On Fri, 18 Nov 2011 18:06:23 -0800, Joe <firstname.lastname@example.org> wrote:
>Please forgive the top posting, it's the quickest way that I can get to
>the points here.
Top posting is evil. Please find a suitable self-inflicted punishment
and vow never to repeat this transgression.
>In what follows below, Jeff, did you mean spherical, and NOT
>hemispherical? If indeed hemispherical, how is that hemisphere oriented
>wrt that Airport Express's case?
Hemispherical. The antenna is located at the "front" of the Airport
Express, away from the power plug. It radiates equally well in all
directions, except into the wall where it's plugged in. If you dangle
it at the end of an extension cord, it will begin to more closely
resemble a spherical pattern. There are probably some holes in the
pattern in the direction of the electronics and in the up/down
direction, but for all intents and purposes, the pattern is roughly a
The antenna is a PIFA PCB antenna. See:
The antenna is the horizontal traces on the PCB in the lower right
edge of the photo below:
Another view, this time on the right edge:
>Also, in the below, Jeff stated
>A typical access
>> point can handle about 100 users doing light web surfing and email,
>> about 5 users doing Netflix, and 1 user doing BitTorrent.
>What kind of method/algorithm does the wifi use to distribute
>transmit/receive bandwidth to the various individuals?
There are many systems running simultaneously. 802.11 prevents an
individual user from hogging all the air time by restricting the time
it can transmit data. That's partly why a 54Mbit/sec association
(connection) will only yield about 25Mbits/sec thruput. CSMA/CA also
provides for distributing the bandwidth. The "CA" stands for
collision avoidance, which does its best with backoff timers to again
prevent monopolizing the channel. Finally, the ethernet backhaul also
has a "fair share" algorithm to prevent the same problem on the wired
part of the network. If the wireless system uses QoS (Quality of
Service) to give priority to time critical packets (VoIP), that will
also redistribute the bandwidth. Most public Wi-Fi system also have
some sort of bandwidth manager which limits the bandwidth of a single
>to Netflix and BitTorrent makes me think that somehow heavy users get
>all/most of the bandwidth and other users are almost shut out.
Netflix is fairly good about not monopolizing bandwidth. Watching an
HD movie will try to grab about 600Kbits/sec. However, BitTorrent
will grab ALL the bandwidth it possibly can. Worse, on an
asymmetrical broadband connection, it will saturate the upstream
bandwidth acting as a server, and prevent all downloading even if
bandwidth is available by delaying ACK's. If misconfigured, it can
also kill the access point by opening too many IP sockets and threads,
running the access point out of buffer memory. It's much like traffic
on the freeway. When things get busy, EVERYONE slows down.
>there some sort of time-allocation slicing to divvy up the bandwidth so
>that one user cannot hog all the bandwidth?
Not with 802.11. However, there are other polling algorithms that
sequentially poll connected users for data. For a large number of
users, that's more efficient than the collision intensive method used
# Jeff Liebermann 150 Felker St #D Santa Cruz CA 95060
# http://802.11junk.com email@example.com