What Is Fixed Wireless?

A fixed radio terminal refers to an application in a wireless local loop, and is usually a terminal that is fixed or can be moved within a small range.

When a wireless terminal starts its wireless network card work, it starts to perform signal scanning on wireless channels in different frequency bands to find all access points APs in its area. There may be multiple APs in each area, and each AP periodically sends a beacon frame to its signal coverage area through its wireless channel. The beacon frame includes synchronization information with the wireless terminal, the MAC address of the AP, and the supported transmission rate. When the wireless terminal receives a beacon frame broadcast by multiple APs on a certain channel, it will automatically or manually select an AP with a strong signal and a low bit error rate, extract relevant information in it, and start to communicate with This AP performs authentication and association.

The authentication process of the IEEE 802.11 wireless local area network is as follows: each wireless terminal first provides identity information to a selected access point AP. Identity information can take two forms, one is each basic service set identifier (

Association of fixed wireless terminals with wireless terminals

After identity authentication is passed, it is necessary to associate. In the process of association, the wireless terminal and the access point AP must negotiate the data transmission rate according to the emphasis of the signal. When the wireless terminal is far away from the AP or there is an obstacle in the middle, the received signal will be weak. Then, at this time, a lower transmission rate should be selected for correlation. Taking IEEE 802.11b as an example, there are two rates of 5.5Mb / s and 11Mb / s. If the signal is weak, the rate of 5.5Mb / s will be automatically selected for correlation.

When the wireless terminal moves from one basic service set to another basic service set in the same extended service set, the wireless network card will rescan and find a new AP, and switch to the wireless channel used by this new AP. Re-certify and associate. In addition, if the communication signal between a wireless terminal and the current AP is too weak, or the bit error rate is too high, the wireless network card will also try to scan other APs to provide selection and replacement.

Roaming of fixed wireless terminals

Similar to the roaming of mobile phones in different cities covered by GSM networks, CDMA networks and 3G networks, roaming of wireless terminals in wireless local area networks refers to allowing wireless terminals to move between different access points APs of wireless local area networks. And during the movement process, it can ensure that network applications will not be interrupted, and normal data transmission and reception are maintained. There are two types of roaming: basic roaming and extended roaming.

Basic roaming means that the mobility of wireless terminals is restricted between different basic service sets within an extended service set. Because the wireless terminal is still in the same extended service set, only the basic service set is changed, and the corresponding network layer information does not need to be changed, so it will not affect the ongoing network applications. But the premise is to be able to effectively find new APs in the basic service set and quickly pass authentication and association. IEEE 802.11 wireless local area networks support this basic roaming within the same extended service set.

Extended roaming refers to the wireless terminal moving from one extended service set to another extended service set, which means that the wireless terminal will move from one LAN to another LAN. At this time, network configuration parameters such as the IP address of the wireless terminal and the IP address of the gateway (router) will change. In addition, not only must the wireless terminal be re-authenticated and associated with a new access point AP, but also all IP packet header information sent and received by the wireless terminal must be changed accordingly. Since the IEEE802.11 series of standards is only a technical standard of the physical layer and the MAC layer, there is nothing that can be done to change the upper layers, so it does not support such extended roaming across extended service sets.

To achieve this kind of extended roaming, it is necessary to use the Mobile IP technology standard released by the IETF in 1996. ^[1]