Wireless Networks first-step

Wireless Networks first-step

Chapter3:RF Fundamentals

A wireless transceiver consists of a transmitter and a receiver. In the transmitter, a process known as modulation converts electrical digital signals inside a computer into either RF or light, which are analog signals. Amplifiers then increase the magnitude of the signals prior to departing an antenna. At the destination, a receiver detects the relatively weak signals and demodulates them into data types applicable to the destination computer.

signal that feeds the antenna has an amplitude, frequency, and phase.

The amplitude indicates the strength of the RF signal. The measure for amplitude is generally power

The frequency describes how many times per second that the signal repeats itself. The unit for frequency is Hertz (Hz), which is the number of cycles occurring each second. For example, an 802.11b wireless LAN operates at a frequency of 2.4 GHz, which means that the signal includes 2,400,000,000 cycles per second.

The phase corresponds to how far the signal is offset from a reference point.A strong advantage of representing data as phase shifts is that impairments resulting from the propagation of the signal through the air don't have much impact. Impairments generally affect amplitude, not the signal phase.

RF signals encounter impairments, such as interference and multipath propagation.Diversity is the use of two antennae for each radio NIC to increase the odds of receiving a better signal on either of the antennae.

A modulator mixes the source information signal, such as voice or data, with a carrier signal. The transceiver couples the resulting modulated and amplified signals to an antenna. The modulated signal departs the antenna and propagates through the air. The receiving station antenna couples the modulated signal into a demodulator, which derives the information signal from the radio signal carrier.

Modulation for RF systems is more complex and covered in the following sections.
Frequency Shift-Keying,Phase Shift-Keying,Quadrature Amplitude Modulation,Spread Spectrum(Direct sequence modulates a radio carrier by a digital code with a bit rate much higher than the information signal bandwidth. Frequency hopping quickly hops the radio carrier from one frequency to another within a specific range.),Orthogonal Frequency Division Multiplexing,Ultrawideband Modulation

The ISM bands are located at 902 MHz, 2.400 GHz, and 5.7 GHz. RF systems operating in the ISM band must use spread spectrum modulation and operate below 1 watt transmitter output power.

Chapter4: Wireless PANs

Wireless PAN technologies utilize both radio frequencies and infrared light, depending on the application.

802.15:Bluetooth transceivers operate at up to 1 Mbps data rate in the 2.4GHz band, using FHSS technology. It constantly hops over the entire spectrum at a rate of 1,600 hops per second, which is much faster than the 802.11 version of frequency hopping.Low-power Bluetooth devices have a range of 30 feet. High-power Bluetooth devices, however, can reach distances of around 300 feet.A critical problem is that Bluetooth and 802.11b neither understand each other nor follow the same rules.

IrDA offers up to 4 Mbps data rates. This version of the standard has up to 3 feet.the IrDA control version of the standard reduces data rates to 75 kbps. In addition, the host computer can communicate with up to eight peripherals simultaneously.

Chapter5:Wireless LANs

802.11 and HiperLAN/2 are the most common standards for wireless LANs.

802.11 defines two forms of medium access: distributed coordination function (DCF) and point coordination function (PCF). DCF is mandatory and based on the CSMA/CA protocol.For supporting time-bounded delivery of data frames such as video, the 802.11 standard defines the optional PCF where the access point grants access to an individual station to the medium by polling the station during the contention-free period. Stations can't transmit frames unless the access point polls them first. The period of time for PCF-based data traffic (if enabled) occurs alternately between contention periods.

The following sections summarize primary 802.11 MAC functions.
Scanning
The 802.11 standard defines both passive and active scanning, Passive scanning is mandatory,Periodically, access points broadcast a beacon, and the radio NIC receives these beacons while scanning and takes note of the corresponding signal strengths,use this information along with the signal strength to compare access points and decide on which one to use.
Optional active scanning is the radio NIC initiates the process by broadcasting a probe frame, and all access points within range respond with a probe response.
Authentication
Authentication is the process of proving identity, and the 802.11 standard specifies two forms: open system authentication and shared key authentication. Open system authentication is mandatory, and it's a two-step process.Shared key authentication is an optional four-step process that bases authentication on whether the authenticating device has the correct WEP key.
Association
Association is necessary to synchronize the radio NIC and access point with important information, such as supported data rates. The radio NIC initiates the association by sending an association request frame containing elements such as SSID and supported data rates. The access point responds by sending an association response frame containing an association ID along with other information regarding the access point.
RTS/CTS
The optional request-to-send and clear-to-send (RTS/CTS) function allows the access point to control use of the medium for stations activating RTS/CTS.
Power Save Mode
The radio NIC indicates its desire to enter a sleep state to the access point through a status bit located in the header of each frame. The access point takes note of each radio NIC wanting to enter power save mode and buffers packets corresponding to the sleeping station.In order to still receive data frames, the sleeping NIC must wake up periodically (at the right time) to receive regular beacon transmissions coming from the access point.After receiving the frames, the radio NIC can go back to sleep.
Fragmentation

802.11 Physical Layers
Initial 802.11
The initial 802.11 standard ratified in 1997 includes frequency hopping spread spectrum (FHSS) and direct sequence spread spectrum (DSSS) physical layers operating in the 2.4-GHz band with data rates of up to 2 Mbps.FHSS doesn't interoperate with any of the other 802.11 physical layers. FHSS does, however, provide a very solution for outdoor, point-to-multipoint systems.DSSS interoperates with the newer 802.11b physical layer.
802.11a
Toward the end of 1999, the IEEE released 802.11a, which defines operation in the 5-GHz band using Orthogonal Frequency Division Multiplexing (OFDM) with data rates up to 54 Mbps.A strong advantage of 802.11a is that it offers the highest capacity because of 12 separate, non-overlapping channels. A potential issue of 802.11a is limited range,less than 100 feet in most facilities.
802.11b
A higher-rate extension to the initial direct sequence standard in the 2.4-GHz band—with data rates up to 11 Mbps,a range of 300 feet.Disadvantages of 802.11b are that you're limited to three non-overlapping channels in the 2.4 GHz band(channel 1,6,11) and the potential for RF interference from other radio devices.
802.11g
IEEE ratified the 802.11g standard in 2003, which is compatible with 802.11b and increases performance up to 54 Mbps in the 2.4-GHz band using OFDM.have the same cons with 802.11b.
HiperLAN/2(802.11h possible)
HiperLAN/2, which stands for High Performance Radio LAN, is a wireless LAN standard developed by the Broadband Radio Access Networks (BRAN) division of the European Telecommunications Standards Institute (ETSI).HiperLAN/2 has a physical layer that is similar to IEEE's 802.11a, which operates at up to 54 Mbps in the 5-GHz band using OFDM. A major difference with HiperLAN/2 is the use of a connection-oriented protocol with time division multiplexing(TDMA) as the basis for supporting data transfer between users.

Chapter6: Wireless MANs

Antennae
directional antennae,omnidirectional antenna,Semidirectional Antennae,Yagi antenna,highly directional antennae.Different types of antennae have different vertical and horizontal beamwidths,the narrower the beamwidth, the longer the range when transmit power is kept constant.Antenna polarization is the physical orientation of the antenna along a horizontal or vertical plane.To maximize the transfer of RF energy from the transmitter to the receiver antenna, both antennae should have the same polarization.

Point-to-Point,Point-to-Multipoint,Packet Radio Systems

802.16 supports point-to-multipoint architecture in the 10–66 GHz range, transmitting at data rates up to 120 Mbps.IEEE published 802.16a in January 2003, which includes support for mesh architecture. 802.16a operates in the licensed and unlicensed frequencies between 2–11 GHz using Orthogonal Frequency Division Multiplexing (OFDM).802.16e is under way.

Wimax ????

Chapter7: Wireless WANs

Take a closer look at the different modulation techniques.

Frequency Division Multiple Access
Frequency division multiple access (FDMA) divides a wide-frequency band into smaller subbands, where each user transmits voice and data over their assigned subband. All users transmit their signals simultaneously.Traditional 1G cellular systems use FDMA for sending data.
Time Division Multiple Access
Time division multiple access (TDMA) keeps users separate by allowing only one user to transmit at any give time. Each user has an assigned time slot for transmission. Some of the older telecommunications operators utilize TDMA to offer voice and data connections over wireless WANs. For example, T1 circuits make use of TDMA for combining separate user connections over the same circuit.
Code Division Multiple Access
Similar to FDMA, code division multiple access (CDMA) allows simultaneous transmissions.The difference, however, is that CDMA users can occupy the entire frequency band at the same time. The users do not experience any interference, because each user modulates her signals using a different code. An advantage of CDMA is that user devices can connect to multiple base stations because of separate codes. This increases performance and reliability. Cellular systems predominately make use of CDMA wireless networks.
Spatial Division Multiple Access (SDMA)
SDMA accommodates multiple users by focusing a beam for each user. This is common in satellite systems.