This work rapidly led to wireless LANs being marketed by a variety of companies.
The trouble was that no two of them were compatible. This proliferation of standards meant that a computer equipped with a brand X radio would not work in a room equipped with a brand Y base station. Finally, the industry decided that a wireless LAN standard might be a good idea, so the IEEE committee that standardized the wired LANs was given the task of drawing up a wireless LAN standard. The standard it came up with was named 802.11. A common slang name for it is WiFi. It is an important standard and deserves respect, so we will call it by its proper name, 802.11. The proposed standard had to work in two modes:
- In the presence of a base station.
- In the absence of a base station.
The first decision was the easiest: what to call it. All the other LAN standards had numbers like 802.1, 802.2, 802.3, up to 802.10, so the wireless LAN standard was dubbed 802.11. The rest was harder.
In particular, some of the many challenges that had to be met were: finding a suitable frequency band that was available, preferably worldwide; dealing with the fact that radio signals have a finite range; ensuring that users' privacy was maintained; taking limited battery life into account; worrying about human safety (do radio waves cause cancer?); understanding the implications of computer mobility; and finally, building a system with enough bandwidth to be economically viable.
At the time the standardization process started (mid-1990s), Ethernet had already come to dominate local area networking, so the committee decided to make 802.11 compatible with Ethernet above the data link layer. In particular, it should be possible to send an IP packet over the wireless LAN the same way a wired computer sent an IP packet over Ethernet. Nevertheless, in the physical and data link layers, several inherent differences with Ethernet exist and had to be dealt with by the standard.
First, a computer on Ethernet always listens to the ether before transmitting. Only if the ether is idle does the computer begin transmitting. With wireless LANs, that idea does not work so well. To see why, examine Fig. 1- 36. Suppose that computer A is transmitting to computer B, but the radio range of A's transmitter is too short to reach computer C. If C wants to transmit to B it can listen to the ether before starting, but the fact that it does not hear anything does not mean that its transmission will succeed. The 802.11 standard had to solve this problem.
The second problem that had to be solved is that a radio signal can be reflected off solid objects, so it may be received multiple times (along multiple paths). This interference results in what is called multipath fading.
The third problem is that a great deal of software is not aware of mobility. For example, many word processors have a list of printers that users can choose from to print a file. When the computer on which the word processor runs is taken into a new environment, the built-in list of printers becomes invalid.
The fourth problem is that if a notebook computer is moved away from the ceiling-mounted base station it is using and into the range of a different base station, some way of handing it off is needed. Although this problem occurs with cellular telephones, it does not occur with Ethernet and needed to be solved. In particular, the network envisioned consists of multiple cells, each with its own base station, but with the base stations connected by Ethernet, as shown in Fig. 1-37. From the outside, the entire system should look like a single Ethernet. The connection between the 802.11 system and the outside world is called a portal.
After some work, the committee came up with a standard in 1997 that addressed these and other concerns. The wireless LAN it described ran at either 1 Mbps or 2 Mbps. Almost immediately, people complained that it was too slow, so work began on faster standards. A split developed within the committee, resulting in two new standards in 1999. The 802.11a standard uses a wider frequency band and runs at speeds up to 54 Mbps. The 802.11b standard uses the same frequency band as 802.11, but uses a different modulation technique to achieve 11 Mbps. Some people see this as psychologically important since 11 Mbps is faster than the original wired Ethernet. It is likely that the original 1-Mbps 802.11 will die off quickly, but it is not yet clear which of the new standards will win out.
To make matters even more complicated than they already were, the 802 committee has come up with yet another variant, 802.11g, which uses the modulation technique of 802.11a but the frequency band of 802.11b. We will come back to 802.11 in detail in Chap. 4.
That 802.11 is going to cause a revolution in computing and Internet access is now beyond any doubt. Airports, train stations, hotels, shopping malls, and universities are rapidly installing it. Even upscale coffee shops are installing 802.11 so that the assembled yuppies can surf the Web while drinking their lattes. It is likely that 802.11 will do to the Internet what notebook computers did to computing: make it mobile.
Wireless LANs: 802.11
4/
5
Oleh
Unknown
