Spectrum Management
European radio regulations for the 5 GHz are stricter, it needs a special procedures to adapt the 802.11 MAC used in Europe.
IEEE 802.11h-2003, or just 802.11h, refers to the amendment added to the IEEE 802.11 standard for Spectrum and Transmit Power Management Extensions. It solves problems like interference with satellites and radar using the same 5 GHz frequency band. The standard provides Dynamic Frequency Selection (DFS) and Transmit Power Control (TPC) to the 802.11a PHY. It has been integrated into the full IEEE 802.11-2007 standard.
Transmit Power Control (TPC)
- To avoid interfering with certain satellite services
- To increase battery life
- To save "wasted" power for shorter range
- To improve throughput by limiting overlap between APs.
TPC attempts to hold the transmit power to the lowest possible productive level.
Standard 802.11h specifies two TPC-related functions.
Standard 802.11h specifies two TPC-related functions.
First, the AP in an infrastructure BSS or a wireless station in an independent BSS advertises the regulatory and local maximum-power level for the current frequency channel in the Beacon (Country element) and Probe Response frames.
- Maximum transmission power (regulatory maximum power) is specified using Country element.
- Power Constraint elements can be used to specify the local maximum power level specific to this network, which is less than or equal to the regulatory maximum. The stations in the BSS can use any transmit power less than or equal to the local max. value.
Second, 802.11h provides a transmit-power reporting mechanism. It defines a TPC Report element that contains Transmit Power and Link Margin fields.
- When a station associates to an AP, it must supply the minim and maximum transmission power in Power Capability IE. AP may reject clients with lower or higher transmit power capability.
- Both AP and clients may dynamically adjust the transmission power on a frame-by-frame basis. Stations may request a radio link measurement by sending an Action frame containing the requesting for a transmission report.
Dynamic Frequency Selection (DFS)
European regulations require that stations avoid interfering with 5 GHz radar systems as well as spread the power load across all available channels.
DFS consists of a number of procedure to enable 802.11 devices to change the radio channel based on measurements and regulatory requirements.
DFS consists of a number of procedure to enable 802.11 devices to change the radio channel based on measurements and regulatory requirements.
When stations first associate to the network, the Association Request frame includes a Supported Channel IE. The AP may reject the association based on the IE.
DFS will periodically test the channel for potential interference from other radar systems, stopping transmission and measuring potential interference, and, advertising that the channel will change if necessary.
Quiet intervals (periods) are the time when all stations in the BSS do not transmit. Quiet IE is included in the Beacon frame and Probe Response frames. During the quiet period, all stations set the NAV to the length of the quiet period to ensure that the vritual carrier sensing algorithm will defer transmissions. In an infrastructure network, channel quiet scheduling is completely under the control of AP.
Action Frames
Action Frames are a type of management frame used to trigger an action in the cell.
Action frame format is as shown below,
- Category Describes the action frame type. Here are the different category type of Action frames (per 802.11-2007 standard where current CWAP exam is based on)
- 0 : Spectrum Management
- 1 : QoS
- 2 : DLS
- 3 : Block ACK
- 4 : Public
- 5 : Radio Measurement
- 6 : Fast BSS Transistion
- 7 : High Throughput (HT)
- 8 : SA Query
- 9 : Protected dual of public action
- 10-125 : Reserved/Unused
- 126 : Vendor specific protected
- 127 : Vendor specific
- 128-255: Error
- Action The action code for this category to perform, it is usually a number represent different actions
- Element Adds additional information specific to the action
留言