Layer 1 (Physical) Connectivity
Before
any two components of your network can talk to each other, they must
share a common physical medium through which they communicate. In the
wired world, this is obvious; you would never try to connect a copper
CAT5 cable to a piece of fiber and expect it all to
"just work."
In the wireless world, every device from your network to your
cordless phone to your garage door opener must share the same
physical medium: electromagnetic waves radiating through the air. It
is possible for all of these devices to communicate without
interfering with each other because they can be made sensitive to a
particular portion of the vast electromagnetic spectrum. This is
analogous to tuning channels on a radio or TV—many channels are
broadcasting simultaneously, but they are well-coordinated and only
use a portion of the available spectrum, to avoid interfering with
each other.
So before any other considerations, devices that need to
intercommunicate on your network must be able to send signals in the
same frequency range. Obviously, an 802.11b card operating at 2.4GHz
doesn't have a chance of carrying on a conversation
with an 802.11a Access Point speaking at 5GHz. In addition to using a
particular frequency range, each wireless protocol also defines a
plan for using that range. For example, the original 802.11
specification defines two RF modulation schemes, FHSS and DSSS. Both
operate at 2.4GHz, but use the spectrum differently.
Frequency Hopping Spread Spectrum (FHSS)
breaks the available spectrum into 77 channels, each 1MHz wide. It
uses a time-based, pseudo-random algorithm to quickly skip between
all of the available channels in an attempt to avoid noise from other
2.4GHz devices. As we saw in Chapter 2,
Direct Sequence Spread Spectrum (DSSS)
breaks the same frequency range into 11 overlapping channels, each
5MHz apart (but 22MHz wide). It uses one channel at a time and
employs more sophisticated encoding techniques to avoid noise and
increase the data rate. Although FHSS and DSSS devices both operate
"at 2.4GHz," they have no hope of
being able to communicate with each other.
Whatever wireless equipment you choose, be sure that both ends are
capable of speaking the same protocol at the same frequency range,
whether that's 802.11b speaking DSSS at 2.4GHz,
802.11a speaking OFDM at 5GHz, 802.11g speaking OFDM at 2.4GHz, or
something altogether different, new, and wonderful. If two pieces of
equipment claim compatibility with the same IEEE standard (such as
802.11b), they should theoretically be able to interoperate. Be sure
to check the fine print on any device that only claims compatibility
with an umbrella term (such as
Wi-Fi), because the
definition of the term can change at the whims of marketing
moguls.
As 802.11b is by far the most common technology used in the wireless
community effort, we will focus on its particulars for the rest of
this chapter.
3.1.1 Layer "1.5" Connectivity
Simply using equipment that adheres
to the same standard on the same channel doesn't
quite fulfill the requirements of Layer 1 (physical) connectivity.
There are a few more protocol requirements that must be met before we
can move on to Layer 2.
802.11b defines two possible (and mutually exclusive) radio modes
that stations can use to intercommunicate. Those modes are
BSS and IBSS.
BSS stands for Basic Service Set. In this operating
mode, one station (the BSS
master, usually a piece of hardware called an
access point, or AP) provides wireless-to-Ethernet bridging. Before
gaining access to the wired network, wireless clients (also called
BSS clients) must first establish communications
with an access point within range, as shown in Figure 3-1. Once the AP has authenticated the wireless
client, it allows packets to flow between the client and the attached
wired network, either routing traffic at Layer 3, or acting as a true
Layer 2 bridge. A related term, Extended Service Set (ESS), refers to
a physical subnet that contains more than one AP. In this sort of
arrangement, the APs can communicate with each other to allow
authenticated clients to "roam"
between them, handing off IP information as the clients move about.
Note that (as of this writing) there are no APs that allow roaming
across networks separated by a router.
IBSS stands for Independent Basic Service
Set, and is frequently referred to as ad-hoc or peer-to-peer mode. In
this mode, no hardware AP is required. Any network node that is
within range of any other can communicate if both nodes agree on a
few basic parameters. If one of those peers also has a wired
connection to another network, it can provide access to that network.
Figure 3-2 shows a model of an IBSS network.
Note that an 802.11b radio must be set to work in either of these
modes, but cannot work in both simultaneously. Both modes support
shared-key WEP encryption (more on that later).
I give specific examples of how to set up BSS and IBSS networks in
Chapter 4 and Chapter 5. Once you have two or more IBSS mode stations,
or a BSS master and one or more BSS clients all within range, you are
ready to move on to actual networking.
3.1.2 Layer 2 and Up
Once
the physical layer is established, a wireless network is very much
like a traditional Ethernet network. Assuming that you want to
connect your wireless clients to the Internet,
you'll want to provide all of the usual TCP/IP
services that make networking so much fun (such as DNS and DHCP). To
the rest of your network, wireless clients look like just another
Ethernet interface, and are treated no differently than the wired
printer down the hall. You can route, rewrite, tunnel, fold, spindle,
and/or mutilate packets from your wireless clients just as you can
with any other network device. Once wireless packets hit the wire,
you use the same hubs, switches, and routers that make up the
majority of traditional wired networks.
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