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Transmission channels

A transmission channel is a narrow frequency band that can be used for communication. In every country, the government generally regulates use of the radio spectrum, as it is the largest user of the spectrum due to military usage.

However, governments also make frequency bands available for unlicensed use. The groups in charge of regulating the use of radio frequencies are:

• The ETSI (European Telecommunications Standards Institute) in Europe
• The FCC (Federal Communications Commission) in the United States
• The MKK (Kensa-kentei Kyokai) in Japan

In Europe, the 890 to 915 MHz band is used for mobile communications (GSM), and only the bands from 2.400 to 2.4835 GHz and 5.725 to 5.850 GHz are available for amateur radio use.

Transmission technologies

Local radio networks use radio or infrared waves in order to transmit data. The technology used for sending radio transmissions is called narrowband transmission, which runs different communication signals through different channels. However, radio transmissions are often subject to numerous limitations, which makes this type of transmission insufficient. Among these limitations:

• Different stations within the same cell involunatarily sharing bandwidth.
• Multipath propagation of radio waves. A radio wave can propagate in different directions and possibly be reflected or refracted by physical objects, so a receiver might receive the same information several instants apart. This would result from those signals taking different paths after being reflected several times.

For that reason, the physical layer of the 802.11 standard defines several transmission techniques for minimising interference problems:

• Frequency hopping spread spectrum,
• Direct-sequence spread spectrum,
• Infrared technology.

Narrowband

The narrowband technique involves using a specified radio frequency for transmitting and receiving data. The frequency band used must be as small possible in order to limit interfering with adjacent bands.

Spread spectrum

The IEEE 802.11 standard allows for two frequency modulation techniques, developed for the military, to transmit data. These techniques, called spread spectrum, involve using wide frequency bands for low-power data transmission. There are two spread spectrum technologies:

• Frequency hopping spread spectrum,
• Direct-sequence spread spectrum

Frequency-hopping

Frequency-hopping spread spectrum, or FHSS, involves splitting the wideband frequency into at least 75 discrete channels (with these "hops" each 1MHz apart), then transmitting it by using a combination of channels known to all stations in the cell. Under the 802.11 standard, the frequency band between 2.4 and 2.4835 GHz allows for 79 discrete 1 MHz channels. The transmission is carried out by broadcasting on one channel after another, using each channel for only a short period of time (about 400 ms), which enables a more easily recognisable signal to be transmitted at a given moment on a given frequency.

Frequency hopping spread spectrum was originally dseigned for military use in order to prevent radio transmissions from being listened to. A station which does not know what frequency combination to use could not listen to the signal, because it would be impossible for it to determine the frequency the signal was being transmitted on and then find the new frequency within the short time window.

Today, local networks with this technology are standard. Because the sequence of frequencies used is universally known, frequency-hopping spread spectrum is no longer a secure way of transferring data. On the other hand, FHSS is still used in the 802.11 standard in order to reduce interference between the various station of a cell.

Direct-sequence spread spectrum

The technique called Direct-sequence spread spectrum (DSSS for short) involves trasmitting a Barker bit sequence (sometimes called pseudo-random noise, or PN for short) for each bit sent. In this operation, each bit which is set to 1 is replaced with a bit sequence, and each bit set to 0 is replaced with its complement.

The physical layer of the 802.11 standard defines an 11-bit sequence (10110111000) to represent 1, and its complement (01001000111) to encode a 0. Each bit which is encoded with this sequence is called a chip or chipping code. This technique (called chipping) modulates every bit with the Barker sequence.

With chipping, redundant information is sent, and this allows error checks, and even error correction, to be performed on transmissions.

In the 802.11b standard, the 2.400-2.4835 GHz frequency band (83.5 MHz wide) has been split into 14 separate channels of 5 MHz each. Only the first 11 may be used in the United States and Canada. Only channels 1 to 13 may be used in the United Kingdom. These are the frequencies associated with the 14 channels:

However, for a proper 11 Mbps transmission, it is necessary to transmit on a 22MHz band because, according to Shannon's theorem, the sampling rate must be at least twice the signal to be digitised. Certain channels overlap neighboring channels. For this reason, isolated channels (1, 6, and 11) which are 25 MHz apart are generally used.

Thus, if two access points using the same channels have overlapping broadcast areas, signal distortions may disrupt transmissions. In order to avoid any such interference, it is recommended to distribute access points and select channels in such a way that two access points using the same channels are never close to one another.

The 802.11a standard uses the frequency bands of 5.15GHz to 5.35GHz and 5.725 GHz to 5.825 GHz, which enables it to define 8 distinct channels each 20MHz wide, a sufficiently wide band to avoid having channels interfere with one another.

Infrared technology

The IEEE 802.11 standard also provides for an alternative to radio waves: infrared light. The primary feature of infrared technology is the use of a light wave to transmit data. These transmissions travel mono-directionally, whether by using a direct line of sight or reflected off a surface. The non-diffuse nature of light waves offers a higher level of security.

With infrared technology, it is possible to send data at 1 to 2 Mbits per second by using a kind of modulation called PPM (pulse position modulation).

PPM modulation involves transmitting constant-amplitude pulses and encoding information based on its pulse position. A transfer speed of 1 Mbps is reached with 16-PPM modulation, while 2 Mbps is reached with 4-PPM modulation, which allows two bits of data to be encoded with four possible positions.

Modulation techniques

While ordinary radio uses frequency modulation (FM ) or amplitude modulation (AM), the 802.11b standard uses a modulation technique called PSK (for Phase Shift Keying). In this process, each bit undergoes a phase shift. A 180° shift is used to transfer at lower speeds (a technique called BPSK for Binary Phase Shift Keying) while a series of four 90° shifts (called QPSK for Quadrature Phase Shift Keying) allows transfers twice as fast.

Optimisation

The 802.11b standard uses other types of encoding to optimise transmission capacity. The two Barker sequences use two complementary 11-bit words and can define only two states (0 or 1).

An alternative method called CCK (complementary code keying) allows several bits of data to be directly encoded on a single chip by using eight 64-bit sequences. Therefore, by coding 4 bits at once, the CCK method can reach maximum speeds of 5.5 Mbps, or even 11 Mbps by coding 8 bits of data.

The technology PBCC (Packet Binary Convolutionnary Code) makes the signal more resilient to multipath distortion. The company Texas Instruments has successfully crafted a sequence that takes advantage of this increased resistance to interference, and allows for speeds of 22 Mbps. However, this technology, called 802.11b+, does not conform to IEEE 802.11b standards, which makes peripherals that support it incompatible with 802.11b devices.

The 802.11a standard operates in the 5 Ghz frequency band, which has 8 distinct channels. This is why an alternative transmission technique that makes use of different channels is available. OFDM (Orthogonal Frequency Division Multiplexing) allows for maximum speeds of 54 Mbps by sending data in parallel on different frequencies. Additionally, OFDM uses spectrum more efficiently.