The IEEE 802.11 Wireless LAN Standard (continued)

Infra-Red Physical Layer
One infrared standard is supported which operates in the 850-to-950nM band with peak power of 2 W. The modulation for infrared is accomplished using either 4 or 16-level pulse-positioning modulation. The physical layer supports two data rates; 1 and 2Mbps.

Direct Sequencing Spread Spectrum (DSSS) Physical Layer
The DSSS physical layer uses an 11-bit Barker Sequence to spread the data before it is transmitted. Each bit transmitted is modulated by the 11-bit sequence. This process spreads the RF energy across a wider bandwidth than would be required to transmit the raw data. The processing gain of the system is defined as 10x the log of the ratio of spreading rate (also know as the chip rate) to the data. The receiver despreads the RF input to recover the original data. The advantage of this technique is that it reduces the effect of narrowband sources of interference. This sequence provides 10.4dB of processing gain which meets the minimum requirements for the rules set forth by the FCC. The spreading architecture used in the direct sequence physical layer is not to be confused with CDMA. All 802.11 compliant products utilize the same PN code and therefore do not have a set of codes available as is required for CDMA operation.

Frequency Hopping Spread Spectrum (FHSS) Physical Layer
The FHSS physical layer has 22 hop patterns to choose from. The frequency hop physical layer is required to hop across the 2.4GHz ISM band covering 79 channels. Each channel occupies 1Mhz of bandwidth and must hop at the minimum rate specified by the regulatory bodies of the intended country. A minimum hop rate of 2.5 hops per second is specified for the United States.

Each of the physical layers use their own unique header to synchronize the receiver and to determine signal modulation format and data packet length. The physical layer headers are always transmitted at 1Mbps. Predefined fields in the headers provide the option to increase the data rate to 2 Mbps for the actual data packet.

The MAC Layer
The MAC layer specification for 802.11 has similarities to the 802.3 Ethernet wired line standard. The protocol for 802.11 uses a protocol scheme know as carrier-sense, multiple access, collision avoidance (CSMA/CA). This protocol avoids collisions instead of detecting a collision like the algorithm used in 802.3. It is difficult to detect collisions in an RF transmission network and it is for this reason that collision avoidance is used. The MAC layer operates together with the physical layer by sampling the energy over the medium transmitting data. The physical layer uses a clear channel assessment (CCA) algorithm to determine if the channel is clear. This is accomplished by measuring the RF energy at the antenna and determining the strength of the received signal. This measured signal is commonly known as RSSI. If the received signal strength is below a specified threshold the channel is declared clear and the MAC layer is given the clear channel status for data transmission. If the RF energy is above the threshold, data transmissions are deferred in accordance with the protocol rules. The standard provides another option for CCA that can be alone or with the RSSI measurement. Carrier sense can be used to determine if the channel is available. This technique is more selective sense since it verifies that the signal is the same carrier type as 802.11 transmitters. The best method to use depends upon the levels of interference in the operating environment. The CSMA/CA protocol allows for options the can minimize collisions by using request to send (RTS), clear-to-send (CTS), data and acknowledge (ACK) transmission frames, in a sequential fashion. Communications is established when one of the wireless nodes sends a short message RTS frame. The RTS frame includes the destination and the length of message. The message duration is known as the network allocation vector (NAV). The NAV alerts all others in the medium, to back off for the duration of the transmission. The receiving station issues a CTS frame which echoes the senders address and the NAV. If the CTS frame is not received, it is assumed that a collision occurred and the RTS process starts over. After the data frame is received, an ACK frame is sent back verifying a successful data transmission. A common limitation with wireless LAN systems is the "hidden node" problem. This can disrupt 40% or more of the communications in a highly loaded LAN environment. It occurs when there is a station in a service set that cannot detect the transmission of another station to detect that the media is busy. In figure 1 stations A and B can communicate. However an obstruction prevents station C from receiving station A and it cannot determine when the channel is busy. Therefore both stations A and C could try to transmit at the same time to station B. The use of RTS, CTS, Data and ACK sequences helps the prevent the disruptions caused by this problem.

Figure 1

Security provisions are addressed in the standard as an optional feature for those concerned about eaves dropping. The data security is accomplished by a complex encryption technique know as the Wired Equivalent Privacy Algorithm (WEP). WEP is based on protecting the transmitted data over the RF medium using a 64-bit seed key and the RC4 encryption algorithm. WEP, when enabled, only protects the data packet information and does not protect the physical layer header so that other stations on the network can listen to the control data needed to manage the network. However, the other stations cannot decrypt the data portions of the packet.

Power management is supported at the MAC level for those applications requiring mobility under battery operation. Provisions are made in the protocol for the portable stations to go to low power "sleep" mode during a time interval defined by the base station.

What the future holds
The IEEE 802.11 WLAN standard will be one of the first generations of standardization for wireless LAN networks. This standard will set the pace for the next generation standard, addressing the demands for higher performance higher data rates and higher frequency bands. Interoperability between WLAN products from different equipment manufacturers will be important to the success of the standard. These products will be implemented on ISA, or PCMCIA cards for use in handheld personal computers, PDAs, laptops or desktop applications. Wireless LAN applications are currently mostly in vertical markets. It is expected that many horizontal applications will follow as 802.11 network infrastructure is installed. Over time the increase in demand for 802.11 products is expected to increase competition and to make wireless LANs more competitive and economical for virtually all applications requiring wireless connectivity. On the horizon is the need for higher data rates, for applications requiring wireless connectivity at 10Mbps and higher. This will allow WLANs to match the data rate of the majority of wired LANs. There is no current definition of the characteristics for the higher data rate signal. However, for many of the options available to achieve it there is a clear upgrade path for to maintain interoperability with 1 and 2 Mbps systems while providing the higher data rate as well.


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