User's Manual

12 Troubleshooting
150 APCD-LM043-4.0
resort. Otherwise, the level and location of the interferer has to be deduced from
measurements available at the CCU and EUM. Several of these measurements are
referenced in the preceding Troubleshooting sections.
Some further clarifications and guidelines are listed here:
• A typical data transmission between the CCU and the EUM requires information
packets to go both ways. For example, a payload transmitted from the EUM to the
CCU will be acknowledged with an ACK packet returned from the CCU. If the
transmission fails, it is difficult to determine directly which direction failed (for an
interferer, the failure will occur at the end of the link which is closer to the interferer).
• With the exception of the received signal level, the CCU radio environment is the
same for all EUMs. If transmissions from one EUM are unreliable, but transmissions
from another EUM with similar received signal strength at the CCU are not, then the
likely problem will be found at the affected EUM. If all EUMs of similar signal strength
are similarly affected, then the problem will likely be found at the CCU.
• If communications exist, the quality of the transmission can be measured by the
transmit retry rate, as indicated in previous sections. This is a powerful diagnostic tool.
• If communications exist, longer packets will suffer more failures (retransmissions)
than shorter packets, which can cause the customer to see some applications work
better than others (e.g. browsing may be less impacted than FTP file transfers). The
ping test, using variable length ping packets, can be a useful device to quantify the
extent of this problem.
• If communications from an EUM to the CCU are not possible, the ARP table can be
used to divide the link into two sections. A ping from the EUM to the CCU that is
received by the CCU will cause the EUM to be entered into the CCU ARP table. Even
if the response from the CCU is lost, verifying the entry in the CCU ARP table will
confirm the EUM-to-CCU link and suggest that the EUM is in a relatively severe
interference environment.
• If the receive signal level at the EUM is above -80dBm, and the EUM has the correct
frequency and valid IP addresses, then if the EUM cannot ping the CCU, it is highly
likely there is an interferer in the vicinity of the EUM.
• A local interferer at the EUM location can usually be managed through proper
placement of the antenna, and if the interferer is in the same residence, judicious
placement of the interfering device.
• For comprehensive diagnosis of an EUM, and determining mitigating actions in severe
cases, a spectrum analyzer may be required.