The IEEE 802.11 Standard — in English

Jul 29, 1999

Now that the iBook has introduced us to yet another technology by way of AirPort, I thought it might interest some readers to find out more about what's involved in the wireless communications it affords iBook users, and what the IEEE 802.11 standard and Direct Sequence Spread Spectrum are all about — in simpler terms.

But before that, here's some information about radio communications.

Radio Wave Propagation
Radio waves in the UHF (300MHz to 3GHz) and VHF (30MHz to 300MHz) bands, are typically dependent on line-of-sight for effective communication, and the manner that they overcome physical obstacles along the transmission path is more by deflection rather than by actually penetrating through the obstacles themselves. In other words, the radio signals emitted from a transmitting antenna will bounce off and around incidental obstacles all along the transmission path until they eventually reach the receiving antenna. More often than not, the antenna of a transmitting station would be arranged so that it sends out radio signals omni-directionally for maximum signal propagation.

I mention this because I believe that in using AirPort's (ostensibly UHF) wireless LAN technology, the ability of the radio signals to actually penetrate through solid walls only appears that way, or is at best limited, although I could be mistaken. In any case, it is academic, and transparent to the user, as the end result is the same: iBooks will typically be able to communicate wirelessly across different rooms up to 150 feet apart, just like a cordless phone handset.

By comparison, lower radio frequencies like HF (3MHz to 30MHz) are less directional and are able to "bend" or refract even through the atmosphere, like a small pebble that you skim across a body of water, due to the difference in air density between various altitudes. This explains how ships or radio ham operators are able to communicate over vast distances beyond the horizon; the HF radio signals can be beamed up into the upper reaches of the atmosphere and refracted eventually back to earth. But I digress.

The IEEE 802.11 Standard
The IEEE 802.11 standard works on a "first listen then talk" basis. This means that an iBook in an AirPort network wishing to send out data must first determine if the radio channel is clear or if another iBook is currently transmitting. If the channel is clear, the data gets sent out. There is a minimum time gap between transmissions permitted from each iBook, meaning no single iBook hogs the channel continuously. Once a transmission has been sent out from an iBook, it must wait till this time gap is up before trying to transmit again. Once the time has passed, a further random amount of time (called a backoff interval) elapses before that iBook "listens" again to verify a clear channel on which to transmit. If the channel is still busy, another backoff interval is selected that is less than the first. This process repeats itself until the waiting time approaches zero and the iBook is eventually allowed to transmit again. Conversely, since only one iBook can send out a transmission at any one time slot, it stands to reason that the others in the network will receive incoming data nonconcurrently, that is, only from one source at a time. Of course, all this traffic management is happening at very rapid speeds among the iBooks in the network as they time-share the same channel, which ensures that the channel is efficiently utilized while at the same time avoiding collisions.

Direct Sequence Spread Spectrum
Direct Sequence Spread Spectrum (DSSS) is one of three transmission techniques under the IEEE 802.11 standard, the others being Frequency Hopped Spread Spectrum (FHSS) and Diffused Infra-red (DFIR). Both DSSS and FHSS (of which DSSS is the preferred choice for AirPort) operate in the 2.4GHz band (as does the AirPort, as confirmed by an Apple Singapore representative), because of its wide availability in many countries and the lower hardware costs in comparison to the higher microwave frequencies.

I shall now try to explain as simply as possible how DSSS works, from the technical information I have gleaned:

Spread spectrum communications is a means of transmitting a signal over a much wider frequency bandwidth than the minimum bandwidth normally required to transmit the information. The minimum is for the spread spectrum to have a bandwidth of at least 10 times the information bandwidth.

A typical radio signal contains both the data itself (which is the useful content) and a carrier frequency, which is modulated or blended with the data signal in order to "carry" the transmission across whatever would be the operating range of the transmitter. In AirPort's case, that would be up to 150 feet.

In DSSS transmissions, another element is introduced, which is a signal called a pseudo-noise (PN) code sequence. This is a binary — and hence digital — code sequence which, when modulated with the carrier frequency and original content, causes the resultant signal to spread across a much wider frequency spectrum, whereas the original radio signal would have occupied only a specific radio frequency. This has the resultant effect of dissipating the signal intensity over a broad range of frequencies, thus shrouding the transmitted signal, and making it indistinguishable from random white noise.

At the receiver end, in a process known as "correlation", a similar pseudo-noise code sequence matching exactly the one used by the transmitter is generated in order to "decode" the transmission by reconstituting the spread spectrum signal into intelligible information again. Naturally, without this code sequence, the spread spectrum signal is useless.

Therein lies the security-enhancing implications of DSSS transmissions, which explains why there is military interest in the technology. Because DSSS transmissions are harder to detect, there is a lower probability of interception. Because it does not occupy specific radio frequencies, it is harder to jam. And because it employs binary code sequences to "encrypt" the transmitted data, it makes it hard for unauthorized parties to "listen in", or to spoof or imitate network members.

Code sequence selection has a bearing on overall link performance. For instance, long codes ultimately provide better reception quality, and possibly better security, but the drawbacks include increased bandwidth which would require more time to transmit (not good for battery-powered devices), and expensive hardware required.

The spread of energy over a wide range of frequencies makes it less likely for spread spectrum signals and typical narrow band communications to interfere with each other, even when they occupy the same frequency range. So DSSS systems could quite safely work in the same environment as other communications devices without too much risk of mutual interference.

The FCC Issue
As you may have read, the launch of iBook in September is contingent upon the FCC granting its approval of the use of AirPort's technology. This article, Basics of a Direct Sequence Spread Spectrum System for Wireless Communications (last updated June 4, 1999), mentions that the FCC licenses a band of frequencies from 902 to 928 MHz for spread spectrum use only and requires no license for transmissions under one watt.

According to a recent article by John Martellaro (The iBook — Young at Heart), the transmission power output of the iBook is a mere 31 milliwatts, so transmission power evidently isn't an issue. As I've mentioned earlier on, the AirPort operats in the 2.4 GHz band, and although that is the prescribed frequency band for DSSS use, interestingly enough, it falls outside the licensed frequency band stipulated by the FCC for spread spectrum use.

Could that be holding things up?

Further reading:

The IEEE 802.11 Standard

Direct Sequence Spread Spectrum (DSSS)

C&K Systems Direct Sequence Spread Spectrum

ABCs of Spread Spectrum

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