[ee122] Clarification on a past midterm question

Anthony Kilman a_kilman at berkeley.edu
Wed Oct 10 15:50:22 PDT 2007

2. Framing.
Traditional Ethernet does not include a length field in its header.
Instead, it uses the absence of an electrical signal on the cable to
deduce the end of a frame.

    (a) Assuming that the physical layer encodes “low” as the absence of
an electrical signal and “high” as the presence of an electrical signal,
for which of the following encodings:

            i. NRZ (Non-Return to Zero)
           ii. NRZI (Non-Return to Zero Inverted)
          iii. Manchester
          iv.  4-bit/5-bit
might the Ethernet controller have difficulty determining the end of the
frame, and under what circumstances? Why not for the others? (8 pts)

My question is, are we still assuming we're referring to the same "high"
and "low" used to encode 0's & 1's, say like the NRZ? Or is this
information somehow encoded with the voltage on the line remaining above
a threshold that would consider it "high"?

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