The 802.11 standard includes a set of amendments for wireless LAN. The amendments mainly differ in modulation techniques, frequency range and quality of service (QoS). Like all standards of the IEEE 802, the IEEE 802.11 covers the first two layers of the OSI model (Open Systems Interconnection), that is to say the physical layer (L1) and data-link layer (L2). The section below will describe what each of those layers implies in terms of wireless standards.
The physical layer has as a role to transport correctly the signal corresponding to 0 and 1 of the data that the transmitter wishes to send to the receiver.
The physical later deals mainly with:
An important parameter that influences the data transfer of certain standard is the choice of modulation technique. The more efficient the data is encoded, the higher bit rate can be achieved. On the other hand, an efficient modulation technique also requires more sophisticated hardware to handle the modulation and de-modulation of the data.
The basic and common idea behind the different modulation techniques used in IEEE 802.11 is to use more bandwidth that is theoretically needed to send one “bit” to achieve resistance against interference. The way that the information is spread leads to different modulation techniques. The most common ones are presented below.
FHSS (Frequency Hopping Spread Spectrum) FHSS is based on the concept of transmitting on one frequency for a certain time, then randomly jumping to another. i.e. the carrying frequency (carrier) changes during the times or that the transmitter periodically changes frequency according to a pre-established sequence. The transmitter synchronizes the receiver thanks to beacons which contain the sequence of jumps and the duration. In the IEEE 802.11 standard, the defined frequency band (ISM) that spans from 2,400 to 2,4835 GHz is divided into 79 channels of 1 MHz and the jump is made every 300 to 400 ms. Hops are made around a central frequency that corresponds to one of the 14 defined channels. This modulation is not common anymore in current products.
DSSS (Direct Sequence Spread Spectrum) DSSS (Direct Sequence Spread Spectrum) implies that for each bit of data, a sequence of bits (sometimes called pseudo-random noise, noted PN) must be transmitted. Each bit that is 1 is replaced by a sequence of bits and each bit being 0 replaced by its complement. The 802.11 physical layer standard defines a sequence of 11 bits (10110111000) to represent a “1” and its complement (01001000111) to represent a “0”. in DSSS, instead of splitting a data signal into pieces send in different frequencies, each data bit is encoded into a longer bit string, called a chip. This modulation technique has been common from 1999 to 2005.
OFDM (Orthogonal Frequency-Division Multiplexing) ODFM, also sometimes called discrete multi-tone modulation (DMT) is a modulation technique based on the idea of frequency division multiplexing (FDM).
FDM, that is used both in radio and TV, is based on the concept that multiple signals are sent out at the same time but on different frequencies. In OFDM, a single transmitter transmits on many (dozens to thousands) different orthogonal frequencies. Orthogonal frequencies, are frequencies that are independent with respect to the relative phase relationship between the frequencies. OFDM involves the usage of advanced modulation techniques in each component which results in a signal with high resistance to interference.
An OFDM carrier signal is the sum of a number of orthogonal sub-carriers, with each sub-carrier being independently modulated commonly using some type of QAM or PSK. This is the most common modulation technique from 2005.
802.11b and 802.11g use the 2.4 Ghz ISM (Industrial, Scientific, Medical) frequency band defined by the ITU. In specific, the "L" BAND ranging from 2 400 to 2 483,5 MHz is used.
The 802.11a standard is using the 5 Ghz band UNII (Unlicensed-National Information Infrastructure) covering 5.15-5.35 GHz and 5.725-5.825 Ghz.
The unlicensed 2.4 Ghz band has lately become very noise in urban areas due to the high penetration of WLAN and other devices that are communicating in the same frequency range, such as microwave ovens, cordless phones and Bluetooth devices. The 5 GHz band gives the advantage of less interference but faces other problems due to its nature. High frequency radio waves are more sensitive to absorption than low frequency waves. Waves in the range of 5 Ghz are specially sensitive to water and surrounding buildings or other objects due to the higher adsorption rate in this range. This means that a 802.11a network is more restricted when it comes to line of sight and more access points might be needed to cover the same area as a 802.11b-based network since 802.11a. for same amount of output power, provides smaller cells.
The data link layer of 802.11, is composed of two parts:
The 802.11 LLC sublayer is identical to layer 802.2 allowing a compatibility with any other network 802, while the MAC sublayer is redefined by standard 802.11 (L2).
MAC characterizes the access to the media in a way common to the various 802.11 standards. It is equivalent to the standard 802.3 (CSMA/CD – Ethernet) for wired networks, with functionalities specific to radio transmissions (the error rate is higher than the copper media) which are normally entrusted to the higher protocols, like fragmentation, error control (CRC), the retransmissions of frames and the acknowledgment of delivery .
802.11b uses a protocol slightly modified compared to the CSMA/CD, called CSMA/CA (Collision Detection vs Collision Avoidance). CSMA/CA can avoid the collisions by using a basic polling method known as RTS/CTS in which the sender sends first a request to send (RTS) and the receiver (usually the AP) acknowledge the request by sending a clear to send (CTS) message when channel is ready to use.
During transmission between two pieces of equipment, the destination station checks the CRC of the frame and returns an ACK (acknowledgment of delivery) to the transmitter. If the transmitting station does not receive this ACK in time, it assumes that a collision occurred and the frame is retransmitted after receiving a new CTS.
The access to the media is controlled by the use of different type of interframe spaces (IFS), which corresponds to the intervals of time that an station needs to wait before sending data. High priority data as ACKs or RTS/CTS packets will wait a shorter (SIFS) time period that normal traffic.
While the CSMA/CA protocol permits to avoid collisions in a shared radio channel, mechanisms as RTS/CTS increases overhead (signaling frames that are necessary for the network but contain no user data) and can therefore never make the performance of 802.11b as good as CSMA/CD (collision detection) or TDMA-based technologies (think of Ethernet in a cable or synchronized E1/T1).
For further reading, see unit “Advanced Wireless Networking”.