The effects of intermittent errors at the data-link layer will depend on the
particular Layer 2 protocol in use. If the Layer 2 protocol is designed to
implement reliable communications such as X.25, then it may be difficult to
identify Layer 2 problems from the behavior of the upper layers. This is
because the Layer 2 protocol will simply retransmit any lost frames. If frames
are lost frequently, then application network performance may be sluggish or
application time-outs may occur.
For Layer 2 protocols that can be classified as best effort delivery, such
as Ethernet, the loss of a frame immediately translates to the loss of a packet
and in the case of TCP, a segment. In this case it is the responsibility of TCP
to retransmit lost segments. Once again, if the frame losses are intermittent
the effects on the application may be that it runs sluggishly or the loss of
performance may not be noticeable at all. However, TCP statistics should show
evidence of the segment loss and retransmissions.
The chart in Figure
outlines the
symptoms that might be experienced at each of the layers of the TCP model when
an intermittent or total Layer 2 failure occurs.
Although these characteristics give some indication that a Layer 2 problem
exists, it will be inconclusive until the devices attached to the
non-functional data link are examined.