Wi-Fi

Wi-Fi is a trademark of the Wi-Fi Alliance denoting any of several IEEE wireless-networking protocols in the 802.11 family, specifically 802.11a, 802.11b, 802.11g, 802.11n, 802.11ac and 802.11ax. (Strictly speaking, these are all amendments to the original 802.11 standard, but they are also de facto standards in their own right.) Like classic Ethernet, Wi-Fi must deal with collisions; unlike Ethernet, however, WiFi is unable to detect collisions in progress, complicating the backoff and retransmission algorithms.

 Unlike any wired LAN protocol we have considered so far, in addition to normal data packets Wi-Fi also uses control and management packets that exist entirely within the Wi-Fi LAN layer; these are not initiated by or delivered to higher network layers. Control packets are used to compensate for some of the infelicities of the radio environment, such as the lack of collision detection. Putting radio-essential control and management protocols within the Wi-Fi layer means that the IP layer can continue to interact with the Wi-Fi LAN exactly as it did with Ethernet; no changes are required. Wi-Fi is designed to interoperate freely with Ethernet at the logical LAN layer.

Wi-Fi MAC (physical) addresses have the same 48-bit size as Ethernet’s and the same internal structure. They also share the same namespace: one is never supposed to see an Ethernet and a Wi-Fi interface with the same address. As a result, data packets can be forwarded by switches between Ethernet and Wi-Fi; in many respects a Wi-Fi LAN attached to an Ethernet LAN looks like an extension of the Ethernet LAN.

Microwave Ovens and Wi-Fi The impact of a running microwave oven on Wi-Fi signals is quite evident if the oven is between the sender and receiver. For other configurations the effect may vary. Most ovens transmit only during one half of the A/C cycle, that is, they are on 1/60 sec and then off 1/60 sec; this may allow intervening transmission time.

Traditionally, Wi-Fi used the 2.4 GHz ISM (Industrial, Scientific and Medical) band used also by microwave ovens, though 802.11a used a 5 GHz band, 802.11n supports that as an option and the new 802.11ac has returned to using 5 GHz exclusively. The 5 GHz band has reduced ability to penetrate walls, often resulting in a lower effective range (though in offices and multi-unit housing this can be an advantage). The 5 GHz band provides many more usable channels than the 2.4 GHz band, resulting in much less interference in “crowded” environments.

Wi-Fi radio spectrum is usually unlicensed, meaning that no special permission is needed to transmit but also that others may be trying to use the same frequency band simultaneously. The availability of unlicensed channels in the 5 GHz band continues to improve.

The table below summarizes the different Wi-Fi versions. All data bit rates assume a single spatial stream; channel widths are nominal. The names in the far-right column have been introduced by the Wi-Fi Alliance as a more convenient designation for the newer versions.

The maximum bit rate is seldom achieved in practice. The effective bit rate must take into account, at a minimum, the time spent in the collision-handling mechanism. More significantly, all the Wi-Fi variants above use dynamic rate scaling, below; the bit rate is reduced up to tenfold (or more) in environments with higher error rates, which can be due to distance, obstructions, competing transmissions or radio noise. All this means that, as a practical matter, getting 150 Mbps out of 802.11n requires optimum circumstances; in particular, no competing senders and unimpeded line-of-sight transmission. 802.11n lower-end performance can be as little as 10 Mbps, though 40-100 Mbps (for a 40 MHz channel) may be more typical.

The 2.4 GHz ISM band is divided by international agreement into up to 14 officially designated (and mostly adjacent) channels, each about 5 MHz wide, though in the United States use may be limited to the first 11 channels. The 5 GHz band is similarly divided into 5 MHz channels. One Wi-Fi sender, however, needs several of these official channels; the typical 2.4 GHz 802.11g transmitter uses an actual frequency range of up to 22 MHz, or up to five official channels. As a result, to avoid signal overlap Wi-Fi use in the 2.4 GHz band is often restricted to official channels 1, 6 and 11. The end result is that there are generally only three available Wi-Fi bands in the 2.4 GHz range, and so Wi-Fi transmitters can and do interact with and interfere with each other.

There are almost 200 5 MHz channels in the 5 GHz band. The United States requires users of this band to avoid interfering with weather and military applications in the same frequency range; this may involve careful control of transmission power (under the IEEE 802.11h amendment) and so-called “dynamic frequency selection” to choose channels with little interference, and to switch to such channels if interference is detected later. Even so, there are many more channels than at 2.4 GHz; the larger number of channels is one of the reasons (arguably the primary reason) that 802.11ac can run faster (below). The number of channels available for Wi-Fi use has been increasing, often as conflicts with existing 5 GHz weather systems are resolved.

802.11ax has preliminary support for additional frequency bands in the 6-7 GHz range, though these are still awaiting (in the US) final FCC approval.

Wi-Fi designers can improve throughput through a variety of techniques, including

1. improved radio modulation techniques

2. improved error-correcting codes

3. smaller guard intervals between symbols

4. increasing the channel width

5. allowing multiple spatial streams via multiple antennas

The first two in this list seem now to be largely tapped out; OFDM modulation (3.6.4.1 OFDM) is close enough to the Shannon-Hartley limit that there is limited room for improvement, though 802.11ax saw fit to move to ODFMA. The third reduces the range (because there is less protection from multipath interference) but may increase the data rate by ~10%; 802.11ax introduced support for dynamic changing of guard-interval and symbol size. The largest speed increases are obtained the last two items in the list.

The channel width is increased by adding additional 5 MHz channels. For example, the 65 Mbps bit rate above for 802.11n is for a nominal frequency range of 20 MHz, comparable to that of 802.11g. However, in areas with minimal competition from other signals, 802.11n supports using a 40 MHz frequency band; the bit rate then goes up to 135 Mbps (or 150 Mbps if a smaller guard interval is used). This amounts to using two of the three available 2.4 GHz Wi-Fi bands. Similarly, the wide range in 802.11ac bit rates reflects support for using channel widths ranging from 20 MHz up to 160 MHz (32 5-MHz official channels).

Using multiple spatial streams is the newest data-rate-improvement technique;

For all the categories in the table above, additional bits are used for error-correcting codes. For 802.11g operating at 54 Mbps, for example, the actual raw bit rate is (4/3)ˆ54 = 72 Mbps, sent in symbols consisting of six bits as a unit.