If there is a problem with sub-optimal operation at the physical layer, the
network may be operational, but performance will be consistently or
intermittently lower than the level specified in the baseline.
If
performance is unsatisfactory all the time, then the problem is probably
related to a poor configuration, inadequate capacity somewhere, or some other
systemic problem. If performance varies and is not always unsatisfactory, then
the problem is probably related to an error condition or is being affected by
traffic from other sources:
Unstable routing due to a marginal port or link somewhere beyond the
broadcast domain, possibly the result of a bad cable
Excessive traffic across a low speed LAN or WAN link, possibly causing
traffic to be discarded or buffer capacity to be exceeded
Overloaded server or service
A physical layer optimization problem occurs when the physical
properties of the connection are substandard, causing data to be transferred at
a rate that is consistently less than the rate of data flow established in the
baseline.