Information Technology Reference
In-Depth Information
22
CHAPTER
Troubleshooting ATM PVCs in a WAN
Environment
This chapter describes how to troubleshoot ATM problems that are seen when transporting L2 frames/L3
packets over a WAN backbone. The following topics will be reviewed:
How are frames or packets segmented into ATM cells?
•
What are the important
show
commands, and how do you interpret them?
•
How can you detect and troubleshoot incorrect shaping or policing?
•
Introduction
Asynchronous Transfer Mode (ATM) is a technology that was defined by the ITU-T (formerly known as
the CCITT) in the early 1990s. The related standards describe a transport technology in which
information is carried in small, fixed-length data units called
cells
.
In an ATM network, a clear distinction can be made between the devices supporting the applications,
called end systems (ES), and the devices that are only relaying the cells. We will call these relaying
devices intermediate systems (IS) or ATM switches. Examples of end systems are routers or LAN
Emulation (LANE) modules. Examples of IS are ls1010, 8540MSR, and BPX. An ATM network can
therefore be represented as shown in Figure 22-1.
Figure22-1 ATM Network Representation
ES 1
IS 1
IS 2
ES 2
IS 3
Among many other things, ATM defines how to segment and reassemble different types of information.
ATM can transport video, voice, and data. Proper quality of service (QoS) is reserved and guaranteed by
the ATM network. Because any type of information can be segmented into cells via the related standard,
ATM is a flexible tool. Therefore, it can be used in many environments. We will categorize those areas
in two main ones:
