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APPENDIX
1. 802.11 Network
From IEEE Std 802.11a-1999[1], IEEE Std 802.11h -2003[2], and 802.11 Wireless Networks[3], we can come out the 802.11 network structure, Media Access Control (MAC), Physical Layer (PHY) and 802.11 Management Architecture as following.
A. 802.11 Network Structure
The basic building block of an 802.11 wireless network is the Basic Service Sets (BSS), which is simply a group of stations that communicate with each other. BSSs come in two flavors, one is independent BSS (IBSS) and another is infrastructure BSS.
Stations (STAs) in an IBSS communication directly with each other without any access point (AP) and thus must be within direct communication range. Typically, IBSSs are composed of a small number of stations (STAs) set up for a specific purpose and for a short period of time. Due to their short duration, small size, and
focused purpose, IBSSs are sometimes referred to as ad hoc BSSs or ad hoc networks.
Infrastructure networks are distinguished by the use of an access point (AP). Access points are used for all communications in infrastructure networks, including communication between mobile nodes in the same service area. If one mobile station in an infrastructure BSS needs to communicate with a second mobile station, the communication must take two steps. First, the originating mobile station transfers the frame to the access point. Second, the access point transfers the frame to the destination station. 802.11 allows wireless networks of arbitrarily large size to be created by linking BSSs into an extended service set (ESS). An ESS is created by chaining BSSs together with a backbone network, such as Figure 1.
Fig.1. Extended Service Set (ESS) B. 802.11 Media Access Control (MAC)
802.11 standards successfully adapt Ethernet-style networking to radio links.
Like Ethernet, 802.11 uses a carrier sense multiple access (CSMA) scheme to control access to the transmission medium. However, collision waste valuable transmission capacity, so rather than the collision detection (CSMA/CD) employed by Ethernet, 802.11 uses collision avoidance (CSMA/CA).Also like Ethernet, 802.11 uses a distribution access scheme with no centralized controller. Carrier sensing is used to determine if the medium is available. Two types of carrier-sensing functions in 802.11 manage this process: the physical carrier-sensing and virtual carrier-sensing functions.
If either carrier-sensing function indicates that the medium is busy, the MAC reports this to higher layers. Physical carrier-sensing functions are provided by thy physical layer. Virtual carrier-sensing is provided by the Network Allocation Vector (NAV).
Most 802.11 frames carry a duration field, which can be used to reserve the medium
for a fixed time period. The NVA is a timer that indicates the amount of time the medium will be reserved. To prevent collisions, 802.11 allows stations to use Request to Send (RTS) and Clear to Send (CTS) signals to clear out an area.
Access to the wireless medium is controlled by coordination functions.
CSMA/CA access is provided by the distribution coordination functions. If contention-free service is required, it can be provided by the point coordination function (PCF). Point coordinators reside in access points, so the PCF is restricted to infrastructure networks.
C. 802.11 Physical Layer
The physical (PHY) layer is divided into two sublayers: the Physical Layer Convergence Procedure (PLCP) sublayer and the Physical Medium Dependent (PMD) sublayer. The PLCP is the glue between the frame of the MAC and the radio transmission in the air. It adds its own header. Normally, frames include a preamble to help synchronize incoming transmissions. The requirements of the preamble may depend on the modulation method, however, so the PLCP adds its own header to any transmitted frames. The PMD is responsible for transmitting any bits it receives from the PLCP into the air using the antenna. The physical layer also incorporates a clear channel assessment (CCA) function to indicate to the MAC when a signal is detected.
Many of the wireless devices that are currently on the market use the 2.4-GHz ISM band, which is rapidly becoming crowded. In an attempt to attain higher data rates and avoid overcrowding, the 802.11 working group is looking at the unlicensed bands around 5 GHz In the United States, the following three bands are called the Unlicensed National Information Infrastructure (UNII) bands:
z 5.15~5.25 GHz z 5.25~5.35 GHz z 5.725~5.825 GHz
Traditional radio communications focus on cramming as much signal as possible into as narrow band as possible. Spread spectrum works by using mathematical functions to diffuse signal power over a large range of frequencies. When the receiver performs the inverse operation, the smeared-out signal is reconstituted as a narrow-band signal, and, more importantly, any narrow-band noise is smeared out so the signal shines through clearly.
The radio-based physical layers in 802.11a use three different spread-spectrum:
z Frequency–hopping spread-spectrum (FH or FHSS)
z Direct–sequence spread-spectrum (DS or DSSS) z Orthogonal Frequency Division Multiplexing (OFDM)
The OFDM system provides 802.11a wireless local area network (WLAN) with data rate communication capabilities of 6, 9, 12, 18, 24, 36, 48, and 54 Mbit/s. The support of transmitting and receiving at data rates of 6, 12, and 24 Mbit/s is mandatory. The system uses 52 subcarriers that are modulated using binary or quadrature phase shift keying (BPSK/QPSK), 16-quadrature amplitude modulation (QAM), or 64-QAM.
Forward error correction coding (convolutional coding) is used with a coding rate of 1/2, 2/3, or 3/4.
D. 802.11 Management Architecture
802.11 management architecture is composed of three component: the MAC layer management entity (MLME), a physical-layer management entity (PLME), and a system management entity (SME). In the wireless world, station must identify a compatible network before joining it. The process of identifying existing networks in the area is called scanning. Several parameters are used in the scanning procedure, such as BSSType (independent or infrastructure), BSSID (individual or broadcast), SSID (“network name”), ScanType (active or passive), ChannelList, ProbeDelay, and Min ChannelTime and MaxChannelTime. A scanning report is generated at the conclusion of a scan. The report lists all the BSSs that the scan discovered and their parameters. After compiling the scan result, a station can elect to join one of the BSSs.
Before this can happen, both authentication and association are required.
2 Dynamic Frequency Selection