Download Using Mac OS X Using wireless client software

Chapter 6: Better Living Through Network Monitoring
Using Mac OS X
Mac OS X doesn’t have a single interface that shows available networks quite
as completely as does Windows XP — the interface on the Mac is a bit less
involved, but probably also a bit easier. To “sniff” out available networks
using OS X, simply open Apple’s Internet Connect application — it’s in your
Applications Folder, and may very well be located on the OS X dock.
When Internet Connect is open, click on the AirPort tab. You see a pulldown
menu that displays each wireless network within range of your Mac (shown
in Figure 6-2). Put another way, it displays each wireless network that has
enough signal strength to reach your Mac. This display provides a green bar
graph display of signal strength — simply choose another network from the
pulldown menu.
Figure 6-2:
Showing
signal
strength in
Mac OS X.
You can streamline this process by putting your AirPort display in the OS X
menu bar. Just go to System Preferences (in your Applications folder or on
the Dock) and click on Network. In Network, click on the AirPort tab and
make sure that the Show AirPort Status in Menu Bar checkbox is selected —
if it’s not, select it and click the Apply Now button.
When the AirPort status is in the menu bar, you always see a list of available
networks in the menu bar, and you can open Internet Connect by simply
using this pulldown menu.
Using wireless client software
Although Windows Zero Config is the easiest way for Windows users to find
and examine wireless networks, it’s not the only way. Most wireless network
adapters include their own client software that handles network configuration, AP selection, and more.
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These client software packages usually offer a network monitoring application that gives you more network information than the software clients that
are built into the operating systems. For example, these clients may provide
more elaborate signal strength and SNR meters and may even offer some raw
data on the actual data throughput across your wireless network (for example, showing bit or packet rates and error rates).
For example, the client software included with NETGEAR’s wireless network
adapters, shown in Figure 6-3, shows transmit, receive, or both transmit and
receive data rates graphically and numerically (in terms of packets of data
per second). It also shows the packet error rate, which, although not identical, is proportional to the BER we discussed earlier in the chapter.
Figure 6-3:
Checking
packet
throughput
using
NETGEAR’s
wireless
client
software.
Using Free “Stumblers”
The Wi-Fi client software included with operating systems and network
adapters can be very handy for tracking down available networks and for
seeing really high-level representations of network performance, but they are
not good tools for doing really precise measurements and monitoring of network performance.
If you’re looking to really examine your wireless network environment, try this
easy (and free!) solution: Download a wireless sniffer or monitoring program.
The most popular (and famous) of these programs is an application known as
NetStumbler — a Windows program — but there are other programs for Mac
OS X, Linux, and even Pocket PC and Palm OS handheld computers.
Chapter 6: Better Living Through Network Monitoring
Network Stumbler
The king of the network monitoring programs, at least for Windows users
(which is to say, for most folks) is Network Stumbler (or NetStumbler),
www.netstumbler.com. This freeware program (version 0.4.0 is current as
we write) actively searches the airwaves for available wireless networks and
displays a wealth of data about them, including
SSID
Channel
Nominal speed (for example, 11 Mbps or 54 Mbps)
Signal strength (in dBm)
Noise (in dBm)
SNR
Encryption (NetStumbler doesn’t differentiate between WEP and WPA; it
simply says “WEP” when encryption is on, and is blank when it’s not.)
System vendor (helpful if you know you’re looking for, say, an Apple
AirPort)
IP address
NetStumbler is easy to use — just download the software from the NetStumbler
Web site and run the installer. A few seconds later, you’re ready to try it out.
NetStumbler is an active scanning network monitoring tool. It finds and monitors networks by actively sending out probe requests on all the Wi-Fi channels
supported by the card in your PC. Most networks respond to these probe
requests, but not all do. A passive scanning monitoring tool (such as Kismet
for Linux computers, or KisMAC for Mac OS X, which we discuss in the next
section) sets your network adapter in a special “listening” mode (called
RFMON mode) that can pick up all of the wireless traffic in your area. We
mention this not because we think NetStumbler is a weak tool (we think it’s a
great tool, in fact), but to let you know that there may be some networks that
it won’t pick up.
NetStumbler doesn’t support all wireless network adapters, but it supports
many. You can find a list of supported cards in the Release Notes file on the
NetStumbler site (www.netstumbler.com/downloads/netstumbler_v0.4.0_
release_notes.pdf), but you may not know which chipset is inside your card.
We recommend that you just download the program and try it out. It’s free, it’s
not a big download, and it won’t mess your system up at all, so try it!
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The first time you run NetStumbler (to do so, just go to your Start menu and
select All Programs➪Network Stumbler), you’ll want to set a few preferences.
To set your NetStumbler prefs, do the following:
1. Select Options in the View menu.
The Options dialog box opens, as shown in Figure 6-4.
Figure 6-4:
Configuring
NetStumbler
preferences.
2. In the General tab, click and drag the Scan Speed slider all the way to
the right to the Fast setting.
3. In this same tab, make sure that the Reconfigure Card Automatically
check box is selected.
This allows NetStumbler to find all the networks in your area, instead of
just the ones you’re already associated with.
4. If you like, you can click on the MIDI tab and select the Enable MIDI
Output of the SNR check box.
Choosing this option causes NetStumbler to emit (through your MIDIenabled sound card) “poor-sounding musical instrument noises.” (We’re
quoting the NetStumbler help file here!) This is mostly annoying, but it
can be useful in situations where you can’t closely watch the screen
while you’re moving around trying to determine signal quality. The pitch
of the musical instrument increases as the SNR increases. (The better
the SNR, the more brain-rattling the pitch!)
5. Click OK to save your settings and close the Options dialog box.
When you run NetStumbler, it automatically turns off Windows Zero Config
(as long as you checked that Reconfigure Card Automatically checkbox in the
preferences) and begins to actively scan all channels in the 802.11a/b/g spectrum (depending upon the network adapter you’re using) — as long as your
wireless network adapter is supported.
Chapter 6: Better Living Through Network Monitoring
Sometimes, NetStumbler does not see any available networks, even though
they are there and “should” be seen. If this happens, your network adapter
may not be supported. Before you give up hope, however, go into the
NetStumbler Device menu and see if there’s more than one driver for your
wireless adapter. For example, for Pat’s NETGEAR RangeMax 802.11g card,
NetStumbler offers a choice of two drivers (Atheros and NDIS 5.1, as shown in
Figure 6-5). For this particular card, the NDIS driver works with NetStumbler,
and the Atheros driver does not. It should say Unsupported next to the
driver if the driver won’t work with NetStumbler, but occasionally it won’t
say that — NetStumbler just won’t see all of your networks. Choose a different driver and see if that makes NetStumbler work for you! You can also
check the following URL to see if your card is compatible: www.stumbler.
net/compat/.
If you select the NDIS driver for your wireless network adapter, NetStumbler
will receive RSSI data instead of signal and noise data (measured in dBm). In
this case, the signal strength data in NetStumbler will be useful, but any noise
or SNR data will not be.
Figure 6-5:
Choosing a
driver in
NetStumbler.
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After NetStumbler has begun monitoring the airwaves, you can find different
ways to display (and therefore analyze) the data. In the left pane of the
NetStumbler window (shown in Figure 6-6), you see several different display
options, each with an expand/collapse plus/minus sign next to it (they’re all
expanded in the figure). You can sort through your networks using these controls by
Channel: You can look at individual 802.11a/b/g channels to see which
access points are on which channels. This can be handy when you’re
trying to figure out to which channel to assign your APs (you can figure
out what the neighbors are using). Click on the plus signs next to individual channels to expand the listing of devices on each channel (listed
by MAC address).
SSID: You can also sort by SSID. This may seem unnecessary, but it actually can be a good tool to see if there are multiple APs using the same
SSID. (Maybe your neighbors all bought identical routers during the last
sale at Fry’s!)
Filters: NetStumbler also provides a range of filters that let you sort
through available networks to find only those that meet certain criteria.
A number of these filters are available, but some of the most interesting
include
• Encryption On or Encryption Off: You can quickly find “open” networks (or ensure that your APs have encryption enabled) by using
these filters. This can be a handy tool when you’re searching for
that “free” AP at the hotel or café to check your e-mail.
• ESS (AP) or IBSS (Peer): Use these filters to sort through the available networks by their status as access points and as peer-to-peer
client networks. (You probably won’t want to try to associate with
peer-to-peer wireless networks unless you’re absolutely sure who
you’re connecting to.)
• Short Slot Time (11g): This filter tells you which 802.11g networks
(if any are available) are set up for “802.11g only” mode. If you’re
looking for the fastest networks around, this is one way (along with
signal strength) to find them.
When you sort NetStumbler using these navigation tools, you see a listing of
each AP or network that fits into that particular category. From this view, you
can read the text columns on the right side of the window (shown in Figure
6-7) to see important network data (like SNR) at a quick glance.
Chapter 6: Better Living Through Network Monitoring
Figure 6-6:
Navigate
NetStumbler
by
expanding
or collapsing
the
navigation
tools on
the left.
Figure 6-7:
Viewing
NetStumbler
data in
column
form.
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To get a graphical representation of any particular network, simply click on
the MAC Address of the device within the channels and SSID views and the
right pane shifts to the display shown in Figure 6-8. From this view, you can
see a running, time-based graphical representation of either signal strength
(for the NSID drivers) or signal strength and noise (for other drivers).
This display can be very handy when you’re changing your network in some
way because you can watch the effects of changes unfold on your computer
screen in real-time.
For example, if you’re trying to find out how well your AP covers your home,
you could use a laptop running NetStumbler and slowly move around the
house (like the Verizon “Can you hear me now?” guy), and watch the signal
strength or SNR dip and peak as you move in and out of good coverage.
If you find a room with poor coverage, have a helper adjust your AP or antenna
placement while you watch the SNR and use NetStumbler as a tool for optimizing placement. You can also turn on the MIDI audio output we mentioned at the
beginning of this section to add an aural dimension to this process.
We talk in more detail about how to do this kind of network optimization in
Chapter 7, but we mention it here so you get an idea about how tools like
NetStumbler can be used.
Figure 6-8:
Using
NetStumbler
to graph
performance
over time.