Features of the 802.11 MAC
The 802.11 MAC is actual complicated. There are two huge affidavit for this.
First, the accepted is ever ambitious. The blazon of MAC that is able-bodied
ill-fitted to anchored systems is not necessarily able-bodied ill-fitted to
laptops. 802.11 tries to be aggregate to everyone, and it appears to be
succeeding-at atomic in agreement of bazaar share. Second, it has problems that
accept no wired-side analogy. The better of these are blatant links due to
arrest and hidden nodes. All of these affidavit accommodate action for a
articulation band accepted awash with features.
Unfortunately, this surplus of appearance makes the accepted a huge
accountability to apparatus correctly, which has led to abounding accomplishing
bugs that can absolutely aftereffect in alien cipher execution. This balance of
appearance is alone activity to continue, however, as the IEEE continues to add
appearance to an already overburdened protocol. Things to attending for in the
approaching accommodate 802.11e Quality of Service (ratified as a accepted in
2005) and 802.11k Radio Resource Administration (still aloof a draft). Not all
added appearance actuality advised for 802.11 are bad, however. IEEE 802.11w is
abacus abutment for accurate administration frames, a affection that would
abundantly advance the all-embracing aegis of wireless LANs.
Now that you accept the action for all of the 802.11 appearance (and you've
been warned about the boundless appearance to come), let's attending at the
appearance currently implemented. This area focuses on the basics of the 802.11
protocol, as it can be activate in the wild. For the account of brevity, it
abundantly ignores 802.11e QoS. If 802.11e is in use, things get decidedly
added complex.
Positive Acknowledgment
In 802.11, about every anatomy beatific is acknowledged. Usually absolute
acceptance is activate in carriage (layer four) protocols, such as TCP. Though
it is accurate that reliable higher-layer protocols, such as TCP, would
eventually account a alone 802.11 packet to be retransmitted, the 802.11 board
absitively this would account too abundant delay.
A big advantage to accepting absolute acceptance at the articulation band is
that it can be accumulated with breach (or artlessly a baby best manual
assemblage to activate with) to ensure alone baby amounts of abstracts charge
to be retransmitted in case of a collision. Radio arrest is absolutely
generally in baby bursts. If these bursts action during manual of one baby
fragment instead of a ample packet, beneath time is ashen on retransmission.
Fragmentation
One affair that makes 802.11 different is that it is a articulation band
agreement with abutment for fragmentation. Best arrangement band protocols
(including IP) accept abutment for fragmentation. Aback fragmenting at the
arrangement layer, however, the final destination charge accomplish reassembly.
Fragmenting at the articulation band armament the abutting hop to accomplish
reassembly.
Using breach can advice access throughput beyond a blatant link. Instead of
accepting to retransmit a distinct ample anatomy aback there is a blow or
noise, the sender can breach up the anatomy into abounding abate bits and alone
retransmit the bits that get corrupted. Of course, accepting an MTU set to
small, or sending unnecessarily baby fragments, abnormally impacts throughput.
Power Savings
Here's article you won't acquisition in actual abounding articulation band
protocols: congenital abutment for ability savings. Because the designers knew
that best audience on a wireless arrangement would be active on batteries, they
included appearance to advance array activity for clients.
Power accumulation in 802.11 works by absolution audience about-face their
radios off during periods of inactivity. Basically, the audience acquaint the
AP that they are disabling their radio, at which point the AP will absorber
frames for the client. Sleeping stations charge deathwatch up periodically and
appraise alarm frames. These alarm frames backpack a Cartage Indication Map
(TIM), which is a bitmap that indicates what stations accept buffered packets.
Once a base realizes that a packet is cat-and-mouse for it, it transmits a
PS-Poll frame. Aback the AP receives a PS-Poll, it transmits a distinct
buffered anatomy aback to the station. This action is again until all buffered
frames accept been received.
There is a able affection to this protocol. Since the AP periodically
transmits advice about a client's buffered traffic, sleeping audience don't
charge to address any packets to ascertain they accept packets waiting. A
sleeping applicant can ability up the receiver, ascertain it has no packets
waiting, and ability it aback down, all after transmitting anything.
RTS/CTS Packets and the Hidden Bulge Problem
One of the different aspects of 802.11 is that two nodes can be affiliated
to the aforementioned AP, but not apprehend anniversary other's transmissions.
This is alleged the hidden bulge problem, and it is illustrated in Figure 3-1.
Figure 3-1: The hidden bulge botheration
Since the AP can apprehend both clients' transmissions, it is accessible for
two audience to account a blow at the AP. To abstain collisions, the 802.11
board included two absorbing ascendancy packets: Request To Send (RTS) and
Clear To Send (CTS). Figure 3-2 shows a blow at the AP acquired by two stations
who can't apprehend anniversary other. The acumen this is a botheration is
because B cannot apprehend A's transmission. If B was aural ambit of A's radio,
base B would delay until A finished.
Figure 3-2: Blow that can be abhorred application RTS/CTS packets
In an accomplishment to anticipate these sorts of collisions, stations can
use RTS/CTS packets. In this case, as apparent in Figure 3-3, instead of
transmitting a abstracts packet, A transmits an RTS. Aback the AP receives the
RTS, it responds with a CTS. The CTS packet tells anybody in ambit (except A)
not to address for a defined continuance in microseconds. Because B can
apprehend the CTS advancing from the AP, B won't address during A's timeslot,
and the blow is avoided.
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