Understanding the network.pdf - Back to Home

you can use the DSU/CSU to collect transmission error statistics. The following list
provides a description of each of the possible WAN error types:
• Bipolar violations (BPV)—Occur when two one-bit pulses of the same polarity
occur in a row. For data transmission over copper transport, bipolar signaling
is used. Binary zeros are represented by an absence of voltage. Binary ones
are represented by alternating + and - DC voltage signals on the line. BPVs
do not occur on fiber optic, microwave, or satellite transmissions because
they use unipolar signaling. Fiber optic and radio wave transmission systems
correct BPVs, if detected. However, BPVs that are "missed" are represented
in the form of bit errors (for example, an incorrectly placed bit
representation).
• Frame errors—Frame errors are discrepancies in what is expected to be the
frame pattern and what is actually received. Frame error measurement is
based on the counting of the framing bits contained in the frame, rather than
examining the entire frame.
• CRC errors—To provide additional data validity checking capabilities,
Extended Superframe (ESF) provides CRC-6 error checking. The CRC-6 tag
is calculated by the transmitting equipment by adding the sum of all the ones
and zeros and dividing it by a prime number. Then, this sum is added to the
end of the data frame. The receiving equipment at the end of the
transmission span (within the circuit path) performs the same calculation
and compares the result. If the two sums do not match, bit errors have
occurred during the transmission over the span. The CRC calculations of the
data frame are recalculated each time the frame passes through a digital
cross-connect, which interconnects the transit segments that make up the
entire data transmission circuit.
• Timing frequency errors (TFEs)—TFEs are "slips" in the circuit timing. These
slips are manifested as the addition or deletion of bits in the data stream.
TFEs result from a poor circuit timing source.
• Signal errors—Result when the signal frequency level of the circuit drops
above or below the tolerances required for proper transmission. Signal levels
vary from segment to segment, depending on the length of the transmission
medium. For proper operation, however, the signal level must be maintained
on each segment consistently. Faulty circuit transmitters, poor or open
cross-connects, an improper LBO setting on the CSU, or a cable length
violation are all possible reasons for signal errors.
Retransmission Errors
Retransmissions are the most common ULP layer error detected. They occur when
an application does not receive a proper response (or no response at all) within a
specific time window. The occurrence of an occasional network retransmission does
not signify any problem per se. Hosts that generate excessive amounts of network