Chapter 15. Leasing a (Network) Roadway Between Lots
of Places
What You Will Learn
After reading this chapter, you should be able to
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Compare and contrast point-to-point WAN link's physical layer
with Frame Relay physical topologies
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Compare and contrast HDLC and PPP with Frame Relay
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Explain the basic forwarding process in Frame Relay using
DLCIs
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Explain the concept behind the term virtual circuit
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List several reasons why Frame Relay is a widely used WAN
technology
The U.S. Department of Transportation (DOT) could plan to build
a road directly between your house and every other place to which you might want
to drive. It could do the same for everyone else as well. That would be
ridiculous, of course, because the DOT would end up paving practically the
entire country! Instead, the DOT built one road to your house, which is in turn
connected to other roads, ultimately allowing you to drive anywhere you want to
drive.
Chapter 14, "Leasing
a (Network) Roadway Between Two Points," covered the basics of how a serial link
could be used between two routers, which can be compared to paving a road
directly between. This chapter covers a WAN technology called Frame Relay. Frame Relay uses one
physical WAN link connected to each site, while allowing each site to send data
to each other site. That's a lot like the DOT paving one road to your house, and
you driving anywhere you want, always leaving your house by driving over that
one road.
When you want to build a network to connect multiple remote
sites, you could just order a lot of serial links. However, Frame Relay requires
less work and less new hardware, making Frame Relay a much more cost-effective
solution compared to leased lines.
In many ways, Frame Relay acts like a network built with an
Ethernet switch. With an Ethernet switch, more than two devices can be cabled to
the switch. To send an Ethernet frame to any of the other devices, the sender
just needs to put the right destination Ethernet address in the frame. As you'll
read in this chapter, a Frame Relay network acts like a big WAN switch, with
routers connecting to it. To send data to another router, the sending router
just needs to send a frame with the right address in it.  |
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