A Field Guide to Wireless LANs for Administrators and Power Users

For certain applications, wired LANs of which the canonical example is Ethernet will always be superior to WLANs if the only means of comparison were raw throughput. For one thing, the wired LAN is much more resistant to radio-frequency (RF) interference, which is a major factor that can affect WLAN throughput. Twisted-pair wiring is extremely immune to RF interference.^[24] The highest-performance wired LANs are based on optical fiber and are the most resistant to both electrical and RF interference. Moreover, any advances in data modulation techniques can be applied equally well to both wired and wireless media.

^[24] However, the fact that there is still an electrical connection between two devices means that one device can still interfere with another; for example, if there is a power surge near one device, that electrical impulse may be carried to the other device and cause significant damage. However, the improved transmission media of twisted pair and optical fiber allow for far higher speeds that are likely to be achievable over a wireless medium.

Therefore, given the inherent advantages of wired media, notably excellent signal-to-noise ratio compared to almost any wireless medium you could imagine, it's very likely that wired media will always be faster than wireless media.

The frequency spectrum in which 11 Mbps WLANs operate is not dedicated to their use. A great benefit to easing deployment of WLANs is that they operate in unlicensed spectrum, but this means that there is no way to prevent nearby "operators" of WLAN devices from interfering with each other. The limited range of WLAN devices helps prevent interference (best case is outdoors, with no line-of-sight obstructions, at 150 meters or so; indoors, usable WLANs rarely have a radius of more than 20 30 meters).

Even if WLAN operators had a guarantee that there were no other WLAN devices operating close enough to interfere with theirs, there would still be naturally occurring radio interference, especially given that modern PC CPUs typically operate near these frequencies and may become a source of RF noise (harmonics from a CPU clock running as slow as 1.2 GHz could generate 2.4 GHz noise, and CPU chips in the 2.4 GHz range are becoming more widely available in PCs targeted at the home user, but luckily, PC cases are usually well-shielded against leakage of RF energy). In fact, there are other sources of RF interference near the 2.4 GHz frequency band besides improperly shielded PC motherboards, including 2.4 GHz cordless telephones and microwave ovens.

Despite the lack of dedicated RF spectrum, the unlicensed nature of WLANs ensures their widespread deployment. The degree to which interference affects throughput will vary depending on the number of devices attempting to operate in the same area, as well as the prevalence of natural or man-made sources of RF interference. WLANs are remarkably robust over a usable radius, so that acceptable performance may be achieved even in less than perfect environments. Later in the book, we will see how the MAC protocol deals with the challenging RF transmission environment, through the use of explicit ACKs, retransmissions, MAC-layer fragmentation, and being able to switch to more robust (albeit slower) modulations.