Troubleshooting ATM PVCs in a WAN Environment - Internetworking Troubleshooting - page 584

• A LAN switched environment, in which LAN Emulation (LANE) is most commonly used.
Typically, there is little QoS in this dynamic environment. (ATM connections are built and torn down
on demand.)
• A WAN environment, in which we have two players:
- The telco, which typically offers very precise quality of service in a static environment. This
ATM network is made of ATM switches. Because a telco offers an ATM service, we will call
him an ATM service provider.
- The enterprise, which asks for an ATM service from the ATM service provider.
This chapter focuses solely on the last type of ATM connections, in an enterprise environment. End
systems in such an environment are routers 99 percent of the time. Therefore, we will use only the word
router in the rest of this document. Those routers will exchange packets (see the following note). We will
use IP as our reference protocol, so all explanations are valid for other Layer 3 protocols such as IPX
and ATALK. As such, from the enterprise point of view, the network is represented in Figure 22-2.

Figure22-2 Network from the Enterprise Point of View

AT M
cloud

Router 1 Router 2

Because ATM is a flexible tool, we can segment pretty much any type of information into
cells. We often talk about packets or frames, Layer 3 or Layer 2 data units. Clearly, we
could use the term protocol data unit , which would allow us to discuss very generally
whatever layer is involved, in sync with the OSI specification. For the sake of clarity, we
will talk about packets, however, and you will understand frames, if necessary.

Note

There must be a traffic contract on the quality of service that will be respected by the enterprise routers
and the ATM service provider. Initially, it looks quite simple, with only two devices in the picture and
an ATM provider's cloud that is simply not visible from the enterprise point of view. Unfortunately, the
problems in this environment are not trivial precisely because you do not have full visibility of the ATM
provider's equipment.

Understanding the Segmentation and Reassembly for AAL5
Frames

AAL here refers to the ATM Adaptation Layer. As the name suggests, it adapts user information (data,
voice, video, and so on) to a format that can be easily divided into ATM cells. The process for AAL5 is
described in Figure 22-3.