Листовка для Cisco Aironet 2702i AIR-CAP2702I-E-K9
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AIR-CAP2702I-E-K9
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Channel access for 40 MHz. Perhaps the fundamental reason for the success of 802.11 is that anyone
can install an AP or use a client, regardless of what other 802.11 devices are already nearby, and mostly it
all “just works.”
all “just works.”
This comes from a MAC design goal that channel access be reasonably efficient and fair to all, regardless
of number of devices, distance to AP, device capability, and so forth -
put succinctly as “your packet is as
important as my packet.
” We see the efficiency goal in the range of MAC techniques to reduce collisions,
such as physical carrier sense (don’t transmit if you hear a lot of energy) and virtual carrier sense (don’t
transmit while someone told you they’d be transmitting or receiving). We see the fairness goal in that each
transmit while someone told you they’d be transmitting or receiving). We see the fairness goal in that each
device is permitted to transmit only after meeting the same carrier sense and collision avoidance
requirements.
However, 40 MHz brings real challenges to both collision avoidance and fairness, since it is either
cost-prohibitive or impossible to maintain accurate physical carrier sense and virtual carrier sense on two
20-MHz subchannels in parallel. Instead,
a “primary” 20-MHz channel is defined with the usual tight
requirements on carrier sense and collision avoidance, augmented by a degraded physical carrier sense on
the “secondary” 20-MHz channel. When a device wants to transmit, it performs channel access in the usual
the “secondary” 20-MHz channel. When a device wants to transmit, it performs channel access in the usual
way
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- all on the primary 20-MHz subchannel. Also, immediately before the device can transmit a 40-MHz
packet, the device inspects the physical carrier sense state of the secondary channel for a short duration to
make sure that the secondary channel is clear too. If clear, the 20 MHz packet is sent; otherwise the device
can either (1) transmit a 20-MHz packet on the primary 20-MHz channel or (2) back off again, then recheck
to determine whether the full 40 MHz is clear. Remarkably, this simple scheme is reasonably fair, and
option (1) is reasonably efficient.
Still, in some topologies, devices on the secondary 20-MHz channel are treated unfairly with respect to the
40-MHz devices, and so 802.11n has additional channel selection rules to try to avoid this scenario in the
first place. These rules work pretty well, given the large number of 40-MHz channels available at 5 GHz.
802.11n was notorious in the standards community for its slow progress. There were three causes: (1) The process
that 802.11n chose to select the winning proposal invited contention. (2) 802.11n was loved nearly to death, in that
many experts wanted to help and to contribute their technology. It took a long time to work through the contention
by adopting many optional modes and then a long time to refine all the optional modes. (3) Operation of 802.11
systems at 2.4 GHz using 40-MHz channel widths in the proximity of 802.15 systems (such as Bluetooth) raised
concerns among parts of the 802.15 community.
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That is, the device checks carrier sense and, if the channel is busy, waits until it is clear, randomly backs off a number of slots,
and waits while it counts off those slots.