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T1: A Survival Guide by Matthew Gast

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Link Layer Problems

Some apparent link layer problems can be traced to physical layer causes, but others are actually problems in the link layer. The tables in this section will lead you to a physical layer solution where appropriate, because the tables are constructed with symptoms. This section assumes that you have addressed all physical layer problems and that no alarms are present on either side of the link.

General Link Layer Problems

When the link layer fails to come up, a common problem is an incorrect clocking setting. One common scenario is that the line has physical connectivity, but no link layer protocol is able to initialize. This usually occurs because the clocking between the CSU/DSU and the router is not configured correctly. Incoming link layer negotiation requests are received by the CSU/DSU and passed out the serial interface to the router. At the router, the requests are processed correctly and responses are sent. If the transmit clocking on the data port is configured incorrectly, however, the response to the incoming configuration request is not received by the CSU/DSU and thus cannot be sent out the T1 network interface. At the remote end, the line appears to be quiet. No physical problems exist and no alarms are present, but no data comes in, either.

Not all routers can supply a transmit clock with the data. If the CSU/DSU is looking for an external transmit clock from the router, it will not read any incoming data, even if that data is supplied according ...

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