What Is PCM Rent?

Identification

• "PCM" is common in Europe, because of the amount of weekly rental rates there. In major British cities, weekly rentals comprise the majority of residential rental properties. An apartment renting on a weekly basis is denoted as "PW" (per week).

Is PCM Better Than PW?

• PCM is better most of the time, because there are 52 weeks a year, but only 12 months. If each month were actually 4 weeks, then the PCM and PW would balance out at 48 weeks each. However, only February is exactly 4 weeks long. To convert PCM to its PW equivalent, add 30 percent. Therefore, a 600 euro PCM rent is 780 euros PW.

A commonly encountered phrase while renting an apartment or room abroad is "PCM," as in "600 euros PCM." In the United States, PCM (per calendar month) is standard practice, so few Americans are familiar with the term. It is usually assumed that rent is PCM. Understanding the difference between PCM rent and weekly rent is important to make informed rental decisions abroad

When is PW Better than PCM?

• PW is better than PCM when leases are short-term, and especially if the renter may have to vacate the property at any time. When a renter might have to leave at any time, a PW arrangement can be made so the lease can be voided on short notice, thus saving the renter the loss of a security deposit and penalties. This makes the arrangement ideal for people who travel a lot for work, military personnel or tourists. However, if the lease really does need to be flexible, rather than simply short-term, it is up to the renter to make sure it includes those terms.

PCM in the United States?

• Most Americans are not familiar with PCM rent, but that does not mean it is completely unheard of. A general rule of thumb is that if you see PCM on rental notices, it is because the local rental market is dramatically overheated and landlords can demand and get PW rent to such an extent that PCM rental terms become an added incentive, rather than just a standard feature. Certain areas in metropolitan New York or San Francisco endure such conditions.

Considerations

• PCM is also often used to describe other money issues. For example, a person who is paid a monthly, as opposed to bi-weekly or weekly salary, may say "I am paid 1,200 pounds PCM." Being paid in PCM terms is usually not as favorable as being paid every week or two weeks, due to the effects of reversing the math and being paid for 48 weeks of work when most people work 50+ weeks.

  
  

  Thanks to ;

  http://www.ehow.com/about_4731021_pcm-rent_.html

  
  

802.11 Frame types

802.11 Frames are devided into three categories.

1. Managment Frames
2. Control frames.
3. Data Frames

1. Management Frames

•    Managment Frames are used by Wireless stations to join and Leave the Basic Service Set
•    Another name for Managment Frames is "MAC protocol Data Unit"(MMPDU)
•    There is no MSDU encapsulated in the MMPDU frame body , which carries only layer2 information fileds and information elements

   Following are the list of all 12 of the Managment frame subtypes as defined by 802.11 standard

    Assosiation Request
    Assosiation Response
    Reassosiation Request
    Reassosiation Response
    Probe request
    Probe Response
    Beacon
    Announcement Traffic Indication Message(ATIM)
    Disassosiation
    Authentication
    Deauthenication
    Action

2. Control Frames

•     802.11 Control frames assit with the delivery of the data frames. 
•     Control frames must be heard by all the stations, therefore they must be transmitted at one of the basic rates
•     Control frames are also used to clear the channel,acquire the channel and provide the unicast frame acknowledgements
•     They contain only header information

    Following are the list of control frame subtypes as defined by 802.11 standard

    Power Save(PS) Poll
    Request to Send (RTS)
    Clear to send (CTS)
    Acknowledgement(ACK)
    Contention-Free(CF)-End (PCF only)
    CF-End+CF-ACK (PCF only)
    Black-ACK(HCF)
    Black Ack Request(HCF)

3.Data Frames

•    Most of the Data Frames carry actual data that is passed down from higher layer protocols
•    Some 802.11 data frames contain no data at all but do have a specfific purpose within BSS
•    There are 15 data frame subtypes 

Data
Data+CF-Ack (PCF only)
Data+CF-Poll (PCF only)
Data+CF-Ack+CF-Poll (PCF only)
Null data (no data transmitted)
CF-Ack (no data transmitted) (PCF only)
CF-Poll (no data transmitted) (PCF only)
Data+CF-Ack+CF-Poll (PCF only)
Qos Data (HCF)
Qos Null (No Data) (HCF)
Qos Data+CF-Ack (HCF)
 Qos Data+CF-Poll (HCF)
 Qos Data+CF-Ack+CF-Poll (HCF)
Qos Cf-Poll(HCF)
Qos CF-ACK+CF-Poll (HCF)


See some of below combination for different frame types.

Thanks to :

http://ilovewifi.blogspot.in/2012/07/80211-frame-types.html

How WLAN CSMA/CA Works

In Wireless LANs CSMA/CA is core concept in comunicating wirelessly. In any shared medium accessing the medium without collision is important part. Its like not talking all at a time, so that remaining people should understood the other's talk.

                       In Ethernet it is achieved through CSMA/CD as it is using full duplex communication. But in wireless it is using Halfduplex communication. The half-duplex constraint applies to devices that share a relative physical area and RF frequency.Several mechanisms are used as a part of 802.11 channel access to minimize the likelihood of frame collisions by multiple STAs attempting to access the transmission medium simultaneously.


Collisions often occur, but the processes used by the 802.11 protocol are in place to minimize the likelihood of collisions and define the appropriate response in the event that a collision is inferred.

Two most common coordination access methods used in WLANs

  1.Distributed Coordination Function (DCF)
  2.Enhanced Distributed Coordination Access (EDCA is part of HCF).

1. Distributed Coordination Function (DCF)

• Using the foundational DCF coordination function logic is active in every station (STA) in a basic service set (BSS) whenever the network is in operation.i.e. each station within a DCF follows the same channel access rules.
• This method is contention-based, which means that each device “competes” with one another to gain access to the wireless medium.
• After a transmission opportunity is obtained and observed, the contention process begins again.
• As the original 802.11 network access method, DCF is the most simple channel access method but it lacks support for quality of service (QoS).
• In order to maintain support for non-QoS devices in QoS-enabled networks, support for DCF is required for all 802.11 networks.

2. Hybrid Coordination Function (HCF)

• As an optional access method that may be used in addition to DCF, HCF was introduced to support QoS. 
• HCF assimilated elements of both DCF and PCF mechanisms, creating a contention-based HCF method, called EDCA, and a contention-free HCF method, called HCCA.
• EDCA inaugurated a means of prioritizing contention-based wireless medium (WM) access by classifying 802.11 traffic types by User Priorities (UP) and Access Categories (AC).
• There are a total of 8 UPs, which map to 4 ACs. 
• EDCA is used by stations that support QoS in a QoS BSS to provide prioritized WM access, but HCF is not used in non-QoS BSSs.

3.Point Coordination Function (PCF)

• As an optional PCF is a contention-free access method.
• PCF provides polling intervals to allow uncontended transmission opportunities for participating client devices.
• In this approach, the AP of a BSS acts as a point coordinator (PC), initiating contention-free periods in which prioritized medium access is granted to clients, one at a time.
• PCF has gone unused in 802.11 WLANs.

Summary

• DCF is the fundamental, required contention-based access service for all networks
• PCF is an optional contention-free service, used for non-QoS STAs
• HCF Contention Access (EDCA) is required for prioritized contention-based QoS services
• HCF Controlled Access (HCCA) is required for parameterized contention-free QoS services

802.11 Channel Access Mechanisms

Both contention-based access methods described previously (i.e. DCF and EDCA) employ similar mechanisms to moderate channel access and to minimize collisions.

Below is Outline for Channel Access:

1. STAs use a physical carrier sense (Clear Channel Assessment—CCA) to determine if the WM is busy.
2. STAs use virtual carrier sense (Network Allocation Vector—NAV) to detect if the WM is busy. When the virtual timer (NAV) reaches zero, STAs may proceed.
3. If conditions 1 and 2 are met, STAs wait the necessary IFS interval, as prescribed by the protocol.
4. If conditions 1 and 2 are met through the duration of condition 3, STAs generate a random backoff number in accordance with the range of allowed values.
5. STAs begin decrementing the backoff timer by one for every slot time duration that the WM is idle.
6. After decrementing the backoff value to zero, with an idle medium, a STA may transmit the allotted frame exchange, in accordance with the parameters of the obtained transmission opportunity.
7. If another STA transmits before Step 6 is completed, STAs observe steps 1, 2, 3, and 5 until the backoff timer is equal to zero.
8. After a successful transmission, repeat as needed.

Carrier Sense

• 802.11 WLANs uses Carrier Sense Multiple Access with Collision Avoidance (CSMA/CA).
• For STAs to cooperate effectively on a half-duplex channel, each STA must be able to determine when the medium is clear, and when another device is actively transmitting. 
• The physical carrier sense uses the physical radio interface to sample the wireless medium to detect transmissions. Virtual carrier sense refers to the use of Duration values in the MAC header of a frame and NAV timers to virtually determine if another STA is transmitting.
• These two mechanisms determine the status of the medium, whether idle or busy.

Physical Carrier Sense

• The physical carrier sense mechanism defined by IEEE is known as clear channel assessment (CCA).
• CCA is the physical measurement taken by a radio interface to determine if the wireless medium is currently in use. The physical layer, which can be divided into two sublayers—the physical medium dependent (PMD, lower sublayer) and the physical layer convergence procedure (PLCP, upper sublayer)—performs this task and communicates this information to the MAC. 
• According to the CCA mode in use, the PMD issues service primitives to the PLCP sublayer indicating whether the wireless medium is in use. 
• The PLCP sublayer then communicates with the MAC layer to indicate a busy or idle medium, which prevents the MAC from attempting to forward a frame for transmission.
• Each PHY within the 802.11-2007 standard dictates the specific operations and signal thresholds used to carry out the CCA mechanism.
• CCA can be divided into two separate processes: Energy detection (ED) and carrier sense (CS). 
• ED functionality is based upon raw RF energy. When an energy level detected in the channel crosses a certain threshold for a certain period of time, the “busy medium” indication will be triggered.
• On the other hand, CS—more precisely, “preamble detection”—monitors and detects 802.11 preambles, which are used to trigger the CCA mechanism and indicate a busy medium.

Virtual Carrier Sense

• Virtual carrier sense mechanism is defined in addition to the physical carrier sense.
• Virtual carrier sense uses information found in 802.11 frames to predict the status of the wireless medium.
• This is performed by means of the network allocation vector (NAV), which is a timer that is set using the duration values in the MAC header of a frame. 
• Each frame contains a duration value, indicating the time required for a station to complete the conversation. 
• All STAs use these duration values to set their NAV, and then they count down the NAV timer, waiting for the medium to become available.

See the Duration Filed in MAC header in below Pic (second filed)

• All STAs attempt to process all frames—and a minimum of the first in a frame exchange—on their channel.
• The first frame in a frame exchange is significant because it can, and sometimes must, be used to determine how long a given transmission opportunity will occupy the wireless medium.

See below wireless capture for Duration value

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• The MAC header of each frame contains a Duration field, which indicates the amount of time necessary to complete the entire frame exchange, or the entire TXOP duration. 
• In DCF, a transmission opportunity only allows for the transmission of one frame, thus the Duration value represents the required IFS interval and the acknowledgement frame (ACK), if one is required.
• The exception to this rule is for networks in which RTS/CTS or CTS-to-self protection is enabled. In this case, the transmission opportunity allows for the use of these frames. 
• In HCF, several frames may be transmitted within a transmission opportunity. Thus, the Duration value refers to the TXOP duration. 
• In either case, non-transmitting STAs must remain idle while the medium is reserved.
• When STAs read the Duration value in a frame, they set their NAV timer accordingly and count down this duration.
• The duration value in the MAC header indicates the time required to complete the transmission opportunity after the current—the frame in which the Duration value resides—frame if completed. 
• If a STA is counting down its NAV and it receives another frame with a longer duration (would increase its NAV), the STA increases its NAV accordingly.
• Conversely, when a STA receives a frame with a shorter duration value (would decrease its NAV), the STA ignores this value and continues to observe the longer NAV duration.

See below RTS / CTS exchange

• In networks where mixed PHY technologies are supported, protection mechanisms are enabled to satisfy the equirements of frame processing and adherence to the common channel access protocol.
• Frames used as a protection mechanism (often an RTS/CTS exchange or CTS-to-Self) are transmitted at a common rate understood by all PHYs in the network.
• Legacy PHY STAs read the Duration value in the protection frame(s), set their NAV timer.

Interframe Spacing (IFS)

• After each frame transmission, 802.11 protocols require an idle period on the medium, called an interframe space (IFS). 
• The length of the IFS is dependent upon a number of factors, such as the previous frame type, the following frame type, the coordination function in use, the access category of the following frame (in a QoS BSS), as well as the PHY type.
• The purpose of an IFS is both to provide a buffer between frames to avoid interference as well as to add control and to prioritize frame transmissions.
• Each IFS “is the time from the end of the last symbol of the previous frame to the beginning of the first symbol of the preamble of the subsequent frame as seen at the air interface

In other words, the IFS interval is observed beginning with the completion of the previous frame. The length of each IFS interval, excluding AIFS, is fixed for each PHY

Short Interframe Space (SIFS)

• SIFS are used within all of the different coordination functions.
• For 802.11-2007, SIFS is the shortest of the IFSs and is used prior to ACK and CTS frames as well as the second or subsequent MPDUs of a fragment burst.
• However, with 802.11n, a shorter IFS (RIFS) was introduced.
• The IEEE explains the use of SIFS accordingly:

                              “SIFS shall be used when STAs have seized the medium and need to keep it for the duration of the frame exchange sequence to be performed. Using the smallest gap between transmissions within the frame exchange sequence prevents other STAs, which are required to wait for the medium to be idle for a longer gap, from attempting to use the medium, thus giving priority to completion of the frame exchange sequence in progress.”

• SIFS is used as a priority interframe space once a frame exchange sequence has begun. 
• This is true when multiple frames are transmitted within a TXOP (as with frame bursting) and it is also true when a single frame is transmitted (as with typical data-ack exchanges).

PCF Interframe Space (PIFS)

• PIFS are used by STAs during the contention-free period (CFP) in PCF mode. 
• Because PCF has not been implemented in 802.11 devices, you will not see PIFS used for this purpose. 
• However, PIFS may be used as a priority access mechanism for Channel Switch Announcement frames, as used to meet DFS requirements. 
• In order to gain priority over other STAs during contention, the AP can transmit a Channel Switch Announcement frame after observing a PIFS.

DCF Interframe Space (DIFS)

• When a STA desires to transmit a data frame (MPDU) or management frame (MMPDU) for the first time within a DCF network, the duration of a DIFS must be observed after the previous frame’s completion.
• The duration of a DIFS is longer than both the SIFS and PIFS.

Arbitration Interframe Space (AIFS)

• The AIFS shall be used by QoS STAs to transmit all data frames (MPDUs), all management frames (MMPDUs), and the following control frames: PS-Poll, RTS, CTS (when not transmitted as a response to the RTS), BlockAckReq, and BlockAck (when not transmitted as a response to the BlockAckReq) With EDCA.
• The basic contention logic is the same as with non-QoS networks, but in order to facilitate QoS, there are some notable differences.
• While DCF can designate a single DIFS value for each PHY, EDCA establishes unique AIFS durations for access categories (AC). 
• For this reason, an AIFS is typically notated as an AIFS[AC]. 
• QoS STA’s TXOPs are obtained for a specific access category, so delineation between ACs must be made.
• For improved control of QoS mechanisms, AIFS values are user-configurable. 
• By default, QoS APs announce an EDCA parameter set in the Beacon frame that notifies stations in the BSS about QoS values.
• By changing these values in the AP configuration, the AP will broadcast a different set of parameters to the BSS.

Extended Interframe Space (EIFS)

• The EIFS value is used by STAs that have received a frame that contained errors. 
• By using this longer IFS, the transmitting station will have enough time to recognize that the frame was not received properly before the receiving station commences transmission.
• If, during the EIFS duration, the STA receives a frame correctly , it will resume using DIFS or AIFS, as appropriate.

Reduced Interframe Space (RIFS)

• RIFS were introduced with 802.11n to improve efficiency for transmissions to the same receiver in which a SIFS-separated response is not required, such as a transmission burst.

See below Pic for IFS comparison

• The graphic demonstrates the relationship between the different IFS intervals.
• You will notice that the initial frame (“Busy Medium”) transmission is preceded by a DIFS or AIFS.
• The graphic shows the relative relationship of the IFS lengths.
• SIFS are the shortest IFS (excluding 802.11n’s RIFS) and PIFS are second shortest, while DIFS and AIFS take up the caboose.
• SIFS, PIFS, and RIFS are used to provide priority access for a given type of frame, which eliminates the need for added contention

Calculating an Interframe Space

The 802.11-2007 specification provides the information necessary for us to calculate the durations for each IFS.
As noted previously, SIFS, PIFS, and DIFS are fixed values for each PHY, while AIFS will vary in accordance with the AC in use.
EIFS are fixed per PHY in DCF networks, but vary when used with EDCA.
The formulas and components used for SIFS, PIFS, DIFS, EIFS, and AIFS calculations are as follows:

aSIFSTime = aRxRFDelay + aRxPLCPDelay + aMACProcessingDelay + aRxTxTurnaroundTime
aSlotTime = aCCATime + aRxTxTurnaroundTime + aAirPropagationTime+ aMACProcessingDelay.

The “aSIFSTime” is the same as a SIFS, measured in microseconds (µs). Similarly, the “aSlotTime” is the same as a slot time. Both of these values are provided for each PHY in the 802.11 specification.

PIFS = aSIFSTime + aSlotTime
DIFS = aSIFSTime + 2 × aSlotTime

Given that the SIFS and slot time values are provided for us in the standard, these calculations are pretty simple.

See below IFS calculations

EIFS (DCF) = aSIFSTime + DIFS + ACKTxTime

In this formula, the “ACKTxTime” is the amount of time it takes to transmit an ACK frame at the lowest mandatory rate in the BSS.

EIFS (EDCA) = aSIFSTime + AIFS[AC] + ACKTxTime

The EIFS (EDCA) formula mirrors the same for DCF, but replaces the DIFS with the appropriate AIFS[AC].

An AIFSN is a number (AIFS Number) value that is user-configurable and determines the brevity (or length) of an AIFS interval. AIFSN values are set for each access category, giving the AIFS[AC] a shorter or longer duration, in accordance with the desired priority.

This is demonstrated by the AIFS[AC] formula:
AIFS[AC] = AIFSN[AC] × aSlotTime + aSIFSTime

Thanks to:

http://ilovewifi.blogspot.in/2012/08/how-80211-csmaca-works.html

802.11n Features

   As we are using more applications on the Clients like Laptops ,Smart phones and other wi-fi enabled devices with Voice and Video. The demand for Higher throughput is going on. Its like seeing some movie on these devices by connecting through the Wi-Fi. So they come up with new ammendment 802.11n to get higher throughputs to overcome growing demand of Higher Throughput.

Below are some of the main Modifications in 802.11n

1. MIMO(multiple input, multiple output antennas)
2. Frame aggregation A-MPDU & A-MSDU
3. Channel bonding (40 Mhz channels)
4. Block Acknowledgments (BLOCK ACK)
5. Modulation & Coding scheme (MCS)
6. Short Guard Interval (short GI)
7. 802.11n interoperability (HT mode)
8. RIFS
9. HT power management


Below are the ways  throughput Improvements are happened in 11n 

•     The number of OFDM data sub-carriers is increased from 48 to 52 which improves the maximum throughput  from 54 to 58.5 Mbps
•    Forward Error Correction (FEC) is a system of error control whereby the sender adds redundant data to allow the receiver to detect and correct errors. 3/4 coding rate is improved with 5/6 boosting the link rate from 58.5 to 65 Mbps
•   The Shorter Guard Interval(GI) between OFDM symbols is reduced from 800ns to 400ns and increases throughput from 65 to 72.2 Mbps Channel
•   Doubling channel bandwidth from 20 to 40 MHz slightly more than doubles rate from 72.2 to 150 Mbps.
•   Support of up to four spatial streams (MIMO) increases throughput up to 4 times 150 to 600 Mbps
•    802.11n maintain backward compatibility with existing IEEE 802.11a/b/g.

1.MIMO(multiple input, multiple output antennas)

  802.11n has ability to receive and/or transmit simultaneously through multiple antennas. 802.11n defines many "M x N" antenna configurations, ranging from "1 x 1" to "4 x 4".

MIMO antennas example

   This refers to the number of transmit (M) and receive (N) antennas – for example, an AP with two transmit and   three receive antennas is a "2 x 3" MIMO device.

 The more antennas an 802.11n device uses simultaneously, the higher its maximum data rate. 

 802.11n uses advanced signal processing techniques

  

a. Spatial Multiplexing (SM)

•      Spatial Multiplexing (SM) subdivides an outgoing signal stream into multiple pieces, transmitted through different antennas. 
•      Because each transmission propagates along a different path, those pieces – called spatial streams – arrive with  different strengths and delays.
•      Multiplexing two spatial streams onto a single channel effectively doubles capacity and thus maximizes data rate.
•      All 802.11n APs must implement at least two spatial streams,up to a maximum of four.
•      802.11n stations can implement as few as one spatial stream.

  b. Space-Time Block Coding (STBC)

•     Space-Time Block Coding (STBC) sends an outgoing signal stream redundantly,using up to four differently-coded spatial streams, each transmitted through a different antenna. 
•   By comparing arriving spatial streams, the receiver has a better chance of accurately determining the original signal stream in the presence of RF interference and distortion.
•   That is, STBC improves reliability by reducing the error rate experienced at a given Signal to Noise Ratio (SNR). This optional 802.11n feature may be combined with SM.

  c. Transmit Beamforming (TxBF) 

•     Transmit Beam-forming (TxBF)  steers an outgoing signal stream towards the intended receiver by concentrating transmitted RF energy in a given direction.
•   This technique leverage additive and destructive environmental impacts
•   This optional 802.11n feature is not yet widely implemented.

2. Frame aggregation A-MPDU & A-MSDU

•     Frame Aggregation increases the payload that can be conveyed by each 802.11 frame, reducing MAC layer overhead from a whopping 83 % to as little as 58 % (using A-MSDU) and 14 % (when using A-MPDU).
•   Legacy 802.11a/g devices can send no more than 2304 payload bytes per frame.
•   But new 802.11n devices have the option of bundling frames together for transmission, increasing payload size to reduce the significance of the fixed overhead caused by inter-frame spacing and preamble. 

  

  There are two aggregation options:

 MAC Service Data Unit Aggregation (A-MSDU):

•   
•  Groups logical link control packets (MSDUs) with the same 802.11e Quality of Service,independent of source or destination.
•     The resulting MAC frame contains one MAC header, followed by up to 7935 MSDU bytes.
•     Whole frame must be retransmitted if no acknowledge .

MAC Protocol Data Unit Aggregation (A-MPDU):

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•  Multiple Ethernet frames for a common destination are translated to 802.11 format and sent as burst.
•  Complete MAC frames (MPDUs) are then grouped into PHY payloads up to 65535 bytes.
•  Elements of an A-MPDUs burst can be acknowledged individually with one single Block-Acknowledge
•  Only not-acknowledged A-MPDUs are retransmitted

 3. Channel bonding (40 Mhz channels)

•    Legacy 802.11 products use channels that are approximately 20 MHz wide.
•    New 802.11n products can use 20 or 40 MHz wide channels in either the ISM or UNII band.
•    802.11n WLANs will uses 40 MHz channels mainly in the 5 GHz UNII band.

4. Block Acknowledgements (BLOCK ACK)

•   Rather than sending an individual acknowledge following each data frame, 802.11n introduces the technique of confirming a burst of up to 64 frames with a single Block ACK(BA) frame
•   The Block ACK even contains a bitmap to selectively acknowledgeindividual frames of a burst (comparable to selective acknowledges of TCP)
•   The use of combined acknowledges can be requested by sending a Block ACK Request(BAR)
•   The Block-ACK options are negotiated and confirmed with ‘Action’ framesdefined in 802.11e (WLAN QoS)

5. Modulation & Coding scheme (MCS)

•   802.11n APs and stations need to negotiate capabilities like the number of spatial streams and channel width.
•   They also must agree upon the type of RF modulation,coding rate, and guard interval to be used. 
•   The combination of all these factors determines the actual PHY data rate, ranging from a minimum 6.5 Mbps to a   maximum 600 Mbps

6. Short Guard Interval (short GI)

•    Guard Interval is the time between transmitted symbols (the smallest unit of data sent at once).
•    This Guard Interval is necessary to offset the effects of multipath that would otherwise cause Inter-Symbol Interference (ISI).
•    Legacy 802.11a/g devices use an 800 ns guard interval, but 802.11n devices have the option of pausing just 400 ns. 
•    Shorter Guard Intervals would lead to more interference and reduced throughput, while a longer Guard Interval would lead to unwanted idle time in the wireless environment. 
•    A Short Guard Interval (SGI) boosts data rate by 11 percent while maintaining symbol separation sufficient for most environments.

7. 802.11n interoperability and coexistence

  

•   Given the fact that millions of legacy 802.11a/b/g devices have been deployed to date, and that those devices operate in the same frequency bands used by 802.11n,enabling coexistence is critical.
•  802.11n deployments must therefore be able to "play nicely" with 802.11a/b/g, both by limiting 802.11n impact on nearby legacy WLANs and by enabling communication with legacy stations. 
•  These goals are accomplished using HT Protection and Coexistence mechanisms.

   a. High Throughput (Greenfield) Mode:

•         There are three 802.11n operating modes: HT, Non-HT, and HT Mixed.
•     An 802.11n AP using High Throughput (HT) mode – also known as Greenfield mode – assumes that there are no nearby legacy stations using the same frequency band.
•     If legacy stations do exist, they cannot communicate with the 802.11n AP. HT mode is optional.

      b. Non-HT (Legacy) Mode :

•    An 802.11n AP using Non-HT mode sends all frames in the old 802.11a/g format so that legacy stations can understand them. 
•    That AP must use 20 MHz channels and none of the new HT features .
•    All products must support this mode to ensure backward compatibility, but an 802.11n AP using Non-HT delivers no  better performance than 802.11a/g.

   c. HT Mixed Mode :

•    The mandatory HT Mixed mode will be the most common 802.11n AP operating mode for the next year or so. 
•    In this mode, HT enhancements can be used simultaneously with HT Protection mechanisms that permit communication  with legacy stations.
•    HT Mixed mode provides backwards compatibility, but 802.11n devices pay significant throughput penalties as compared to Greenfield mode.

8. RIFS 

•    RIFS is a means of reducing overhead and thereby increasing network efficiency.
•    RIFS may be used in place of SIFS to separate multiple transmissions from a single transmitter, when no  SIFS-separated response transmission is expected. 
•    RIFS shall not be used between frames with different RA values.
•    The duration of RIFS is defined by the aRIFS PHY characteristic .
•    A STA shall not allow the space between frames that are defined to be separated by a RIFS time, as measured on the medium, to vary from the nominal RIFS value (aRIFSTime) by more than ± 10% of aRIFSTime. 
•    Two frames separated by a RIFS shall both be HT PPDUs.

9. HT powermanagment

•  802.11n introduces two powersaving mechanisms that can be used by HT clause radios
•  802.11n radios still support legacy power save mode.
•  The first new power save management is called Spatail Multiplexing PowerSave mode (SM power save)
•  The purpose of SM powersave is powerdown the all but one of its radios
•  For example a 4*4 MIMO device with four radio chains would power down three of its four radios
•  The second Power save managment is Power save Multipoll (PSMP).
•  PSMP is extention to the automatic power save mode
•  U-PSMP is similar to U-APSD and uses trigger and deliver enabled mechanisms.
•  Scheduled PSMP is similar to the S-APSD Power save 

Hope this will help as brief introduction.

Thanks to :

http://ilovewifi.blogspot.in/2012/08/80211n-features.html

Step by step process in connecting client to AP with dot1x

For connecting the client to AP with 802.1x involves sequence of steps. 
Below are sequence of steps.


1. Client started with Active scanning . Sends "Probe request"
2. AP responds with "Probe Response"
3. Client sends "Auth Request"
4. AP responds with "Auth Response"
5. Client sends "Assosiation Request"
6. AP responds with "Assosiation Resonse"
7. The station sends an "EAP - Start" message to the AP.This initiates the process of "EAP authentication".
8. The AP sends an "access request" on behalf of the client to the RADIUS server.
9. The AP replies with an "EAP Request/Identity" message.
10. The station sends an "EAP  Response/Identity" message containing its credentials (such as username) to the AP. This
 message will contain ID based on the EAP type such as "EAP-TLS", "EAP-TTLS", "EAP-PEAP", "EAP-LEAP", or "EAP-FAST".In a password-based EAP, the user.s password is NOT part of this
message.
11. The AP forwards the "user ID" to the "RADIUS server".
12. The "RADIUS server" responds with a "challenge message", which the access point forwards to the station as an EAP message.
13. The station encrypts the challenge message using its password (or other credential) as a secret key and sends the resulting value back to the AP.
14. The access point forwards the "encrypted challenge" to the "RADIUS server".
15. The RADIUS server uses the password (or other credential) that it has stored for the user to encrypt the same challenge message it sent to the station. If the resultant value and the value returned by
the station match, the RADIUS server sends a success message to the AP.
16. The AP forwards the "success/failure"  message to the station.
17. The station now sends a "challenge" to the "RADIUS server" to authenticate the AP (the network), and proceeds through the reverse authentication process.
18. If the network is successfully authenticated, the station passes a success message through the AP to the RADIUS server, which opens a port. The user is now LIVE on the network.
19. The station and RADIUS server each generate a dynamic unicast  key (which will match) from key material exchanged during the mutual authentication phase.
20. The RADIUS server sends the unicast WEP key to the AP in a RADIUS attribute. The attribute is encrypted using the shared key used between the AP and RADIUS server.
21. Now Client sends "DHCP" discover message if it is configured for DHCP IP address.
22. DHCP servers will respond with "DHCP offer " messages.
23. Client will respond for one of the "DHCP offer" Messages with "DHCP request"
24. Then corresponding "DHCP server" responds with "DHCP ACK" message for confirmation.
25. Now client is able to send data to the AP network.

Please see the above frame exchange for the sample 

Thanks to :

http://ilovewifi.blogspot.in/2012/07/step-by-step-process-in-connecting.html

Gopalswamy Doraiswamy Naidu - The Legend

G. D. Naidu (Gopalaswamy Doraiswamy Naidu

History
	Late Shri G.D.Naidu, a successful industrialist and a keen educationalist of South India, started two premier institutions in the year 1945, “Sir Arthur Hope College of Technology” and “Sir Arthur Hope Polytechnic”. He donated both the institutions to the Government. Both colleges are now known as “The Government College of Technology” and “The Government Polytechnic of Coimbatore” respectively.
	Later in 1946, he started another institute in the name of Industrial Labour Welfare Association (ILWA), now known as G.D.Naidu Charities. His aim was to impart higher education with more emphasis on practical training so that students can get employment immediately.
G.D.Naidu Charities conduct short term courses on ,Automobile servicing and maintenance, WEDM, EDM, CAD (AutoCAD, Pro-E, Inventor, Power Shape, CATIA, ANSYS, SOLIDWORKS, etc.), CAM (Keller, Delcam, Unigraphics), Metrology, CMM etc. It has trained more than 35000 students over the past 62 years.
G D Naidu Charities has set up a new training centre known as Gedee Technical Training Institute (GTTI) at Coimbatore, South India to train Precision Machining Technicians, Tool & Die Engineers, Tool Designers, Mechatronics Technicians and Mould Polishers.
Objective
To run a high-tech training centre that can:
Discipline the young minds and bring out the hidden talents to create technocrats of tomorrow. Select and train students in all possible latest technologies. Conduct full time courses in the area of technical skills and other allied trades at craftsman level. Provide consultancy for the local small and medium scale industries. Follow a modern syllabus, which is designed to systematically educate students on CAD/ CAM and CNC machining. Build up self-esteem, pride, sense of social belongingness and cleanliness in the trainees during the process of training.

Gedee Technical Training Institute

734, Avinashi Road, (Next to President Hall) Coimbatore – 641 018, INDIA Phone : 91 - 422 - 22 222 43     91 - 422 - 22 22 058 Fax : 91 - 422 - 22 42 760 Email : administration@gttiinfo.com     gtti@rediffmail.com

http://www.gttiinfo.com/courses_offered.html

G. D. Naidu (Gopalaswamy Doraiswamy Naidu (23 March 1893 – 4 January 1974) was an Indian inventor and engineer who is also referred to as the Edison of India.^[1][2] He is credited with the manufacture of the first electric motor in India. His contributions were primarily industrial but also span the fields of electrical, mechanical, agricultural (Hybrid cultivation) and automobile engineering.^[3] He had only primary education but excelled as a versatile genius. Among his hobbies was train travel to nearby cities.

G. D. Naidu (Gopalswamy Doraiswamy Naidu) who is sometimes referred as the "Edison of India.His contribution spans the fields of electrical, mechanical, agricultural (Hybrid cultivation) and automobile engineering . Mostly at an Industrial level than the Academia.

If there is one name that best symbolises Coimbatore's spirit of entrepreneurship, it is that of G D Naidu. Born on March 23rd, 1893, in Kalangal near Coimbatore, this school dropout began his Transport business in 1920, with the purchase of a passenger auto-coach, which he himself drove for the service between Pollachi and Palani. In a matter of a few years, his United Motor Service (UMS) owned the most efficient fleet of public transport vehicles in the country. In 1937, the first motor to be produced in India, was brought out at G D Naidu's UMS factory.

As an inventor, G D Naidu was one-of-a-kind in the country. He invented an Electric Razor - Rasant, that gave users far more shaves than other existing options in the international market. Among his other inventions were the super-thin shaving blades, a distance adjuster for film cameras, a special fruit juice extractor, a tamper-proof vote-recording machine and a kerosene-run fan. In 1941, he announced that he had the ability to manufacture five-valve Radio sets in India at a mere Rs 70/- a set. In 1952, his brainchild - the indigenously built Petrol engine two-seater Car (costing a mere Rs 2,000/-) rolled out. But production was stopped subsequently, because of the Government's refusal to grant the necessary license. His inventiveness was not confined to machinery alone. He is said to have grown ten feet high Cotton plants, millet plants with high yields and several injections for plants that made possible what Sir C V Raman called "Botanic marvels". t On his trips abroad, Naidu always seemed to draw appreciation for his innovations and his personal drive. In 1935, he personally filmed the funeral of King George V at London. In 1936, he met Adolf Hitler in Germany (even taking Still Photographs of the Fuhrer)[citation needed]. Among the Indian stalwarts that GD Naidu's camera captured were Mahatma Gandhi, Pandit Jawarharlal Nehru and Subash Chandra Bose. GD Naidu remained an outsider to Politics, despite having contested and lost in the 1936 Provincial General Elections.

In 1944, Naidu retired from active involvement with his automobile combine and announced several philanthropic measures including grants for Research scholarships and welfare schemes for his employees and the depressed sections of society. Through Naidu's efforts and his donations the Arthur Hope Polytechnic and the Arthur Hope College of Engineering were set up. In 1967, the G D Naidu Industrial Exhibition, conceptualised, designed and built by the great man himself, was established.

With his demise on the 4th of January, 1974, Coimbatore lost its greatest ambassador to the world. There have been several tributes paid to this legend, but none seems as apt as that by Sir C V Raman: "A great educator, an entrepreneur in many fields of engineering and industry, a warm-hearted man filled with love for his fellows and a desire to help them in their troubles, Mr Naidu is truly a man in a million - perhaps this is an understatement!"

He is credited in manufacturing the first electric motor in India. An Industrial Exhibition in Coimbatore is held in his name. He started the first Engineering college at Coimbatore (now known as Government College of Technology). He provided employment in engineering and manufacturing sector to many individuals in fifties and sixties (early for a home grown entreprenuer in India). He was considered as a visionary in Coimbatore and rest of Tamil Nadu as well.

There is a school in his hometown Coimbatore named after him (G.D Matriculation Higher Secondary School) and is managed by his daughter-in-law Mrs. Chandra Gopal. His grandson Mr. Rajkumar now runs the Gedee industries that has seen better days during his lifetime.

G.D.Naidu Museum & Industrial Exhibition in Coimbatore

Posted on January 2, 2011 by Rajesh K

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G.D.Naidu (1893 – 1974) was an eminent inventor and industrialist from Coimbatore, India. What’s interesting about this inventor is he never studied more than the primary level! In fact, even at an young age, he hated the system so much that the threw mud in the face of his teachers (Twice. Back then, there were no slates but one needed to write using fingers on sand). So, naturally he was sent out of school and perhaps that should have been a very happy incident for our young G.D.Naidu! It seems he was quite mischievous during his early years and he is even believed to have lit a whole stack of hay which was kept over a cart, just for fun!

The above picture is of the Motor bike that he bought from an English revenue official because he was very impressed on seeing it for the first time. He was inquisitive to learn how it works and hence he disassembled it and reassembled it several times to understand how the various parts of the vehicle works!

The above picture is of the first electric motor in India which was made by G.D.Naidu’s UMS Group company called National Electric Works in 1937. These exhibits and many more are kept at the G.D.Naidu Museum and Industrial Exhibition (Near to Nilgiris Super Market – Avinashi Road in Coimbatore, the same building also hosts the Indo-German Goethe Institute & UMS Training center among others).

The G.D.Naidu museum and industrial exhibition in Coimbatore contains an impressive collection of various electrical/electronic/scientific devices and gadgets that has been in existence since very early years to the recent times. In fact, one visit to this museum would make you familiar with the different types of film cameras, projection TV’s, ball point pens, floppy disks, calculators, type writers, PA systems, printers, and many many more devices that were used right from 1930′s/40′s till today! The photos below show a few of the interesting equipments kept on display.

Have you heard of a mechanical calculator? Well, you have just seen one!! 

Well this is how the earliest computers looked! In fact, there is one more computer kept there which was so huge that it did not fit in to my camera view! Its specifications should interest you though – 48 Kilo Bytes of RAM/ 8.6 Mega Bytes of 12″ Hard Drive/ 8″ Floppy drive cabinet  Somehow, I managed to take a photo that covered half of this whole computer – check it out.

In certain sections of this museum, the lights would go on automatically as you walk near that part of the exhibits and then switch off automatically when you go away from them! There were so many exhibits arranged in multiple long rows and a long shot of one of the rows is shown below.

In fact, there is a separate area where quite a few scientific exhibits are kept for encouraging curiosity in Science for children and perhaps even the grown up ones! For example, Try to say the colour of each of the below word (without struggling, that is) – 

Below picture is of a cool early petrol car that G.D.Naidu built in India. But sadly, the Government rejected the License to him for manufacturing them commercially (In those days, you could manufacture something commercially only if you got a license from the Government  ).

He wanted to make certain precision blades/knives too (he learnt the technology during one of his visits to Germany) but the Government gave the license for manufacturing it to someone else! It seems multiple such rejections made him destroy some of his inventions in frustration and perhaps even discouraged the great innovator who could have done much more. It seems, he had the hands of a wizard and what ever he touched became Gold! He even grew some plants in his garden which were unusual (Like cotton plants that grew to ten feet, coconut trees that were 3 feet, etc using perfect organic methods). But sadly, we do not have any documentation of all those methods and perhaps we don’t deserve them as well!

It seems in-spite of such things, he was warm, always smiling, helping others and kept a positive outlook towards life. He founded a school and a college and his company – Universal Motor Service provided a lot of jobs to engineering people back then in Coimbatore.  Below are some of the awards received by him, his company and the museum.

G.D.Naidu was an avid photographer and photography was his hobby and passion. If you visit his museum, you can see the photos of a number of dignitaries who have visited the museum (A.B.Vajpayee, Indira Gandhi, etc). In fact, he was personally invited to the funeral procession of King George V at London and he filmed it too…

In 1940, a Phonograph was assembled and made by G.D.Naidu himself with some help from an American firm and his own UMS. By the way, a phonograph is a device that can record and playback sound, originally invented by Edison. Perhaps that’s why he was called as the Edison of India!

This museum is open from morning 8:00 AM to 4:30 PM (Except on Sundays) and is a must visit for all science history enthusiasts.