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CCNP Practical Studies: Troubleshooting by Donna L. Harrington

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Frame Relay at the Physical Layer

Although Frame Relay is a Layer 2 technology, it obviously depends on Layer 1, too. How about things such as clocking, cables, controllers, and channel service units/data service units (CSUs/DSUs)? These are all things you take for granted, but cannot forget in the support world. Although in lab scenarios you use back-to-back serial cables with one end wired to be a physical DTE and the other end to be a physical DCE, this works a little differently in the practical world.

The router may directly connect to the Frame Switch or to a CSU/DSU that connects to the frame switch. The CSU/DSU is like a modem in many respects. It converts the V.35 or EIA/TIA-449 signals to a properly coded transmission signal necessary ...

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