. Narrow-band modulated WLAN generally uses special frequency bands, which need to be approved by the national radio management department before it can be used. Of course, you can also choose the ISM band, which can avoid applying to the Radio Management Committee. However, the problem is that when the adjacent instruments or communication equipment are also using this frequency band, the communication quality will be seriously affected and the reliability of communication cannot be guaranteed.
3. Infrared mode
The transmission technology based on infrared has made great progress in recent years. At present, almost all the widely used home appliance remote controllers adopt infrared transmission technology. As a transmission mode of WLAN, the biggest advantage of infrared ray is that it is not interfered by radio, and its use is not restricted by the National Radio Regulatory Commission. However, the transmission distance of infrared rays to opaque objects is limited.
Second, the network topology
The extended structure of WLAN can be divided into two categories: non-central or peer-to-peer topology and hub-based topology.
1, no central topology
A network without a central topology requires that any two sites in the network can communicate directly.
Generally speaking, this topological network uses public broadcast channels, and all stations can compete for the public channels. Most channel access control (MAC) protocols use CSMA (Carrier Sense Multiple Access) multi-access protocol.
The advantages of this structure are good network invulnerability, easy networking and low cost. However, when there are too many users (sites) in the network, channel competition becomes the key to limit the network performance. And in order to meet the requirement that any two sites can communicate directly, the site layout in the network is greatly restricted by the environment. Therefore, this topology is suitable for workgroup networks with relatively few users.
2. Central topology
In the central topology, a wireless station is needed as the central station, and all wireless stations' access to the network is controlled by it.
In this way, when the network traffic increases, the deterioration of network throughput performance and network delay performance is not serious. Because each station can only communicate with other stations within the coverage of the central station, the layout of stations in the network is less limited by the environment. In addition, the central station provides a logical access point for accessing the wired backbone network.
The weakness of centralized network topology is poor invulnerability. The failure of the central point can easily lead to the paralysis of the whole network, and the introduction of the central site increases the network cost.
In practical applications, wireless networks are often combined with wired backbone networks. At this time, the central station acts as an adapter between the wireless network and the wired backbone network.
Third, the network interface
This relates to which layer a station in a wireless network accesses the network system. Generally speaking, the network interface can be selected in the physical layer or data link layer of the OSI reference model.
The so-called physical layer interface refers to replacing the usual wired channel with wireless channel, while the layers above the physical layer remain unchanged. The biggest advantage of this is that the upper network operating system and the corresponding drivers cannot be modified. This interface is generally used as a hub and wireless repeater of wired networks to realize interconnection between wired LANs or expand the coverage of wired LANs.
Another interface method is to access the network from the data link layer. This interface mode does not follow the MCA protocol of wired LAN, but adopts the MAC protocol which is more suitable for wireless transmission environment. In implementation, the MAC layer and its lower layer are transparent to the upper layer, and the corresponding drivers are configured to complete the interface of the upper layer of the domain, which can ensure the normal operation of the existing wired LAN operating system or application software on the wireless LAN.
At present, most WLAN manufacturers adopt data link layer interface mode.