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THIS PAGE EXPLAINS, IN SIMPLE TERMS, HOW FROTTLE [1] WORKS, AND WHY IT
IS NEEDED.
THE HIDDEN NODE PROBLEM
The WiFi [2] protocol, the collective name for the 802.11a/b/g suite
of protocols for Wireless Ethernet, was only ever designed to be used
indoors over a relatively small area. In a long-range outdoor
environment, the problems with this design become apparent and we
experience the Hidden Node problem. Frottle is an application that can
reduce or eliminate this problem.
COLLISION AVOIDANCE
On any network, computers and other networked devices (called hosts)
cannot make sense of multiple messages if they arrive at the same
time. On a wired network, a host will first check that there is no
other traffic on the network before transmitting its data. This is
called Carrier Sense Multiple Access with Collision Detection
(CSMA/CD) ( Explainer [3] ). On a wireless network, a host cannot
transmit and receive data at the same instant, so CSMA/CD will not be
effective. 802.11b/g use the Carrier Sense Multiple Access/Collision
Avoidance (CSMA/CA) method whereby a host will first transmit a
message, called a Request To Send (RTS) to the intended recipient.
(Explainer [4] ). In a BSS [5] network, this is the Access Point [6]
(AP). If the AP senses that the airwaves are clear, it sends a Clear
To Send (CTS) message back to the host. The host then sends its data.
AN INDOOR DEPLOYMENT
In an indoor environment where all wireless hosts and the AP are
close to each other, the CSMA/CD method works well for coordinating
traffic. Every host can receive the radio transmissions of every other
host, and so traffic is coordinated.
HIDDEN NODE INDOORS
Even in an indoor environment, Hidden Node can be a problem. If one
host, or node, wanders out of range of all the other nodes on the
network, it cannot coordinate its traffic directly with the other
nodes. It must rely directly on the AP and use RTS/CTS packets to know
when to transmit. In a network with a small number of nodes, RTS/CTS
packets are generally successfully in preventing the Hidden Nodes from
interrupting traffic flow. But when many hosts are connected, and
those hosts are transmitting/receiving a large volume of traffic, the
RTS/CTS packets themselves start to collide.
HIDDEN NODE OUTDOORS
In a long-range, outdoor environment, in almost all cases no node can
hear the transmissions of any other node, except the AP itself. All
nodes are relying on the RTS/CTS method. The more nodes connected
means more traffic volume at the AP and a greater probability of
RTS/CTS packets colliding. When RTS/CTS packets collide, all nodes
stop transmitting for a moment, and then send their RTS packets again.
In very congested situations, this can cause all traffic to stop for
considerable periods of time, making the network unusable.
WHAT FROTTLE DOES
Frottle uses the Linux Netfilter firewall in all hosts on the network
to delay traffic. Frottle uses a Media Access method similar to that
of a Token Ring [7] network. The trasmission of all traffic on all
hosts, including control packets, is done in a coordinated fashion, so
no collisions should ever occur.
Links:
------
[1] http://melbournewireless.org.au/?FROTTLE
[2] http://melbournewireless.org.au/?WiFi
[3] http://bucarotechelp.com/networking/97090740.asp
[4] http://www.wispfaq.com/searchfaq.html?id=88
[5] http://melbournewireless.org.au/?BSS
[6] http://melbournewireless.org.au/?AccessPoint
[7] http://foldoc.hld.c64.org/foldoc.cgi?token+ring
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