What is gaurd Interval
It makes sure that Inter Symbol Interference at the receiver is minimized, if the channel exhibits delay spread.
Delay spread is caused by dispersive medium, Ex:
Why is 802.11ac requires every frame to be sent in AMPDU format ?
What are the differences between 11n & 11ac
Removed features:
1. green Field
2. RIFS
3. PCO
4. LSig protection
5. Unequal modulation
6. Dual CTS protection
Added
1. Enhanced CCA on Secondary channels by increasing the sensivity to -72dBm from -62 dBm.
2. Static & Dynamic Band width selection.
The basic Request-to-Send (RTS) and Clear-toSend (CTS) mechanism of 802.11 is modified to
improve dynamic channel width operation.
Consider an interference scenario illustrated in
Figure 3, whereby STA2 is transmitting to AP2 and
AP1 is communicating with STA1. AP2 is
occupying overlapping channels of the secondary 40
MHz channel of AP1. STA1 and STA2 can interfere
with each other, but the interference is not heard by
the two APs. To address this situation, bandwidth
signaling is added to the RTS and CTS frames. As
illustrated in Figure 4, AP1 sends an RTS with the
bandwidth of the intended transmission, which is 80
MHz comprised of channels 36, 40 44, and 48 in this
example. Before STA1 replies with a CTS frame, it
senses the medium on all secondary channels for
PIFS. If the secondary 40MHz channel is not free,
STA1 sends a CTS response with the BW of the
clear channels, i.e. 40 MHz comprised of channels
36 and 40 in this example. Then AP1 sends data to
STA1 only on the clear channels and STA1 replies
with Block Ack (BA) frames that are duplicated over
the clear channels.
However, if the interference in this example is
frequent, another new mechanism may be employed.
In such a case, STA1 can send a Operating Mode
Notification frame to AP1 to tell the AP that the
client is changing its bandwidth on which it
operates. For example, STA1 can change its
operating bandwidth from 80 MHz to 40 MHz with
the constraint that the client still needs to use the
same primary channel as the AP. Subsequently the
AP will only send data frames at this reduced
bandwidth.
What is VHT-Single MPDU
Normal Ack or Implicit Block Ack Request.
In a frame that is a non-A-MPDU frame or VHT single MPDU:
The addressed recipient returns an ACK or QoS +CF-Ack frame after a short
interframe space (SIFS) period, according to the procedures defined in 9.3.2.8 and
9.19.3.5. For a non-DMG STA, this is the only permissible value for the Ack Policy
subfield for individually addressed QoS Null (no data) frames.
In a frame that is part of an A-MPDUOtherwise:
The addressed recipient returns a BlockAck MPDU, either individually or as part of
an A-MPDU starting a SIFS after the PPDU carrying the frame
PLCP Signal _ legacy (Length field):
The PLCP LENGTH field shall be an unsigned 12-bit integer that indicates the number of octets in the PSDU that the MAC is currently requesting the PHY to transmit.
IN VHT
802.11ac devices set the data rate to 6 Mbps and derive a spoofed length in bytes so that when any receiver calculates its length, it matches the time duration required for the 802.11ac frame.
The MPDU Delimiter for each sub frame contains the length of the AMPDU sub frame.
It makes sure that Inter Symbol Interference at the receiver is minimized, if the channel exhibits delay spread.
Delay spread is caused by dispersive medium, Ex:
Why is 802.11ac requires every frame to be sent in AMPDU format ?
An MPDU contains only packet duration but not length information. Because the delimiter in an A-MPDU contains MPDU length information, 802.11ac requires that even a packet with a single MPDU is transmitted in an A-MPDU format to provide packet length information in bytes.
What are the differences between 11n & 11ac
Removed features:
1. green Field
2. RIFS
3. PCO
4. LSig protection
5. Unequal modulation
6. Dual CTS protection
Added
1. Enhanced CCA on Secondary channels by increasing the sensivity to -72dBm from -62 dBm.
2. Static & Dynamic Band width selection.
The basic Request-to-Send (RTS) and Clear-toSend (CTS) mechanism of 802.11 is modified to
improve dynamic channel width operation.
Consider an interference scenario illustrated in
Figure 3, whereby STA2 is transmitting to AP2 and
AP1 is communicating with STA1. AP2 is
occupying overlapping channels of the secondary 40
MHz channel of AP1. STA1 and STA2 can interfere
with each other, but the interference is not heard by
the two APs. To address this situation, bandwidth
signaling is added to the RTS and CTS frames. As
illustrated in Figure 4, AP1 sends an RTS with the
bandwidth of the intended transmission, which is 80
MHz comprised of channels 36, 40 44, and 48 in this
example. Before STA1 replies with a CTS frame, it
senses the medium on all secondary channels for
PIFS. If the secondary 40MHz channel is not free,
STA1 sends a CTS response with the BW of the
clear channels, i.e. 40 MHz comprised of channels
36 and 40 in this example. Then AP1 sends data to
STA1 only on the clear channels and STA1 replies
with Block Ack (BA) frames that are duplicated over
the clear channels.
However, if the interference in this example is
frequent, another new mechanism may be employed.
In such a case, STA1 can send a Operating Mode
Notification frame to AP1 to tell the AP that the
client is changing its bandwidth on which it
operates. For example, STA1 can change its
operating bandwidth from 80 MHz to 40 MHz with
the constraint that the client still needs to use the
same primary channel as the AP. Subsequently the
AP will only send data frames at this reduced
bandwidth.
What is VHT-Single MPDU
Normal Ack or Implicit Block Ack Request.
In a frame that is a non-A-MPDU frame or VHT single MPDU:
The addressed recipient returns an ACK or QoS +CF-Ack frame after a short
interframe space (SIFS) period, according to the procedures defined in 9.3.2.8 and
9.19.3.5. For a non-DMG STA, this is the only permissible value for the Ack Policy
subfield for individually addressed QoS Null (no data) frames.
In a frame that is part of an A-MPDUOtherwise:
The addressed recipient returns a BlockAck MPDU, either individually or as part of
an A-MPDU starting a SIFS after the PPDU carrying the frame
PLCP Signal _ legacy (Length field):
The PLCP LENGTH field shall be an unsigned 12-bit integer that indicates the number of octets in the PSDU that the MAC is currently requesting the PHY to transmit.
IN VHT
802.11ac devices set the data rate to 6 Mbps and derive a spoofed length in bytes so that when any receiver calculates its length, it matches the time duration required for the 802.11ac frame.
The MPDU Delimiter for each sub frame contains the length of the AMPDU sub frame.
