Comparison of 802.11 with 802.16

At this point you may be thinking: Why devise a new standard? Why not just use 802.11?
There are some very good reasons for not using 802.11, primarily because 802.11 and 802.16 solve different problems. Before getting into the technology of 802.16, it is probably
worthwhile saying a few words about why a new standard is needed at all.

The environments in which 802.11 and 802.16 operate are similar in some ways, primarily in
that they were designed to provide high-bandwidth wireless communications. But they also
differ in some major ways. To start with, 802.16 provides service to buildings, and buildings
are not mobile. They do not migrate from cell to cell often. Much of 802.11 deals with mobility,
and none of that is relevant here. Next, buildings can have more than one computer in them, a
complication that does not occur when the end station is a single notebook computer. Because
building owners are generally willing to spend much more money for communication gear than
are notebook owners, better radios are available. This difference means that 802.16 can use
full-duplex communication, something 802.11 avoids to keep the cost of the radios low.

Because 802.16 runs over part of a city, the distances involved can be several kilometers, which means that the perceived power at the base station can vary widely from station to station. This variation affects the signal-to-noise ratio, which, in, turn, dictates multiple
modulation schemes. Also, open communication over a city means that security and privacy are essential and mandatory.

Furthermore, each cell is likely to have many more users than will a typical 802.11 cell, and
these users are expected to use more bandwidth than will a typical 802.11 user. After all it is
rare for a company to invite 50 employees to show up in a room with their laptops to see if
they can saturate the 802.11 wireless network by watching 50 separate movies at once. For
this reason, more spectrum is needed than the ISM bands can provide, forcing 802.16 to
operate in the much higher 10-to-66 GHz frequency range, the only place unused spectrum is
still available.

But these millimeter waves have different physical properties than the longer waves in the ISM bands, which in turn requires a completely different physical layer. One property that
millimeter waves have is that they are strongly absorbed by water (especially rain, but to
some extent also by snow, hail, and with a bit of bad luck, heavy fog). Consequently, error
handling is more important than in an indoor environment. Millimeter waves can be focused
into directional beams (802.11 is omnidirectional), so choices made in 802.11 relating to
multipath propagation are moot here.

Another issue is quality of service. While 802.11 provides some support for real-time traffic
(using PCF mode), it was not really designed for telephony and heavy-duty multimedia usage. In contrast, 802.16 is expected to support these applications completely because it is intended for residential as well as business use.

In short, 802.11 was designed to be mobile Ethernet, whereas 802.16 was designed to be wireless, but stationary, cable television. These differences are so big that the resulting standards are very different as they try to optimize different things.

A very brief comparison with the cellular phone system is also worthwhile. With mobile phones, we are talking about narrow-band, voice-oriented, low-powered, mobile stations that
communicate using medium-length microwaves. Nobody watches high-resolution, two-hour
movies on GSM mobile phones (yet). Even UMTS has little hope of changing this situation. In short, the wireless MAN world is far more demanding than is the mobile phone world, so a
completely different system is needed. Whether 802.16 could be used for mobile devices in the future is an interesting question. It was not optimized for them, but the possibility is there. For the moment it is focused on fixed wireless.