Please help with these questions

1. When the "default Gateway" fails to respond, what OSI layer
would be the first to experience this problem?

2. To what layers do the following belong?

a. Router serial line encapsulation failure?
b. Dialup authentication failure?

Thanks.

With every reservation about the correctness, here's what I think:

1) Network Layer
- Gateways are an IP construct.
2a) Physical Layer
- Just a reasonable guess. It makes sense to me that the serial encapsulation would fail, if connectivity at the physical layer failed.
2b) Datalink Layer
- To my understanding authentication mechanisms in PPP work at the datalink layer, since they are part of the LCP (Link Control Protocol).

Again I haven't verified the answers, they may be wrong, especially since the questions are theoritical. Perhaps someone else can say for sure.

I too agree with NSKE, but am not sure about 2a.

This could also be a layer 2 problem due to the fact serial line encapsulation failure can be the result of an incorrect 'clock rate' setting at the serial interface.

The question is, does this setting involve the physical or data-link layer?