U-APSD or WWM-PS (802.11e)

Introduction to IEEE 802.11e In IEEE 802.11e, two types of QoS are identified – prioritized QoS and parameterized QoS. Prioritized QoS is a weak requirement that enforces relative priority between traffic classes. Parameterized QoS, on the other hand, is a strict requirement expressed quantitatively in terms of the QoS parameters. A new access mechanism (called as HCF – Hybrid Coordination Function) has been defined with EDCA for contention based and HCCA for contention free access methods. Please recall that DCF is contention based while PCF is contention free access methods in 802.11 A few additional mechanisms have been added to improve channel utilization and efficiency. These are “Block Acknowledgement”, “Direct Link Setup”, “Automatic Power Save Delivery” among others. The 802.11 header has been modified to add a new field to classify the type of traffic. The TID (traffic ID) is used to select a UP (user priority) for prioritized QoS or a TSPEC (traffic specification) for parameterized QoS. TID values between 0 – 7 are considered user priorities and these are identical to the IEEE 802.1D priority tags. TID values between 8 – 16 refer to TSPECs. A TSPEC between an AP and an STA is negotiated by means of new management commands – ADDTS Request, ADDTS Response and DELTS. After a TSPEC is successfully negotiated, an STA can get a TXOP by one of two ways: Using EDCA, an STA can contend for the medium, and if it acquires the medium can use the medium for TXOP time limit. The STA must observe the TXOP time limit as specified in an IE in beacons. During CP/CFP, an AP can grant an STA a TXOP using the QoS CF-Poll (called as “polled TXOP” in contrast to “EDCA TXOP”). The TXOP limit is specified in the CF-Poll frame. While the spec allows an AP to grant a “polled TXOP” to an STA during either CFP or CP, it is recommended that it not be issued during CFP, but only during CP for reasons of simplicity in implementation. Wireless Media Extensions (WME) Wireless Media Extensions (WME), also referred to as WiFi Multimedia (WMM), is an industry driven initiative to ensure that a basic subset of IEEE 802.11e QoS mechanisms are interoperable. As such, WME supports only EDCA but not HCCA. Without HCCA, parametererized QoS can not be supported. As mentioned earlier, prioritized QoS identifies four traffic classes (or Access Categories) with differing priorities. The 8 user priorities of 802.1D map to these 4 ACs. Also, the Atheros code maps TOS fields in IP headers to these 4 ACs. AC Number Name Description 0 BE Best effort 1 BK Background 2 VI Video 3 VO Voice Atheros’ MAC uses the following parameters to set QoS priorities. The parameters CWmin, CWmax and AIFS together control the priority of the 4 ACs. The parameter TxOp lets the AP take control of the medium for an extended time so as to transmit data frames in quick succession and this is sometimes referred to as frame bursting. QoS Parameter Default Value Valid Range Description CWmin AC-BE 4 0 - 15 Minimum size of contention window AC-BK 4 AC-VI 3 AC-VO 2 CWmax AC-BE 6 0 - 15 Maximum size of contention window AC-BK 10 AC-VI 4 AC-VO 3 TxOp AC-BE 0 0 - 8192 Transfer opportunity limit in units of 32 microseconds. A value of 0 implies one MPDU. AC-BK 0 AC-VI 3008 AC-VO 1504 AIFS AC-BE 3 0 - 15 Arbitration Inter Frame Space AC-BK 7 AC-VI 1 AC-VO 1 NoAckPolicy 0 0 - 1 When set, 802.11 ACKs are disabled U-APSD 1 0 - 1 Unscheduled Automatic Power Save Delivery

U-APSD UAPSD enables an STA to retrieve unicast QoS traffic buffered in the AP by sending trigger frames. During association/re-association, an STA indicates (in the QoS Info field) which Access Categories are UAPSD enabled. For non-UAPSD enabled ACs, an STA shall continue to use legacy mechanisms, namely PS-Polls, as a means to retrieve the legacy power-save buffered frames. WMM Power-Save Advertisement The UAPSD enabled AP advertises its capability by setting the UAPSD (bit 7) of the QoS Info field in either the WMM Information Element or the WMM Parameter Element. This capability is advertised in Beacons, Probe Responses and (Re)Association Responses. U-APSD enabled STA Association The QoS Info field in the Association request from STA indicates which ACs shall use the UAPSD mechanism. It also specifies the maximum SP length which indicates the number of buffered frames the AP may send per service period. As WMM/WME does not support parametrized QoS, these parameters can only be negotiated during (re)association time. STA sets Power Management An STA sets the PM bit in the frame control field to indicate that it is in power-save mode. The AP will then start to buffer frames of delivery enabled ACs in the UAPSD queue and non-delivery enabled ACs in legacy power-save queue. The AP will not buffer frames for an STA in Active mode, as indicated by a 0 in the PM bit. STA sends UAPSD Trigger An STA in power-save mode can send a QoS NULL or QoS Data frame to trigger the AP to send buffered frames. The AP acknowledges the trigger and then follows it up with up to "maxSP" number of frames. The last frame transmitted in the SP will have the EOSP bit set. All transmitted frames will have MORE bit set with the exception of the last frame that has the EOSP set. The last frame will have the MORE bit set only if there are more buffered frames in the delivery enabled queue. If there are no buffered frames on the AP when the trigger is received, the AP will respond with a QoS NULL frame with EOSP set to end the current Service Period. QoS Commands for Atheros 802.11 based radios As of now, there is no support for parameterized QoS or HCCA for Atheros based radios. Accordingly, features of prioritized QoS are only described. Enable WMM: iwpriv ath0 wmm 1 Disable WMM: iwpriv ath0 wmm 0 Query WMM setting: iwpriv ath0 get_wmm Configuring and Querying EDCA Parameter Set Set CW¬¬min iwpriv ath0 cwmin 0 ac = 0, 1, 2, 3 cwmin = 0 - 15 Set CW¬¬max iwpriv ath0 cwmax 0 ac = 0, 1, 2, 3 cwmax = 0 - 15 Set TxOp Limit iwpriv ath0 txoplimit 0 ac = 0, 1, 2, 3 txop = 0 - 8192 Set AIFS iwpriv ath0 aifs 0 ac = 0, 1, 2, 3 aifs = 0 - 15 Query CW¬¬min iwpriv ath0 get_cwmin 0 ac = 0, 1, 2, 3 Query CW¬¬max iwpriv ath0 get_cwmax 0 ac = 0, 1, 2, 3 Query TxOp Limit iwpriv ath0 get_txoplimit 0 ac = 0, 1, 2, 3 Query AIFS iwpriv ath0 get_aifs 0 ac = 0, 1, 2, 3 Query all WMM Parameters Wlanconfig ath0 list wme Configuring Other 802.11e Parameters Set NoAckPolicy iwpriv ath0 noackpolicy 0 ac = 0, 1, 2, 3 enable = 1, disable = 0 Query NoAckPolicy iwpriv ath0 get_noackpolicy 0 ac = 0, 1, 2, 3 Enable U-APSD iwpriv ath0 uapsd 1 Disable U-APSD iwpriv ath0 uapsd 0 Query NoAckPolicy iwpriv ath0 get_uapsd Simple Test to Verify QoS The ICMP application “ping” may be used to set TOS in IP headers and may be used to quickly verify QoS functionality. The following command can be issued to send the ICMP packets with appropriate TOS settings. ping –Q –C tos = 0 or 96 (AC-BE), 32 or 64 (AC-BK), 128 or 160 (AC-VI), 192 or 224 (AC-VO) Thanks to : http://wifi-insider.com/wlan/wmm.htm

How to Design Solar PV System

What is solar PV system? Solar photovoltaic system or Solar power system is one of renewable energy system which uses PV modules to convert sunlight into electricity. The electricity generated can be either stored or used directly, fed back into grid line or combined with one or more other electricity generators or more renewable energy source. Solar PV system is very reliable and clean source of electricity that can suit a wide range of applications such as residence, industry, agriculture, livestock, etc. Major system components Solar PV system includes different components that should be selected according to your system type, site location and applications. The major components for solar PV system are solar charge controller, inverter, battery bank, auxiliary energy sources and loads (appliances).   •   PV module – converts sunlight into DC electricity.   •   Solar charge controller – regulates the voltage and current coming from the PV panels going to       battery and prevents battery overcharging and prolongs the battery life.   •   Inverter – converts DC output of PV panels or wind turbine into a clean AC current for AC       appliances or fed back into grid line.   •   Battery – stores energy for supplying to electrical appliances when there is a demand.   •   Load – is electrical appliances that connected to solar PV system such as lights, radio, TV, computer,       refrigerator, etc.   •   Auxiliary energy sources - is diesel generator or other renewable energy sources. Solar PV system sizing 1. Determine power consumption demands The first step in designing a solar PV system is to find out the total power and energy consumption of all loads that need to be supplied by the solar PV system as follows:      1.1 Calculate total Watt-hours per day for each appliance used.            Add the Watt-hours needed for all appliances together to get the total Watt-hours per day which            must be delivered to the appliances.      1.2 Calculate total Watt-hours per day needed from the PV modules.             Multiply the total appliances Watt-hours per day times 1.3 (the energy lost in the system) to get             the total Watt-hours per day which must be provided by the panels. 2. Size the PV modules Different size of PV modules will produce different amount of power. To find out the sizing of PV module, the total peak watt produced needs. The peak watt (Wp) produced depends on size of the PV module and climate of site location. We have to consider “panel generation factor” which is different in each site location. For Thailand, the panel generation factor is 3.43. To determine the sizing of PV modules, calculate as follows:      2.1 Calculate the total Watt-peak rating needed for PV modules            Divide the total Watt-hours per day needed from the PV modules (from item 1.2) by 3.43 to get               the total Watt-peak rating needed for the PV panels needed to operate the appliances.      2.2 Calculate the number of PV panels for the system            Divide the answer obtained in item 2.1 by the rated output Watt-peak of the PV modules available            to you. Increase any fractional part of result to the next highest full number and that will be the            number of PV modules required. Result of the calculation is the minimum number of PV panels. If more PV modules are installed, the system will perform better and battery life will be improved. If fewer PV modules are used, the system may not work at all during cloudy periods and battery life will be shortened. 3. Inverter sizing An inverter is used in the system where AC power output is needed. The input rating of the inverter should never be lower than the total watt of appliances. The inverter must have the same nominal voltage as your battery. For stand-alone systems, the inverter must be large enough to handle the total amount of Watts you will be using at one time. The inverter size should be 25-30% bigger than total Watts of appliances. In case of appliance type is motor or compressor then inverter size should be minimum 3 times the capacity of those appliances and must be added to the inverter capacity to handle surge current during starting. For grid tie systems or grid connected systems, the input rating of the inverter should be same as PV array rating to allow for safe and efficient operation. 4. Battery sizing The battery type recommended for using in solar PV system is deep cycle battery. Deep cycle battery is specifically designed for to be discharged to low energy level and rapid recharged or cycle charged and discharged day after day for years. The battery should be large enough to store sufficient energy to operate the appliances at night and cloudy days. To find out the size of battery, calculate as follows:      4.1 Calculate total Watt-hours per day used by appliances.      4.2 Divide the total Watt-hours per day used by 0.85 for battery loss.      4.3 Divide the answer obtained in item 4.2 by 0.6 for depth of discharge.      4.4 Divide the answer obtained in item 4.3 by the nominal battery voltage.      4.5 Multiply the answer obtained in item 4.4 with days of autonomy (the number of days that you            need the system to operate when there is no power produced by PV panels) to get the required            Ampere-hour capacity of deep-cycle battery. Battery Capacity (Ah) = Total Watt-hours per day used by appliances x Days of autonomy (0.85 x 0.6 x nominal battery voltage) 5. Solar charge controller sizing The solar charge controller is typically rated against Amperage and Voltage capacities. Select the solar charge controller to match the voltage of PV array and batteries and then identify which type of solar charge controller is right for your application. Make sure that solar charge controller has enough capacity to handle the current from PV array. For the series charge controller type, the sizing of controller depends on the total PV input current which is delivered to the controller and also depends on PV panel configuration (series or parallel configuration). According to standard practice, the sizing of solar charge controller is to take the short circuit current (Isc) of the PV array, and multiply it by 1.3 Solar charge controller rating = Total short circuit current of PV array x 1.3 Remark: For MPPT charge controller sizing will be different. (See Basics of MPPT Charge Controller) Example: A house has the following electrical appliance usage: One 18 Watt fluorescent lamp with electronic ballast used 4 hours per day. One 60 Watt fan used for 2 hours per day. One 75 Watt refrigerator that runs 24 hours per day with compressor run 12 hours and off 12 hours. The system will be powered by 12 Vdc, 110 Wp PV module. 1. Determine power consumption demands Total appliance use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 24 x 0.5 hours)   = 1,092 Wh/day Total PV panels energy needed  = 1,092 x 1.3   = 1,419.6 Wh/day. 2. Size the PV panel 2.1 Total Wp of PV panel capacity       needed = 1,419.6 / 3.4   = 413.9 Wp 2.2  Number of PV panels needed = 413.9 / 110   = 3.76 modules                                                                          Actual requirement = 4 modules           So this system should be powered by at least 4 modules of 110 Wp PV module. 3. Inverter sizing     Total Watt of all appliances = 18 + 60 + 75 = 153 W     For safety, the inverter should be considered 25-30% bigger size.     The inverter size should be about 190 W or greater. 4. Battery sizing     Total appliances use = (18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)     Nominal battery voltage = 12 V     Days of autonomy = 3 days     Battery capacity = [(18 W x 4 hours) + (60 W x 2 hours) + (75 W x 12 hours)] x 3                                                 (0.85 x 0.6 x 12)     Total Ampere-hours required 535.29 Ah     So the battery should be rated 12 V 600 Ah for 3 day autonomy. 5. Solar charge controller sizing     PV module specification     Pm = 110 Wp     Vm = 16.7 Vdc     Im = 6.6 A     Voc = 20.7 A     Isc = 7.5 A     Solar charge controller rating = (4 strings x 7.5 A) x 1.3 = 39 A     So the solar charge controller should be rated 40 A at 12 V or greater. ----------------------------------- First, you need to inventory all your loads: laptop 60W 12 hours  lamp CFL 20W 6 hours lamp CFL 20W 6 hours coffee maker 1400W 0.1hr internet modem 30W 24 hours Fridge 350W 6 hours Once you have a listing of your loads, you total the daily watt hours for each load: lamp CFL 20W 6 hours = 120wh tea pot 1400W 0.1hr = 140wh then you add all the watt hours up for the daily total = 1,220 wh [ or 1.22KWh ]  Then you need the number of good sun hours in your area pvwatts.org lets use 5 as a average number. Double your consumption : 2440 and divide by sun hours 5 = 488W 488w is the amount of solar PV needed to run your loads for the caculated time. Then we can figure batteries later. ----------------------------------- Thanks to :  http://www.solarpaneltalk.com/showthread.php?2275-Help-on-design-on-Solar-power-plant http://www.leonics.com/support/article2_12j/articles2_12j_en.php