WiMAX Technology - Architecture

Next, it is worth mentioning the architecture and construction of the system. When it comes to its architecture, the system, in the basic version, operates in the point-multipoint architecture. We therefore obtain a centrally located base station and terminals connecting to it via radio are situated around it. Optionally, the system can also operate in the crate architecture without base station, where every user connects his neighbor. The system construction covers two first layers of the ISO/OSI system, that is the physical PHY layer and MAC layer.

The physical layer uses the already-mentioned techniques for obtaining maximum reach with simultaneous provision of maximum capacity. The system basically uses two frequency ranges, namely 10-66GHz and 2-11GHz (for instance in the unlicensed band 2,4GHz), while width of the channel is changeable depending on the required transmission speed. Duplex is realized in the system basing on two proven techniques:

- time duplex TDD, in which for upwards and downwards transmission, the same frequencies are used. The upwards transmission takes place in one time space and downwards transmission in the other and so on.

- frequency duplex FDD, in which one frequency is attributed to the upwards link and another to the downwards link.

Next, we need to pay attention to the channel access method:

- In the downwards link (from the base station to the terminal) TDM technique is used. The base station transmits incessantly in the time spaces. Each terminal receives the whole transmission but it processes only the data that are destined for it. The station uses special CID identifiers that make it possible for the terminal to obtain information on which data are destined for it.

- In the upwards link (from the terminal to the base station) TDMA access technique is used. The channel is divided into time spaces, whereas every terminal can transmit only in the time space previously assigned by the base station.

The MAC layer guarantees intelligent access to the physical layer, not found in the hitherto used systems, thanks to which it was possible to achieve a very high QoS level. Its greatest advantage is the possibility of dynamic band allocation to every user in order to guarantee proper quality of the rendered services. This layer is divided into three functional sub-layers:

- Security sub-layer is responsible for authentication and authorization of the terminal, as well as for encoding of movement in the network

- The MAC CPS sub-layer, whose task is to steer access to the transmission by reserving links designed for various purposes.

- The MAC CS sub-layer, whose task is to allocate data coming in from the higher layers (those that are not covered by the standard) to the right MAC links so that the assumed QoS level is maintained in the properly available band..

One of the major achievements of system`s creators is obtaining the QoS quality of the services on a level unmatched in other wireless networks. There are basically four classes of services to which the movement is directed. Each class uses different transmission mechanisms, thanks to which it is for instance possible to guarantee minor delays for applications that especially require it. The classes are as follows:

- UGS support applications requiring a constant CBR bit rate, such as emulation T1/E1 or VoIP without silence.

- rtPS support applications operating in the real time and periodically generating packages with changeable length, such as MPEG or VoIP video streams with silence detection.

- nrtPS support applications that do not require operation in real time, but require a changeable data package size generated in a periodical manner, ex. FTP service.

- BE (Best Effort) supporting applications that do not require QoS. For example, a www service can be realized in this.

Finally, we should briefly compare WiMAX and WiFi as these technologies are often recognized as competitive. Comparison of both of these standards is all the more difficult due to the fact that they were created for different purposes. Both solutions are obviously wireless, but they, however, differ in both the technologies, as well as application area. When it comes to application areas, the basic difference is that WiFi are LAN type networks, that is directed at the recipients who do not want to use the cabling in construction of local networks on short distances. WiMAX, on the other hand, is a MAN metropolitan network that is supposed to provide a broadband access to its users in areas of whole cities. It seems that WiFi`s position remains unthreatened due to its price, mass scale and relative quality. In the future, however, it is highly probable that Wifi devices will be totally replaced by WiMAX. WiMAX offers far greater transmission safety, greater capability and has QoS mechanisms that, in WiFi standard, do not practically function. When it comes to the technical aspect, WiMAX is undoubtedly more technically advanced than WiFi. Among others, the applied modulation techniques are characterized by almost twice as greater spectrum efficiency and resistance to disturbances. What also deserves attention is the fact that modulation can be chosen in WiMAX depending on client`s needs. There is a number of other differences, however, finally it is not particularly difficult to notice than WiMAX technology considerably outruns WiFi.

See also: WiMAX Technology