1. Basic overview
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1. Wired and wireless networks

At present, the most famous wired network is Ethernet (Ethenet), but wireless network WLAN is a promising development field. Although it may not completely replace Ethernet, it is having more and more users. The most promising is Wifi. This article introduces wireless network related content.

Compared with wired networks, wireless networks still have many disadvantages:

(*) Because the two parties communicate through wireless communication, a connection needs to be established before communication; while the wired network is directly connected by cable, without this process.

(*) The communication mode of the two parties is half-duplex; the wired network can be full-duplex.

(*) The probability of errors below the network layer during communication is very high, so the probability of frame retransmission is very high. It is necessary to add a retransmission mechanism to the protocol below the network layer (you cannot rely on the above TCP/IP delay waiting for retransmission. Transmission and other overhead to ensure); while the probability of error in the wired network is very small, there is no need for such a complicated mechanism at the network layer.

(*) The data is carried out in a wireless environment, so it is very easy to capture packets, and there is a security risk.

(*) The power consumption is relatively high because of sending and receiving wireless signals, which is a test for the battery.

(*) Relative to wired network throughput is low, this is gradually improving, 802.11n protocol can reach 600Mbps throughput.

2. Agreement

The protocols used by Ethenet and Wifi belong to the IEEE 802 protocol set. Among them, Ethenet uses the 802.3 protocol as its protocol below the network layer; and Wifi uses 802.11 as its protocol below the network layer. Regardless of whether it is a wired network or a wireless network, the parts above the network layer are basically the same.

The main concern here is related content in the Wifi network. Wifi’s 802.11 protocol contains many sub-parts. Which develops in chronological order, mainly:

(1) 802.11a, established in September 1999, works in the frequency range of 5gHZ (band width 325MHZ), and the maximum transmission rate is 54mbps, but it was not very popular at that time, so it was not used much.

(2) 802.11b, established in September 1999, is slightly later than 802.11a, works in the 2.4g frequency range (band width 83.5MHZ), and the maximum transmission rate is 11mbps.

(3) 802.11g, formulated in June 2003, works in the 2.4gHZ frequency range (band width 83.5MHZ), and the maximum transmission rate is 54mbps.

(4) 802.11n, only approved by IEEE in 2009, can work in both 2.4gHZ and 5gHZ, and the maximum transmission rate is 600mbps.

These protocols are the basic protocols required for wireless network communication. The latest developments are generally better than the original ones.

It is also worth noting that 802.11n has made some important improvements on the MAC layer, which has led to a great improvement in network performance. For example:

(*) Because the transmission rate depends to a large extent on the ChannelWidth of the Channel (channel), a technology is adopted in 802.11n to combine the two channels into one when transmitting data, and then proceed. Transmission, which greatly improves the transmission rate (this is also called HT-40, high through).