users-guide.tex

Linux下wifi实现

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\begin{document}
\title{Madwifi/Atheros Wireless Linux Driver Users Guide}
\author{Protocols Group}
\maketitle
\tableofcontents
\section{Configuring MadWifi using Wireless Extensions}
This section describes the configuration of the Atheros wireless
driver using the Wireless Extension Tools.

\subsection{Using \mytt{iwconfig}}
The generic \mytt{iwconfig} tool is used to set parameters which
common across most drivers.  For a detailed description of
\mytt{iwconfig}, please use \mytt{man iwconfig}.  In this Section, we
will describe the use of iwconfig in the Madwifi driver.  The formats
of the \mytt{iwconfig} command is:
\bv
\begin{tabular}{lll}
\mytt{iwconfig} & --help & \\
\mytt{iwconfig} & --version & \\
\mytt{iwconfig} & [\textit{interface}] & \\
\mytt{iwconfig} & \textit{interface} & [\mytt{essid X}] [\mytt{freq
  F}] [\mytt{channel C}] [\mytt{sens S}] [\mytt{ap A}] [\mytt{rate R}]
\\
& & [\mytt{rts RT}] [\mytt{frag FT}] [\mytt{txpower T}] [\mytt{enc E}] [\mytt{key K}] [\mytt{retry R}]\\
\end{tabular}
\ev

The first form of the \mytt{iwconfig} command gives a brief help
message. The second form of the \mytt{iwconfig} command returns the
current version of \mytt{iwconfig} along with the version of the
wireless extensions with which it was built.

In the third form of the \mytt{iwconfig} command, the current wireless
status of the \textit{interface} is returned.  If no
\textit{interface} is specified, the current wireless status of every
network interface is returned.  Non-wireless devices will not return
any wireless status.

The last form of the \mytt{iwconfig} command allows the user to change
any of the optional parameters. Only the parameters which you wish to
change need to be specified.  Unspecified parameters will not be
modified.  Each parameter is described below.

\subsubsection{\mytt{essid} - ESSID or Network Name}
Set the ESSID (also known as network name) to the given value.  In
station mode, the driver will attempt to join the network with the same
ESSID.  In AP mode, the driver will use the parameter as the ESSID.
\begin{example}
  The following command sets the ssid to ``Atheros Wireless
  Network'' on \mytt{ath0}:
  \bv
  \cmd{iwconfig ath0 essid "Atheros Wireless Network"}\\
  \ev
\end{example}

\subsubsection{\mytt{freq}/\mytt{channel} - RF Frequency or Channel}
Set the frequency or channel of the device to the given value.  Values
below 1000 are interpreted as channel numbers.  Values above 1000 are
interpreted as frequency measured in Hz.  For frequency values, the
suffix \mytt{k}, \mytt{M}, or \mytt{G} can be appended to the value to
specify kilohertz, Megahertz, and Gigahertz, so that \mytt{2.412G},
\mytt{2412M}, and \mytt{2412000000} refer to the same frequency. 
Setting the channel to a specific value will override the private
\mytt{mode} control described in Section~\ref{sec:iwprivs}.
\begin{example}
  The following command sets the operating frequency to 5.2GHz:
  \bv
  \cmd{iwconfig ath0 freq 5.2G}
  \ev
  Either of the following commands set device to operate on channel
  11:
  \bv
  \cmd{iwconfig ath0 freq 11}\\
  \cmd{iwconfig ath0 channel 11}
  \ev
\end{example}

\subsubsection{\mytt{sens} - Sensitivity Threshold}
Set the sensitivity threshold to the given value.  This is the lowest
signal strength at which the packets are received.  Currently, this
threshold is not implemented and any returned value is meaningless.

\subsubsection{\mytt{ap} - Use a Specific AP}
Specify which AP the device should associate with.  The supplied value
should be the MAC address of the desired AP or any of the keywords
\clival{any}, \clival{auto}, or \clival{off}.
\begin{example}
  The following command instructs the \mytt{ath0} device to associate
  with the AP that has MAC address 00:03:7f:03:a0:0d.
  \bv
  \cmd{iwconfig ath0 ap 00:03:7f:03:a0:0d}
  \ev
\end{example}

\subsubsection{\mytt{rate} - Set the Data Transmit Rate}
Set the bit rate for transmitted packets.  The value is specified in
bits per second and the values can be suffixed by \mytt{k}, \mytt{M}, or
\mytt{G} for kilobits, megabits, and gigabits respectively.  The value
\mytt{auto} is also valid and causes the device to use the bit rate
selected by the rate control module.  It's also possible to supply the
bit rate followed by \mytt{auto}, in which case the driver will
automatically select from the bit rates not exceeding that rate.
\begin{example}
  The following commands sets the maximum bit rate to 36Mbs.  Thus,
  the driver will automatically select the best rate less than or
  equal to 36Mbs.
  \bv
  \cmd{iwconfig ath0 rate 36M auto}
  \ev
\end{example}

\subsubsection{\mytt{rts} - Set the RTS/CTS Threshold}
Set the minimum packet size for which the device sends an RTS using the
RTS/CTS handshake.  The parameter is the threshold in bytes or the
\clival{off} keyword.  If it's set to \clival{off} or the maximum packet
size, RTS/CTS will be disabled.
\begin{example}
  The following command sets the minimum packet size to use the
  RTS/CTS handshake to 40.
  \bv
  \cmd{iwconfig ath0 rts 40}
  \ev
\end{example}

\subsubsection{\mytt{frag} - Set the Fragmentation Threshold}
Set the maximum fragment size.  The parameter is either the threshold in
bytes, or the \clival{off} keyword, which disables fragmentation.
\begin{example}
  The following command sets \mytt{ath0} to fragment all packets to at
  most 512 bytes.
  \bv 
  \cmd{iwconfig ath0 frag 512}
  \ev
\end{example}

\subsubsection{\mytt{key/enc} - Manipulate WEP Encryption Keys and Mode}
This parameter is used to manipulate the WEP key and authentication
mode.  It can be used to set the key, change the key, select the active
key, enable and disable WEP, and set the authentication mode.  The
driver can store up to 4 keys.  Each instance of the \mytt{key} command
manipulates only one key.  Thus, to change all 4 keys, 4 separate
commands must be used.

The key value can be specified as is in the hexadecimal form.  If an
ASCII string is used for the key value, prepend ``\mytt{s:}'' to the
key.  To change a key other than the current key, prepend
``[\mytt{index}]'' to the key value, where \mytt{index} is the number of
the key you wish to change.  To select which key is active, use
``[\mytt{index}]'' without supplying any keys, where \mytt{index} is the
desired key number.   Including the keywords \mytt{open} or
\mytt{restricted} changes the authentication mode between open
authentication and restricted authentication.  Use the \mytt{off}
keyword to disable WEP.
\begin{example}
  The following command sets the default key to be key 3:
  \bv
  \cmd{iwconfig ath0 key [3]}
  \ev
  The following command sets the default key to be the hex key
  0xDEAD-BEEF-AA:
  \bv
  \cmd{iwconfig ath0 key DEAD-BEEF-AA}
  \ev
  The following command sets key 2 to the ASCII phrase ``password''
  and sets the authentication type to open:
  \bv
  \cmd{iwconfig ath0 key [2] s:password open}
  \ev
  The following command disables WEP:
  \bv
  \cmd{iwconfig ath0 key off}
  \ev
\end{example}

\subsubsection{\mytt{txpower} - Set Transmit Power}
Set the transmit power for data packets.  Bare numbers are interpreted
as values in dBm.  If the number is followed by \clival{mW}, the value
is interpreted in milliwatts.  The value can also be \clival{auto} for
automatic power control and \clival{off} for disabling the radio
transmission.
\begin{example}
  The following command sets all data packets to transmit at either 30
  dBm or the maximum allowed in the current regulatory domain:
  \bv
  \cmd{iwconfig ath0 txpower 30}
  \ev
\end{example}

\subsubsection{\mytt{retry} - Set Retry Limit}
This parameter sets the maximum number of retries used in the software
retry algorithm.  Currently, the driver does not implement software
retry, thus this parameter is meaningless.

\subsection{Using \mytt{wlanconfig}}
\label{sec:wlanconfig}
The current MadWifi driver supports multiple APs and concurrent
AP/Station mode operation on the same device.  The devices are
restricted to using the same underlying hardware, thus are limited to
coexisting on the same channel and using the same physical layer
features.  Each instance of an AP or station is called a Virtual AP
(or VAP). Each VAP can be in either AP mode, station mode, ``special''
station mode, and monitor mode.  Every VAP has an associated
underlying base device, which is created when the driver is loaded.

Creating and destroying VAPs are done through the \mytt{wlanconfig}
tool found in the MadWifi tools directory.  Running the
\mytt{wlanconfig} utility with no arguments returns a brief help
line.  The format of the \mytt{wlanconfig} command takes two forms:
\bv
\mytt{wlanconfig} \clival{VAP} \mytt{create wlandev}
\clival{Base Device} \mytt{wlanmode} \clival{mode}
[\mytt{bssid}$\|$\mytt{-bssid}] [\mytt{nosbeacon}]\\
\mytt{wlanconfig} \clival{VAP} \mytt{destroy}
\ev
Every Linux network device consists of a prefix followed by a number
indicating the device number of the network device.  For instance, the
ethernet devices are named \mytt{eth0}, \mytt{eth1}, \mytt{eth2},
etc.  Each VAP which is created is also registered as a Linux network
device.  The value \clival{VAP} can be either a prefix name of the Linux
network device, or it can be the entire device name.  For instance,
specifying \clival{VAP} as \mytt{ath} lets the Linux kernel add the
network device as the next device with the prefix \mytt{ath}.  Thus,
the Linux kernel appends the proper number to the end to form the full
device name, e.g., \mytt{ath1} if \mytt{ath0} already exists.
However, the full device name can also be specified.  For instance,
\clival{VAP} can also be \mytt{ath2}.  In this case, the network
device \mytt{ath2} is registered, regardless of whether \mytt{ath1}
exists.

The \clival{Base Device} is the underlying wireless network device
name created when the driver is loaded.  The MadWifi driver creates
\mytt{wifi0}, \mytt{wifi1}, etc.\ as the underlying devices.  By
specifying the \clival{Base Device}, the VAP is created with the
\clival{Base Device} as the parent device.

The \clival{mode} is the operating mode of the VAP.  The operating
mode of the VAP cannot be changed once it is created.  In special
cases, the operating
mode of the VAP can be different from the operating mode of the
underlying parent device.  The first VAP which is \emph{created} sets
the operating mode of the underlying device.  If the first VAP is
deleted and a new VAP is created with a different operating mode than
the original VAP, then the operating mode of the underlying device is
changed to the new operating mode.  The valid operating modes and
their descriptions are given in Table~\ref{tab:wlanOpMode}.
\begin{table}[h*]
  \centering
  \begin{tabular}{|l|l|} \hline
    Mode & Description \\ \hline
    Auto &  Auto select operating mode \\
    Managed & Station mode for infrastructure networks\\
    Master & AP mode \\
    Monitor & Passive monitor (promiscuous) mode \\ \hline
  \end{tabular}
  \caption{\mytt{wlanconfig} Operating Modes}
  \label{tab:wlanOpMode}
\end{table}

Only one station VAP can exist on a device.  If the station VAP is the
first VAP created, then no other VAPs are allowed to be created. If
the first VAP created is in AP (Master) mode, then one station VAP is
allowed to be created.  In this case, other AP VAPs can also be
created after the station VAP.  When AP and station VAPs coexist, the
\mytt{nosbeacon} flag must be used when creating the station.  This
flag disables the use of hardware beacon timers for station mode
operation.  This is necessary because concurrent AP and station
operation implies the station should not modify the TSF clock for the
APs.

Creating multiple VAPs typically implies that the MAC address of each
VAP is different.  However, if the \mytt{-bssid} flag is used, then
the MAC address of the underlying wireless device is cloned for the
VAP being created.

To destroy a VAP, the \mytt{wlanconfig} command is used with the
\mytt{destroy} parameter.  In this case, the full device name must be
used, i.e.\ you must specify the entire name, not just the device
prefix.

\begin{example}
  If we wish to use the system as a station only, we would create a
  single station VAP once the driver is loaded.  The following
  command creates a single station VAP named \mytt{ath0} on device
  \mytt{wifi0}:
  \bv
  \cmd{wlanconfig ath create wlandev wifi0 wlanmode sta}
  \ev
  Note that no other VAPs can be created since the we are assuming
  this is the first VAP created on \mytt{wifi0}.  Since this is the
  first VAP created, we only need to specify \mytt{ath}, not
  \mytt{ath0}.  However, the following command would also be correct:
  \bv
  \cmd{wlanconfig ath0 create wlandev wifi0 wlanmode sta}
  \ev
  The MAC address of the station VAP is the same as the underlying
  device's MAC address since it is the first VAP created.
\end{example}
\begin{example}
  Now, we wish to create two AP VAPs on device \mytt{wifi0}.  The first
  device will have a cloned MAC address taken from the underlying
  device. The second VAP will have a ``virtual'' MAC address formed
  from the underlying device's MAC address.  The first VAP will be
  \mytt{ath0} and the second device will be \mytt{ath2}.
  \bv
  \cmd{wlanconfig ath create wlandev wifi0 wlanmode ap}\\
  \cmd{wlanconfig ath2 create wlandev wifi0 wlanmode ap}
  \ev
\end{example}
\begin{example}
  Now, we wish to create two AP VAPs on device \mytt{wifi0}.  Both
  devices will have the same MAC address cloned from the underlying
  device.  The first VAP will be \mytt{ath0} and the second VAP will
  be \mytt{ath1}.
  \bv
  \cmd{wlanconfig ath create wlandev wifi0 wlanmode ap -bssid}\\
  \cmd{wlanconfig ath create wlandev wifi0 wlanmode ap -bssid}
  \ev
\end{example}
\begin{example}
  Now, we wish to create two AP VAPs and one station VAP.  The AP VAPs
  will be \mytt{ath0} and \mytt{ath2} and the station VAP will be
  \mytt{ath1}.
  \bv
  \cmd{wlanconfig ath create wlandev wifi0 wlanmode ap}\\
  \cmd{wlanconfig ath create wlandev wifi0 wlanmode sta nosbeacon}\\
  \cmd{wlanconfig ath create wlandev wifi0 wlanmode ap}
  \ev