past issues of the column, we have
focused on bridges operating in
the 2.4 and 5 GHz band, using 802.11
technologies. These bridges wring
the most out of802.11, but even
with 802.11n, there is a limit
to the capacity they can provide.
BridgeWave’s gigabit wireless
bridges operate in the 60 GHz and
80 GHz frequency bands using custom
modulation methods to provide gigabit
speeds for a true alternative to
802.11-based bridges, where the
link speed is half-duplex, and
so is split between the upstream
and downstream side of the link,
BridgeWave’s solutions are
true full-duplex, so the aggregate
speed of the link is 2 Gbps (1 Gbps
up and 1 Gbps down).
60 GHz band is totally unlicensed
and can provide gigabit speeds up
to about 3/4 mile, or 1.5 miles with
a high-gain antenna. The 80 GHz band
requires “light” licensing
(much cheaper and faster than full
FCC licensing) and provides gigabit
speeds up to about 4-7 miles.
is not just about throughput, though.
It’s about reliability.
BridgeWave’s links can be engineered
to the desired level of availability:
up to 99.999%. BridgeWave’s
28-year Mean Time Between Failures
means that the equipment will probably
outlive the usefulness of the link
(ten-gig wireless, anybody?). Finally,
all BridgeWave radios undergo HALT/HAAS
testing before they are shipped.
This means that they are put into
an environmental chamber and cycled
through temperature extremes and
vibration all while powered on and
maintaining a link. This means that
any units with manufacturing defects
fail in the factory, not in the field.
contact Connect802 Sales at 925.552.0802
to learn more about BridgeWave’s
products. We look forward to hearing
two 802.11 devices transmit data
at the same time, on the same channel,
within range of each other, their
data is likely to be corrupted.
In this issue, you’ll learn
about Carrier Sense Multiple Access
with Collision Avoidance (CSMA/CA),
an algorithm in 802.11 that compensates
for this occurrence.
its name suggests, the goal of
CSMA/CA is to avoid collisions—that
is, occurrences when two devices
on the same channel, within range
of each other, transmit at the same
time. CSMA/CA in 802.11 is often
compared to its Ethernet counterpart,
CSMA/CD, which uses collision detection
instead of collision avoidance. All
other things being equal, collision
detection is a better strategy, because
it detects the actual occurrence
of a collision and takes action.
Collision avoidance does its best
to make sure that collisions don’t
happen, but it can’t directly
detect that one has occurred. So
why doesn’t 802.11 use collision
detection? Because in a wireless
network, it is impossible to directly
detect that a collision has occurred.
Wireless signals lose strength rapidly
as they propagate through the environment.
There’s no way for a station
to detect the very-weak signals of
other stations everywhere in its
coverage area, so there’s no
way for a station to know for sure
whether its transmission is causing
a collision. Collision avoidance
is the best 802.11 can do.
starts with “Carrier
Sense.” This means that, before
a station transmits, it tries to
detect whether another station is
already transmitting. If a transmission
is already occurring, the station
enters a mode called deferring.
The station continues to defer until
the original transmission completes.
802.11 requires a certain minimum
period of silence between each transmission,
known as the inter-frame spacing (IFS).
Once the air has gone silent, the
station waits the IFS.
the “Collision Avoidance” comes
into play. Imagine that the network
was silent. What are the chances
that two stations, purely through
chance, would start to transmit at
the same time, and have a collision?
The chances are pretty small. Now,
imagine the scenario where one station
is sending a long transmission. During
that transmission, any station that
wants to transmit will enter the
deferring state. This means that,
at the end of a transmission, there
are likely to be many stations deferring.
If all of those stations were to
wait the IFS and then immediately
transmit, a collision would be guaranteed.
Instead, what 802.11 does is this:
if a station is deferring, then after
the air goes silent and after the
IFS has passed, the station chooses
a random time period, known as the backoff
timer, and continues to wait
at least that long. This means that
if there are multiple stations deferring,
they will all pick different random
times. One of them will get to go
first and the others will go back
to deferring. The collision will
have been avoided. Of course, there
is still the possibility that some
other station, that was not deferring,
will spontaneously transmit and create
a collision, but as previously mentioned,
this is relatively unlikely.
a station chooses a backoff timer,
it continues to count down that
same backoff timer, even if it
goes back to deferring because
some other station’s backoff
timer has expired. This means that,
eventually, every station will get
a chance to transmit its data. From
this perspective, 802.11 provides
fair access to the medium.
Collectively, the behaviors described
above are known as the Distributed
Coordination Function (DCF). It
can be a little confusing to hear the
terms CSMA/CA and DCF both used. They
are, functionally speaking, equivalent
within the context of 802.11.
In “Essential Wi-Fi,” we
discussed 802.11’s Distributed
Coordination Function (DCF) and how
it provides fair access to the medium
while reducing the probability of
collisions. But DCF is not the end
of the story. In this article, we’ll
discuss the other side of the coin, Point
Coordination Function (PCF).
DCF provides fair access to the
medium, a station can be denied
access to the medium in the short
term. For example, imagine that you
have a packet to transmit. You detect
that another station is currently
transmitting, so you defer. After
the air goes silent, you wait the
IFS and then you choose a backoff
timer. So do several other stations
who were also deferring. Let’s
say that, just by chance, you chose
a particularly long backoff timer.
You’ll end up waiting while
all of the other stations transmit
their packets. In the long term,
all stations will choose the same average backoff
timer, because they’re all
using the same random algorithm.
But in the short term, a station
that chooses a long backoff timer
will end up waiting for a long time,
at least in computer terms.
many types of application, this
is not a problem. When we say, “waiting
for a long time,” we’re
probably not even talking about seconds.
We’re probably talking about
tens or hundreds of milliseconds,
depending on the length of the packets
that are being transmitted. When
doing a file transfer or downloading
a web page, it doesn’t really
matter if a few packets go very quickly
and a few packets go a little more
slowly, as long as the overall transfer
is sufficiently fast. But for applications
like Voice Over IP, there’s
an expectation that all packets arrive
with approximately the same spacing.
There’s even a name for the
degree to which the timing between
packets varies: jitter.
And if jitter is too high, call quality
The Point Coordination Function (PCF)
is 802.11’s answer for applications
that need regular, guaranteed, priority
access to the medium. The way it
works is this.
Every so many beacon intervals,
the AP will initiate the contention-free
period (CFP). It does this by
sending a special beacon frame that
causes all of the stations in range
to go silent. Any station that attempts
to transmit a frame during this period
will detect that the network is busy
and will enter the deferring state.
Next, the AP sends CF-Poll frames
to stations which have indicated
that they want to participate in
point-coordination. When a station
receives a CF-Poll frame, it may
send a single data frame. Stations
indicate whether they want to participate
in point-coordination when they associate
with the AP. When all stations have
been polled, or when the beacon interval
is over, the AP sends a CF-End frame,
which indicates that the contention-free
period is over. All stations resume
normal DCF operation, using CSMA/CA
to determine which one gets to transmit.
This process repeats itself periodically,
every so many beacon intervals.
of PCF is that certain stations get
guaranteed, priority access to the
medium. A voice-over-IP phone, for
example, could be configured to use
point-coordination, meaning that it
would be guaranteed to get to send
its data every 2 nd or 3 rd beacon
interval. Unfortunately, a disadvantage
of PCF is that it’s an optional
part of the 802.11 standard, and, to
our knowledge, no manufacturer has
ever implemented it in a production
access point or client. For all of
its potential benefits, it’s
only of theoretical interest.
for encrypting your Wi-Fi network
abound, but here’s
a new one: a man in Buffalo, NY, got
a visit from Immigrations and Customs
Enforcement after his IP address
was used to download illegal materials.
It was later determined that the
homeowner was innocent, but that
was probably cold comfort after having
his door kicked in, being thrown
on the ground, and having guns pointed
at him. Sharing your Wi-Fi may be
a neighborly thing to do, but leaving
the network unprotected opens the
door to all sorts of activities for
which law enforcement may try to
hold you responsible.
Some people argue that by leaving your
Wi-Fi network open, you can deny responsibility
for any illegal activity that occurred
on the network. This defense has been
used in P2P file sharing cases. Defenses
like this have worked
in a few cases , but it still seems
better to simply prevent the illegal
activity in the first place.