CWAP - Chapter 7
What IFS is used for QoS devices?
AIFS AIFS is used when arbitration has yet to be completed. AIFS is equal to SIFS plus a varying number of slot times. The varying slot times depends on which access category is being used. The higher the priority the shorter the slot times. The number of slot times used in the AIFS is called the arbitration interframe space number (AIFSN) Voice and Video - 2 Slot times Best Effort - 3 slot times background - 4 slot times
How do STA's set their NAV timer?
AP's and STA's set their NAV value according to the duration value inside the 802.11 header. The duration value is contained within the Duration/ID field of all 802.11 frames except the power save poll. The duration value will set the NAV only if the AP or STA viewing the header is not the receiver frame.
What is the contention window?
After a STA has waited while performing both virtual and physical carrier senses, the STA may contend for the medium during a window known as the contention window. The upper limit for the random backoff is always equal to the contention window (CW). The CW is derived from the equation 2 to the X -1 where X is a value that increments with each failed frame. For DSSS X starts at 5 which results in a CW of 31. OFDM networks X starts at 4 which results in a CW of 15 The design of the CW gives OFDM WLANS a major boost in efficiency compared to DSSS based networks. Not only is the slot time shorter but the Random sample is half as large. This random backoff design should also illuminate why it is so important to monitor retransmissions on a WLAN. Failed frames cause the CW to grow exponentially because of incrementing x values. A larger CW results in more quiet time on the channel. More quiet time means a less efficient channel, thus resulting in lower throughput
Describe Random Backoff Timer
Another mechanism that prevents collisions by differentiating 802.11 channel access is a quiet period of time before a frame transmission, similar to the IFS. Unlike IFS, the random backoff is not static. It is a period of time that changes based on a random number chosen by each AP. AP's and STA's stay quiet during the random backoff by randomly choosing a number of slot times and then counting down until the number of slot times is equal to zero. Once the device hits zero the STA can transmit
What are two important factors to remember about CCA?
CCA only applies to 802.11 modulation. Interference from non-802.11 devices does not cause the CCA to go into a busy state CCA may not keep ALL devices within a BSS quiet. If an AP or STA is too far away to detect data transmissions at the requisite energy level, the CCA may go into the idle state even though the channel is still occupied If an AP or STA sees the CCA as idle while the channel is still occupied, it could lead to collisions. That's why 802.11 WLANS have the NAV, which acts as a virtual alternative
How does Clear Channel Assessment work?
CCA simply involves listening to the channel. If the channel is considered occupied, then the CCA indicates a state of busy. If the channel is considered clear, then the CCA indicates a state of idle. The CCA is set to busy if a high enough level of energy is detected coming from a valid, modulated 802.11 bits. Energy detection level varies by transmit power
What do 802.11 STA, including both AP STA and non-AP STA use to contend for the wireless channel?
CSMA/CA
In regards to medium contention which protocol is used for non QoS STA's to contend for the medium?
Distributed Coordination Function (DCF). This also means that these STA's can only send null and data frames rather than QoS data and QoS null frames. Also note that AP's must also follow the same contention rules as STA's
Hybrid Coordination Function (HCF) involves two channel access methods, what are they?
HCF controlled channel access - Designed to be an improvement for contention-free access (specifically, PCF) Enhanced distributed channel access - Designed to be an improvement for contention-based access (specifically DCF)
The 802.11e amendment introduced QoS to 802.11 networks. 802.11e defines a new channel access method called?
Hybrid Coordination Function which improves upon both DCF and PCF.
What are the 4 rules that define which IFS will be used before a frame transmissions
If the arbitration has been completed, then a reduced IFS (RIFS) or short IFS (SIFS) will be used. In most cases, the SIFS is used. The RIFS is only used between consecutive frames transmitted by the same 802.11n device If arbitration has not been determined, then an arbitration IFS (AIFS) or DCF IFS (DIFS) will be used. The AIFS is used for WLAN's that support 802.11e QoS, and the DIFS is used for WLANs that do not support 802.11e QoS If an AP or STA has received a corrupted frame - as defined by having an incorrect FCS, then an extended IFS (EIFS) will be used The PCF IFS (PIFS) is part of PCF and therefore not used in the real world.
To prevent all STA's from transmitting the moment the medium is free another protocol is used for medium contention
Interframe Space
Describe one problem that EIFS can have on a network
Near/far can be an unfortunate side effect of EIFS. When data is sent to the near STA, it may be transmitted at a data rate that is too high for the far STA to correctly demodulate. The frame therefore will appear corrupt even though the transmission was successful. The far STA will then have to stay quiet for an EIFS while the near STA will be allowed to use the short DIFS. The use of DIFS will give the near STA higher priority, thus giving the near STA tow or more consecutive opportunities to transmit while the far STA is quiet. This can lead the far STA with little opportunity to transmit
Explain Block Acknowledgements and how they are used?
Originally introduced in the 802.11e standard although not used until 802.11n. The block ACK process is designed to make the TXOP more efficient. When block ACK are used, the data frames of a CFB may be sent consecutively without interruption by ACK. At the end of a CFB, the data transmitting STA will simply send a block ACK request frame and receive a single block ACK frame in return. In the real work a CFB is rarely used but block ACK are used with 802.11n and frame aggregation (A-MPDU, A-MSDU). When multiple frames are aggregated by an 802.11n STA or AP into an A-MPDU, a block ACK can be used to ACK those frames more efficiently
What two carrier sense protocols are used by STA's to indicate whether a channel is busy or idle
Physical Carrier Sense - also known as clear channel assessment Virtual Carrier Sense - also known as Network Allocation Vector Both QoS and non-QoS devices use the CCA and the NAV in the same way. So before a STA attempts to transmit data, both the CCA and the NAV must be checked in order to verify that the channel is idle.
What are two other protocols that use CSMA/CA but have never been implemented in the real world
Point Coordination Function (PCF) and HCF-controlled channel access (HCCA) Both protocols involve the AP taking over control of the wireless channel and dictating which STAs are allowed to send through the use of polling.
AP's and STA's that support QoS are called?
QoS AP's and QoS STA's
When a QoS STA associates to a QoS AP, they join a?
QoS BSS When a QoS AP and QoS STA form a QoS BSS, channel access is altered. Instead of having channel access being equal for all devices, as in the case in a non-QoS BSS
What IFS is used when a TXOP, CFB, and block ACK are all in use?
Reduced Interframe Space (RIFS). This is a new IFS that is used in 802.11n and is even shorter in time than an SIFS. A RIFS interval can be used in place of an SIFS interval, resulting in less overhead during a frame burst 2 microseconds long
What are Slot times?
Slot times are quiet periods, just like IFS. Slot times are 9 microseconds in length for 802.11a/n devices operating in 5Ghz. The same slot time length for 802.11g/n devices operating in the 2.4Ghz band, but only when HT or ERP is used with the short preamble. For 802.11 b/g/n devices on 2.4 Ghz band using DSSS, a 20 microsecond slot is used instead. 20 microsecond slot time is also used if HT or ERP is used with the long preamble. 802.11 FHSS networks use a 50 - microsecond slot time
What is the process if CCA detects modulated energy levels?
The CCA will go busy for 15 microseconds if DSSS modulation is being used or for 4 microseconds if OFDM modulation is being used. If at the end of that period modulated bits continue to be detected on the channel at the requisite energy levels, the CCA will continue to stay busy and 802.11 devices, both AP's and STA's, will continue to stay quiet.
Describe Extended Interframe Space (EIFS)
The EIFS is designed to give AP's and STA's a chance to retransmit after a failed frame If a previously received frame contains an error, then the transmitting STA must wait an EIFS duration instead of DIFS before transmitting For 802.11b/g/n devices using DSSS in the 2.4Ghz band an EIFS is 364 microseconds. 802.11a/n devices in the 5Ghz band is 160 microseconds. EIFS is by far the longest IFS
What is Interframe Space (IFS)
The IFS is a quiet period that APs and STA's must wait before any 802.11 frame transmission. There are many different IFS's used depending on the situation
Descirbe Point Coordination Function Interframe Space (PIFS)
The PIFS is equal to one slot time plus one SIFS, and it is designed to give AP's the chance to send a beacon in order to begin a CFB. The design of the PIFS is that it has a shorter length than the DIFS, thus giving it a higher priority In real-world 802.11 devices, the PIFS is only used with the Channel Switch Announcement. When a STA has a channel switch announcement to transmit, the PIFS gives them priority over all other frame types that must be sent after arbitration
What function does the Random backoff timer serve in relation to the 802.11e amendment?
The random backoff timer keeps the channel free of collisions, which is the same purpose it had in the original 802.11 standard (DCF) but rather than giving equal access to all traffic, the Backoff time that is apart of 802.11e it only gives equal access to all traffic in a given access category (AC) If different ACs are operating on the same channel, channel access will not be the same. The way this is manipulated is by the contention window (CW). Each AC has a min and max CW value defined by the AP
What is Transmit opportunity (TXOP)
This is a limited duration controlled access phase, providing contention free transfer for QoS Data. The data frame transmissions within a TXOP are called a contention free burst (CFB). No DIFS or AIFS may happen and no random backoff time may interrupt the CFB as long as the TXOP has not expired
Describe Reduced Interframe Space (RIFS)
This is a new interframe space that is used in 802.11n and is even shorter in time than SIFS. A RIFS interval can be used in place of an SIFS interval, resulting in less overhead during a frame burst The length is 2 microseconds. Only one type of device uses it, 802.11n devices using MIMO. It precedes only one type of frame:data. RIFS is used only between frames of a contention free burst (CFB). It also is used only between frames sent by a single device. It cannot be used, for example, between a data/ACK pair because those frames are transmitted by two different STA's
Describe Distributed Coordination Function Interframe Space (DIFS)
This is a period of time used with DCF clear channel assessment. During a contention period, if the medium is continuously idle for a DIFS duration, only then should it transmit a frame DIFS is designed to force AP's and STA's with ordinary data in the queue to stay quiet for enough time to allow higher priority frames to have access to the channel. It is used when the arbitration process has yet to be completed. The DIFS is made long - equal to the length of an SIFS plus two slot times DIFS is used by AP's and STA's that do not use QoS
Describe Short Interframe Space (SIFS)
This is a short gap or period of time that is used during the transmission of data Depending on the type of WLAN, the SIFS may be either 10 microseconds(b/g/n) or 16 microseconds long(a/n). SIFS is a commonly used IFS and is used whenever arbitration has been completed. It is not used if an 802.11n AP or STA is using MIMO to transmit frames. If an 802.11n device is transmitting multiple frames as part of a contention free burst (CFB) then the RIFS will be used between those frames.
What are some of the reasons CCA may fail to keep other devices on the channel quiet
Too far from the transmitting device obstructions blocking the RF path from the transmitting device Interference in the RF path to the transmitting device
A QoS BSS has four access categories (ACs) with varying levels of channel access. They are?
Voice, Video, Best Effort and Background
CSMA/CA causes WLAN's to have a much_____throughput-to-data rate ratio than wired LANS
lower
