Leasing the Cable When You Can't Run the Cable
If two routers in two cities need to forward packets to each
other, they need some sort of physical medium over which to
send the packets. Back in Chapter 4,
"How to Build a Local (Network) Roadway," you learned that you could just run an
Ethernet cross-over cable between two devices, and the two devices could
communicate with Ethernet. That works well, but we aren't allowed to run
Ethernet cables through other people's property.
This section introduces the general idea of a WAN link, which
conceptually is equivalent to an Ethernet cross-over cable. Figure 14-1 depicts the general idea.
If you remember much about the physical details of Ethernet, Figure 14-1 should look familiar. When R1
sends out an electrical signal over the cable, R2 needs to receive that same
signal on the wires that it expects to receive data. Likewise, R1 needs to
receive what R2 transmits. Because the physical interfaces on the routers use
the same pins to transmit, the cable connects the twisted pair used for
transmitting by R1 over to R2's receive pins, and vice versa. In effect, the
cable in the figure looks like an Ethernet cross-over cable. All that the two
routers need is a cable between them, with transmit and receive pairs of wires,
so that they can send and receive anytime they want.
The temptation might be to simply use Ethernet. However, two
problems stop you from using Ethernet in this case:
-
You aren't legally allowed to run a cable between Atlanta and
Cincinnati (the sites in the figure).
-
Those who can run the cablenamely, the telephone companies
(telcos)don't lease or sell 450-mile Ethernet cross-over cables.
As you might imagine, there is a solution that acts like using
an Ethernet cross-over cable between two routers; the details are just a little
different. The next section describes the details.
|
 |
Many telephone companies are beginning to offer WAN services
based on Ethernet. However, the vast majority of existing WAN connections work
like what is described in this chapter and Chapters 15, "Leasing a (Network) Roadway Between Lots
of Places," and 16, "Driving from
Home onto the Globally Interconnected (Internet) Roadway." | |
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