Hardware Reference
In-Depth Information
long as the number of objects connected to each of these
is small, the effect on the whole network is minimal. It may
not seem minimal when you're using the object whose
connector fails, but the rest of the network remains stable,
so it's easy to reconnect when your connector is working
again.
If you're using the Internet as your network, you can
take this model for granted. If you're building your own
network, however, it's worth comparing all these models
to see which is best for you. In simpler systems, one of the
three networks shown in Figure 3-2 might do the job just
fine, saving you some complications. As you get further
into the topic, you'll see some examples of these; for the
rest of this chapter, you'll work with the multitiered model
by relying on the Internet as your infrastructure.
X
Figure 3-3
A complex, multitiered network.
Multitiered network
Modems, Hubs, Switches, and Routers
The connectors in Figure 3-3 represent several different
types of devices on the Internet. The most common among
these are modems, hubs, switches, and routers. Depending
on how your network is set up, you may be familiar with one
or more of these. There's no need to go into detail as to the
differences, but some basic definitions are in order:
All the devices attached to a hub receive all the messages
that pass through the hub, and each one is responsible for
filtering out any messages that aren't addressed to it. Hubs
are cheap and handy, but they don't really manage traffic.
A
switch
is like a hub, but it's more sophisticated. It keeps
track of the addresses of the objects attached to it, and it
passes along messages addressed to those objects only.
Objects attached to a hub don't get to see messages that
aren't addressed
to them.
A
modem
is a device that converts one type of signal into
another, and connects one object to one other object. Your
home cable or DSL modem is an example. It takes the digital
data from your home computer or network, converts it to
a signal that can be carried across the phone line or cable
line, and connects to another modem on the other end of
the line. That modem is connected to your Internet Service
Provider's network. By this definition, the Bluetooth radios
from Chapter 2 could be considered modems, as they
convert electrical signals into radio signals and back.
Modems, hubs, and switches generally don't actually have
their own addresses on the network (though most cable and
DSL modems do). A
router
, on the other hand, is visible to
other objects on the network. It has an address of its own,
and it can mask the objects attached to it from the rest
of the net. It can give them private addresses, meaningful
only to the other objects attached to the router, and pass
on their messages as if they come from the router itself. It
can also assign IP addresses to objects that don't have one
when they're first connected to the router.
A
hub
is a device that multiplexes data signals from several
devices and passes them upstream to the rest of the net.
It doesn't care about the recipients of the messages it's
carrying—it just passes them through in both directions.
